STRATIGRAPHIC NOMENCLATURE OF THE SIRTE BASIN, LIBYA by F. T. BARR and A. A. WEEGAR PUBLISHED BY THE PETROLEUM EXPLORATION SOCIETY OF LIBYA TRIPOLI, LIBYA 1972 Drilling rig located in sand sea, Waha Field, central Sirte Basin (photograph by R.C. Schwalbe) INTRODUCTION In 1960, the Stratigraphic Lexicon for Libya was published under the auspices of the Commission of Stratigraphy of the International Geologic Congress. It was prepared by the Names and Nomenclature Committee of the Petroleum Exploration Society of Libya under the chairmanship of P. F. Burollet. This lexicon was completed during the early years of intensive petroleum exploration in Libya. Considerable surface geological mapping had been done by oil company geologists during the 1950's and a number of exploratory wells had been drilled in northern Cyrenaica, western Libya and the Sirte Basin. The first post-war wildcat, the Libyan American A1-18, was spudded in April 1956, on the Jabal al Akhdar, northern Cyrenaica, and two and a half years later, in the autumn of 1968, the Oasis B1-32 well discovered Libya's first commercial oil in the D ahra Field in the northwestern Sirte Basin. Consequently, the 1960, Stratigraphic Lexicon was based mainly on pre-war publications dealing with surface exposures and on lat er u n pu blish ed oil c om pan y su r fac e dat a. Refer enc e m ade to unpublished company reports, however, was unfortunate as it led to the introdu ction of a nu m ber of new form ational nam es that were n ever adequately defined or documented. The subsurface formations of the Sirte Basin were not dealt with. Nevertheless, this Stratigraphic Lexicon has been one of the main source hooks to the large number of petroleum geologists engaged in the Petroleum Industry of Libya over the past decade. In 1963, it was decided by the Petroleum Exploration Society of Libya to revise the Stratigraphic Lexicon. Committees were formed and work proceeded intermittently over several years. During this period, however, there was a great turnover in oil company personnel which precluded any continuity in this rather long term project. In addition, there was some question as to value of a revision of the 1960 lexicon at that time, as almost nothing had been published on the Sirte Basin. There is still very little in print on the subsurface formations of the Sirte Basin, bu t several papers on the geology of Libyan oil fields have recently been published, which inclu de the Am al field (Roberts, 1970), Augila Field (Williams, 1968), Intisar "A" Field (Terry and Williams, 1969), Sarir field (Sanford, 1970) and Zelten Field (Fraser, 1967). These studies discuss the subsurface stratigraphy of their particular field areas in varying detail, but usually with emphasis on the producing reservoirs. They are the main published works mentioning the subsurface stratigraphy of the Sirte Basin and as such are extremely important.However, it was not their objective to establish a scheme of formational names for the stratigraphic succession encountered, and consequently, few names were formally proposed and given adequate documentation. In addition to the oil field studies, there have been a number of significant publications since 1960 dealing with the formational n om en c l atu r e of su r face ex posu r es som e o f whi c h ext en d in t o th e subsurface. These studies include Jordi and Lonfat (1963) and Gohrbandt (1966b) on the Hon graben area along the west flank of the Sirte Basin, Desio et al. (1963), Burollet ( 1963) and Magnier (1963) on the Jabal Nefusa; and Kleinsmeide and van den Berg (1968), Pietersz (1968) and Barr and Hammuda (1971) on northern Cyrenaica. It was decided that the greatest need was for the establishment of a subsurface formational nomenclature for the Sirte Basin. Hundreds of wells have been drilled in this basin over the last decade and hu ndreds of geologists have devoted years to the study of the sedimentary sequence penetrated by these wells. Because of the confidential nature of much of the petroleum exploration, there was not a complete exchange of data and ideas within the industry. This resulted in the election of independent systems of formational names by each operating company. As none of these systems was pu blished an d m ade available t o t h e pr ofession , t h ey had no recognised validity outside the confines of the particular company. The advantages of a single system of formational names was obvious so it was in this direction that the present Nam es and Nomenclature Com m ittee decided to proc eed. It was fu rt h er decided th at becau se m an y n ew formations would have to be proposed and documented, this special publicati on of the Petroleum Exploration Society of Libya would best serve our purposes, rather than a volume in the series of international lexicons. This volume is mainly concerned with the Sirte Basin and names are proposed for forty new groups, formations and members recognised in the subsurface. However, three columns on the enclosed correlation chart are devoted to rock units exposed at the surface in the Hon graben area, Jabal Nefusa and northeastern Libya. These formations are briefly described in the text. This was done to relate the well known surface formations with those known only in the subsurface. Much of the resulting system of formational nomenclature shown on the correlation chart has been successfully used for many years. It is not a system er ec ted by a single oil c om pan y bu t has c om e fr om variou s companies, although principally from Mobil Oil Libya - Gelsenberg A.G. and Oasis Oil Com pany of Libya(1) an d represents the efforts of num erous petroleum geologists. It is a stratigraphic nomenclature that has evolved and matured over the last decade and a half. We thank our many colleagues in Tripoli for their considerable aid in the preparation of this volume. D.W. Baird, G.K. Baker, A.S. Campbell, E. Kling, W.G. Frost and C.H. Squyres reviewed the manuscript and offered m any h elpfu l su ggest ion s. L.W. Fahr m eier prepar ed th e lith ological descriptions of most of the photomicrographs. Mobil Oil Libya - Gelsenberg A.G. and Oasis Oil Company of Libya are acknowledged in particular for releasing publication portions of numerous electric-logs and for providing the necessary time and drafting facilities to produce this volume. F.T. BARR Chairman - Names and Nomenclature Committee February 10, 1972 (1) Oasis is the operator for Amerada Petroleum Corp. of Libya, Continental Oil Co. of Li b ya, M ar at ho n Pet ro l eu m L ib ya an d S hel l E xp lo r at i o n en P r odu c t i e, Maatschappij (Libya) N.V. AIN TOBI LIMESTONE Age: Cenomanian The Ain Tobi Limestone was originally proposed by Christie (1955, pp. 18, 19) for a prominent cliff forming limestone unit along the northern escarpment of Jabal Nefusa. The type section is exposed along the TripoliGarian highway near the top of the Jabal Nefusa escarpment. Lithology This formation consists of a light gray or yellow-gray, massive, dolomitic limestone which is often oolitic near its base. The lower portion of the formation often contains rounded quartz grains. Desio et al. (1963) report that the Ain Tobi Limestone has a relatively constant thickness of about 460 to 525 feet. Contact Relationships The Ain Tobi Limestone represents a transgressive unit deposited as the Cenomanian seas advanced across a sequence of non marine older Mesozoic rocks. The Chicla Formation-Ain Tobi Limestone contact is well exposed along the Bu Gheilan-Garian road about half way up the northern escarpment of Jabal Nefusa. No angular discordance can be seen at the contact; in fact, there is a gradational appearance that is probably the result of the uppermost few feet of Chicla sands being reworked by the initial Cenomanian transgression. East of Ras el Tahuna, Desio et al. (1963) report that the Chicla Formation is missing and the Ain Tobi Limestone directly overlies the Lower Jurassic Bu Sceba Formation. The Ain Tobi Limestone is conformably overlain by the Cenomanian Jefren Marls. Palaeontology and Age Much of the Ain Tobi Limestone is unfossiliferous and when fossils are recovered they are usually poorly preserved. When Christie (1955) described the Ain Tobi Limestone 11 he listed a number of pelecypods including the rudist Ichtyosarcolites which is concentrated in a traceable band. These pelecypods indicate a very shallow marine environment of deposition and a Cenomanian age for the Ain Tobi Limestone. Origin of Name This formation derives its name from a water well (= ain in Arabic ) near Kaf Tobi at the type section a few miles northeast of Garian. 12 AKHDAR GROUP Age: Eocene Kleinsmeide and van den Berg ( 1968) proposed the name Ak h dar Formation to include all of the Eocene rocks of Jabal al Akhdar. The Akhdar Formation included the Apollonia Limestone, Derna Limestone and Slonta Limestone of Gregory (1911) which were lowered to the rank of members. The present authors contend that Gregory's terms are valid mappable rock-units over much of Jabal al Akhdar and should be retained as formations. The Akhdar Formation is therefore raised in rank to group. Lithology See Apollonia Limestone, Derna Limestone and Slonta Limestone. Contact Relationships The Akhdar Group unconformably overlies either the Jardas Formation, Atrun Limestone or Wadi Ducchan Formation The Oligocene Cyrene Formation usually unconformably overlies the Akhdar Group. Origin of Name This group derives its name from the Jabal al Akhdar where it is extensively exposed. 13 AMAL FORMATION Age: Cambro-Ordovician (?) The name Amal Formation has recent]y been used for a formation in the subsurface of the Amal Field area (Roberts, 1970). However, it was not formally proposed as a new formation nor was a type section established. Therefore, the Amal Formation is here proposed as a new formation in the subsurface of the eastern Sirte Basin, however, it is identical to the rock unit used by Roberts. The type section is located in the Mobil B1-12 Well at a drill depth of 9829 to 11,290 feet (total depth), which corresponds to a subsea depth of 9688 to 11,149 feet. The base of the formation was not reached. Lithology The Amal Formation is predominantly a sandstone sequence. It is heterogeneous in color, grain size and sorting. In color, the sandstones range from white to red, purple, tan and gray. The tan, white and gray colors are dominant. The grain size ranges from very fine sand to cobbles with the medium to coarser grain sizes being more common. Sorting is generally poor, and conglomerate beds are frequent. Quartz is the dominant detrital material as well as the most important cementing agent . Accessory constituents are feldspar, mica, pyrite, hematite and various dark minerals. Clays, sericite and rarely dolomite are found as cementing materials in much of the formation. Usually, the sandstones are firmly cemented, and orthoquartzites are common, particularly in the upper part of the formation. Interbedded with the sandstone, but comprising a much lesser part of the formation, are gray silty clays and gray, green and red, brittle, micaceous shales. In addition to the sedimentary rocks, volcanic rocks in the form of dikes, sills or flows are found at a number of horizons in the upper part of the formation. Contact Relationships In the B1-12 Well, the Amal Formation is unconformably overlain by the Maragh Formation. The base of the formation was not reached in the B1 -12, but in other wells, s uch as B2-12, the Amal Formation unconformably overlies volcanic rocks. In the type section, the top of the Amal Formation is placed at the change from the friable dolomitic sandstone of the Maragh 14 Formation to the more firmly cemented quartzitic Amal sandstone. Because of onlapping relationships around the Amal high, the Amal Formation can be overlain by various Rakb Group clastics or carbonates. The base of the formation, where reached, occurs at the change from sandstone to volcanic rocks. Paleontology and Age The Amal Formation is mainly barren of fossils. Some plant remains have been recovered, but thus far they have not provided a reliable age determination. Radiometric determinations on volcanics within and beneath the Amal Formation have given a broad and inconsistent spread of ages ranging from Cambrian to Jurassic. Regional considerations, however, suggest that the most likely age of the Amal Formation is Cambro-Ordovician. The coarse grained, unsorted nature of the Amal Formation, along with the lack of marine fossils, suggests a continental environment of deposition for this unit. Distribution The Amal Formation, or its equivalents, are widely spread in the eastern Sirte Basin. Although physical continuity has not been demonstrated, it may be equivalent to the Hofra Formation in the western Sirte Basin. In the Amal Field area, the Amal Formation varies greatly in thickness from the B6-12 and B8-12 wells where it is absent as a result of truncation, to over 1400 feet in the B1-12 Well (see Roberts, 1970, textfigs. 6-8). Because of the unconformity at the top of the formation, a complete section has not been penetrated by a single well. A composited section would have a thickness in excess of 3000 feet. Origin of Name This formation is the principal reservoir of the Amal Field from which it derives its name. Additional References Roberts (1970) provides a valuable discussion of the lithology of this formation in the Amal Field, its regional structural setting, and its importance as a major petroleum reservoir. 15 16 17 18 APOLLONIA LIMESTONE Age: Lower to Middle Eocene Gregory (1911) subdivided the Eocene rocks of northern Cyrenaica into three formations, the oldest being the Apollonia Limestone. This formation is especially well developed along the base of the northernmost coastal escarpment of Jabal al Akhdar. Lithology The lower part of the Apollonia Limestone consists mainly of thin bedded, tan, chalky calcarenite. Layers containing dark gray chert nodules are present every two to five feet. Most of these nodules are only a few inches in diameter, but occasionally bands are present which contain nodules several feet in length. Marl seams are rare. The middle and upper portions of the formation consist of alternating thin beds of soft, white, chalky, calcilutite and harder, tan calcilutite. Chert nodules are less common in the upper part of the formation. Contact Relationships In the Ras Hilal area, the Apollonia Limestone unconformably overlies the Upper Cretaceous Atrun Limestone. The Middle Eocene Derna Limestone conformably overlies the Apollonia Limestone over much of the Jabal al Akhdar. Paleontology and Age In some areas, the basal Apollonia Limestone contains a planktonic foraminiferal fauna including Globorotalia subbotinae Morozova which is indicative of the Ypresian (Lower Eocene), while much of the overlying formation contains nummulites and other foraminifera including Truncorotaloides rohi Bronnimann and T. topilensis (Cushman) of Middle Eocene age. 19 Origin of Name This formation derives its name from the ancient Greek city of Apollonia located at the site of the modern village of Susa. Additional References Burollet, 1960 Pietersz, 1968 Kleinsmeide and van den Berg, 1968 20 ARGUB CARBONATE Age: Upper Cretaceous The name Argub Carbonate is here proposed for a new formation located in the subsurface of the northwestern Sirte Basin. The type section is located in the Oasis D1-32 Well at a drill depth of 6672 to 6885 feet, which corresponds to a subsea depth of 5860 to 6073 feet. Lithology In its type section, the Argub Carbonate consists mainly of a hard, tan, finely crystalline, slightly glauconitic dolomite with subordinate interbeds of white to gray, argillaceous, glauconitic limestone and very rare, thin, sandy stringers. The dolomite is dense or sometimes vugular. Limestone becomes more common southwards, becoming the dominant lithology in the Bahi Field and further south. This formation is characterized by a very high electric-log resistivity. The Argub Carbonate, which has an average thickness of between 150 and 250 feet, is especially well developed in the Bahi Field area. Contact Relationships The Argub Carbonate usually overlies the Lidam Formation or other Cenom anian roc ks with ap parent conformity . This for mation is conformably overlain by the Rachmat Formation. Both contacts (leaving the Argub Carbonate) are usually marked by sharp decreases in resistivity on the electric-log. Paleontology and Age No diagnostic fossils have been recovered from the Argub Carbonate. However, its stratigraphic position indicates that it is from the early part of the Upper Cretaceous, probably Turonian in age. The Argub Carbonate may be a basinward equivalent of the Etel Formation. Origin of Name The Argub Carbonate derives its name from Argub en Naal located about 44 miles east of the type section. Argub is an Arabic word for ridge or spur. 21 22 ARIDA FORMATION Age: Oligocene The name Arida Formation is here proposed for a new formation located in the subsurface of the south-central Sirte Basin. The type section is located in the Oasis E3-59 Well at a drill depth of 2178 to 2578 feet, which corresponds to a subsea depth of 1851 to 2251 feet. Lithology The Arida Formation in its type area consists of two members: an upper shale unit, and a lower unit consisting mainly of sandstone. The upper unit consists of a soft, light gray to dark green, glauconitic shale, which is sometimes silty. The thickness of this member varies from about 10 to 150 feet. It has a thickness of 65 feet in the type section of the Arida Formation. The lower member consists of three or four quartz sandstone units which are separated by thin (5 to 20 feet) intervals of soft, blue-green, laminated shale. The sandstones are generally very friable, fine grained, glauconitic, very well to poorly sorted. Some of the sandstones have a clay matrix while other portions of them are almost clay free. This member has a thickness of 330 feet in its type section. Contact Relationships The basal sands of the Arida Formation unconformably overlie the upper limestone of the Augila Formation There is an abrupt lithologic change at this contact. The upper shale member of the Arida Formation is conformably overlain by the Oligocene basal sands of the Diba Formation. This contact is usually abrupt. Paleontology and Age The bulk of the Arida Formation is not very fossiliferous; however, sparse planktonic and benthonic foraminifera have been recovered from the upper shale unit of this formation. These species have indicated that the Arida Formation is Oligocene in age. 23 24 Petroleum Significance The sandstones of the lower Arida Formation form important reservoirs in the Gialo Field. Origin of Name This formation derives its name from Al Arida, a geographic location about 48 miles northeast of Marada (approximately 28° 33’ N lat.: 19° 25’ E long.). 