ABSTRACT

The Early and Middle Jurassic High Atlas sea extended as a marine trough across what is now the central and eastern portions of the High Atlas Mountains, southern Morocco. The sedimentary infilling of this sea is recorded by a thick sequence of limestones and marls punctuated by spectacular coral-algal reef horizons. A 1,200 m-thick section of Aalenian and Bajocian sediments near Rich, Morocco, was studied in detail to document the upward-shoaling, basin-to-reef transition.

The lower 900 m of the section represents deeper-water deposition: it consists of monotonously interbedded, dark-hued marls and lime mudstones and wackestones that hear a pelagic fossil fauna, and scattered turbidites of lime grainstone. An overlying transitional sequence about 200 m thick records progressive shallowing, as shown by 1) increased fossil diversity and abundance, 2) an increase in algally micritized particles, 3) a change from a pelagic to a benthic shelled fauna, 4) an increase in limestone-to-marl ratio, and 5) the appearance of small bioherms with scleractinian coral framework. The sequence is capped by a 100 m-thick horizon of very fossiliferous limestones and superbly preserved coral-algal reefs that grew in shallow water.

Several lines of evidence suggest that some of the reefs grew on submarine ridges that were uplifted by Middle Jurassic tectonic movements.