SEDIMENTOLOGY AND ICHNOLOGY OF FLOODPLAIN PALEOSURFACES IN THE BEAUFORT GROUP (LATE PERMIAN), KAROO SEQUENCE, SOUTH-AFRICA

Floodplain paleosurfaces are depositional surfaces that were exposed o n ancient floodplains and have been preserved in the rock record. Thre e types of floodplain paleosurfaces have been recognised in the Late P ermian Beaufort Group strata of the southwestern Karoo Basin of South Africa. They occur in sedimentary sequences which represent point bar proximal floodplain, and distal floodplain facies. The distribution of paleosurfaces in the Beaufort strata was ultimately controlled by fla sh-floods which transported sand out of the large meandering river cha nnels into the floodplains. Preservation of the vertebrate and inverte brate traces was enhanced by a silty-clay veneer that accumulated on t he sand surfaces during waning flood. The surfaces were buried without significant modification in parts of the floodplain where water becam e ponded. These were most commonly in crevasse splay channels, on dist al crevasse splay lobes prograding into axial floodbasin lakes and in swales on the downstream portion of point bars. Ichnotaxa that appear to be environmentally specific include arthropod trackways of Umfolozi a type on proximal crevasse splays, algal matted textures on distal cr evasse splays and large 'septate' traces of Beaconites type on point b ar ridges. Trackways of synapsid reptiles are present on many of the p aleosurfaces and do not appear to be environmentally specific. Cross-c utting relationships of the ichno-fossils and desiccation features are compared with those of modern flash-flood deposits to reconstruct som e time constraints on the formation, exposure, and burial of these sur faces.