25 ATRUN LIMESTONE Age: Upper Cretaceous The name Atrun Limestone was proposed by Barr and Hammuda (1971) for the Upper Cretaceous chalky white limestone in the Wadi al Atrun - Marsa al Hilal area of northern Cyrenaica. The type section of this formation is located near the mouth of Wadi al Atrun. Lithology The Atrun Limestone consists of thin bedded, tan white, moderately hard, chalky calcilutite and biocalcarenites. In its type locality, marl seams are present every few feet. These seams weather more easily than the limestone accentuating the marly bedding planes. Bands containing broad, flat, dark gray, chert nodules, two to eight inches in length are found about every two to six feet. This formation has a thickness of 160 feet in Wadi al Qalah, but thickens considerably eastwards. Contact Relationships The Atrun Limestone conformably overlies the Upper Cretaceous Hilal Shale and is unconformably overlain by Eocene Apollonia Limestone. There is slight angular discordance at the latter contact. Paleontology and Age Foraminifera are abundant throughout the Atrun Limestone and the planktonic faunas have been recently documented (Barr 1968a, 1972; Barr and Hammuda 1971). Coccoliths are common, while only rare ostracodes and radiolaria have been recovered. Inoceramus, echinoids and a few brachipods have been recognized. The foraminifers indicate that the Atrun Limestone range s fr om uppermos t Coniacian to upper Maastrichtian in age. The following planktonic foraminiferal zones have been recognized (from top to bottom): Abathomphalus mayaroensis Zone Globotruncana gansseri Zone Globotruncana tricarinata Zone Globotruncana elevate Zone Globotruncana concavata concavata Zone 26 The high percentage of planktonic specimens suggests an open sea, neritic environment of deposition. Origin of Name This formation derives its name from Wadi al Atrun where its type section is located. 27 28 AUGILA FORMATION Age: Upper Eocene The name Au gila Formation is here proposed for a new formation located in the subsurface of the eastern and central Sirte Basin. The type section is located in the Oasis E1-59 Well at a drill depth of 2444 to 2707 feet, which corresponds to a subsea depth of 2103 to 2366 feet. Lithology The Augila Formation in the southeastern and south-central Sirte Basin can often be subdivided into three lithologic units or members. The lowest member consists of a light gray to green, soft shale with thin argillaceous limestone or dolomite interbeds. This shale sometimes becomes sandy and slightly glauconitic at its base. This unit varies in thickness from about 100 to 250 feet. The middle member is a soft, friable, porous, glauconitic, quartz sandstone. The sand is fine grained and subangular to subrounded. The distribution of this unit is more erratic than the others, varying from 0 to about 100 feet in thickness within two or three miles. In the type section of the Augila Formation, this sandstone member is missing. The uppermost unit consists of a light to dark gray, hard, sandy, slightly glauconitic limestone that is occasionally very argillaceous. Molluscan debris is common. This member is 100 feet thick in the Augila Formation's type section. Along the eastern margin of the Sirte Basin, the lower part of the Augila Formation sometimes consists of a predominantly limestone facies which is termed the Rashda Member. Contact Relationships The Augila Formation overlies the Gialo Limestone with apparent disconformity. The contact is sharp usually changing from a shale or sandy shale to a massive limestone. The Augila Formation is unconformably overlain by sands of the Oligocene Arida Formation. 29 Paleontology and Age The lower shale member of the Augila Formation contains both benthonic and planktonic foraminiferal assemblages indicative of an Upper Eocene age. The upper limestone member contains N u m m u l i t es fabianii and N striatus which are also indicative of the Upper Eocene. The shale member was probably deposited under inner to middle neritic, open sea conditions, while the sandstone and limestone units were deposited in shallower, more restricted environments. Origin of Name The Augila Formation derives its name from the Augila oasis which is about 30 miles north of the Gialo Field where the formation's type section is located. 30 Rashda Member The name Rashda Member is here proposed for a new member of the Augila Formation found in the subsurface in the Concession 12 area. The type section is ill the Mobil D4-12 Well at a drill depth of 2522 to 2689 feet, which corresponds to a subsea depth of 2258 to 2425 feet. Lithology The Rashda Member in its type section consists of a predominantly limestone sequence with a thin shale unit at the top. The limestone beds consist of white to light-gray, fossiliferous, tan calcilutites and calcarenites which are sometimes chalky, pyritic and occasionally glauconitic. The shales are olive green, soft, blocky, fossiliferous, pyritic, and silty to sandy in part. This member grades laterally from the type area into soft, green, calcareous claystone and green shale with thin limestone and chalk interbeds. Contacts In the type section, the Rashda Member is conformably overlain by the upper portion of the Augila Formation and it disconformably overlies the Gialo Limestone. The upper contact occurs at the change from limestone to the green Rashda shales. The basal contact is placed at the change from the chalky Rashda Limestone to the tan, very nummulitic limestone of the Gialo Limestone. Paleontology and Age Based on the occurrence of Nummu lites fabianii, and other small benthonic and planktonic foraminifera, the Rashda Member is considered to be Upper Eocene in age. The rich faunal assemblages found in the Rashda Member indicate that it was deposited in a shallow marine environment, mainly in the inner to middle neritic zones. An abundance of algae occurs in the lower part of the member in some areas. 31 32 Distribution The Rashda Member has a thickness of 167 feet in its type section. The typical unit is restricted to the "D" or Rakb area of Concession 12. The equivalent, dominantly shale, unit can be recognized in the greater Concession 12 area. Petroleum Importance The Rashda Member is productive in the Rakb Field of Concession 12. Origin of Name The Rashda Member derives its name from a geographic locality in northwestern Concession 12. 33 34 BAHI FORMATION Age: Cretaceous The name Bahi Formation is here proposed for a new formation located in the subsurface of the northwestern Sirte Basin. The type section is located in the Oasis A3-32 Well at a drill depth of 6400 to 6763 feet, which corresponds to a subsea depth of 5264 to 5627 feet. Lithology The Bahi Formation consists of interbedded sandstone, siltstone, conglomerate and shale. Sandstone and pebbly sandstone are the most common lithologies in the Bahi Field area. The sandstones usually consist of medium to coarse, subangular to angular quartz grains with a clay matrix. Glauconite is common in the uppermost 10 to 20 feet and is apparently absent in the lower part of the formation. The siltstones and shale are mainly red, sometimes mottled. The conglomerates contain rounded quartzite pebbles with a sand or sand and shale matrix, often with red staining. Usually there is a basal conglomerate 10 to 20 feet in thickness. The Bahi Formation occurs in the northwestern part of the Sirte Basin and is especially well developed in the Bahi Field area. The thickness of the Bahi Formation varies from a few feet to a maximum of over 400 feet. Contact Relationships The Bahi Formation is overlain by the Cenomanian Lidam Formation or other Upper Cretaceous formations with an abrupt contact. The formation unconformably overlies various lower Paleozoic formations. In its type area, the Bahi Formation overlies quartzites of probable Cambro-Ordovician age. Paleontology and Age The Bahi Formation is barren of fossils and consequently its age is uncertain. This formation occurs at the base of the marine 35 Upper Cretaceous, often being overlain by Cenomanian sediments. The upper few feet of the Bahi Formation contain glauconite and are probably part of the overlying marine cycle. However, the lower part of the formation appears to be of non-marine origin and may be considerably older. The older portion may be equivalent to the "Nubian" Formation elsewhere. Origin of Name The Bahi Formation derives its name from the Bahi Field where it is well developed and its type section is located. 36 BEDA FORMATION Age: Middle Paleocene The name Beda Formation is here proposed for a new formation located in the subsurface of the western Sirte Basin. The type section is located in the Oasis BBB1-59 Well at a drill depth of 5525 to 5950 feet, which corresponds to a subsea depth of 4762 to 5187 feet. Lithology In much of the southwestern Sirte Basin, the Beda Formation consists mainly of various interbedded limestone lithofacies with subordinate dolomite and calcareous shale. The main rock types include argillaceous calcilutite and skeletal and oolitic calcarenites. Oolites are sometimes common in the upper part of the formation, and dasycladacean algae are often abundant in much of the lower part. These constituents have been considered characteristic of this unit, and this formation in the past has informally been called the Dasycladacean-Oolitic Limestone. Fenestral fabric is sometimes common, especially in the dolomites (see plate 1, fig. 1). In much of the southwestern Sirte Basin, this formation has been informally subdivided into an upper and lower member. The subdivision is based mainly on electric-log character In the northwestern part of the basin, the Beda Formation becomes more shaly and is subdivided into a lower Thalith Member and an upper Rabia Shale Member. The Beda Formation varies in thickness from a few feet to a maximum of over 800 feet. It has a thickness of 45 feet in its type section. Contact Relationships Over much of the western Sirte Basin, the Beda Form ation conformably overlies the Danian Hagfa Shale. This contact is usually abrupt, but sometimes transitional. In the Defa Field area, the Beda Formation conformably overlies the Defa Limestone and in the HofraDahra-Bahi area it conformably overlies the Satal Formation. The lower boundary of the Beda Formation usually corresponds closely to the top of the Danian Stage. 37 38 The Beda Formation is conformably overlain by the shales of the Khalifa Formation or Dahra Formation. This contact may be sharp but is often transitional, being characterized by an interbedding of limestone and calcareous shale. The top of the Beda Formation is placed at the position where limestone predominates over shale. Paleontology and Age Dasycladacean algae and benthonic foraminifera, including common miliolids, represent some of the most common types of fossils found in this formation. Molluscs, echinoids and corals are also present. These fossil assemblages represent a variety of shallow marine environments of deposition. Foraminiferal faunas indicate a Middle Paleocene age, probably Montian. Petroleum Significance This formation forms reservoirs in the Ora and Zaggut Fields. 39 Rabia Shale Member The name Rabia Shale is here proposed for a new member of the Beda Formation in the subsurface of the northwestern Sirte Basin. The type section is located in the Mobil Al-l I Well at a drill depth of 3288 to 3502 feet, which corresponds to a subsea depth of 2343 to 2557 feet. Lithology In the Al-11 Well, the Rabia Shale is predominantly a shale and mudstone sequence with minor limestone interbeds. The shales are dark gray to dark green, calcareous, fossiliferous, fissile to splintery and occasionally waxy. The mudstones are gray to gray-green, soft, gummy and very calcareous. The limestones are light gray, fine textured and very argillaceous to marly. Contact Relationships In its type section, the Rabia Shale Member is conformably overlain by the Dahra Formation and it conformably overlies the Thalith Member. The top of the member is placed at the change from the light gray fossiliferous limestones of the Dahra Formation to the darker Rabia shales and clays. The base occurs at the passage from the Rabia shales to the gray microcrystalline fossiliferous limestones of the Thalith Member. On the electric-log, the top of the Rabia Shale is characterized by a resistivity decrease while its base occurs at a resistivity increase. Distribution The Rabia Shale Member is 214 feet thick in the type section. It is restricted to an area in the northwestern basin near the Dahra 40 and Hofra Fields and loses its lithologic homogeneity off the Dahra-Hofra high where it cannot be differentiated from the remainder of the Beda Formation. Origin of Name Rabia is a place name from the greater Dahra-Hofra area. 41 42 Thalith Member The name Thalith Member is here proposed for a new member of the Beda Formation in the subsurface of the northwestern Sirte Basin. The type section is located in the Mobil Al-11 Well at a drill depth of 3502 to 3620 feet, which corresponds to a subsea depth of 2557 to 2675 feet. Lithology The Thalith Member is predominantly limestone with minor amounts of shale or chalky marl. The limestones ar e light t o dark gray, microcrystalline to microgranular, fossiliferous, argillaceous to very argillaceous, chalky in part, soft to medium hard with rare pyrite and glauconite. The shales are light to dark gray, occasionally gray-green or black, calcareous, fossiliferous, fissile, but occasionally soft and waxy, and contain rare pyrite. Contact Relationships In the Al-11 Well, the Thalith Member is conformably overlain by the Rab ia Shale Member and is conformably underlain by the Satal Formation. The top of the member is placed at the change from the calcareous Rabia Shale, to the light gray to brown Thalith limestones. The base of the unit is picked at the top of the white to light gray, very chalky limestone of the Satal Formation. Elsewhere, the Thalith Member conformably overlies the Hagfa Shale with the contact being marked by the first occurrence of the typically gray, fissile, slightly calcareous Hagfa Shales. In the type section, the top of the Thalith Member is marked on the electric-log by a resistivity decrease at 3502 (-2557) feet and the base is similarly characterized by a resistivity decrease at 3620 (-2675) feet marking the top of the Satal Formation. This latter point coincides with a sharp negative deflection of the S.P. curve. 43 Distribution In the type area, the Thalith Member is 115 feet thick. It extends over the Dahra-Hofra area into Concessions 57, 13, 72 and western 59. To the east, it loses its identity and is undifferentiated from the Beda Formation. 44 CHICLA FORMATION Age: Lower Cretaceous The Chicla Formation was described by Christie (1955, p. 17) for a predominately sandstone and shale sequence which crops out along the northern escarpment of Jabal Nefusa. Lithology The Chicla Formation consists mainly of light colored, crossbedded, friable, quartz sandstone, conglomerates, and brown, yellow and red silts and clays. Thin lignite beds are also present. The Chicla Formation varies greatly in thickness from a few feet to over 850 feet at its type section north of Jefren (Desio et al, 1963, pp. 50, 51). Contact Relationships The Chicla Formation unconformably overlies the Jurassic Bu Gheilan Limestone and Chameau Mort Sandstone in the Garian-Jefren area and the non-marine Cabao Sandstone of probable Lower Cretaceous age further to the west near Giado. The Chicla Formation is overlain by the Cenomanian Ain Tobi Limestone with apparent conformity. Paleontology and Age Plant fragments and silicified wood are fairly common in this formation testifying to its non-marine origin. This environmental interpretation is supported by numerous sedimentary structures. In addition, several species of freshwater pelecypods have been identified (Desio et al, 1963, p. 58). Origin of Name This formation derives its name from the village of Chicla located on the Jabal Nefusa between Jefren and Garian. 45 CYRENE FORMATION Age: Oligocene The Cyrene Formation was first proposed by Gregory (1911) for the limestone and marls overlying the Slonta Limestone at the ancient Greek ruins of Cyrene. The Cyrene Formation covers a broad area on the upper plateau of Jabal a] Akhdar. In the vicinity of Cyrene, this formation is conveniently subdivided into three members: the Shahhat Marl, Algal Limestone and Labrak Calcarenite. Lithology The lowermost member of the Cyrene Formation, the Shahhat Marl Member, consists of highly fossiliferous, glauconitic, soft, gray or yellowish marls and marly limestone. This member contains common Pectens, other molluscs and bryozoa. Nummulites also have been recovered. This impermeable marl stops downward percolating ground water and has produced a line of springs along the face of the upper escarpment, the largest of which is the 'Fountain of Apollo' (or Ain Shahhat) at Cyrene. Pietersz (1968, p. 128) records a thickness of 70 ft. for this member. The Algal Limestone Member directly overlies the Shahhat Marl and consists of a massive white or cream colored, highly fossiliferous, algal calcarenite which becomes harder and denser near its top. Corals, echinoids and nummulites have been recovered, in addition to abundant algae. This member has a thickness of 87 ft. ( Pietersz, 1968, p. 128). The Labrak Calcarenite Member conformably overlies the Algal Limestone at the top of the upper escarpment. It consists of white to yellow, soft, friable, calcarenites and marls. This member is very fossiliferous although most fossils have been badly weathered. This member is extensively quarried along the Al Faidia road south of Shahhat for building blocks. In this area the Labrak Calcarenite has a thickness of 50 ft. (Pietersz, 1968, p. 129). Contact Relationships The Cyrene Formation usually unconformably overlies the Eocene Slont lime stone. However , in 46 the Marawa a nd Jard as al Abid areas it sometimes unconformably overlies older formations (e.g. Derna Limestone and Jardas Formation). The Cyrene Limestone is disconformably overlain by the Miocene Faidia Formation or other Miocene units. Paleontology and Age Over the years there has been some uncertainty as to the age of the Cyrene Formation. Kleinsmeide and van den Berg ( 1968, p. 121), however, record Nu m m u lit es int erm ediu s and N. vasc u s from this formation (although they use the term Al Kuf Formation) and these species are diagnostic of the Lower Oligocene. Much of the Cyrene Formation appears to have been deposited in very shallow marine, high energy environments. Origin of Name The Cyrene Formation derives its name from the ancient Greek city of Cyrene where its type section is located. 47 48 DAHRA FORMATION Age: Paleocene The name Dahra Formation is here proposed for a new formation located in the subsurface of the western Sirte Basin. The type section is located in the Oasis F1-32 Well at a drill depth of 3040 to 3350 feet, which corresponds to a subsea depth of 1886 to 2196 feet. Lithology The Dahra Formation in its type area consists mainly of white to light gray, chalky, calcarenite and calcilutite which are sometimes slightly argillaceous or pyritic, and subordinate tan to brown microcrystalline dolomite and thin interbeds of dark shale. Minor anhydrite is sometimes present in the upper part of the formation. In the southwestern part of the Sirte Basin, the lower part of this formation becomes more shaly consisting mainly of interbedded shale and argillaceous calcilutite (see plate 1, fig. 2). The Dahra Formation is restricted to the western portion of the Sirte Basin. In its type section it has a thickness of a little over 300 feet. Contact Relationships The Dahra Formation conformably overlies the Beda Formation and in turn is conformably overlain by the upper argillaceous limestone member of the Khalifa Formation. In its type section, the Dahra Formation conformably overlies the Rabia Shale Member of the Beda Formation. In the central part of the basin, this formation changes laterally to a completely shale sequence which is the lower shale member of the Khalifa Formation. Paleontology and Age Benthonic foraminifera consisting of miliolids and textularids are some of the most common fossils found in much of this formation. 