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.