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Tectonics, basin evolution and evaporites |
1 School of Geography, Earth and Environmental Sciences, The University of Birmingham, Birmingham B15 2TT, UK (e-mail: turnerpetergeos{at}btinternet.com)
2 Present address: Turner Geoconsultants Ltd, 7 Carlton Croft, Streetly, West Midlands B74 3J7, UK
This paper describes the Late Triassic and Early Jurassic evaporites of the Berkine/Ghadames Basin in eastern Algeria and western Libya, which is important because it forms the regional seal to a number of hydrocarbon reservoirs in the Triassic Argilo-Gréseux Inférieur. This evaporitic succession spans the time interval Norian to Late Liassic and is divided into three contrasting units (S1, S2 and S3). S1 consists of five evaporitic cycles dominated by mudstone and halite, which represent the main salt deposits of the Berkine Basin. At this time the Berkine/Ghadames basin was a restricted evaporitic basin with a barrier to the north (Medenine High) separating the basin from the developing peri-Tethys and North Atlantic. The succession thickens to the west, thins around the Hass Messouad Ridge and continues across the Maghreb into the western High Atlas of Morocco, a distance of some 1500 km. S2 is represented by the basin-wide development of a carbonate platform in Pliensbachian times. This is the product of a relative sealevel rise concomitant with the developing extension of the region and may reflect the opening of an Atlantic gateway. S3 comprises five cycles that are predominantly mudstones, fine-grained carbonates and anhydrites. Mounded structures, evident on seismic sections, may represent the development of patch reefs and the whole succession is interpreted as a shallow-marine carbonate–evaporite ramp. Similar thickness variations to those observed in Sequence 1 continue through Sequence 3 and reflect the on-going influence of differential subsidence along basement lineaments. A number of factors controlled the development of the evaporite basin. These include Late Triassic sealevel rise and flooding of a sub-sealevel basin, combined with globally warm climatic conditions and on-going extensional tectonics in the peri-Tethyan and Atlantic areas. Restricted circulation was caused by an east–west oriented sill to the north (Medenine High in central Tunisia) and the basin was episodically refilled as global sealevel rose during the Early Mesozoic and extensional tectonics continued.
