ABSTRACT Upper Devonian and lower Carboniferous deposits exposed along the north flank of the Tindouf Basin in southern Morocco are assigned to 12 sedimentary facies belonging to four interacting depositional environments: progradational deltaic, marine shelf, beach and tidal flat. These environments together form an ancient delta system which records regressive-transgressive episodes due to phases of constructive and destructive delta building. The constructive phase of delta building occurs when an actively depositing fluvial system progrades a platform of deltaic plain deposits onto a marine shelf, without significant contemporaneous reworking of sediment by marine processes. The first destructive phase of delta building occurs when a less actively depositing fluvial system enables marine p ocesses of a microtidal and moderate wave energy sea, to build a beach on the margin of the deltaic plain. The final destructive phase of delta building occurs when detrital influx is almost absent and the deltaic plain converts to a back-barrier tidal flat which is eventually transgressed by the barrier beach and marine shelf.
ABSTRACT A depositional model is presented for the gold-rich Precambrian deposits within the Witwatersrand basin. It is a closed basin inset in a now stable craton of the southern African Shield with an areal extent of approximately 39,000 km2 The clastic fill has a maximum thickness of 7,500 m and comprises laterally persistent conglomerates, sandstones and shales occurring within thick (up to 800 m), widespread, and continually alternating coarser and finer-grained sequences. The succession as a whole coarsens upward and is divided into a sandstone and shale dominated lower division and a sandstone and conglomerate dominated upper division. Mineralogical maturity also increases upward, the sandstones of the lower division being mainly sub-greywackes, those of the upper division be ng feldspathic quartzites. Analysis of the abundant sedimentologic data gathered during exploration and development of the gold deposits demonstrates that sedimentation within the basin fits a braided alluvial plain and lacustrine model. The origin and lateral relationships of the different rock types are explained by this depositional setting. The thick alternating coarser and finer grained sequences are explained by a shifting depositional system (sourceward and basinward) controlled by tectonism in the basin and source area. The upward increase in mineralogical maturity and grain size is explained by migration inward of the basin margins, contemporaneous with sedimentation, causing marginal reworking of the lower division sequence as well as regression of the sedimentary succession. The lower division become a distal environmental equivalent of the upper division.
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