Depositional Sequence Evolution, Paleozoic and Early Mesozoic of the Central Saharan Platform, North Africa

SPRAGUE, ANTHONY R. G., Exxon Production Research Company, Houston, TX

Over 30 depositional sequences have been identified in the Paleozoic and lower Mesozoic of the Ghadames basin of eastern Algeria, southern Tunisia, and western Libya. Well logs and lithologic information from more than 500 wells were used to correlate the 30 sequences throughout the basin (total area more than 1 million km2). Based on systematic changes in the log response of strata in successively younger sequences, five groups of sequences with distinctive characteristics have been identified: Cambro-Ordivician, Upper Silurian-Middle Devonian, Upper Devonian, Carboniferous, and Middle Triassic-Middle Jurassic. Each sequence group is terminated by a major, tectonically enhanced sequence boundary that is immediately overlain (except for the Carboniferous) by a shale-prone interval dep sited in response to basin-wide flooding. The four Paleozoic sequence groups were deposited on the Saharan platform, a north-facing, clastic-dominated shelf that covered most of North Africa during the Paleozoic. The sequence boundary at the top of the Carboniferous sequence group is one of several Permian-Carboniferous angular unconformities in North Africa related to the Hercynian orogeny. The youngest sequence group (Middle Triassic to Middle Jurassic) is a clastic-evaporite package that onlaps southward onto the top Paleozoic sequence boundary. In this group, the lowstand systems tract in each sequence is characterized by a well-developed basinward facies change from sandprone, nonmarine (brain-plain) deposits to an anhydrite-halite succession deposited in a restricted shallow shelf nvironment. The progressive changes, from the Cambrian to the Jurassic, in the nature of the Ghadames basin sequences is a reflection of the interplay between basin morphology and tectonics, vegetation, eustasy, climate, and sediment supply.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)