49 Some shale interbeds especially near the Dahra Formation - Khalifa Formation lateral boundary contain planktonic species indicative of a Paleocene (lower Landenian) age. Petroleum Significance The Dahra Formation forms the main reservoirs in the Dahra (F) and Hofra Fields. Origin of Name The Dahra Formation derives its name from the Dahra Field where its type section is located. 50 DEFA LIMESTONE Age: Lower Paleocene (Danian) The name Defa Limestone is here proposed for a new formation located in the subsurface of the Defa Field area in the south central Sirte Basin. The type section is located in the Oasis B4-59 Well at a drill depth of 5621 to 6184 feet, which corresponds to a subsea depth of 4970 to 5533 feet. Lithology The Defa Limestone consists of a variety of interbedded limestone lithofacies. The main types include chalky calcilutite, and pelletal, algal, and coralline calcarenites. This formation is highly fossiliferous, locally containing abundant molluscs, algae, benthonic foraminifera and corals. Burrows and fenestral fabric (algal mat) are also common in some of the chalky calcilutite facies (see plate 2). The Defa Limestone is restricted to an area about 6 miles in breadth (E-W) and 12 miles in length (N-S) in the Defa Field and adjacent areas. The Defa Limestone has an average thickness of about 400 feet. Contact Relationships The Defa Limestone usually overlies the Upper Cretaceous Waha Limestone with apparent conformity. The Defa Limestone is the lateral equivalent of the Hagfa Shale and near this facies change a wedge of Hagfa Shale occasionally underlies the Defa Limestone. The Defa Limestone is conformably overlain by the Paleocene Beda Formation. Paleontology and Age The abundance of pellets, miliolids, dasycladacean algae, corals and various molluscs indicates that the Defa Limestone was deposited under very shallow marine conditions. Few of these fossils, however, are diagnostic of a precise age. Nevertheless, this formation 51 overlies 52 the upper Maastrichtian Waha Limestone and has been demonstrated to be the lateral equivalent of the Hagfa Shale which as been dated by planktonic foraminifera as Danian in age. Origin of Name This formation is the principal reservoir of the Defa Field from which it derives its name. 53 DERNA LIMESTONE Age: Middle Eocene The name Derna Limestone was proposed by Gregory (1911) for a prominent limestone unit well exposed along the coastal escarpment near Derna, and present over a large portion of Jabal al Akhdar. Lithology The lower part of the Derna Limestone consists mainly of a thickbedded to massive, white to cream colored, hard, crystalline limestone. A few bands with chert nodules are found near its base and become increasingly rare upwards. The middle and upper parts of the Derna Limestone are more chalky and thin bedded with occasional marl seams. Pietersz ( 1968, p. 127) reports a total measured thickness of 370 ft. for this formation; however, the Derna Limestone appears to be considerably thicker in the Susa-Shahhat area. Contact Relationships The Derna Limestone conformably overlies the Eocene Apollonia Limestone and is conformably overlain by the Eocene Slonta Limestone. Paleontology and Age This formation is quite fossiliferous, locally containing common algae, echinoids and pelecypods. Abundances of large nummulites including Nu mm u lites gizehen sis, N. cu rvispira, N. beaum ontiand N . su b b eau m o n t iare characteristic of this formation. These nummulites indicate a Lutetian (Middle Eocene) age for the Derna Limestone. Origin of Name This formation derives its name from the town of Derna where it was originally described. Additional References Burollet, 1960 Pietersz, 1968 Kleinsmeide and van den Berg, 1968 54 DIBA FORMATION Age: Oligocene The name D iba Form ation is here proposed for a new formation located in the subsurface of the south-central Sirte Basin. The type section is located in the Oasis E3-59 Well at a drill depth of 1584 to 2178 feet, which corresponds to a subsea depth of 1257 to 1851 feet. Lithology The Diba Formation consists of an alternating sequence of thick sandstone units and thin shales. A few sandy limestone beds are sometimes present near the top of the formation. The shale is soft, silty and gray to green in c olor. The sand stone is p redom in antly unconsolidated and consists of fine to coarse, subangular to subrounded quartz grains. Glauconite is present in many of the sands. The Diba Formation is recognized in the south-central Sirte Basin. Sometimes, however, when the upper shale member of the underlying Arida Formation is not present, it is difficult to separate these two Oligocene formations. The Diba Formation has a thickness of 594 feet in its type section. Contact Relationships The Diba Formation conformably overlies the upper shale member of the Arida Formation and is overlain by the Miocene Marada Formation. The upper contact is usually placed at the base of a thin shale bed which often overlies the uppermost Oligocene sandstone or sandy limestone unit. Although an unconformable relationship between the Diba Formation and Marada Formation has not been recognized in the subsurface, regionally Oligocene and Miocene strata are often separated by an unconformity. Paleontology and Age No diagnostic fossils have been recovered from the Diba Formation. However, it is co nsidered Oligocene in 55 age as it conformably 56 o v e r l i e s th e O l i g o ce n e A r i d a Fo r m a ti o n a n d i s o v e r l a i n b y Lo w e r Miocene beds of the Marada Formation. Origin of Name Diba is the Arab ic word for female wolf. 57 58 ETEL FORMATION Age: Upper Cretaceous The name Etel Formation is here proposed for a new formation located in the subsurface of the Sirte Basin. The type section is located in the Oasis 02-59 Well at a drill depth of 8730 to 9264 feet, which corresponds to a subsea depth of 8070 to 8604 feet. Lithology The Etel Formation typically consists of thin bedded carbonates, shale, anhydrite, and siltstone or very fine grained sandstone. The carbonates are predominantly dolomite with subordinate limestone. The dolomite is gray to more commonly brown and dark brown, very finely crystalline to microcrystalline, sucrosic in part, and well indurated. The limestones are mainly white, gray and tan, argillaceous and occasionally silty, calcilutites, and to a lesser extent, calcarenites. The anhydrites, which characterize the formation, are the usual off-white to medium-gray, microcrystalline, well indurated varieties. The shales are gray-green to brown, occasionally red, calcareous, and in part contain pyrite. Rare graygreen siltstone and very fine grained, calcareous, glauconitic sandstone have been observed in the shales. Contact Relationships In the 02-59 Well, the Etel Formation is conformably overlain by the Rachmat Formation and in turn conformably overlies the Lidam Formation. The upper contact is placed at the change from the shales or carbonates of the Rachmat Formation to the first occurrence of anhydrite which characterizes the top of the Etel Formation. The base of the formation is placed at the top of the characteristically dolomitic sequence of the Lidam Formation. In some areas the Etel Formation is transgressive onto older formations and unconformably overlies various rocks of pre-Upper Cretaceous age as well as the Cretaceous Bahi Formation The Etel Formation is wide spread across the central and southern Sirte Basin, but is absent on the regionally high areas su ch as Dah ra- 59 Hofra, Waha and Amal. It attains its greatest thicknesses in the major trough areas. In its type section, it has a thickness of 529 feet, while further into the Marada trough it thickens to over 1300 feet. Paleontology and Age Fossils are very rare in the Etel Format on and no diagnostic forms have been recognized. In some areas the Etel Formation conformably overlies the Cenomanian Lidam Formation and is overlain by Coniacian beds of the Rachmat Formation. Therefore, because of its stratigraphic position alone, the Etel Formation is considered Turonian in age. Origin of Name This formation derives its name from Graret al Etel located about 40 miles southeast of Marada and 12 miles northwest of Bir Zelten along the southwestern base of Jabal Zelten. 60 FAIDIA FORMATION Age: Miocene Pietersz (1968, p. 129) used the name Faidia For m ation for a sequence of shales and limestones overlying the Cyrene Formation on the upper terrace of eastern Jabal al Akhdar. The type section is located along the road below the ruins of the old fortress near the entrance to the village of Al Faidia, about 10 miles south of Shahhat. Lithology This formation is subdivided into an upper limestone member and a lower shale member. The upper member consists of white to brownish, medium to coarse-grained, partly crystalline limestones, with a few intercalations of brownish marly limestones. The lower member of the Faidia Formation consists of a soft, yellow brown, glauconitic marl with intercalated green clays, having a thickness of 12 ft. Pietersz (1968) reports that the Faidia Formation has a total thickness of 143 ft. Contact Relationships The Faidia Formation overlies the Oligocene Labrak Calcarenite Member of the Cyrene Formation. There is some disagreement, however, as to the nature of this contact. This contact is usually not well exposed, but appears to be abrupt. Pietersz (1968) considers it to be disconformable. In contrast, Kleinsmeide and van den Berg (1968, p. 121) state that they have seen no clear indication of a stratigraphic break between the Labrak Calcarenite and the Faidia clays and therefore regard both units as members of their Al Kuf Formation, which incorporates the Cyrene Formation and Faidia Formation into a single unit. Paleontology and Age The Faidia Formation contains common Pecten, oysters, echinoids and algae. Pietersz (1968) considers the age to be Lower to Middle Miocene (Aquitainian to probably Helvetian). Origin of Name This formation derives its name from the village of Al Faidia. 61 GARGAF GROUP Age: Cambro-Ordovician Burollet (1960, pp. 24, 25) originally proposed the name G ar gaf Group to include a thick sandstone sequence in the Jabal Gargaf area of the Fezzan. This sequence was subdivided into four formations (from bottom to top): Hassaouna Formation, Haouaz Formation, Melez Chograne Formation and Memouniat Formation. The type section was designated from igneous basement on Jabal Gargaf (Jabal Hassaouna: 28° 15' N; 14° 00' E) to the Silurian (Gothlandian) outcrops at Aouinet Quenine (28° 20' N; 12° 55' E) and Bir al Gasr (27° 30' N; 12° 55' E). A similar lithologic sequence has b een encountered in the subsurface of a large portion of the Sirte Basin which apparently is of the same age. These subsurface rocks are thought to be equivalent to those cropping out in the Fezzan and are assigned to the Gargaf Group although the same sequence of formations has not been recognized. In the DahraHofra area and other portions of the western Sirte Basin, this thick sequence of quartz sandstones and quartzites has been assigned to the newly defined Hofra Formation. The Hofra Formation is included in the Gargaf Group, but its relationship to the surface formations of this group is uncertain. Lithology See Hofra Formation. See Burollet (1960) for the component formations exposed in the Fezzan. Contact Relationships See Hofra Formation. Origin of Name The Gargaf Group derives its name from Jabal Gargaf where the lower portion of its type section is located. 62 GARIAN LIMESTONE Age: Cenomanian The Garian Limestone was originally described by Christie (1955, pp. 19, 20) for a dolomitic limestone unit exposed near the top of the northern escarpment of Jabal Nefusa. Lithology The Garian Limestone consists of light gray or whitish, hard, massive crystalline dolomitic limestone and dolomite. Chert is fairly common especially in the upper part of the formation. The Garian Limestone, like the Ain Tobi Limestone, is often a resistant cliff forming unit. Both Christie (1955, p. 19) and Desio et al. (1963, p. 67) report that this formation has a thickness of 180 feet in its type section located about 4 miles east of Garian. Contact Relationships The Garian Limestone conformably overlies the Jefren Marl and is conformably overlain by the Tigrinna Marl Member of the Mizda Formation. Both contacts are usually abrupt. Paleontology and Age The Garian Limestone was deposited in very shallow marine water, probably not too unlike the Ain Tobi seas. Subsequent diagenetic alteration appears to have destroyed many of the fossils of this formation. Consequently, diagnostic fossils have not been recovered and it is because of its stratigraphic relationship with the Jefren Marl that the Garian Limestone is considered upper Cenomanian or possibly Turonian in age. Desio et al. (1963, pp. 67, 68) list the following fossils from this formation collected in a small quarry near Garian: Neithea (Neitheops) quinquecostata(Sowerby) Exogyra conica (Sowerby! Exogyra flabellata (Goldfuss) Lopha diluviana (Linnaeus) Ichthyosarcolites triangularisDesmarest Origin of Name The Garian Limestone derives its name from the city of Garian located about 4 miles west of the type locality. 63 GEDARI FORMATION Age: Middle to Upper Eocene The name Gedari Formation is here proposed for a new formation which extends from outcrop into the subsurface of the western Sirte Basin. The type section is located in the Mobil AA1-11 Well at a drill depth of 586 to 3218 feet, which corresponds to 372 feet above sea level to 2260 feet below sea level. Lithology The Gedari Form ation comprise s a co mple x ass emblage of interbedded shale, clay, limestone, dolomite, sandstone. chalk and gypsum. The shale and clay are gray to green, calcareous, soft and fossiliferous. The limestone is usually medium hard, occasionally chalky, calcilutite. Biocalcarenites are sometimes present. The dolomite is tan to dark brown, microcrystalline hard and tight. The sandstone is gray, very fine grained to coarse grained, silty, shaly and fossiliferous. It is occasionally calcareous and glauconitic. Contact Relationships Over much of the area where it is present, the Gedari Formation is exposed at the surface. Occasionally, it is unconformably overlain by the Najah Group. In the type section at the Mobil AA1-11 Well, the upper contact is placed at the base of a massive, fine-grained to coarse-grained, silty, quartz sandstone and siltstone above a soft, green, sandy clay. In the type area, the Gedari Formation is apparently conformably underlain by the Hon Evaporite Member of the Gir Formation. The contact occurs at the top of the massive Hon anhydrite. This contact is considered to be unconformable in the extreme western part of the Sirte Basin. Paleontology and Age The Gedari Formation is mainly of Middle Eocene age, but may range from Lower Eocene to Upper Eocene. The faunal assemblage 64 of foraminifera, pelecypods, gastropods, echinoids and bryozoans indicate that for the most part, the formation was deposited in a shallow marine environment. The occasional presence of planktonic foraminifera near the base of the unit suggests, however, more open marine conditions. Distribution The Gedari Formation crops out in western Concessions 11 and 57. In the type section, it is 2632 feet thick, which is near the maximum for the formation. Eastward, it grades into its lateral equivalent, the Gialo Limestone. Origin of Name The formation takes its name from the Wadi al Gedari which is located at 29° 30' N lat., 18° 00' E long. 65 66 67 68 69 GIALO LIMESTONE Age: Eocene The name Gialo Lim estone is here proposed for a new formation located in the subsurface over much of the Sirte Basin. The type section is located in the Oasis E91-59 Well at a drill depth of 2750 to 4337 feet, which corresponds to a subsea depth of 2395 to 3982 feet. Lithology The Gialo Limestone consists of a thick sequence of gray or tan to brown, shallow marine limestone. The main limestone lithofacies include muddy nummulitic calcilutite and calcarenite. This formation is highly fossiliferous containing concentrations of several species of nummulites, which in parts of the sequence make up a high percentage of the limestone. Molluscan, echinoid, and bryozoan debris are locally common. Portions of this formation are chalky and very friable especially near the top, while more common]v the limestone is fairly hard and well indurated (see plate 3). The Gialo Limestone is widespread in the subsurface of the Sirte Basin. Parts of the Gialo Limestone are very similar lithologically and paleontologically to the Derna Limestone of northern Cyrenaica. In fact, similar shelf facies are found over broad areas of North Africa. The Gialo Limestone is 1575 feet thick in its type locality. This thick sequence is usually subdivided into several members in those areas where it has been studied in detail. Contact Relationships The Gialo Limestone overlies the Lower Eocene Gir Formation with apparent conformity. This contact is usually placed at the top of the sequence containing common thin dolomite and evapor ite beds. Sometimes a very thin marly or chalky unit is encountered at this contact which can be correlated on electric-logs. Shale of the basal Augila Formation often overlies the Gialo Limestone. This contact is abrupt and probably disconformable. 70 Paleontology and Age Concentrations of large nummulites, including N u m m u l i t e s gizehensis, N cuvispira, N discorbina, N bullatus and N. partischi tauricus, occur at different stratigraphic levels within this formation. They indicate that most of the Gialo Limestone is Middle Eocene in age; however, there is some evidence to suggest that the upper few feet of the formation is sometimes Upper Eocene. Arni (1967) has illustrated several of the common species of Nummulitesfrom sequences in the Sirte Basin that are now assigned to the Gialo Limestone. The foraminifera C o s k i n o l i n a, D ic tyocon oides and miliolids are also locally common. Planktonic foraminifera are rare or absent. These abundances of large benthonic foraminifers indicate that the Gialo Limestone was deposited under shoal marine conditions. Petroleum Significance The upper Gialo Limestone forms the principal reservoir in the Gialo Field. Origin of Name The Gialo Limestone derives its name from the Gialo oasis which is located about 30 miles north of the Gialo Field where this formation's type section is located. 71 72 73 74 GIR FORMATION Age: Lower Eocene The name Gir Formation is here proposed for a new formation on the surface and in the subsurface of the Sirte Basin. The type section is located in the Oasis Y1-59 Well at the drill depth of 1082 to 3085 feet, which corresponds to the subsea depth of 255 to 2258 feet. The name Gir Gypsum has sometimes been used for a member of the Gebel Uaddan Formation in the Uaddan area (Burollet, 1960). However, this unit was never adequately documented and a type section was never proposed. As such, it is considered an invalid name. Nevertheless, as nearly as can be ascertained, it was probably equivalent to at least the upper portion of the newly named Gir Formation. Lithology The Gir Formation is typically a sequence of interbedded dolomites and anhydrites with subordinate amounts of limestone and shale. Halite is present in the southwestern part of the basin. In the type section, the uppermost part of the formation is composed of a thin interval of buff to tan, nummulitic limestone (Mesdar Limestone Member) which is underlain by a thick sequence of interbedded, massive, white to tan, dense anhydrites and tan to brown, fine-crystalline dolomites (Hon Evaporite Member). This is in turn underlain by tan, argillaceous, massive dolomite of the Facha Dolomite Member. For details of lithology see descriptions of these component members. Contact Relationships The Gir Formation, for the most part, occupies a conformable position with both the overlying and underlying units. Over the greater part of the Sirte Basin, the Gialo Limestone is conformably superjacent to the Gir Formation, but in the northwest, this position is occupied by the Gedari Formation with a probable unconformity separating it from the Gir Formation. 