Coral-Reef and Bioherm Morphology--Criteria for Interpretation of Evolution of Jurassic Basin of Central High Atlas Mountains, Morocco: ABSTRACT

Abstract

The Jurassic sedimentary basin of the High Atlas Mountains is an elongate northeast-southwest trough 100 km wide and 800 km long in which sedimentation was initiated across continental Triassic sediments and terminated with Upper Jurassic continental sediments. More than 75% of the rocks of this sequence are exposed and provide a unique opportunity for the study of the evolution of a sedimentary basin; some of the best tools for this analysis are the coral reefs and related bioherms common to the basin. Biohermal structures include the following. (1) Micrite and sponge bioherms range from a few meters to more than 1 km in diameter and are approximately 100 m thick. Their lack of sorting and geographic position in the center of the basin suggest formation below wave base. (2) Barrier coral reefs are several hundred meters in diameter and thickness. The position on the shelf margin, sorting of the interbedded sediments, and the presence of flanking and capping beach sediments suggest formation just below and within the intertidal zone. (3) Coral-reef pinnacles are 100 m in diameter and thickness. The bases of the pinnacles lack evidence of wave sorting, but their tops are sorted and are covered by lag deposits, suggesting, together with their central position in the basin, that the pin acles formed just below and up to wave base. (4) Patch reefs consist of (a) microatolls of mud and brachiopod has up to 100 m in diameter and a few meters thick (sorting of the related sediments suggests that they formed above wave base, but lack of beach features or supratidal evidence indicates a lagoonal environment) and (b) coral-lined channels several tens of meters across and a few meters deep. Their occurrence within crossbedded and sorted sands without sorting of the coral suggests formation within the subtidal zone. By using the coral reefs and other sedimentary features, it can be shown that the margins of the basin remained shallow platforms throughout the basin history, and that the center of the basin deepened progressively. End_of_Article - Last_Page 788------------