Jurassic sediments of the Central High Atlas of Morocco: Deposition, burial and erosian history

Abstract

During the beginning of the Jurassic, the Central and Eastern High Atlas of Morocco was a sea way open to the East and connected with the Tethys but not with the newly formed Atlantic. Limestones and marls were deposited in a narrow trough (40 km) where up to 9 km of sediments have been locally mesured. There, subsidence was very active (average : 100–200 m/m y). On the bordering platform of that narrow basin, the sediments are much thinner (0,8 km) and composed essentially of pure limestone (subsidence : env. 20 m/m y). In the trough sedimentation starts with thick intertidal limestones, but with time becomes more influenced by slumps, mudflows, turbidites and local erosion. At the end of basin filling (Mid-Jurassic) shallow terrigenous sediments and red sandstones indicate the transition to continental deposits. As erosion has been active, ever since the Upper Jurassic, it is often difficult in many regions to estimate the original sedimentary thickness. Illite crystallinity, IC, has proved to be a reliable method for tracing the burial evolution of these carbonate rocks, from diagenesis to the beginning of metamorphism. Our results suggest that the lower part of the sedimentary pile in the synclines have reached 300° C with a normal gradient. In the anticlines, where the sedimentary pile was 4–5 km thick, an abnormal gradient of 60° C /km was probably related to the circulation of hot fluids.