Claude Bobier

Subsurface Mesozoic basins in the central Atlas of Tunisia: Tectonics, sequence deposit distribution, and hydrocarbon potential

The Mesozoic and Cenozoic outcrops of the central and meridional Atlas of Tunisia have been studied by structural, biostratigraphic, and sedimentologic approaches. We extend these studies to the subsurface, employing seismic and well data addressing structures, sequence stratigraphy, and hydrocarbon potential. The basin structures are related to strike-slip fault systems associated with Triassic evaporites. Mesozoic sedimentary sections in the areas of the Gafsa, Sidi Ali Ben Oun, Sidi Aich, and Majoura blocks were studied by an integrated seismic sequence stratigraphic and tectonic approach calibrated to wells and surface geologic controls. We identified tectonic corridors where Upper Triassic evaporites intruded younger Mesozoic rocks beginning in the Jurassic. The organization of the tectonic blocks is characterized by the distribution of subsiding basin zones of graben and rim synclines and resistant areas of platforms. Triassic intrusions induced by the strike-slip movements formed salt pillows and domes that accentuate the border elevations between the rim syncline depocenters and the platforms. Mesozoic sequence deposits are distributed along the flanks of the basin-platform borders according to lowstand prograding downlap systems tracts overlapped by aggrading and retrograding onlap and toplap deposits. The time-space distribution of the systems tract deposits indicates the possibility of formation of Triassic, Jurassic, and Cretaceous carbonate and sandstone prograding turbidites, reefs, structural and stratigraphic pinch-outs, and unconformities in the flanks of salt domes and basins. The geometry and distribution of reservoir and seal systems tracts permit the consideration of new Triassic, Jurassic, and Cretaceous combined stratigraphic and structural hydrocarbon traps around the basins and the platforms. This (Begin page 886) is a new tectonic model of the central Atlas of Tunisia that shows new potential objectives and traps.

Mud diapirism, fan sedimentation and strike-slip faulting, Caribbean Colombian Margin

Abstract A closely spaced seismic survey carried out during the Diapicar cruise off the Caribbean coast of Colombia permitted study of the interaction between thick deltaic sedimentation and tectonic stress. The consequence of these combined tectonic and sedimentary processes is the formation of large mud diapirs. The Colombian accretionary wedge is divided into two parts by a more than 100 km long lateral ramp. This observation, replaced in a plate tectonic framework, indicates that the northern South America is divided in several blocks which move to the northwest. These blocks may overthrust the Caribbean oceanic crust but we exclude the formation of a true subduction zone.

Subsurface Geodynamics and Petroleum Geology of Transform Margin Basins in the Sahel of Mahdia and El Jem (Eastern Tunisia) (1)

Geophysical, structural, and seismic-stratigraphic analysis of the Mahdia sahel of Tunisia, based on seismic data and sedimentologic and microtectonic field measurements, reveals a dynamic basin formation from the Aptian to the Quaternary. Seismic-structural and isopach mapping of seismic horizons of Cretaceous and Cenozoic megasequences shows the partition of the mobile graben depocenter and carbonate and siliciclastic platform basins. These basins are limited by synsedimentary restrictions along strike-slip faults oriented east-west and north-south. The interplay of these wrench faults, in regional transtension and transpression during the Cretaceous and Cenozoic, induced the synchroneous opening and closing of the graben and platform basins, producing large accumulatio s of black shales and turbidites, and reef, oolite, and carbonate platform deposits. This was associated with the dragging, rotation, and blocking of dekakilometer-size blocks assimilated to lithospheric microplates. These movements were accompanied by volcanism and Triassic salt intrusions along the bordering active faults in the east-west-trending and north-south-trending corridors. In these basins, sequential inversion of basin subsidence and the superposition of rhombic basins and wrench depocenters created migrating depocenters and provided the conditions in rapidly subsiding areas for source rock maturation and, later, hydrocarbon migration into stratigraphic and tectonic traps. The Tunisian Sahel domain at the northern border of the African plate is characterized by tectonic mecha isms which provide a model for tectono-sedimentary basin evolution in a wrench context.

Clay diapiric structures within the strike-slip margin of the southern leg of the Barbados prism

Abstract The study area is situated 200 km to the east of Trinidad; in the southern sector of the Barbados accretionary wedge. Analysis of seismic profiles and Seabeam bathymetric data, permit the identification of an important dextral wrench fault system oriented N95°E which induced morphostructural elements, characteristic of this type of kinematics in a thick sedimentary cover, separated from its basement by a decollement level. This decollement level is due to the presence of Miocene clay beds, supersaturated with water which under the effect of stresses, trigger clay diapiric processes in this transtensional sector. We shall demonstrate that the boundary between the Caribbean Plate and the South American plate continues as far as 250 km to the east of Trinidad.

Significance of the E-W fault system in the geodynamic evolution of the Tunisian Alpine Chain foreland. Example of the Sbiba-Cherichira fault system in Central Tunisia

The recent sedimentary basins in Central Tunisia correspond to a set of depocenters with complex geometry which are bounded by E-W, N070 and N-S brittle structures. These bordering faults, active during Eocene and Cretaceous times, have been rejuvenated at the end of the Neogene and during Quaternary in a relay pattern system associated with compressive and extensive deformations according to the alternance of extension and compression phases (Tortonian Atlasic Phase of compression, post tectonic top Miocene-early Pleistocene extension associated to the rifting of the Tyrrhenian Basin, and Pleistocene Phase of compression). These tectonic regime changes involve subsidence inversions. Moreover, the neotectonic study carried out along the strike-slip faults corridories and their associated structures enable us : -- to precise the timing of the tectonic deformations ; -- to establish tectono-sedimentary relationships of Mio-Plio-Quaternary age.