75 The Gir Formation normally overlies the Upper Paleocene Harash Limestone or the Kheir Formation. Paleontology and Age Most of the Gir Formation contains very few fossils. Benthonic foraminifera including nummulites, however, are occasionally present especially in the Mesdar Limestone Member. These microfossils, along with the stratigraphic position of the formation, indicate that the Gir Formation is Lower Eocene in age. Distribution The Gir Formation crops out in the Jabal Uaddan area and extends into the subsurface across much of the Sirte Basin. In the type section, it has a thickness of about 2000 feet and reaches a maximum thickness of approximately 3000 feet in western Concession 11. In the western Sirte Basin, the Gir Formation is subdivided into two members: an upper Hon Evaporite Member and a lower Facha Dolomite Member. locally, the Hon Evaporite Member comprises the major part or all of the Gir Formation. To the east, these members become indistinguishable in a limestone, dolomite and anhydrite sequence so that the Gir Formation is not subdivided. Also, in the east-central Sirte Basin, more open marine influences become dominant which are represented by the Mesdar Limestone Member. The Mesdar Limestone constitutes varying amounts of the Gir Formation, ranging from a thin interval at the top of the formation, to the entire Gir Formation. Petroleum Significance The Gir Formation contains oil in the Ed Dib and Facha Fields and in the QQ-11, G-72 and B-78 areas. Origin of Name This formation derives its name from Jabal Gir located in the southwestern Sirte Basin, southeast of the oasis of Uaddan. 76 Hon Evaporite Member The name Hon Evaporite is here proposed for a new member of the Gir Formation in the subsurface of the western and central Sirte Basin. The type section is located in the Oasis Y1-59 Well at a drill depth of 1160 to 2774 feet, which corresponds to a subsea depth of 333 to 1947 feet. Lithology The Hon Evaporite Member of the Gir Formation consists of a sequence of interbedded anhydrite and dolomite with minor shale. The anhydrites are massively bedded, white to tan, hard to dense, and are microcrystalline to finely crystalline. The dolomites are light tan to brown in color, very fine to finely crystalline, often of sucrosic texture, commonly vugular, and occasionally mottled or nodular. Fossils are rare. The shales occur in thin interbeds and are gray to green, soft, and calcareous. Halite is present in this member in southwestern Concession 11 and western Concession 57. Contact Relationships In the type section, the Hon Evaporites are conformably overlain by a thin sequence of Mesdar Limestone and they conformably overlie the dolomites of the Facha Member of the Gir Formation. Elsewhere in the Sirte Basin, the Hon Evaporites conformably underlie the Gialo Limestone or the equivalent Gedari Formation. Its basal contact is conformable with the Facha Dolomite Member, the Kheir Formation or the Jabal Zelten Group. Distribution The Hon Evaporites are 1614 feet thick in the type section and they reach a maximum thickness of about 3500 feet in western Concession 11. This member is well developed in western Concessions 11, 57 and 72, and it is in thes e areas where the 77 lar ger percentages of halite occur. The Hon Evaporites lose their identity north of Concession 11 and in northeastern and eastern Concession 13 where they intertongue with the more open sea facies of the Mesdar Limestone Member. The Hon Evaporites crop out along the Uaddan uplift in the area around the C1-44 Well. Petroleum Importance The Hon Evaporite Member forms the reservoir of the Ed Dib, Facha and QQ-11 fields. Origin of Name The member derives its name from the oasis of Hon located in the southwestern Sirte Basin. 78 Facha Dolomite Member The name Facha Dolomite M em ber is here proposed for a new member of the Gir Formation in the subsurface of the western Sirte Basin. The type section is located in the Oasis Y1-59 Well at a drill depth of 2774 to 3085 feet, which corresponds to a subsea depth of 1947 to 2258 feet. Lithology The Facha Dolomite Member of the Gir Formation is predominately a tan, argillaceous, finely crystalline, massive, commonly vugular dolomite with minor amounts of anhydrite. Occasional limestone beds occur within the unit, which consist of white to cream, soft argillaceous calcilutite. Contact Relationships The Facha Dolomite is conformably overlain by the Hon Evaporite Member of the Gir Formation and conformably overlies either the Harash Formation or Kheir Formation. The top of the Facha Dolomite Member occurs at the change from the basal massive anhydrite of the Hon Evaporite Member to a dominantly dolomite section. The base of the unit is placed at the change from dolomite or anhydrite to limestone or marl of the underlying formations. Distribution The Facha Dolomite is restricted to the western Sirte Basin. In its type section, it has a thickness of 341 feet. This thickness remains fairly constant over the entire area in which the Facha Dolomite occurs. 79 Petroleum Important The Facha Dolomite Member is productive in the Ed Dib and Facha Fields. Origin of Name The name of the member is derived from the Facha Field in Concession 11. 80 81 82 83 Mesdar Limestone Member The name M esdar Limestone Member is here proposed for a new member of the Gir Formation in the subsurface of the central and eastern Sirte Basin. The type section is located in the Mobil B1-13 Well at a drill depth of 4183 to 5970 feet, which corresponds to a subsea depth of 3862 to 5649 feet. Lithology In the B1-13 Well, the Mesdar Limestone Member consists of massive limestone with rare thin shale and dolomite beds. The limestone varies from white to tan and brown, from calcilutite to biocalcarenite, from hard to soft, and is moderately to poorly cemented. It is argillaceous in part, sometimes pyritic or cherty, often with a chalky character and is fossiliferous. Nummulites are sometimes abundant and constitute the rock framework of some beds. The shales are greenish-gray to brown, very calcareous, soft, and grade to argillaceous calcilutite. The dolomites are tan, very finely granular and medium hard. In the eastern Sirte Basin, the dolomite becomes more common and has some associated anhydrite. Contact Relationships In the type section, the Mesdar Limestone Member is conformably or possibly disconformably overlain by the Gialo Formation, and conformably overlies the Kheir Formation. In some areas the base of the formation rests on an unconformable surface with some of the underlying Upper Paleocene section missing. The upper contact in the B1-13 does not coincide with a distinct lithologic break and it is difficult to pick from samples alone. In most cases, the presence of glauconite, pyrite, occasional phosphate pellets and sand grains in the basal Gialo Limestone serves to differentiate it from the Mesdar Limestone. When this occurs the contact may be disconformable. In the B1-13 Well, the base of the Mesdar Limestone is marked by the change from limestone to gray calcareous shale. In other areas it occurs within a limestone sequence and the contact must be picked by electriclo g character. In the type section, the top of the Mesdar Limeston e is placed at a character istic decr ease in 84 electric resistivity, and the base of the member is marked by a distinctive decrease in resistivity accompanied by a pronounced negative deflection in the S.P. curve. Distribution In the type section, the Mesdar Limestone has a thickness of 1787 feet. It attains its thickest development in the central and eastern parts of the Sirte Basin where it may constitute the entire Gir Formation. It changes facies laterally into the dolomites and evaporites of the other Gir Formation members. Origin of Name The name Mesdar is a geographic term from the southeastern Concession 13 area. 85 86 87 88 HAGFA SHALE Age: Lower Paleocene The name Hagta Shale is here proposed for a new formation located in the subsurface of the Sirte Basin. The type section is located in the Oasis Y1-59 Well at a drill depth of 5710 to 6733 feet. which corresponds to a subsea depth of 4883 to 5906 feet. Lithology This formation consists mainly of a shale sequence with occasional thin limestone interbeds. The shale is dominantly gray, but ranges through gray-brown, gray-green and black. It is soft to medium hard, calcareous, fossiliferous, fissile, splintery and slightly silty in part. Occasional beds of light brownish gray, sticky, very calcareous clay occur in the shales. The limestone beds which are more common in the upper part of the formation, are gray, tan and brown, very fine grained, hard, dense, fossiliferous and rarely glauconitic. Contact Relationships In Y1-59, the Hagfa Shale is conformably overlain by the Beda Formation and conformably overlies the Maastrichtian Kalash Limestone. The top of the formation is marked by the change from the limestone of the Beda Formation to shale or argillaceous limestone, whereas the base is placed at the abrupt change from shale to limestone. In some areas both contacts become gradational and are difficult to recognize from well samples. In the type section, the top of the Hagfa Shale is marked by a characteristic reduction in electric-log resistivity at 5710 (-4893) feet. The base is placed at a similarly characteristic increase in resistivity at 6733 (-5905) feet. Paleontology and Age The Hagfa Shale contains a diverse foraminiferal assemblage. Planktonic species are common especially in the lower part of the 89 formation, including Globoconusa daubjergensis, Globorotalia compressa, Globigerina pseu dobu lloides, and G t rilocu linoides. This microfauna indicates that at least most of the Hagfa Shale is Danian in age. The abundance of planktonic forms also indicates that the Hagfa Shale was deposited in a fairly deep, open sea environment. Distribution This formation is widespread in the central and western Sirte Basin. It varies greatly in thickness reaching its maximum thicknesses in some of the troughs, as in its type section where it attains a thickness of over 1000 feet. It is thin or absent on some of the regionally high areas. In the eastern Sirte Basin, the Hagfa Shale becomes increasingly calcareous and then abruptly passes into the Lower Sabil Carbonates. Origin of Name This formation derives its name from Wadi al Hagfa, located about 15 miles southeast of the oasis of Tagrifet. 90 91 92 HARASH FORMATION Age: Upper Paleocene The name Harash Formation is here proposed for a new formation located in the subsurface of much of the central and western Sirte Basin. The type section is located in the Oasis AA1-59 Well at a drill depth of 4182 to 4470 feet, which corresponds to a subsea depth of 3589 to 3877 feet. Lithology The Harash Formation consists mainly of soft, chalky, white to brown, argillaceous calcilutite and muddy calcarenite with thin interbeds of gray to green, calcareous, fissile shale. The shale often becomes the predominant lithology in the lower part of the formation. This widespread formation varies from a few feet to over 500 feet in thickness. The Harash Formation has a thickness of 288 feet in its type section. Contact Relationships The Harash Formation conformably overlies the Upper Paleocene Zelten Limestone or Upper Sabil Carbonates. This contact is usually abrupt. The Harash Formation is usually conformably overlain by the Kheir Formation or more rarely the Lower Eocene Gir Formation. Paleontology and Age The limestone beds of the Harash Formation often co ntain abundances of the foraminifer Operculina. Bryozoa and algae are also common. The basal part of the overlying Kheir Formation has been dated with planktonic foraminifera as uppermost Paleocene (upper Landenian) in age. Therefore, this formation which also contains an Upper Paleocene fauna is also considered upper Landenian. 93 94 HILAL SHALE Age: Albian to Coniacian The name Hilal Shale was proposed by Barr and Hammuda (1971) for the Cretaceous shales exposed along the southwestern shore of the Marsa al Hilal embayment in Northern Cyrenaica. Lithology The Hilal Shale consists chiefly of dark gray to greenish gray shales which weather to a very light gray color. The shales are often glauconitic and sometimes contain small pyrite crystals and clusters. The uppermost 10 to 20 ft. are usually calcareous and lighter in color representing a transition from the overlying Atrun Limestone. Thin white limestone beds are also present in the uppermost few feet of this formation. The basal contact of the Hilal Shale has not been observed so an exact thickness cannot be given; however, this formation has a thickness in excess of 1000 ft. Contact Relationships The Hilal Shale is conformably overlain by the Upper Cretaceous Atrun Limestone, the contact being gradational. Paleontology and Age The Hilal Shale contains abundant, well preserved foraminiferal faunas with high percentages of planktonic specimens many of which recently have been documented by Barr (1972) and Barr and Hammuda (1971). Rare ostracodes and occasional pyritized nassellarian radiolaria have been observed. Calcispheres are sometimes abundant. Macrofossils are rare, echinoid fragments and Inoceramus prisms being the most common. A few fragments of very small pyritized ammonites have been recovered formation. f rom The the subsurface foramin iferal in the faunas lower part indicate of that the the Hilal Shale ranges in age fr om Albian to Coniacian. This 95 formation can be subdivided into the following planktonic zones (from top to bottom): Globotruncana concavata concavata Zone Globotruncana concavata cyrenaica Zone Globotruncana sigali Zone Praeglobotruncana helvetica Zone Rotalipora cushmani Zone Rotalipora appenninica Zone Ticinella roberti Zone Origin of Name This formation derives its name from the Marsa al Hilal area where it is exposed. 96 HOFRA FORMATION Age: Cambro-Ordovician (?) The name Hofra Formation is here proposed for a new formation located in the subsurface of the northwestern Sirte Basin. The type section is located in the Mobil Al-11 Well at a drill depth of 4352 to 7977 feet, which corresponds to a subsea depth of 3407 to 7032 feet. Lithology The Hofra Formation comprises primarily a sequence of relatively clean quartz sandstones with minor amounts of shale, siltstone, and conglomerates. The sandstones are normally white to light gray in color, firmly cemented with silica, very o ften to the point of becoming orthoquartzites. Quartz overgrowths are common. Quartz is the dominant detrital component, commonly occuring as moderately sorted, fine to coarse, occasionally frosted, subrounded grains; however, all variations in size, degree of rounding and sorting exist. Accessory constituents are pyrite, biotite and muscovite micas, sericite, some chlorite and rare feldspars. Dark colored heavy mineral grains are present throughout. Near the base of the Hofra Formation in the Al-l1 Well, smoky and milky quartz clasts are seen which are identical to the material observed in the subjacent phyllite. The shales and siltstones occur throughout in thin beds and laminae and are gray to brown-gray and green rarely red in color, micaceous to very micaceous, varying from blocky to fissile, and sometimes soft, waxy and lustrous. The dominant colors are white to light gray. However, varicolored sandstones or orthoquartzites do exist in the sequence. These are red, often hematitic, yellow and green. In addition to the above sedimentary rocks, the Hofra Formation includes some thin interbeds of volcanic rocks such as olivine basalt in the Al-11 Well and rhyolite tuff (?) in the I1-13 Well. Contact Relationships In the type section, the Hofra Formation is unconformably overlain by the Satal Formation and is unconformably underlain by phyllite. The to p of the unit is at the abrup t passage fro m t he chalky 97 limestone of the Satal Formation to a light-gray siliceous quartz sandstone; and the base is at the bottom of a red-brown orthoquartzite and siltstone sequence. The Hofra Formation is characterized by high electrical resistivities. The interface between the Hofra Formation and superjacent units is considered to be a major unconformity, being the product of the Hercynian orogeny and ensuing erosion. At various localities, the Hofra Formation is overlain by Nubian, Bahi or younger formations which range in age from Cretaceous to Lower Paleocene. In the Oasis D1-32 Well, it underlies Silurian shale, however, the degree of conformity is not known. The top of the Hofra Formation can normally be recognized by the lithologic change from carbonate, shale or sandstone of the overlying rocks to relatively clean, hard, silica-cemented orthoquartzite. Paleontology and Age Diagnostic fossils have not been recovered from the Hofra Formation and the age of this formation is uncertain. However, in the Oasis D1-32 Well, the Hofra Formation underlies Silurian graptolitic shales, and in the Mobil Al-11 Well it rests on phyllites which are possibly the result of Cambrian metamorphism. Furthermore, this formation is very similar lithologically to the Cambro-Ordovician formations of the Gargaf Group in the Fezzan (Burollet, 1960, pp. 24, 25). The Hofra Formation is therefore tentatively considered Cambro-Ordovician in age and correlative to a portion of the Gargaf Group. Distribution Over most of its occurrence, the Hofra Formation has been only superficially penetrated and little is known of its thickness. Complete penetration has been made in the Al-l 1 Well where the sequence was found to be 3625 feet thick. To the northwest, the formation has a minimum thickness of 4490 feet. The Hofra Formation has widespread distribution throughout the western Sirte Basin. It is probably equivalent to at least part of the rocks of the Gargaf Group exposed on the surface in western and southern Libya. Lithologic similarities and stratigraphic position also suggest that the Hofra is equivalent to the Amal Formation, although physical continuity cannot be demonstrated. 98 Petroleum Importance This formation is productive in the Ora, Samah, Balat, Belhedan and Raguba Fields. Origin of Name The Hofra Formation derives its name from the Hofra Field, in which the type well was drilled. 