Basin Development and Compressional Deformation in the Central High Atlas Mountains Northwest of Errachidia (Morocco)--Rejuvenated Tectonics

Rainald Brede

The development of the central High Atlas Mountains from the Triassic until today was controlled by a set of pre-existent basement faults which were reactivated in various manners.

During the Triassic a graben began to subside at the northwestern border of the African craton. The graben's development was at least temporarily linked to the opening of the Atlantic Ocean. The growing basin was filled by red detrital material from the adjacent elevated regions. Evaporites were only locally formed. Mostly the red beds show an intercalation of basic volcanics. During the early and middle Liassic, a shallow shelf with reefs existed at the southern rim of the Atlas-Gulf, from which much carbonate material was delivered into the basin. This deposition is proved by turbiditic limestones of considerable thickness in the Pliensbachian. After a further deepening of the Atlas-Gulf in the late Liassic and the early Dogger, a trend toward shallowing began in the Bajocian, enabl ng reefs to grow from the south into the basin. Toward the end of the middle Dogger, the trough began to fill with red detrital material. This period of red-bed sedimentation lasted until the early part of the late Cretaceous and had abundant intercalation of gypsum near the top of the sequence. Due to a transgression a short marine period followed, documented by the Cenomanian-Turonian limestones. This marine period was succeeded again by the deposition of red beds in the middle of late Cretaceous. The Jurassic sediments are sometimes penetrated by small doleritic dikes.

The development of the basin was controlled by east-northeast-striking faults. During compression in the Tertiary, the ^sgr₃ direction (north-northwest) of the Mesozoic basin development changed into the ^sgr₁ direction of compressional deformation and the synsedimentary normal faults were reactivated as upthrusts. This change is proved by the fact that the axis of slump folds shows the same direction as the axis of the younger compressional folds.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.