Biostratigraphy and Ostracod Faunas of the Miocene Marada Formation of the Eastern Sirt Basin, Libya

An attempt is made to define the biostratigraphy of the Marada Formation using the ostracod faunas for age determination and palaeoenvironmental analysis. The material studied is ditch cutting samples from the Miocene Marada formation of three wells drilled in the eastern Sirt Basin by the Wintershall company in concession 97, zone 2. The Marada Formation has previously been studied by several authors such as Desio, (1935) who dated as Early-Middle Miocene, with fewer studies on the ostracods, e. g. Innocenti and Pertusati (1984) and Szchechura (1989). 28 genera and 55 species of ostracods have been recorded; six species are new: Actinocythereis sirtensis sp. nov, Bythocypris tripoliensis sp. nov, Cyprideis maradaensis sp. nov, Cytheridea joshensis sp. nov, Hermanites zaltanensis sp. nov, Paijenborchellina keeni sp. nov; 22 species have previously been described; 9 can be closely compared with described species; and 18 left under open nomenclature. Many of the species previously described have a wide ranging distribution in the Miocene to Pliocene of the Mediterranean region and North Africa; some of these are restricted to the Lower Miocene, others to the Upper Miocene while no species diagnostic of the Middle Miocene have been found. Four ostracods biozones have been recognised in the Marada Formation: Biozone A is defined by the total range of Pokornyella deformis minor and indicates Lower Miocene (Aquitanian); Biozone B is defined as the interval between the last appearance stratigraphically of Pokornyella deformis minor and last appearance of Aurila soummamensis and is Lower Miocene (Burdigalian); Biozone C is an interval zone probably of Middle Miocene age, Biozone D is an assemblage zone indicating the base of the Upper Miocene. The Biozones were present in all three Wells. The Burdigalian biozone can be correlated with biozones of the Miocene in Turkey. The palaeoenvironments indicated by the ostracods are shallow marine (Infralittoral zone) with significant brackish levels (probably lagoonal) in the Burdigalian

Lower Palaeogene Ostracoda, From the Sirt Basin of Libya

Ostracods have been studied from 5 wells from the Sirt Basin, two from trough areas, 3 from platform areas, together with 2 outcrop sections on the western edge of the basin. The El-Fogha section, previously studied by Barsotti (1963) was resampled. 122 species and subspecies belonging to 58 genera have been recorded; 35 new species and subspecies are proposed. 39 species have been previously recorded from west Africa, North Africa and the Middle East, 10 species are certainly contaminants and come from younger sediments, the remainder are left under open nomenclature; although some of these show similarities to earlier described faunas. Most of the species recorded in this study are widely distributed in the lower Palaeogene of west, north Africa and the Middle East. The Palaeocene/Eocene boundary is marked by a large faunal turnover with the extinction of many typical Palaeocene ostracod species. The extinction of Paracosta bensoni has been used to define this boundary; studying the wells, the boundary is marked downhole by the sudden appearance of a rich ostracod fauna both in numbers of individual and in species. Four ostracod biozones have been recognised: early Eocene, late Palaeocene Paracosta bensoni zone, middle Palaeocene Paracosta Paleomokattamensis zone, and an early Palaeocene biozone. Graphic correlation techniques have been used for correlating the wells. Filtered gamma ray logs has been used to establish a L. O. C (line of correlation) between platform well YY1-6 and trough well KK1-6. This technique has given a high resolution to solve the boundary problem between the Haagfa and Khalifa formation in well KK1-6. This approach, together with the ostracod biostratigraphy, indicates that the Hagfa Shale and Khalifa Formations include younger (i.e early Eocene) strata than previously thought. The biofacies described by Bassiouni and Luger, (1990) have been recognised in the wells; these show a shallowing upwards sequence through the Palaeocene from outer shelf to near shore biofacies, with slight deepening at the start of the Eocene. Cluster analysis has helped in recognising different ostracoda associations within Bassiouni and Luger's shallow marine Afro-Tethyan type biofacies in the outcrop sections. The wide distribution of the Libyan fauna in North and West Africa, and the Middle East (Barsotti 1963, Reyment 1963, Berggren 1974 and Bassiouni and Luger 1990). The Trans-Saharan Seaway connecting Libya and North Africa with Nigeria via Mali and Niger had maximum flooding during the late Palaeocene transgression