99 100 101 102 103 104 105 JABAL ZELTEN GROUP Age: Upper Paleocene The name Jabal Zelten Groupis here proposed as a new group in the subsurface of the central and western Sirte Basin. The type section is located in Oasis AA1-59 Well at a drill depth of 4182 to 4804 feet, which corresponds to a subsea depth of 3589 to 4211 feet. Component Formations The Jabal Zelten Group usually consists of two formations, the Harash Formation and the Zelten Limestone. In some areas, such as Waha and Defa, the group cannot be differentiated. Lithology See Harash Formation and Zelten Limestone. The Jabal Zelten Group, undifferentiated, comprises a dominantly carbonate sequence with subordinate shale interbeds. The limestone varies from calcilutite to calcarenite and biocalcarenite, and is occasionally chalky or dolomitic. The dolomite, which is more common in the middle part of the group, is buff to tan, and fine-grained. The shales are gray to green, calcareous, subfissile and pyritic. Contact Relationships The Jabal Zelten Group is conformably overlain by the Facha Dolomite Membe r of the Gir Formation, the Gir Form ation undifferentiated, or the Kheir Formation. It conformably overlies the Khalifa Formation. Distribution This group has wide distribution across the central and western Sirte Basin. To the east, it passes into the equivalent Upper Sabil Carbonates. In the type section, the Jabal Zelten 106 Group has a thick- ness of 621 feet, and this remains fairly constant over much of the area where it is present. Origin of Name The group derives its name from the Jabal Zelten which is located in the central Sirte Basin. 107 JARDAS FORMATION Age: Upper Cretaceous The Jardas Formation of Kleinsmeide and van den Berg (1968) and the Jardas al Abid Formation of Pietersz (1968) are identical so the shorter name is preferred. I n addition, "Gerdes el Abid Limestone" is listed in the Stratigraphic Lexicon (Burollet, 1960, p. 28), but had never been formally proposed as a formational name. Nevertheless, this unit as used would probably be the exact equivalent of the Jardas Formation. The type locality of the Jardas Formation is in the vicinity of the village of Jardas al Abid, located about 15 miles south of Al Marj in northern Cyrenaica. This formation has an elliptical outcrop pattern there, extending along the deeply eroded axis of the northeast-southwest trending Jardas al Abid anticline for about 14 miles with a breadth of 7 miles. The Jardas Formation is also fairly well exposed in similar fashion along the eroded axis of the Marawa anticline. Kleinsmeide and van den Berg (1968) have subdivided the Jardas Formation into four members: Gasr al Abid Marl, Benia Limestone, Got Sas Marl and Al Feitah Limestone. Lithology The Gasr al Abid Marl, the oldest member, crops out along the anticlinal axis near Jardas al Abid village. It consists of light colored, sometimes dolomitic, limestone, yellow and white marls, and dark green calcareous shale. The base of this member is not exposed. The Benia Limestone Member overlies the Gasr al Abid Marl Member in the Jardas al Abid and Marawa areas and consists of light colored crystalline limestone, which is sometimes chalky or marly, and gray massive dolomite. The Got Sas Marl Member consists of yellow marls interbedded with harder, more resistant argillaceous limestone beds. Overlying this member is a white or light gray to yellow limestone unit which is often crystalline, but sometimes chalky or marly. This is the Al Feitah Limestone Member. The base of the Jardas Formation is not exposed so the exact thickness is unknown. Pietersz (1968, p. 126) estimates however, that approximately 3300 ft. of this formation is exposed in the Jardas al Abid area. 108 Paleontology and Age The Gasr al Abid Marl contains a fairly rich planktonic foraminiferal assemblage which is indicative of the Upper - Cenomanian. F.T. Barr has identified the following species from this member: Rotalipora cushmani (Morrow) Rotalipora greenhornensis (Morrow) Praeglobolruncana stephani (Gandolfi) Hedbergella delrioensis (Carsey) Hedbergella brittonensis Loeblich & Tappan Heterohelix moremani (Cushman ) In addition, Marchetti (1935, p. 27) identified a number of molluscs and echinoids from this member which were also indicative of the Upper Cenomanian. The Benia Limestone Member appears to be rather unfossiliferous; however, Kleinsmeide and van den Berg (1968, p. 117) record occasional beds rich in large gastropods and pelecypods including rudists and oysters. They also record from the lower part of the member C u n eo l i n a pavon ia parva Henson and Rotalipora greenhornensis (Morrow) (=R . b r o t zen i). The latter species is restricted to the Upper Cenomanian. The presence of planktonic species suggests open marine conditions, but with probably shallow depths for a portion of the formation, while some of the other carbonate facies suggest more restricted bank and lagoonal environments. The Got Sas Marl Member contains abundant macrofossils that have been studied by Marchetti (1935) and Naldini (1949) and are indicative of a Senonian age. Barr (1968b) has recovered microfaunas from these marls containing abundant ostracodes and both planktonic and benthonic foraminifera. G lobot ru nc ana cor onat aBolli and G. an gu stic arin at a Gandolfi, which are common in the lower and middle Senonian, have been recovered from these marls in the Got Sas valley. Origin of Name The Jardas Formation derives its name from the village of Jardas al Abid. 109 JEFREN MARL Age: Cenomanian The Jefren Marl was originally described by Christie (1955, p. 19) for a sequence of marls exposed along the northern escarpment of Jabal Nefusa. Christie mentioned a section at El Mehla, between Rumia and Jefren, that is usually considered the type section (Desio et al., 1963, pp. 65, 66). Lithology The Jefren Marl consists mainly of soft, brown to greenish yellow marls with thin limestone beds and occasional bands of gypsum. Outcrops are poorly exposed. The soft Jefren Marl, lying between the much more resistant and cliff forming Ain Tobi Limestone and Garian Limestone, forms a topography of hilly plains that are used for agriculture. This formation has a thickness of 260 feet in its type section. Contact Relationships The Jefren Marl conformably overlies the Ain Tobi Limestone and is conformably overlain by the Garian Limestone. These contacts are abrupt in the Jefren-Garian area, but are more gradational to the east, in the area of Kussabat. Paleontology and Age Fossils are rare, but Christie (1955, p. 19) and Desio et al. (1963, p. 66) list a few macrofossils which suggest a Cenomanian age for the Jefren Marl. More recently, Gohrbandt (1966a) recovered a foraminiferal fauna from this formation near Kussabat which included the following shallow marine benthonic species: Pr aealveoli n a c r et ac ea t en u is R e i c h e l , Thomasinella punicaSchlumberger; Buccicrenata libyca Gohrbandt. These foraminifers confirm the Cenomanian age assigned to the Jefren Marl. Origin of Name The Jefren Marl derives its name from the Berber village of Jefren located near this formation's type locality. 110 KALASH LIMESTONE Age: Upper Cretaceous (Maastrichtian) The name Kalash Limestone is here proposed for a new formation located in the subsurface of much of the Sirte Basin. The type section is located in the Mobil E1-57 Well at a drill depth of 6890 to 7200 feet, which corresponds to a subsea depth of 6251 to 6561 feet. Lithology The Kalash Limestone consists predominantly of a white, tan or gray, argillaceous calcilutite, with some dark gray calcareous shale interbeds. Occasionally the upper part of the formation becomes calcarenitic. Glauc onite is rare. Both planktonic and benthonic foraminifera are common and there are rare molluscan fragments. In the eastern Sirte Basin the Kalash Limestone is white and increasingly chalky, becoming quite similar to the Atrun Limestone of northern Cyrenaica (see plate 4, fig. 7). The Kalash Limestone varies from a few feet to over 500 feet in thickness; however, it usually ranges from about 100 to 300 feet. This formation occurs over much of the Sirte Basin. In the DahraHofra area of the northwestern Sirte Basin, however, it is replaced by the lower Satal Formation. Contact Relationships The Kalash Limestone often conformably overlies the Sirte Shale. This contact may be sharp, as in its type section, or gradational (e.g. O259). The Kalash Limestone more rarely overlies other Upper Cretaceous formations such as the Samah Dolomite and Waha Limestone with apparent conformity, or it unconformably overlies older rocks such as the Nubian Formation, Hofra Formation or igneous basement. The Kalash Limestone is most often conformably overlain by the Paleocene Hagfa Shale or Lower Sabil Carbonates. The contact with the Hagfa Shale is usually sharp while its contact with the Lower Sabil Carbonates is more gradational, often being placed at the base of the lowest dolomitic beds which are apparent 111 by their high electric- 112 log resistivity. The top of the Kalash Limestone coincides with the Maastrichtian-Danian contact. The Kalash Limestone is the lateral equivalent of the Waha Limestone and lower Satal Formation. Paleontology and Age The Kalash Limestone contains abundant foraminiferal faunas in cluding common plan ktonic spe cies belonging to th e ge ner a Globotru ncana, Rugoglobigerinaand Heterohelix. These assemblages indicate that the Kalash Limestone was deposited in open sea, probably neritic conditions during the Maastrichtian. Origin of Name The Kalash Limestone derives its name from the Kalash area in southwestern Concession 12. 113 114 KHALIFA FORMATION Age: Paleocene The name Khalifa Formation is here proposed for a new formation located in the subsurface of the western and central Sirte Basin. The type section is located in the Oasis AA1-59 Well at a drill depth of 4804 to 5333 feet, which corresponds to a subsea depth of 4211 to 4740 feet. Lithology In its type section, the Khalifa Formation consists of an upper argillaceous limestone unit (196 ft. thick) and a lower shale sequence (333 ft. thick). The limestone consists mainly of a very argillaceous, dark gray, moderately indurated calcilutite with some interbedded gray calcareous shale. The thicker lower unit is a dark gray to black, fissile, slightly pyritic shale with occasional thin calcareous stringers. The Dahra Formation is the lateral equivalent of the lower shale unit of the Khalifa Formation. Therefore, in those areas where the Dahra Formation is present (in contrast to the Khalifa Formation's type area), the Khalifa Formation consists only of the thin upper argillaceous limestone unit. In a few areas, such as Waha Field, the Beda Formation is not present, the interval having changed to a shale. This shale is then included in the lower Khalifa Formation. Contact Relationships The Khalifa Formation conformably overlies the Dahra Formation, Beda Formation or Hagfa Shale. Often this is a gradational contact. The Khalifa Formation is conformably overlain by the Upper Paleocene Zelten Limestone. At this contact there is a sharp change from the light colored chalky limestone of the Zelten to the darker more argillaceous Khalifa Formation. This contact is reflected by a sharp shift of the S.P. curve of the electric-log. 115 Paleontology and Age The Khalifa Formation contains abundant foraminiferal faunas. The argillaceous limestone unit contains an assemblage dominated by shallow marine benthonic forms including common miliolids. The shale often contains numerous planktonic species indicative of a more open sea environment. These foraminiferal faunas indicate that the Khalifa Formation is Upper Paleocene (Landenian) in age. Origin of Name The Khalifa Formation derives its name from the Khalifa Field in western Concession 59. 116 KHEIR FORMATION Age: Upper Paleocene to Lower Eocene In the Stratigraphic Lexicon (Burollet, 1960, pp. 26, 27), the Kheir Marl is described as a member of the Gebel Uaddan Formation in the Jabal Uaddan-southern Hon graben area. This designation was based on stratigraphic sections described by Chiesa.(1940) in Wadi Amur, Wadi Tar and Wadi Zmam. The sequence in Wadi Amur (29° 18' N; 16° 10' E) was designated the type section by Burollet (loc. cit.). This rock unit has been recognized in the subsurface of much of the Sirte Basin and it is proposed that it be elevated to the rank of formation. A section in the basin that has been used as a subsurface reference is located in the Oasis E1-59 Well at a drill depth of 5998 to 6272 feet, which corresponds to a subsea depth of 5657 to 5931 feet. Lithology In the subsurface, the Kheir Formation is predominately shale with some clay, marl and limestone. The shale is gray to dark-gray and green, fissile and calcareous. The clay is gray, soft, calcareous to very calcareous. The marl is gray, soft and argillaceous, while the limestones are gray calcilutites containing common fossils and pyrite. The Kheir Formation is extremely variable in lithologic composition, and the predominate rock types range from shale through marl to limestone. Surface exposures consist of yellow to greenish, often gypsiferous, marl. Contact Relationships In the E1-59 Well, the Kheir Formation is conformably overlain by the Lower Eocene Gir Formation, and it in turn, conformably overlies the Paleocene Upper Sabil Carbonates. The upper contact is placed at the abrupt change from limestone to the shale of the Kheir Formation, and the basal contact at the abrupt change from shale to the limestone of the Upper Sabil Carbonate. 117 118 Paleontology and Age The Kheir Formation in the subsurface often contains common microfossils, including Operculina and other smaller benthonic and planktonic foraminifera. Globorotalia velascoensis has been recovered from the lower Kheir Formation while G. subbotinae occurs in the uppermost portion. These stratigraphically important index fossils suggest that the Kheir Formation straddles the Eocene-Paleocene (YpresianLandenian) boundary, the lower Kheir Formation being Upper Paleocene and the upper part being Lower Eocene. Distribution The subsurface Kheir Formation reference section is 274 feet thick. The Kheir Formation is present across a large portion of the Sirte Basin. Origin of Name This formation derives its name from Wadi Kheir located along the southwestern margin of the Hon graben. Additional References Chiesa, 1940 Burollet, 1960 Jordi and Lonfat, 1963 Gohrbandt, 1966b 119 120 LIDAM FORMATION Age: Upper Cretaceous (Cenomanian) The name Lidam Formation is here proposed for a new formation located in the subsurface of various portions of the Sirte Basin. The type section is located in the Mobil G1-57 Well at a drill depth of 7012 to 7376 feet, which corresponds to a subsea depth of 6338 to 6702 feet. Lithology The Lidam Formation consists of well indurated, light brown to light gray, sucrosic dolomite, sometimes vugular, or light gray to brown, well cemented pelletal calcarenite, or a combination of these two lithologies. The lower part of the formation is sometimes sandy indicating a local mixing with underlying sands. Quartz grains become increasingly rare upwards in the formation. Glauconite and oolites are also rare. Much of the formation is barren of fossils, but molluscan, algal and echinoid fragments are locally common (see plate 4, fig. 8). The Lidam Formation is found mainly in the principal trough areas of the Sirte Basin where it attains a maximum thickness of about 600 ft. Contact Relationships The Lidam Formation represents the earliest marine deposit in many parts of the Sirte Basin, unconformably overlapping various units including the "Nubian" Formation, Paleozoic rocks, and igneous and metamorphic basement. It is usually overlain by the Upper Cretaceous Etel Formation with apparent conformity; however, it is sometimes unconformably overlain by younger formations of the Rakb Group. Paleontology and Age Although macrofossil fragments occasionally are recognized, most fossils in this formation are poorly preserved often having been altered by dolomitization. Microfossils are also rare c onsisting m ainly 121 of ostracods, miliolids and the large benthonic foraminifer, O val veo l i n a ovum (d'Orbigny). This latter species is diagnostic of a Cenomanian age. Both the lithofacies and biofacies of the Lidam Formation are indicative of very shallow marine conditions, in part lagoonal and intertidal. Origin of Name This formation derives its name from the Mobil Lidam Well (B1-57), located in the vicinity of Graret Umm Lidam, which penetrated a well developed sequence of this formation. 122 MARADA FORMATION Age: Miocene The Marada Formation was originally described by Desio (1935 for a sequence of rocks exposed along the southern escarpment of Dor Marada in the central Sirte Basin. Lithology This formation consists of a large number of lithofacies including interbedded green and gray laminated shales, sandstones, sand' limestones and calcarenites. Gypsiferous beds are locally common Selley (1969) has measured a large number of sections of the Marad Formation of the central Sirte Basin providing considerable detailed sedimentological data on this unit including paleoenvironmental interpretations. This formation has a maximum thickness of about 490 feet in the Marada area. Contact Relationships The Marada Formation appears to disconformably overlie various Oligocene rocks including the Diba Formation in the southeaster Sirte Basin. Paleontology and Age Plant remains and terrestrial vertebrate fossils have been recovered in abundance from the sandy beds exposed along the base of Jaba Zelten, while higher in the section and further north, shallow marine fossils are common (e g oyster banks, corals, etc.). The Marad. Formation contains a number of rapidly changing facies which re present an interfingering of various continental littoral and shallow marine environments. See Selley (1969) for an environmental interpretation of these sediments. The mammalian faunas recovered from the lower part of Jaba Zelten suggest a Burdigalian age (Lower Miocene) (Savage and White 1965). The age of the upper part of the formation remains uncertain Origin of Name The Marada Formation derives its name from the oasis of Marada in the central Sirte Basin. 123 MARAGH FORMATION Age: Upper Cretaceous to Lower Paleocene The name Maragh Formation is here proposed for a new formation in the subsurface of the eastern Sirte Basin. The type section is located in the Mobil N1-12 Well at a drill depth of 10,096 to 10,237 feet, which corresponds to a subsea depth of 10,017 to 10,158 feet. Lithology The Maragh Formation is primarily a clastic unit composed of sandstone, conglomerate and shale with minor carbonates. In texture and composition the Maragh Formation is extremely heterogeneous, varying from well-sorted, well-rounded, friable quartz sandstone to argillaceous, poorly sorted, tight arkosic conglomerate. The formation is often well cemented with calcite or dolomite, and commonly contains a high percentage of red or green interstitial clay and thin interbeds of gray, red or green non-calcareous fissile shale. Glauconite and hematite are very common constituents. The carbonates consist of dolomite which is sandy to pebbly, containing occasional anhydrite and carbonaceous material. Contact Relationships The Maragh Formation is a basal marine transgressive unit which unconformably overlies the Nubian Formation, Amal Formation, volcanics, or granite rocks, and is conformably overlain by various, predominantly carbonate rocks of the Rakb Group. In the type section, the Maragh Formation overlies the Amal Formation and is overlain by the Rachmat Formation. Regionally, the Maragh Formation does not have distinctive electriclog characteristics. The sandstones of the formation may become dolomitic or calcareous near the top of the unit and grade upward through sandy carbonates into the pure carbonates of the overlying units. In this case, the contact is placed at the top of the dominantly clastic sequence. 