VERTEBRATE TAPHONOMY OF LATE PERMIAN FLOODPLAIN DEPOSITS IN THE SOUTHWESTERN KAROO BASIN OF SOUTH-AFRICA

The predominantly fluvial strata of the Late Permian Beaufort Group in the southwestern Karoo Basin contain numerous fossils of therapsid reptiles. This study investigates the taphonomy of these fossils and the sedimentology of the floodplain deposits in which they are most commonly preserved. Taphonomic assessment of 329 in situ fossils along 3 cliff exposures included data on disarticulation, bone weathering, attitude of skulls, bone color, type of perimineralization and degree of pre-fossilization damage. This led to the recognition of 6 ''taphonomic pathways'' along which bones entered the rock record. Three floodplain facies, channel-bank, proximal floodplain and distal floodplain, that were previously defined on sedimentological and pedological criteria also have taphonomically discrete attritional bone assemblages. Localized concentrations of fossil bones are interpreted as evidence for the presence of waterholes on the inner-bank levees and playa margins. Taphonomically anomalous, fully-articulated and paired Diictodon skeletons are preserved in burrow casts. A major influence on the taphonomic signature of each floodplain facies was the proximity of the burial site to the main channel, especially during episodes of channel avulsion, and the effect this had on the frequency and intensity of flooding and the overall rate of floodplain accretion. The former influenced disarticulation, transport and embedding of the bones whereas the latter determined their ''residence time'' on the floodplain before burial Attritional fossil assemblages in the proximal flood-plain mudrocks probably accumulated over 100-1000 years during which time floodplain accretion was more than 5.5 mm/y. However, paleosols superimposed on these strata record periods of floodplain stasis lasting from 5000-10,000 years, when sediment accumulation was considerably less, more in the order of 0.4 mm/y. This study demonstrates the value of vertebrate taphonomy in providing additional information to support or contradict the paleoenvironmental interpretation of fluvial facies.