Sedimentology of an intra-montane rift-controlled fluvial dominated succession: The Upper Triassic Oukaimeden Sandstone Formation, Central High Atlas, Morocco

Abstract

Triassic successions in the High Atlas of Marrakech (Morocco) provide excellent outcrop analogues for continental fluvial systems within an intra-montane rift basin and allow the study of facies distribution and controls on deposition. This paper focuses on the analysis of the Oukaimeden Sandstone Formation (F5), a fluvial dominated formation deposited in an active rift setting. Combination of traditional sedimentological field analysis with modern digital data capture techniques (e.g. spectral gamma-ray, LIDAR terrestrial scanner imaging) allows a detailed description and interpretation of the facies. The Oukaimeden Sandstone Formation is composed of channel sandstone bodies alternating with lenticular shaped fluvial bar sandstones and overbank mudstone deposits. Alternating with the fluvial facies, aeolian sandstones and alluvial fan deposits are also observed. Changes in architectural style are used to subdivide the formation into three members. The lower member (Lower Oukaimeden ) was deposited by an ephemeral braided system. The middle member (Middle Oukaimeden) records a change to perennial braided fluvial conditions in response to tectonics combined with a change in climate towards more humid conditions. The upper member (Upper Oukaimeden) is characterized by the return to ephemeral conditions, which combined with the occurrence of aeolian dunes, is interpreted to record increased aridity. The upper part of the member exhibits tidal influence, related to the first marine incursion into the basin. The Oukaimeden Sandstone Formation provide a well documented outcrop example of deposition within an intra-montane setting influenced by a combination of tectonic and climatic controls.