Structural style and evolution of a Late Triassic rift basin in the Central High Atlas, Morocco: controls on sediment deposition

Abstract

AbstractLate Triassic continental sediments deposited in an active‐rift setting are exposed in the Oukaimeden‐Ourika Valley, located in the Central High Atlas Basin of Morocco. The Oukaimeden Sandstone Formation is dominated by ephemeral and perennial braided fluvial facies, and is an outcrop analogue for Triassic sandstone hydrocarbon reservoirs found in Atlantic margin and North African basins. This paper documents detailed analysis of the outcrop data to investigate the influence of tectonics on deposition and the interplay with climatic controls. The geodynamic evolution of the basin is interpreted to be influenced by Atlantic rifting to the west and the formation of the Tethys Sea to the north, which led to the development of ENE and NNE striking normal faults. The present‐day fault and outcrop geometry reflects later inversion due to the Alpine compression that led to uplift of the High Atlas and subsequent erosion.The ENE trending rift‐basin is bound by normal faults, which are probably in part reactivated older Hercynian structures. The facies distribution was controlled by a complex interplay of tectonic and climatic controls. Evidence for syn‐sedimentary movement of both fault sets is observed, with stratigraphic thickening and associated progressive change in bedding dip. These faults controlled the basin dimension, geometry of the half‐graben and created the accommodation for sediment deposition. The presence of breccia deposits close to the main ENE bounding faults indicates footwall erosion and deposition of basin margin fans. The location and orientation of the main fluvial system was controlled by these structures, and the main channel belts ran parallel and proximal to the controlling faults, whereas away from the main syn‐depositional faults overbank deposits dominate. Smaller contemporaneous NNE oriented faults are generally shorter with less throw, and had only a limited influence on gross sedimentation patterns. These faults are interpreted to be syn‐sedimentary, displaying characteristics that suggest basal detachment within the Triassic, and as such provide evidence for the extension direction during late Triassic time. A periodic change from ephemeral to perennial systems, with associated changes in architectural style, is potentially attributed to climatic control, although a structural influence cannot be dismissed. Copyright © 2009 John Wiley & Sons, Ltd.