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Graben-Fill Facies and Paleoenvironments: Example from East African Rift System

Carl F. Vondra, Daniel R. Burggraf, Jr.

During the past two decades, the rift valley system of eastern Africa has been the focus of many interdisciplinary scientific studies because of the peculiar tectonic setting that it represents and the abundant evidence of early man enclosed by graben-fill sediments. Geologic studies have largely been confined to individual basins where detailed stratigraphic and sedimentologic investigations have led to the identification of paleoenvironments and paleogeographic reconstructions important to the better understanding of graben processes and sedimentation. Through integration of many of these investigations, unifying aspects of tectonic and sedimentary phenomena allow a generalized graben-fill depositional model to be developed. This model consists of seven major lithofacie and corresponding depositional environments: (1) interbedded conglomerate and pebbly mudstone facies (alluvial fan); (2) lenticular conglomerate and sandstone facies (fluvial channel); (3) interbedded sandstone, siltstone, tuff, and claystone facies (flood plain); (4) lenticular fine-grained sandstone and lenticular bedded siltstone facies (distributary channel and interdistributary flood basin); (5) arenaceous bioclastic carbonate facies (littoral-lacustrine beach and lagoon); (6) laminated siltstone facies (shallow-shelf lacustrine and prodelta); and (7) evaporite and/or diatomite facies (inland sabkha and lacustrine). The facies of this model interfinger complexly and attain variable thicknesses depending on the individual structural and sedimentologic history of each graben basin. T e association of depositional environments and relationships between basin development and tectonics are unmistakably similar.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.