Abstract

During Liassic time, carbonate sediments were deposited in a northeast-southwest oriented marine trough that now forms part of the High Atlas mountains, Morocco. Shelf and deep basin deposits of this trough consist of marls, micrites and marly micrites deposited during Late Pliensbachian and Early Toarcian time. Rocks of both shelf and deep basin origin are rhythmically arranged in vertical sequence. Carbonate rich micrite units alternate with carbonate poor marls and marly micrite units. Carbonate poor units deposited on the shelf contain a terrigenous fraction (illite, chlorite and silt-sized quartz) that can account for as much as 79% of the rock. Carbonate poor units of the deep basin also contain clays, illite and chlorite, but in smaller total amounts than occur on the shelf, accounting for only 30% of the rock. Detrital quartz is absent. The rhythmicity can be explained by variations in supply of siliciclastics, with the shelf environments receiving larger volumes and coarser grained materials than the offshore, basinal environment. On the shelf, rhythmicity is most probably related to climatic patterns that were responsible for controlling the supply of siliciclastics from a mature hinterland into a dominantly low energy environment of deposition on the shelf. Deep water rhythmicity can be explained in terms of 1) storms that caused entrainment of shelf clays and their subsequent redeposition in deeper, offshore areas; and 2) turbidity currents that originated at the shelf/trough margin and carried clays out into the axial part of the trough as part of the suspension load of the current, the bed load being deposited nearer the shelf/trough margin.

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