124 No difficulty occurs in placing the lower contact of the Maragh Formation where it overlies granitic or volcanic rocks; however, when the underlying rocks are older clastics, similar to the red-bed sequence in the Mobil Well S1-12, or the Amal Formation, the contact with the lower unit may appear to be gradational and is difficult to pick. It is then placed at the base of the lowest marine clastics beds or at the top of the highest quartzite or orthoquartzite. Paleontology and Age The Maragh Formation is a time-transgressive rock unit which apparently ranges in age from Cenomanian to Danian, with the greater part of it probably being older than Campanian. The formation is essentially barren of f ossils othe r than palynomorphs. Long-ranging pollen grains indicative of the Albian to Upper Cretaceous have been recovered from this formation. The Maragh Formation is believed to have been deposited in a littoral or shallow marine environment during the transgression of Upper Cretaceous seas. The clastic material was probably derived from local sources and deposited after being transported only short distances. The Maragh Formation may in part represent a littoral facies, equivalent to other formations of the Rakb Group. Distribution The distribution of the Maragh Formation is irregular, and its thickness and distrib ution are assumed to be controlled by the paleotopography of the pre-Upper Cretaceous surface. It appears to be thicker on the flanks of both regional and local highs, and it ranges in thickness from zero on the Amal-Nafoora high to over 500 feet on the northeastern flank of the Amal high. The Maragh Formation is similar to the transgressive Bahi Formation in the northwestern Sirte Basin. Petroleum Importance The Maragh Formation produces oil in the northeastern part of the Amal Field and in the Amal "U" Field. 125 126 Origin of Name The name of the formation is derived from the Bir Maragh water well north of Concession 12, at 29° 50’ N latitude; 21° 08’ E longitude. Additional Reference In a paper on the geology of the Amal Field, Roberts (1970) briefly describes this formation and discusses its importance as a reservoir. No attempt, however, was made to formally define the Maragh Formation. 127 MIZDA FORMATION Age: Upper Cretaceous The Mizda Formation is a name that is in common usage for the Upper Cretaceous rocks exposed in the vicinity and south of the oasis of Mizda, located about 55 miles south of Garian. It has never been adequately described, but Burollet (1960, pp. 21, 22) included it in the Stratigraphic Lexicon and provided a brief description. Jordi and Lonfat (1963, p. 116) expanded the concept of this formation to include the Gasr Tigrinna Formation of Christie (1955, p. 20) as the lowermost of the three members of the Mizda Formation. These members are (from bottom to top) the Tigrinna Marl Member, Mazuza Limestone Member and Thala Member. Lithology Christie (1955, p. 20) recorded a sequence of at least 285 feet of soft, buff to greenish marls, bands of red, pink and yellow limestone, and white chalky limestone conformably overlying the Garian Limestone in the Jabal Nefusa area. He proposed the name Gasr Tigrinna Formation for this sequence; Desio et al. (1963, p. 69) suggested a simplification of this double name to Tigrinna Formation, and Jordi and Lonfat (1963, p. 116) incorporated this unit in the Mizda Formation. The type section for the Tigrinna Marl Member is located on the southwest side of the hill on which the ruins of a Berber fortress (or gasr) stands, about one mile south of the village of Tigrinna. At Mizda, the lower part of the Mizda Formation consists mainly of shales, with some interbedded limestone, dolomite and gypsum. The middle part of the sequence is the Mazuza Limestone Member and consists of a hard, medium to thick-bedded limestone unit with some chert. This unit is overlain a few miles further to the south by soft marls, containing considerable gypsum, dolomite and limestone which are considered the Thala Member. Contact Relationships The Mizda Formation conformably overlies the Garian Limestone in the Jabal Nefusa area and is conformably overlain by the Zmam Formation further to the south. 128 Paleontology and Age The type Tigrinna Marl Member contains a rich assemblage of fossil gastropods, brachiopods, pelecypods, and echinoids. Ostracods are common and a few benthonic foraminifers have been recovered. These fossil assemblages are indicative of a very shallow marine environment of deposition and are suggestive of a Turonian age. Origin of Name This formation derives its name from the oasis of Mizda. 129 NAJAH GROUP Age: Oligocene to Miocene The name Najah Group is here proposed as a new group located at the surface and in the subsurface of the central and eastern Sirte Basin. The type sections of the three component formations, the Arida, Diba and Marada Formations, serve as a composite type section for this group. Component Formations In the central Sirte Basin, the Najah Group comprises in descending order, the Marada Formation, Diba Formation and Arida Formation. Along the eastern margin of the Sirte Basin, the Regima Formation replaces at least part of the Marada Formation. Lithology See Marada Formation, Regima Formation, Diba Formation and Arida Formation. Contact Relationships Over much of the central and eastern Sirte Basin, the Najah Group unconformably overlies the Augila Formation. The top of this group is exposed at the surface or is unconformably overlain by Pleistocene or Recent deposits. Origin of Name Najah (Arabic) is an encampment of bedouin tents. 130 NEFUSA GROUP Age. Upper Cretaceous The name Nefusa Group was proposed by Burollet (1960, p. 37, 38) to include most of the Upper Cretaceous rocks exposed along the northern escarpment of Jabal Nefusa. Component Formations The Nefusa Group comprises the following formations in descending order: the Garian Limestone, Jefren Marl and Ain Tobi Limestone. Lithology See Garian Limestone, Jefren Marl and Ain Tobi Limestone. Origin of Name This group derives its name from the Jabal Nefusa which includes its type locality. 131 "NUBIAN" FORMATION Age: Mesozoic (mainly Lower Cretaceous) The term Nubian Formationor Nubian Sandstone was first used for a widespread, non-marine sandstone sequence that is well exposed in Nubia, in the Nile Valley of Upper Egypt (Russegger, 1837). Subsequently, this formation has been applied to the non-marine Mesozoic rocks that cover a large portion of southern Egypt and extend across into the southern half of Libya. The most recent Geologic Map of Libya (Conant and Goudarzi, 1964) shows this formation covering thousands of square miles in southeastern and southwestern Libya. However, because of some divergent usages of the term Nubian over the last century, this formational name is not without its ambiguity and recently has become the focus of controversy. Consequently, some authors have recommended abandoning the term (Pomeyrol, 1968). This name is commonly used by petroleum geologists in Libya, and following the usage of Conant and Goudarzi (1964), we regard the Nubian as a worthwhile name. It is a broad term, however, and perhaps should be considered a group. When this sequence has been studied in detail locally, it usually can be subdivided into smaller rock units. When enough work has been done on this widespread and broadly defined unit, perhaps it will eventually be replaced by many more precisely defined and geographically restricted new formations. In this report, we are therefore using the term "Nubian" Formation in an informal sense. However, until the Nubian can be replaced by smaller, more meaningful units, the term "Nubian" Formation will continue to be of value. Lithology The "Nubian" Formation of the Sirte Basin consists of a variety of interbedded non-marine lithofacies including sandstones, siltstones, shales and conglomerates. Lignite beds are occasionally present. The sandstones are very fine to coarse grained, quartzitic, tan to gray, p redominantly poorly sorted, often with a clay matr ix. The finer sandstones tend to be more poorly sorted, while some of the coarser sandstones have little matrix material and are very porous. Crossbedding and ripp le marks are com mon. The shale s are red, ma roon, 132 green and gray, commonly micaceous and laminated. They sometimes contain considerable carbonaceous debris. The conglomerates usually contain rounded quartz pebbles with a sand and clay matrix. Contact Relationships In the Sirte Basin, the " Nubian " Formation unconformably overlies granitic basement, volcanics or various Lower Paleozoic formations. Often a basal conglomerate is present. The Nubian Formation is overlain by various marine Upper Cretaceous formations ranging in age from Cenomanian to Maastrichtian. Paleontology and Age Much of this formation is unfossiliferous; however, fossil plants and spores have been recovered from a number of wells. This paleobotanical evidence usually suggests a Lower Cretaceous age. Viterbo (1969) records the Aptian charaphyte Cavator harrisi Peck along with the brackish water ostracods Cytherella, Bisulcocypris and Theriosynoccum from the "Nubian" penetrated by wells in Concession 82, southeastern Sirte Basin. Further to the west, the Neocomian fern Weichselia mantelli, which is common in the British Wealden, has been recovered from the subsurface "Nubian" Formation. This fern is also found at a number of surface exposures in southwestern Libya including the Messak cliffs near Germa. Sedimentary and paleontological studies suggest that much of this formation was deposited under a variety of continental environments including fluviatile and alluvial conditions. 133 RACHMAT FORMATION Age: Upper Cretaceous The name Rachmat Formation is here proposed for a new formation located in the subsurface of the Sirte Basin. The type section is located in the Oasis 02-59 Well, at a drill depth of 8272 to 8730 feet, which corresponds to a subsea depth of 7612 to 8070 feet. Lithology The Rachmat Formation is typically a shale section with minor limestone, sandstone and occasional dolomite interbeds. The shales are dominantly dark gray and sometimes green or brown, fissile to slightly blocky, slightly to non-calcareous, and often glauconitic and pyritic. The limestones occur more frequently in the lower part of the formation and are light to medium gray, microcrystalline to finely granular, dense, fossiliferous and argillaceous in part. The dolomites are commonly found in the basal par t of the f ormation and they are tan to b rown, microcrystalline to finely crystalline, anhydritic and often porous. Contact Relationships The Rachmat Formation is typically overlain by the Sirte Shale which may be a conformable to disconformable relationship. In the eastern Sirte Basin, the Rachmat Formation has a gradational contact with the overlying Tagrifet Limestone. Depe nding on basin position, the Rachmat Formation may conformably overlie the Argub Carbonates, Etel Formation, Bahi Formation or Maragh Sandstone, or unconformably rest on the Amal Formation or granite. The top of the Rachmat Formation can normally be placed at the change from the characteristically brown, waxy, calcareous shales of the Sirte Shale to the more gray, fissile, less calcareous shales of the Rachmat Formation. In many localities this contact is marked by a silty to sandy, pyritic, glauconitic zone with phosphatic pellets at the base of the Sirte Shale. Where the Rachmat Formation overlies the Etel Formation, the contact is picked at the first occurrence of massive anhydrite. 134 135 Distribution The Rachmat Formation is widely spread across the Sirte Basin and reaches its thickest development in the major trough areas. It is absent on the regional highs such as the Dahra-Hofra, Waha and Amal areas and attains a thickness in excess of 1200 feet in the Mobil F1-13 well. Paleontology and Age The Rachmat Formation contains common foraminifera and ostracods over much of the Sirte Basin. In the northern part of the basin, this formation contains the planktonic foraminifera G l o b o t r u n c a n a concavata, G. coronataand G. angusticarinata. These microfossil assemblages indicate that the Rachmat Formation is Santonian to Coniacian in age. Origin of Name The Rachmat Formation derives its name from the Ramlet er Rachmat, an area of sand about 18 miles southeast of Marada. Additional References Williams (1968, p. 200, text-figs. 1-3) uses the name Rachmet Shale on several stratigraphic cross sections through the Augila Field and gives a brief description of the formation; however, he used the name informally and did not propose it as a new formation. Nevertheless, his usage of the name is the same as that proposed in this paper. 136 RAKB GROUP Age: Upper Cretaceous The name Rakb Group is here proposed as a new group present in the subsurface of the Sirte Basin. The type section is located in Oasis 0259 Well at a drill depth of 7702 to 9264 feet, which corresponds to a subsea depth of 7042 to 8604 feet. Component Formations In the northwestern Sirte Basin, the Rakb Group comprises in descending order, the Sirte Shale, the Rachmat Formation, and the Argub Carbonate. In the southern part of the basin, the formational sequence remains the same except that the Argub Carbonate is replaced by the Etel Formation. To the northeast, the Rakb Group consists of the Sirte Shale, Tagrifet Limestone, Rachmat Formation and Maragh Formation; however, on some of the regionally high areas in both the west and east, all but the Sirte Shale may be absent with the Sirte Shale directly overlying preUpper Cretaceous rocks. Lithology See Sirte Shale, Rachmat Formation, Etel Formation, Argub Carbonate, Tagrifet Limestone and Maragh Formation. Contact Relationships Throughout the basin, where it is present, the Rakb Group is conformably overlain by the Kalash Limestone or the Satal Formation. This contact may be gradational as in the 02-59 Well, or sharp as in the G1-57 Well. Since the Rakb Group represents the major part of the Upper Cretaceous transgressive sequence, its various components may rest conformably on the Lidam Formation, or lie unconformably on older formations ranging in age from Precambrian to Lower Cretaceous. Distribution The Rakb Group is widely distributed across the Sirte Basin, with its thick ness being c ontrolled by the paleotopography which 137 existed during Upper Cretaceous times. It is thin or absent on high areas such as Dahra-Hofra and Waha, and it reaches its thickest development in the major trough areas. In the type section it is 1562 feet thick. Origin of Name T he Rakb Group derives its name from the Rakb F ield of southeastern Concession 12. 138 REGIMA FORMATION Age: Middle Miocene The Regima Formation was originally described by Desio (1935) in the Benghazi area of northern Cyrenaica. The type locality is located along the face of the escarpment below the village of Regima, about 18 miles east of Benghazi. Lithology This formation consists of a well-bedded, highly fossiliferous, hard, white to dark gray, chalky, slightly crystalline limestone, in part dolomitic, and with occasional dark gray marl seams. Pietersz (1968, p. 130) states that the Regima Formation has a thickness of 205 ft. at its type section, while only 6 miles to the northeast a thickness of 285 ft. was measured along the escarpment. Contact Relationships Along the western flank of Jabal al Akhdar, the Regima Formation unconformably onlaps several formations of different ages including the Eocene Apollonia and Derna Limestones and the Oligocene Cyrene Formation. Paleontology and Age This formation is very fossiliferous, but often the fossils have been moderately weathered. Stefanini (1923), Desio (1935) and others have recorded numerous fossils from this formation including molluscs, echinoids, corals, bryozoa and foraminifera. The large foraminifer Borelis melo (Fichtel and Moll) is fairly common indicating a Middle Miocene age for this formation. Origin of Name This formation derives its name from the village of Regima. Additional References Burollet, 1960 Pietersz, 1968 Kleinsmeide and van den Berg, 1968 139 SABIL CARBONATES LOWER SABIL CARBONATES Age: Lower to Middle Paleocene The name Lower Sabil Carbonates is here proposed for a new formation in the subsurface of the eastern Sirte Basin. The type section is located in the Mobil C1-12 Well at a drill depth of 8630 to 9958 feet, which corresponds to a subsea depth of 8267 to 9595 feet. Lithology The Lower Sabil Carbonates consist mainly of dolomite with lesser amounts of limestone and occasional chalk and anhydrite. The dolomites are tan to brown in color, mainly very finely crystalline, occasionally sucrosic, medium hard, rarely fossiliferous and carbonaceous. The limestone occurs at the top and base of the formation as calcilutites. These are white to tan in color, fossiliferous and occasionally chalky. The chalk is associated with the limestone section at the top of the formation. The anhydrite occurs throughout the dolomite section. Outside of the type area, particularly to the east, the formation may consist almost entirely of dolomite with increased anhydrite content. Contact Relationships In the C1-12 Well, the Lower Sabil Carbonates are conformably overlain by the Sheterat Formation, and they conformably overlie the Upper Cretaceous Kalash Limestone. The upper contact is normally determined by the change from green to gray, calcareous, fossiliferous, pyritic shale of the Sheterat Formation to the light-colored Lower Sabil Limestone. The bottom contact usually occurs where microcrystalline dolomite overlies the white microcrystalline, occasionally fossiliferous Kalash Limestone. This contact is often gradational and difficult to pick from samples alone. Where the Sheterat Shale is not present, the Upper and Lower Sabil Carbonates may be vertically contiguous making the contact difficult to recognize. 140 Distribution The Lower Sabil Carbonates have a thickness of 1328 feet at the C112 Well, and they thicken from east to west across Concession 12. They comprise the eastern equivalent of a heterogeneous succession of shales and carbonates of the western Sirte Basin. Origin of Name This formation derives its name from Matan as Sabil located about 7 miles north of the Augila oasis. 141 142 143 UPPER SABIL CARBONATES Age: Upper Paleocene The name Upper Sabil Carbonates is here proposed for a new formation located in the subsurface of the eastern Sirte Basin. The type section is located in the Mobil C1-12 Well at a drill depth of 7700 to 8485 feet, which corresponds to a subsea depth of 7337 to 8122 feet. Lithology The Upper Sabil Carbonates consist predominantly of limestones with lesser dolomite and some chalk. In the type section, the limestones are light in color, ranging from pink, white and gray to tan. They are mainly calcilutites, often dolomitic or chalky, fossiliferous, soft to more usually hard and dense, and only occasionally contain chert, pyrite, and carbonaceous material. The dolomite is generally light to medium brown, fine to medium crystalline, anhydritic, and of medium hardness; while the chalk is soft and white to pink in color. East of the Amal Field, the sequence becomes increasingly dolomitic and anhydritic. Contact Relationships The Upper Sabil Carbonates are conformably overlain by the Kheir Formation, and they in turn conformably overlie the Sheterat Formation. Where the Sheterat shales are not present, the Upper Sabil Carbonates rest directly on the Lower Sabil Carbonates. In the C1-12 Well, the upper contact is placed at the change from the gray, green to brown shale of the Kheir Formation to pink fossiliferous calcilutite. The bottom contact is placed at the change from the Upper Sabil Carbonates to the gray to tan pyritic calcilutite and gray to green shale of the Sheterat Formation. The Upper Sabil Carbonates bear distinctive electric-log characteristics in the type section, and the top contact occurs at a resistivity decrease accompanied by a sharp negative deflection in the S.P. curve. The bottom contact is at a resistivity increase accompanied by a positive deflection in the S.P. curve. 