Near-shore marine and continental sediments of the Sirte basin, Libya

Abstract

At the end of the Cretaceous period the Sirte basin developed in North Africa between the Tethyan foredeep and the Sahara Shield. In-filling of the basin throughout the Tertiary era resulted in the deposition of a thick sequence of clastic rocks and carbonates with minor evaporites. The Sirte basin contains large quantities of oil and gas. This paper describes mixed carbonate and clastic Miocene shoreline deposits of a type which prevailed during much of the history of the Sirte basin. These rocks have been studied in the field using sedimentological logging techniques. Forty-seven detailed sections have been recorded totalling some 3000 m of sediment over an area of about 1500 sq km. Some 26000 field observations have been made. These have been used to interpret the depositional environments of these rocks by comparison with studies of recent sediments. Five major facies have been recognized; each has been given a type section and is defined by five parameters, viz: geometry, lithology, sedimentary structures, palaeontology and palaeocurrent pattern. Four of these facies trend parallel to the shore of the Sirte basin. They are locally cross-cut by the fifth. In the north bioclastic limestones were deposited by the shoreward migration of offshore bars and barrier beaches. These interfinger southwards with laminated lagoonal shales with oyster banks. The shales in turn pass shorewards into interlaminated fine sands, silts and shales with abundant bioturbation and micro-cross-lamination. These beds are cut by channels of inclined laminated sand and shale. This third facies is interpreted as inter-tidal, the channels originating as creeks which drained the mud flats. The most southerly (landward) of these four facies consists of crossbedded sands and shales, lignites, rootlet beds, with a spectacular fauna of continental vertebrates. These beds are thought to be the deposits of a fluviatile coastal plain. The four facies belts just described are replaced at two places by n–s trending calcareous sandstone shoe-string complexes interpreted as estuarine channels. Palaeocurrent analysis shows that carbonate sediment, generated in the offshore zone, was transported up-slope towards the land, while clastic sediment was carried seawards from the Sahara Shield. Bipolar palaeocurrents in the calcareous sandstone channels indicate mixing of these two sediment types within the estuarine environment. This Miocene shoreline was similar to the present-day coast of south west Texas, though in this analogue the barriers are made of non-carbonate sediment. The Miocene limestones are broadly comparable to the present-day barrier carbonates of the Trucial coast and have certain points of similarity with recent carbonate sands of the Bahamas.