Tectonically Controlled Facies Changes in the Jurassic of the Northern Border of the Central High Atlas East of Midelt (Morocco)

Manfred K. Hauptmann, Werner Heinetz

Jurassic carbonate sedimentation and facies changes at the northern border of the Central High Atlas mountain range east of Midelt are mainly controlled by synsedimentary tectonics which took place during the development of the Central High Atlas rift. There is no evidence of the influence of global eustatic sea level changes. During early Liassic time the carbonate facies documents an extended platform with numerous isolated mud mounds surrounded by biodetritic limestone fans. From middle Liassic to early Dogger (Aalenian) time, different kinds of mass-flow deposits coincide with the maximum extension of the Central High Atlas trough. Large-scale slide blocks, conglomerates, mudflow and debris-flow sediments, etc, show the existence of a platform-basin border formed by a preexisting fault system acting as downthrusts during Jurassic time. A sudden diminution of the subsidence at the Aalenian-Bajocian border is indicated by nearshore carbonates (tidal flats, tidal channels). Already at middle Bajocian time red beds were deposited.

From the Oligocene onward during a compressive phase most likely due to the Rif orogenesis, these faults were reactivated. As a result, the east-northeast-striking faults which oriented at right angles toward the direction of compression were transformed to inverse faults and the north-northeast-striking faults became sinistral strike-slip faults. At the fault junction, areas of higher compression were created, inducing a cleavage of the Mesozoic sediments. Whereas upfolds predominantly occur over preexisting fault zones, wide flat downfolds developed above the basement blocks.

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