144 Distribution The Upper Sabil Carbonates have a thickness of 785 feet in the C112 Well and thicken to about l000 feet in the P1-12 Well. They thin rapidly, except for local biohermal build-ups into the trough area west of Concession 12. Further to the west these carbonates pass laterally into the rocks of the Jabal Zelten Group. Petroleum Importance Upper Sabil Carbonates are productive in the Sahabi and Gialo Fields. Origin of Name The name is derived from Matan as Sabil located about 7 miles north of the Augila oasis. 145 146 147 148 SAMAH DOLOMITE Age: Upper Cretaceous The name Samah Dolomite is here proposed for a new formation located in the subsurface of the Samak Field area of the south central Sirte Basin. The type section is located in the Oasis L3-59 Well at a drill depth of 6382 to 6468 feet, which corresponds to a subsea depth of 5618 to 5704 feet. Lithology The Samah Dolomite consists predominantly of a hard, light to dark brown or gray, mottled, crystalline dolomite, sometimes limey, becoming in part a dolomitic limestone. Thin shale interbeds are rare. The dolomite is often vugular, sometimes with fractures filled with calcite or dolomite. Quartz grains are found in the basal few feet. The Samah Dolomite is restricted to the Samah Field and adjacent areas. This formation has a maximum thickness of about 300 feet. Contact Relationships An argillaceous facies of the Kalash Limestone overlies the Samah Do lomite. The contact is sharp and is sometimes mar ked by concentrations of pyrite which suggest that it may be disconformable. The Samah Dolomite unconformably overlies quartzite of probable lower Paleozoic age. This quartzite is tentatively assigned to the Hofra Formation of the Gargaf Group. Paleontology and Age Fossils are rare in the Samah Dolomite and those that do occur are poorly preserved. Pelecypod fragments including rudists are recognized. Rare foraminifera also have been recovered from thin shale interbeds. These fossils are indicative of a Late Cretaceous age and a shallow marine environment of deposition. Origin of Name The Samah Dolomite is the principal reservoir of the Samah Field from which it derives its name. 149 SATAL FORMATION Age: Lower Paleocene to Upper Cretaceous (Danian to Maastrichtian) The name Satal Formation is here proposed for a new formation located in the subsurface of the Dahra-Hofra area of the northwestern Sirte Basin. The type section is located in the Oasis B2-32 Well at a drill depth of 3782 to 4811 feet, which corresponds to a subsea depth of 2593 to 3622 feet. Lithology The Satal Formation is subdivided into an upper member of Lower Paleocene age and a lower member of Upper Cretaceous age. The lower member of the Satal Formation consists of a massive gray to white, moderate to well indurated argillaceous calcilutite that grades to a calcarenite near the top of the unit. Few thin dolomitic intervals may be present throughout the section. Shale is absent, except for thin beds that may be present at the base. The lower member of the Satal Formation may be subdivided into three main zones: (l) a lower zone containing abundant pelagic foraminifers; (2) a middle zone with sparse pelagic and benthonic microfossils; and (3) an upper zone containing abundant benthonic foraminifers. Elsewhere, off the Dahra-Hofra platform area, this lower member of the Satal Formation grades into the Kalash Limestone which consists of a deeper, more open sea facies (see plate 5, fig. 9). The contact between the lower and upper members of the Satal Formation is placed at a stratigraphic position where the underlying unit suddenly becomes more chalky and less dolomitic. This lithologic change coincides with the Danian-Maastrichtian boundary. The upper member of the Satal Formation is characterized by being light gray to white, moderately indurated, composed of fine grained calcarenite in association with calcilutite. On the higher shelf areas these sediments are most often dolomitized and they can include thin beds of anhydrite. Shales are only encountered along platform margins where they interfinger with calcilutites. Most common constituents are pellets and benthonic foramin ifers, including miliolids that ar e pr esent throughout the unit. Less c ommo nly found are mol- 150 luscs, echinoderms, algae and corals. A diagnostic feature denoting the top of the upper member is a thin (5-15 ft.) zone composed of dense, gray, pelecypod calcilutite. The Satal Formation is restricted to a broad platform in the northwestern Sirte Basin which extends for over 120 miles in an east-west direction and with a north-south breadth of about 40 miles. Contact Relationships The Satal Formation conformably overlies the Sirte Shale; however, in some areas where the Sirte Shale is missing, it unconformably overlies the Cambro-Ordovician (?) quartzite of the Hofra Formation (e.g. B1-32). The contact with the overlying Thalith Member of the Beda Formation is generally sharp, and apparently conformable. Paleontology and Age Although molluscs, corals, echinoderms, ostracods and algae are locally common, benthonic foraminifera are the most abundant fossils recognized throughout much of the Satal Formation. In the upper member, miliolids and rotalids are the most common. At the top of the lower member Siderolites calcitrapoides is often abundant Orbitoides and O m ph aloc yc lu s also occur, but are not as common Occasionally, planktonic forms have been recovered from the basal part of the lower member. The foraminiferal faunas indicate that the upper member is Lower Paleocene (Danian) and the lower member is Upper Cr etac eous (Maastrichtian) in age. Petroleum Significance The Satal Formation forms the principal reservoirs in the Bahi and Dahra 'B' Fields. Origin of Name The Satal Formation derives its name from Wadi es Satal located about 30 miles west of the Dahra 'B' Field. 151 152 153 154 SHETERAT FORMATION Age: Middle Paleocene The name Sheterat Formation is here proposed for a new formation in the subsurface of the eastern Sirte Basin. The type section is located in the Mobil C1-12 Well at a drill depth of 8485 to 8630 feet, which corresponds to a subsea depth of 8122 to 8267 feet. Lithology The Sheterat Formation typically consists of light-gray to tan, fossiliferous calcilutite and some chalk with interbeds of gray, green to brown, pyritic, fossiliferous, fissile to blocky, calcareous shale. Contact Relationships The Sheterat Formation is conformably overlain by the Upper Sabil Carbonate s and it is conformably underlain by the L ower Sabil Carbonates. The upper contact is placed at the change from the calcilutites of the Upper Sabil to the shales of the Sheterat Formation. The base of the formation occurs at the change from shale to the calcilutites of the Lower Sabil Carbonates. In areas where this formation is very thin, it may exist only as a band of argillaceous limestone. In the C1-12 Well, the upper contact is marked by an increase in resistivity as well as by a positive deflection of the S.P. curve. The base is placed at a resistivity decrease and a prominent negative deflection of the S.P. curve. Distribution The Sheterat Formation is 145 feet thick in its type section. It is absent in eastern Concession 5~, east of Gialo Field. It is 20 feet thick at Amal Field, and appears to thicken rapidly to the west. Origin of Name The formation derives its name from the Wadi Umm al Sheterat along the western edge of Concession 12. 155 156 SIRTE SHALE Age: Upper Cretaceous (Maastrichtian to Campanian) The name Sirte Shale is here proposed for a new formation located in the subsurface of the Sirte Basin. Its type section is located in the Oasis 02-59 Well at a drill depth of 7702 to 8272 feet, which corresponds to a subsea depth of 7042 to 7612 feet. Lithology The Sirte Shale in its type section is dominantly a shale sequence with thin limestone interbeds. The shales range in color from dark gray to mainly dark brown and are subfissile to splintery and often waxy in character. They are carbonaceous and calcareous for the most part and grade into shaly limestone. Foraminifera are common and occasionally pyrite is present. In the type section, the shales are only occasionally silty, but in adjacent areas, they become increasingly silty and sandy towards the base which is often characterized by the presence of pyrite, glauconite, and small phosphatic nodules as well as a sandy zone. The thin limestone beds are characteristically tan to dark brown, argillaceous calcilutites. In the eastern Sirte Basin, the Sirte Shale assumes a darker color and consists of black, with lesser brown, pyritic calcareous shales with dolomite and sandy interbeds. Contact Relationships In the type section, the Sirte Shale is conformably overlain by the Kalash Limestone, and it disconformably overlies the Rachmat Formation. The upper contact is somewhat gradational, and it is placed at the change from gray cryptocrystalline, often chalky limestone to shale. In other areas, such as southern Concession 57, this contact is sharply defined both lithologically and by electric-log character. The base is marked lithologically by a well defined change from brown waxy carbonaceous, calcareous shale to the gray, slightly to non-calcareous Rachmat shales. The electric-log characteristically shows a sharp decrease in resistivity at this contact. 157 Regionally, the Sirte Shale conformably underlies the Kalash Limestone or Satal Formation, and the contact can be picked either by lithologic or electric-log characteristics. Depending on basin position, the basal contact can be gradational or unconformable. In the western Sirte Basin, the Sirte Shale rests on rocks as old as the Bahi Formation. Where the unconformity is present, the base of the formation is commonly marked by pyrite, glauconite, phosphate pellets and quartz sand grains. In the eastern Sirte Basin, the time equivalent of the Sirte Shale becomes calcareous in the lower part, and in this area the name Sirte Shale is restricted to the upper shale sequence. In the northeastern part of the basin, the Sirte Shale grades laterally into and rests on the Maragh Formation. On the Amal high it often unconformably overlies the Amal Formation. Paleontology and Age The Sirte Shale contains abundant foraminifera. In the northern Sirte Basin, planktonic species are present throughout the formation. Further to the south, they are found only in the upper part of the Sirte Shale, with common benthonic foraminifera. including floods of several small bulimines, occurring in the lower part of the formation. Several of the more characteristic planktonic species in the Sirte Shale include: Globotruncana fornicata, G. arca, G. tricarinata, G linneiana and G. obliqua. The foraminifera recognized in the Sirte Shale indicate that this formation ranges in age f rom lower Maastrichtian to lowermost Campanian or Santonian. Origin of Name The Sirte Shale derives its name from the town of Sirte, located along the southwestern shore of the Gulf of Sirte. Additional References Williams (1968) uses the name "Rakb Shale" in several stratigraphic cross sections through the Augila Field. He used the name informally, however, and did not propose it for a new formation. The name "Rakb Shale" is here considered an invalid synonym of the Sirte Shale. 158 SLONTA LIMESTONE Age: Middle to Upper Eocene The Slonta Limestone is the youngest of the three Eocene formations proposed by Gregory (1911). This formation is best developed in the central and eastern Jabal al Akhdar area in the vicinity of Slonta, Shahhat and Derna. Lithology In the Slonta area, this formation consists of white to tan, mediumbedded, hard calcarenites containing common nummulites. To the northeast near Derna, this formation is a coral algal limestone. The Slonta Limestone has a thickness of 30 to 60 ft. at Shahhat and increases southwards to a maximum of about 200 ft. in its type section near the village of Slonta. Contact Relationships The Slonta Limestone conformably overlies the Middle Eocene Derna Limestone and is unconformably overlain by the Oligocene Cyrene Formation. Paleontology and Age Pietersz (1968, p. 128) records Nummulites gizehensis, N. striatus and Echinolampas from the Slonta Limestone which are indicative of a Middle Eocene age, and Nummulites fabianii from the upper part of the formation which is diagnostic of the Upper Eocene. Origin of Name This formation derives the name from the village of Slonta in the central Jabal al Akhdar. Additional References Burollet, 1960 Pietersz, 1968 159 SURFA FORMATION Age: Upper Paleocene The name Surfa Formation was originally proposed by Jordi and Lonfat (1963) for a sequence of shale, marl and limestone exposed near Wadi Tar along the southwestern margin of the Hon graben. Jordi and Lonfat subdivided this formation into three members (from bottom to top): Bu Ras Marlstone Member, Gelta Chalk Member and O p e r c u l i n a Limestone Member. The upper two members had previously been described in the Stratigraphic Lexicon ( Burollet, 1960, p. 27). Lithology The Bu Ras Marlstone Member consists mainly of alternating thin beds of light colored limestone, marls and calcareous shales. It has a thickness of about 100 feet in the Wadi Tar area. The Gelta Chalk Member consists of a thick-bedded to massive, light gray to white, soft chalk and chalky limestone. Some harder limestone, marls and gypsum beds are also present. This member has a thickness of about 165 feet. The Operculina Lim estone Memberconsists of thin-bedded, dark brown to gray limestone and dolomite with some marls and chalky marls. This member is about 40 feet thick. Contact Relationships The Surfa Formation conformably overlies the Zmam Formation and is conformably overlain by the Kheir Formation. There appears to be a minor disconformity between the Bu Ras Marlstone Member and the Gelta Chalk Member. Paleontology and Age The Surfa Formation contains common benthonic foraminifera suggestive of a shallow marine environment of deposition. Operculina and Lockhartia are sometimes common in the uppermost member. 160 TAGRIFET LIMESTONE Age: Upper Cretaceous The name Tagrifet Limestone is here proposed for a new formation located in the subsurface of the southeastern Sirte Basin. The type section is located in the Oasis N1-59 Well at a drill depth of 8390 to 8690 feet, which corresponds to a subsea depth of 8259 to 8559 feet. Lithology The lower part of the Tagrifet Limestone consists mainly of a well indurated, porous, gray to brown calcarenite. Pellets are sometimes common. Quartz grains are often present in the basal part, becoming rare higher in the section. Fossil debris including rudistid and other molluscan fragments is occasionally abundant. The upper part of the Tagrifet Limestone is usually a more argillaceous calcilutite, darker brown and less porous with occasional shale interbeds. Macrofossil debris is less common although foraminifera increase in abundance. The Tagr ifet Limestone is restricted to an are a alo ng the southeastern margin of the Sirte Basin including the Augila-Nafoora Field area. The thickness varies from a few feet to a maximum of about 600 feet. It has a thickness of 300 feet in its type section. Contact Relationships The Tagrifet Limestone is conformably overlain by the Sirte Shale. This contact is usually sharp; however, limestone stringers occasionally develop in the basal Sirte Shale giving the contact a gradational appearance. The Tagrifet Limestone overlies the Rachmat Formation with apparent conformity, or unconformably overlies granitic basement. This formation appears to be the lateral equivalent of the lower Sirte Shale in other parts of the basin. 161 162 Paleontology and Age The lower part of this formation especially, is highly fossiliferous containing rudistids, Inoceramus, other pelecypods, gastropods, bryozoa, algae, foraminifera and ostracods. These assemblages, along with the characteristic sediments, indicate a very shallow marine environment of deposition. Williams (1968) in his study of the Rakb Carbonates (a synonym of the Tagrifet Limestone) provides considerable detail on the depositional environment and diagenesis of this formation in the Augila Field. Petroleum Significance The Tagrifet Limestone is one of the principal reservoirs in the Augila and Nafoora Fields. Origin of Name The Tagrifet Limestone derives its name from the Tagrifet oasis located about 50 miles north of Zella. Discussion Williams (1968) gives a detailed lithologic description of the "Rakb Carbonates" from the Augila Field and includes three photomicrographs of different lithofacies found in the lower part of this formation. The term "Rakb Carbonates" had never been formally proposed as a formational name and Williams makes it clear that he was using the name informally and did not intend to propose a new formation. Although the name "Rakb Carbonates" is therefore considered a nom en nu du m and an invalid synonym of the Tagrifet Limestone, Williams' description and discussion of this unit are of considerable value. 163 WADI DUCCHAN FORMATION Age: Upper Cretaceous to Paleocene (?) The Wadi Ducchan Formationwas described by Pietersz (1968, pp. 126, 127) and Kleinsmeide and van den Berg (1968, pp. 118, 119, pl. 1) with its type locality between Got Sas and Wadi Ducchan approximately 6 miles west of Jardas al Abid in Northern Cyrenaica. The formation is known only from the Jardas al Abid and Aweilia areas. Lithology T his formation co nsists mai nly of h ard, gray to bro wn microcrystalline dolomite and dolomitic limestone. Bedding is often obscure. These dolomites are sometimes vugular and locally brecciated. Pietersz (1968, p. 127) estimates that 2000 feet of this formation is exposed in the Jardas al Abid area. A much thinner sequence is exposed in the Aweilia area. Contact Relationships The Wadi Ducchan Formation appears to conformably overlie the Upper Cretaceous Jardas Formation and is unconformably overlain by the Middle Eocene Derna Limestone. Paleontology and Age This formation is usually barren of fossils, although leached fossil molds are sometimes recognized. Kleinsmeide and van den Berg (1968, p. 118) state that rudists have been found in the lower portion of the formation while poorly preserved nummulite casts have been observed in the upper part. Based on its stratigraphic position and this weak paleontologic evidence a tentative age assignment of Maastrichtian to Paleocene was made for the Wadi Ducchan Formation by the original authors. Origin of Name This formation derives its name from Wadi Ducchan where its type section is located. 