Changing fluvial environments across the Permian-Triassic boundary in the Karoo Basin, South Africa and possible causes of tetrapod extinctions

The Karoo Supergroup is a 12 km thick succession of sedimentary rocks that accumulated in a large intracratonic retroarc foreland basin in southwestern Gondwana. The strata record 100 million years of almost continuous sediment accumulation from the Permo-Carboniferous (300 Ma) through to the Early Jurassic (190 Ma) under a range of climatic regimes and within several tectonically controlled sub-basins. Alluvial sediments dominate the succession from the Late Permian onwards. Fossils of synapsid reptiles and early dinosaurs are sufficiently common to be used in a ten-fold biostratigraphic subdivision of these strata. The Permian/Triassic boundary in the Karoo succession is marked by a major extinction of the herbivorous dicynodonts which co-incides with a rapid change in fluvial facies. This study uses field observations of the sedimentary facies, palaeosols and in situ fossils of well exposed PTr boundary sequences in the southern Karoo Basin to provide evidence of environmental changes that may have caused the tetrapod extinctions in the main Karoo Basin. Below the PTr boundary sequence, the sediments comprise drab greenish-grey mudrocks with a few thick (3-12 m) multistoried, laterally-accreted fine-grained sandstone bodies. The mudrocks host hydromorphic palaeosols with palustrine carbonate horizons and mostly disarticulated skeletal remains of Dicynodon Assemblage Zone fauna. Above a transition zone of approximately 20 m thickness these strata change into reddish-brown mudrocks with numerous thin (0.5-5 m) ribbon sandstone bodies and more continuous sheet sandstones, calcic palaeosols with rhizocretions, nodular horizons and pervasive calcareous encrustation of articulated skeletons of Lystrosaurus and associated fauna of the Lystrosaurus Assemblage Zone. Within a 20-m thick interval of the transitional strata fossils of Dicynodon and Lystrosaurus occur together. The facies transition that co-incides with the latest Permian dicynodont extinction is interpreted as a change in fluvial style from meandering to low sinuosity channels with general drying of the floodplain habitats and redistribution of vegetation belts. The horsetails in particular became significantly reduced in areal distribution. The changes in fluvial style in the Karoo Basin at the end of the Permian were triggered by a pulse of thrusting in the southerly source area dated at 247 Ma (±2 Ma) which brought about rapid progradation of a large sandy braided fan system (the Katberg Sandstone Fm.) into the central parts of the basin. A concomitant basinward migration of depositional environments allowed the Lystrosaurus communities that were adapted to dry floodplains to colonise the entire basin to the detriment of most of the wet floodplain Dicynodon fauna that lived there. © 1995.