Structural Evolution of the Mejerda Zone, Northern Tunisia

BAIRD, ALASTAIR, and JOHN GROCOTT, Kingston Polytechnic, Kingston-upon-Thames, Surrey, United Kingdom, GEOFFREY GRANT,* Geco Geophysical Co., Woking, United Kingdom, RICHARD MOODY, and ROSS SANDMAN, Kingston Geological Services, Kingston-upon-Thames, Surrey, United Kingdom

The Mejerda zone lies southeast of the Tellian Atlas and has been regarded as part of a reactivated foreland to this late Palaeogene to Neogene thrust belt. Folds have a northeast-southwest (Atlas) trend and the zone contains Triassic, Jurassic, Cretaceous, and Tertiary sediments, including source and reservoir rocks. Northeast-southwest trending Miocene-Pliocene-Pleistocene basins in the zone have attracted petroleum exploration.

In the Mejerda zone, Triassic rocks are generally held to have been emplaced diapirically. However, they often outcrop in thin, fault-bounded sheets and not in diapiric domelike structures. Furthermore, there is no unequivocal evidence for the existence of extensive salt deposits in the zone, either at outcrop or in boreholes.

Instead, the Triassic rocks are red clastics with some gypsum and black dolomites.

Balanced cross sections show that a thin-skinned thrust model with a decollement in the Triassic is consistent with the surface geology of the Mejerda zone. In this model the Triassic rocks decorate low-angle thrust faults. Shortening is estimated to be 40-50%, and the zone is part of the Atlas thrust belt and not a reactivated foreland.

As well as thrust-generated old-over-young stratigraphic superpositions, many tectonic contacts in the Mejerda zone place young rocks on old. Such superpositions may be caused by out-of-sequence thrusts but more likely reflect northwest-southeast extension following thrusting. This extension formed the longitudinal Miocene-Pliocene-Pleistocene basins which conceal thrust-belt structures.

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