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ABSTRACT: Geologic Setting for Hydrocarbons in Tunisia

Steven Schamel

Hydrocarbon habitats on the Tunisia-Sicily shelf result from the complex interplay of three factors: (1) proximity to a source of terrigenous sediments, (2) eustatic sea level changes resulting in major transgression/regression events, and (3) a varied and ever-changing structural mosaic involving successive transtensional, transpressive, and compressional tectonic phases. The stratigraphy, structure, and petroleum resources of the shelf are linked to the crustal template created during the middle Mesozoic rifting of the Tethyan margin of North Africa. Transtensional stretching and crustal fragmentation forming the Tunisia-Sicily passive margin occurred in the Late Triassic-Jurassic at the juncture of the South Saharan and Gibraltar shear zones, creating a complex array o ridges and furrows and localized pull-apart basins. During the

Cretaceous and early Tertiary, the subsident block-faulted shelf was buried beneath a varied stratigraphy ranging from a thin pelagic limestone succession devoid of terrigenous components in Sicily to a considerably thicker neritic Tunisian succession composed of mixed terrigenous and carbonate strata. Differences in stratigraphic character across the shelf relate to the relative position of Siciliy and Tunisia between the unstable and subsident Tethyan margin and the stable and emergent Saharan platform. Beginning in the middle Cretaceous, the region experienced localized tectonic instability expressed as transtensional faulting, crustal inversion, salt diapirism, and submarine volcanism. These events profoundly altered bathymetry and facies distribution on the shelf. As the North Af ican continental margin subducted northward beneath the Kabyle-Calabrian accretionary complex in the late Cenozoic, the thin competent carbonate successions of Sicily responded by detaching in a series of southward migrating thrust sheets. The thrusts die out along strike in Tunisia. Here, displacement was accommodated by folding in the thick marly succession weakened by an array of evaporite diapirs. Since the end of the Miocene, the entire shelf has been subjected to major right-lateral wrenching. The principal post-Paleozoic hydrocarbon habitats are (1) Triassic-Jurassic restricted pull-apart basins, (2) Cretaceous-early Tertiary structurally controlled shallow shelf edges, and (3) the late Cenozoic foredeep containing synorogenic clastic facies.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990