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Keywords:

  • Climate;
  • continental rift basin;
  • fluvio-aeolian sedimentology;
  • Morocco;
  • Upper Triassic

Abstract

The continental Upper Triassic Tadrart Ouadou Sandstone Member was deposited in an extensional setting on the Pangaean continent, strongly influenced by a low-latitude climatic regime (10° to 20° north). Complex interaction of basin subsidence and climatically driven processes led to high facies variability and a lack of correlatable units across the Argana Valley exposures. A process-orientated approach integrating detailed facies with architectural element analysis was undertaken, which resulted in a multistage depositional model for the Tadrart Ouadou Sandstone Member. The basin-scale model shows that basal alluvial fan and braided river systems are confined to the centre of the Argana Valley exposures. Aeolian deposits occur throughout the sequence, but dominate in the north. After a phase of playa deposition, prominent basin-wide fluvial incision of up to 8 m marks the onset of perennial fluvial flow. These well-sorted, internally complex and locally highly amalgamated fluvial sandstones are widespread throughout the basin and are focused in a north to south (south-west) flowing channel system. After a final stage of aeolian sedimentation, sandstone deposition of the Tadrart Ouadou Sandstone Member in the Argana Valley is terminated rapidly by the onlap of lacustrine mudstones of the Sidi Mansour Member. The study revealed that, except for one pronounced period of perennial conditions, sedimentation is controlled largely by ephemeral fluvial flow, alternating ground water tables, deflation processes and periods with limited periodic local run-off. The study highlights that facies architecture in the basin is the result of complex interaction of local syn-sedimentary tectonics and the climatic regime within the basin, but also the climate of the catchment area to the east. The data suggest a proximal to mid-distal basin setting in the rain-shadow to the west of a mountain range (Massif Ancien), which exerted a strong control on the depositional environments of Triassic deposits exposed in this part of South-west Morocco.