164 WAHA LIMESTONE Age: Maastrichtian The name Waha Limestone is here proposed for a new formation located in the subsurface of the south-central Sirte Basin. The type section is located in the Oasis A29-59 Well at a drill depth of 6450 to 6525 feet, which corresponds to a subsea depth of 5969 to 6044 feet. Lithology The Waha Limestone consists predominantly of a tan to white, grain supported, skeletal calcarenite. Lime mud is present in varying amounts, usually ranging between 15 and 35 percent. The grains consist mainly of fossil debris including fragments of rudistids, other molluscs, echinoids, algae and benthonic foraminifera. Quartz grains are common in the basal formation, but become rare higher in the section (see plate 5, fig. 10). The Waha Limestone varies in thickness from a few feet to a maximum of about 300 ft. This formation is restricted to the region of the Waha and Defa Fields and adjacent areas in the central Sirte Basin. Contact Relationships In the Waha Field area, the Waha Limestone is conformably overlain by an unusually thin Maastrichtian Kalash Limestone. In the Defa area, the Waha Limestone is conformably overlain by the Danian Defa Limestone. The Waha Limestone unconformably overlies non-marine sandstones of probable earlier Cretaceous age (Nubian Formation?) or igneous basement. Laterally the Waha Limestone is a facies equivalent of the Kalash Limestone. Paleontology and Age In addition to molluscan and echinoid fragments, the large benthonic foraminifera Om phalocyclu s macroporu s, Siderolites calci- 165 166 t rapoides and O rbit oides m edia are some of the most common and diagnostic fossils in this formation. These species indicate a Maastrichtian age and a very shallow marine environment of deposition. Petroleum Significance The Waha Limestone is the principal reservoir in the Waha Field and is also an important reservoir in the Defa Field. Origin of Name The Waha Limestone derives its name from the Waha Field. 167 168 ZELTEN LIMESTONE Age: Upper Paleocene The name Zelten Limestone is here proposed for a new formation located in the subsurface of the central and western Sirte Basin. The type section is located in the Oasis AA1-59 Well at a drill depth of 4470 to 4804 feet, which corresponds to a subsea depth of 3877 to 4211 feet. Lithology The Zelten Limestone consists predominantly of a limestone sequence with very subordinate amounts of shale. In its type section, this formation consists of cream, tan and gray-colored, argillaceous to shaly, chalky, fossiliferous calcilutite and some calcarenite with thin stringers of gray-green, very soft, fissile, pyritic shale. Occasionally, the unit becomes biohermal in character. In other areas, the formation consists of tan to br own, very fossiliferous, arenaceous, glauconitic, vuggy calcilutite and calcarenite with subordinate amounts of white to brown; finely crystalline to medium crystalline, hard, dense dolomite. In some wells (e .g. B1-13) the Zelten Limestone is primarily a sequence of anhydritic dolomite. Contact Relationships The Zelten Limestone normally overlies the Khalifa Formation and underlies the Harash Formation with both contacts being conformable. In the Harash area, it is in conformable contact with the underlying Lower Sabil Carbonates. In the type section, the upper contact is placed at the passage from the dominantly shaly basal Harash Formation to the calcilutites of the Zelten Limestone. The base of the unit is placed at the abrupt change from calcilutite to the shales of the Khalifa Formation. On the electric-log the top of the formation is normally characterized by a sharp increase in resistivity and a similar increase is encountered when passing into the Khalifa Formation. A large negative deflection of the S.P. curve is typical of the Zelten Limestone. 169 Distribution In the type section, the Zelten Limestone has a thickness of 333 feet which remains fairly constant over a large area where it is typically developed. It has a widespread distribution in the western and central Sirte Basin. In the south-central part of the basin, however, it cannot be differentiated from the Harash Formation, and its equivalent there forms part of the undifferentiated Jabal Zelten Group. Petroleum Significance The Zelten Limestone forms the principal reservoir of the Zelten Field, and it contains lesser reserves in several smaller fields. Origin of Name The Zelten Limestone derives its name from the Jabal Zelten in the central Sirte Basin. 170 ZMAM FORMATION Age: Maastrichtian to Lower Paleocene The Zmam Formation was originally described by Jordi and Lonfat (1963) as a shale, marl and limestone sequence exposed along the southwestern border of the Hon graben (=Djofra Graben) on the western margin of the Sirte Basin. The type section of the Zmam Formation is located on an isolated hill near the entrance of Wadi Tar, about 30 miles northwest of the oasis of Socna. Jordi and Lonfat (op. cit.) subdivided the Zmam Formation into the following members: Had Limestone Member Upper Tar Marl Socna Mollusc Bed Lower Tar Marl Lithology The Lower Tar Marl consists mainly of thin bedded, dark gray and greenish gray shales. These shales are often gypsiferous. Near the top of this member, beds of light tan, moderately soft marls become increasingly common. Jordi and Lonfat (1963, p. 117) record 260 feet of Lower Tar Marl in Wadi Tar, but the base of this member is not exposed. They estimate that the total thickness based on surface and subsurface data is approximately 755 to 790 feet in this area. Locally the Socna Mollusc Beds separate the Upper Maastrichtian Lower Tar Marl from the Danian Upper Tar Marl. This unit is a highly fossiliferous marly limestone. The Socna Mollusc Beds are especially well developed in the vicinity of Socna where they have a maximum thickness of about 40 feet (Gohrbandt, 1966b, p. 36) and thin northwards until they are no longer recognizable enough to separate the Tar Marl into two members. The Upper Tar Marl is similar lithologically to the Lower Tar Marl. It is well exposed in Wadi Tar where it has a total thickness of 230 feet. In its type section, the Had Limestone Member forms a prominent escarpment and consists mainly of dolomitic limestone and dolomite in three thick layers separated by chalky marl which is occasionally dolomitic. It has a thickness of approximately 165 feet in Wadi Tar. 171 Contact Relationships The Zmam Formation conformably overlies the Upper Cretaceous Mizda Formation in the Hammada al Hamra and is conformably overlain by the Paleocene Surfa Formation. Paleontology and Age The Lower Tar Marl is richly fossiliferous both in macrofossils and microfossils. Pelecypods, gastropods, echinoids, and a small button coral are common. The calcareous beds in the upper part of the section contain concentrations of exceptionally large specimens of the benthonic foraminifers Om phaloc yc lu s m ac ro por u s(Lamarck) and S i d e r o l i t e s c al c i t r ap o i desLamarck. These species are excellent index fossils to the upper Maastrichtian and are indicative of very shallow marine conditions. The underlying interbedded shales, in contrast, contain abundant, well preserved planktonic foraminifers. F.T. Barr has recovered the following species from the type section: Globotruncana gansseri Bolli Globotruncana stuarti(de Lapparent) Globotruncana arca(Cushman) Globotruncana contusa(Cushman) Globotruncana marieiBanner and Blow Rugoglobigerina rugosa(Plummer) Heterohelix spp. These pelagic faunas are indicative of deeper open sea conditions. Most of the section exposed in Wadi Tar is assigned to the Globotruncana g a n s s e r i Zo ne including some of the interbe dded marls with Omphalocyclus macroporusand Siderolites calcitrapoides. Approximately the uppermost 60 ft., however, contain no planktonics, but have the forementioned large benthonics in abundance and may be equivalent to the Ahathomphalus mayaroensis Zone. This seems quite probable even though the index marker to this zone, A m ayaroensis, has not been observed, as no break in deposition between the Maastrichtian and Danian is recognized, although the Omphalocyclus bearing marls indicate a marked shallowing of the seas at the end of Maastrichtian time. The Socna Mollusc Beds are highly fossiliferous containing abundances of molluscs including Venericardia desori (Ronchetti) and Nerinea sp. Gohrbandt (1966b, p. 36) reports that he has recovered 172 abundant specimens of the shallow marine benthonic foraminifer Discorbis pseu doscopus from shaly intercalations in the Socna Mollusc Beds in Wadi Tar. This species is widely distributed in the Danian rocks of the Sirte Basin suggesting a lowermost Tertiary age for the Socna Mollusc Beds. The Upper Tar Marl contains some macrofossils, and an abundant foraminiferal fauna which has never been documented. This member is probably Danian in age. Origin of Name The Zmam Formation derives its name from Wadi Zmam located along the southwestern margin of the Hon graben. 173 BIBLIOGRA PHY Arni, P., 1967, "A comprehensive graph of the essential diagnostics of the Nummulites". Micropaleontology, vol. 13, no. 1, pp. 41-5a, pls. 1-4. Barr, F.T., 1968a, 'Late Cretaceous Planktonic Foraminifera from the Coastal Area East of Susa (Apollonia), Northeastern Libya". Jour. Paleo., vol. 42, no. 2, pp. 308-321, pls. 37-40. Barr, F.T., 1968b, "Upper Cretaceous Stratigraphy of the Jabal al Akhdar, Northern Cyrenaica". In Geology and Archaeology of Northern Cyrenaica, Libya, F.T. Barr, ed. Petrol. Explor. Soc. Libya pp. 131-147, pls. 1-3, Tripoli. Barr, F.T., 1972 ( i n p ress), "Cretaceous Biost ratigraphy and Pl anktonic Foraminifera of Libya". Micropaleontology, vol. 18, no. 1. Barr, F.T., and Hammuda, O.S., 1971, "Biostratigraphy and Planktonic Zonation of the Upper Cretaceous Atrun Limestone and Hilal Shale, Northeastern Libya". Proc. 2nd Internat. Conf. Planktonic Microfossils, Rome. Burollet, P.F., 1960, "Lexique Stratigraphique International, vol. IV: Afrique. Part IV: Libya". Cong. Geol. Internat. - Commission de Strat., Rech. Sci., pp. 162, 2 maps, Paris. Burollet, P.F., 1963, "Excursion to Jebel Nefusa". Guidebook of Petrol. Explor. Soc. Libya, 19 pp., Tripoli. Chiesa, C., 1940, "La Serie Eocenica del Gebel Uaddan nella Giofra (Libya)". Ann. Museo Libico Storia Naturale, vol. 2, pp. 189-202, Tripoli. Christie, A.M., 1955, " Geology of the Garian Area " . U.N. Tech. Assist. Program, file No. TAA 173/87/06, Rp. No. TAA/LIB/2, pp. 1-60, geol. map, New York. Conant, L.C., and Goudarzi, G.H., 1964, "Geologic Map of Kingdom of Libya". U.S.G.S. Mis. Geol. Invest. Map I-35()A, Wash. D.C. Desio, A., 1935, "Missione scientifica della Reale Accademia d'Italia a Cufra (1931IX) Vol. 1: Studi geologici sulla Cirenaica, sul Deserto Libico, sulla Tripolitania e sul Fezzan Orientali". Reale Accad. d'Italia, Viaggi di Studio ed Esplorazioni, pp. 1-464, pls. 1-6, map, Rome. Desio, A., Ronchetti, C.R., Pozzi, R., Clerici, F., Invernizzi, G., Pisoni, C., and Vigano, P.L., 1963, " Stratigraphic studies in the Tripolitanian Jebel (Libya)". Riv. Ital. Paleon. Strat., Mem. 9, pp. 1-126, geol. map, Milan. Fraser, W.W., 1967, "Geology of the Zelten Field, Libya, North Africa". Proc. 7th World Petr. Cong., vol. 2, Elsevier Publ., pp. 259-264, Amsterdam. Gohrbandt, K.H.A., 1966a, "Some Cenomanian Foraminifera from Northwestern Libya". Micropaleontology, vol. 12, no. 1, pp. 65-70, pl. 1. Gohrbandt, K.H.A., 1966b, "Upper Cretaceous and Lower Tertiary Stratigraphy along the western and southwestern edge of the Sirte Basin, Libya " . I n South-central Libya and Northern Chad, a Guidebook to the Geology and Prehistory, J.J. Williams, ed. Petrol. Explor. Soc. Libya, pp. 331-341, Tripoli. Gregory, J.W., 1911, "The Geology of Cyrenaica". Quart. Jour. Geol. Soc., vol. 67, no. 268, pp. 572-615, London. 175 Hecht, F., Furst, M., and Klitzsch, E., 1963, "Zur Geologie von Libyen". Geol. Rdsch., vol. 53, pp. 413-470, Stuttgart. Jordi, H.A., and Lonfat, F., 1963, "Stratigraphic subdivision and problems in Upper Cretaceous and Lower Tertiary deposits in Northwestern Libya " Riv. Inst. Francais Petrole, vol. 18, nos. 10-11, pp. 114-122, Paris. Kleinsmiede, W.F.J., and van den Berg, N.J., 1968, " Surface geology of the Jabal al Akhdar, Cyrenaica, Libya " . In: Geology and Archaeology of Northern Cyrenaica, Libya, F. T. Barr. ed. Petrol. Explor. Soc. Libya, pp. 115-123, pls. 1-2, geol. map, Tripoli. Magnier, P., 1963, "Etude stratigraphique dans le Gebel Nefousa et le Gebel Garian (Tripolitaine, Libye )" . Bull. Soc. Geol. France, ser. 7, vol. 5, pp. 8994. Marchetti, M., 1934, "Note illustrative per un abbozzo di carta geologica della Cirenaica". Boll. Soc. Geol. Ital., vol. 53, pp. 309-325. Marchetti, M., 1935, "Sulla presenza del Cretaceo medio in Cirenaica". Rend. R. Accad. Nazion. Lincei, vol. 21 ser. 6, fasc. 1, pp. 25-29, Rome. Pietersz, C.R., 1968, "Proposed nomenclature for rock units in Northern Cyrenaica " . In: Geology and Archaeology of Northern Cyrenaica, Libya, F.T. Barr, ed. Petrol. Explor. Soc. Libya, pp. 125-130, Tripoli. Pomeyrol, R., 1968, "Nubian Sandstone" . Bull. Amer. Assoc. Petrol Geol., vol. 52, no. 4, pp. 589-600. Roberts, J.M. , 1970, "Amal Field, Libya". In Geology of Giant Petroleum Fields, M.T. Halbouty, ed. Amer. Assoc. Petrol. Geol., Mem. 14, pp. 438-448, Tulsa. Russegger, J., 1837, "Kreide und Sandstein". Einfluss von Granit auf letztern. Neues Jahrb. Mineral., pp. 665-669. Sanford, R.M., 1970, "Sarir Oil Field, Libya - Desert Surprise". In Geology of Giant Petroleum Fields, M.T. Halbouty, ed. Amer. Assoc. Petrol. Geol., Mem. 14, pp. 449-476, Tulsa. Savage, R.J.G., and White, M.E., 1965, "Two Mammal faunas from the early Tertiary of Central Libya". Proc. Geol. Soc. London, no. 1623, pp. 89-91, London . Selley, R.C., 1969, "Near-shore marine and continental sediments of the Sirte Basin, Libya". Quart. Jour. Geol. Soc., vol. 124, pp. 419-460, pls. 21-25, London . Stefanini, G., 1923, " Fossili terziari della Cirenaica". Paleontografia Ital. ( 1921 ), vol. 27, pp. 101-146, Pisa. Terry, C.E., and Williams, J.J., 1969, "The Idris "A" Bioherm and Oilfield, Sirte Basin, Libya - its Commercial Development, Regional Palaeocene Geologic Setting and Stratigraphy". In The Exploration for Petroleum in Europe and North Africa, P. Hepple ed. Instit. Petrol., pp. 31-48, London. Viterbo, I., 1969, Lower Cretaceous Charophyta from the subsurface "Nubian Complex" of the Sirte Basin (Libya). Proc. 3rd African Micropal. Colloq., pp. 393-402, pl. 1, Cairo. Williams, J.J., 1968, "The Sedimentary and Igneous Reservoirs of the Augila Oil Field, Libya " . I n Geology and Archaeology of Northern Cyrenaica, Libya, F.T. Barr, ed. Petrol. Explor. Soc. Libya, pp. 197-205, Tripoli. 176 ALPHABETICAL INDEX Page Ain Tobi Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Akhdar Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Al Feitah Limestone Member (Jardas Fm.) . . . . . . . . . . . . . . . . . .108 Algal Limestone Member (Cyrene Fm.) . . . . . . . . . . . . . . . . . . . . .46 Amal Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Apollonia Limestone Argub Carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 Arida Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 Atrun Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26 Augila Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29 Bahi Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 Beda Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 Benia Limestone Member (Jardas Fm.) . . . . . . . . . . . . . . . . . . . .109 Bu Ras Marlstone Member (Surfa Fm.) . . . . . . . . . . . . . . . . . . . . .160 Chicla Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45 Cyrene Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46 Dahra Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49 Defa Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51 Derna Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54 Diba Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55 Etel Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59 Facha Dolomite Member (Gir Fm.) . . . . . . . . . . . . . . . . . . . . . . . . .79 Faidia Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61 Gargaf Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62 Garian Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 Gasr Al Abid Marl Member (Jardas Fm.) . . . . . . . . . . . . . . . . . . . .108 Gedari Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64 Gelta Chalk Member (Surfa Fm.) . . . . . . . . . . . . . . . . . . . . . . . . .160 177 Page Gialo Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70 Gir Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75 Got Sas Marl Member (Jardas Fm.) . . . . . . . . . . . . . . . . . . . . . . .109 Had Limestone Member (Zmam Fm.) . . . . . . . . . . . . . . . . . . . . . .171 Hagfa Shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89 Harash Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93 Hilal Shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95 Hofra Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97 Hon Evaporite Member (Gir Fm.) . . . . . . . . . . . . . . . . . . . . . . . . . .77 Jabal Zelten Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106 Jardas Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .108 Jefren Marl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110 Kalash Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111 Khalifa Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115 Kheir Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117 Labrak Calcarenite Member (Cyrene Fm.) . . . . . . . . . . . . . . . . . . .46 Lidam Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121 Lower Sabil Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .140 Lower Tar Marl Member (Zmam Fm.) . . . . . . . . . . . . . . . . . . . . . .171 Marada Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123 Maragh Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .124 Mazuza Limestone Member (Mizda Fm.) . . . . . . . . . . . . . . . . . . .128 Mesdar Limestone Member (Gir Fm.) . . . . . . . . . . . . . . . . . . . . . . .84 Mizda Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .128 Najah Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .130 Nefusa Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .131 'Nubian' Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .132 Operculina Limestone Member (Surfa Fm.) . . . . . . . . . . . . . . . . .160 Rabia Shale Member (Beda Fm.) . . . . . . . . . . . . . . . . . . . . . . . . . .40 Rachmat Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .134 Rakb Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .137 Rashda Member (Augila Fm.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 Regima Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .139 Sabil Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .140 178 Page Samah Dolomite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149 Satal Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .150 Shahhat Marl Member (Cyrene Fm.) . . . . . . . . . . . . . . . . . . . . . . .46 Sheterat Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .155 Sirte Shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .157 Slonta Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .159 Socna Mollusk Beds (Zmam Fm.) . . . . . . . . . . . . . . . . . . . . . . . . .171 Surfa Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .160 Tagrifet Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .161 Tar Marl Member (Zmam Fm.) . . . . . . . . . . . . . . . . . . . . . . . . . . .171 Thala Member (Mizda Fm.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .128 Thalith Member (Beda Fm.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43 Tigrinna Marl Member (Mizda Fm.) . . . . . . . . . . . . . . . . . . . . . . .129 Upper Sabil Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .144 Upper Tar Marl Member (Zmam Fm.) . . . . . . . . . . . . . . . . . . . . . .171 Wadi Ducchan Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .164 Waha Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .165 Zelten Limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .169 Zmam Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .171 179