Seismic sequence stratigraphy, basin structuring, and hydrocarbon implications of Cretaceous deposits (Albian–Maastrichtian) in central Tunisia

Abstract

Albian–Maastrichtian successions of the central and southern Atlas Mountains of Tunisia were investigated using seismic data, complemented by well and outcrop information. Basin-wide deformation can be related to tectonics and appears to have been the major control on variable distribution of Albian–Maastrichtian strata. The northeast–southwest Albian–Cenomanian extension was focussed on major faults and minor antithetic and synthetic faults creating structural depressions and grabens. Transtensional inversion was started during the Turonian. It was characterized by change of the field stress that changed from northeast–southwest to east–west then northwest–southeast during the Coniacian–Campanian. These transtensional events were followed by a northwest–southeast Campanian–Maastrichtian transpression as evidenced by the reactivation of preexisting faults and formation of new reverse faults and associated blind subtle anticlines. The structural rejuvenations were also influenced by holokinesis, which caused the block rotation. This diapirism coupled with sea-level fluctuations has controlled the sediment distributions. Seismic stratigraphic schemes developed for two adjacent sub-basins and two depocenters suggest coeval changes in deposition. Six second-order seismic-sequences are identified in the southern Gafsa sub-basin. These sequences are unequally distributed between depocenters. The Mèjel Bel Abbès sub-basin and Ali Ben Khalifa depocenter contain four second-order seismic-sequences, whereas only three seismic-sequences are identified in the Gamouda depocenter. Third-order sequences are defined by unconformities along the slopes and on the platforms where Upper Cretaceous sediment volume and sedimentation rates decrease. The major sequence boundaries recognized in the subsiding basins are correlated with outcrop unconformities and are considered to have regional significance, possibly controlled by eustatic sea-level changes. Each sequence constitutes the interaction and the dual signatures of the eustasy and the regional or local tectonics. The results of this integrated seismic stratigraphic and structural analysis have implications for evaluating the hydrocarbon potential. The distribution of Upper Cretaceous stratigraphic and structural traps and seals enable us to assess the petroleum potential of central Tunisia.