Abstract

Stratigraphical and sedimentological studies of Lower Jurassic deposits at the western end of the Central High Atlas Mountains, Morocco, has led to a reappraisal of the tectonic significance of this linear feature during the critical period of the opening of the Atlantic Ocean. Lower Jurassic deposits south and east of Marrakech represent an extremely shallow water and supratidal marine carbonate accumulation that formed in fault-controlled basins at the western end of the trough. The basins were aligned east-northeast and the character and distribution of the deposits strongly suggests that no connection with the basins of the Western High Atlas existed. To the east, sediments indicate more open conditions, but no evidence of bathyal or abyssal conditions was seen.

Sedimentation was probably structurally controlled by vertical tensional and compressional movements as the result of reorientation of stress between major strike-slip faults. Volcanic flows and shallow intrusions, which occur at the base of the Jurassic sequence, and associated carbonate and clastic sediments show no evidence of being part of ophiolite sequences; they more closely resemble extrusive basalt sequences associated with rifting as seen in the Triassic of the Eastern United States and the Tertiary of Northwest Europe. Evidence from this area suggests that the High Atlas seaway represented only an arm of the Tethyan Ocean, which extended along a long-standing zone of crustal weakness.

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