TAPHONOMIC TRENDS OF MACROFLORAL ASSEMBLAGES ACROSS THE PERMIAN-TRIASSIC BOUNDARY IN THE KAROO BASIN, SOUTH AFRICA

The terrestrial crisis that reportedly parallels the P/Tr marine mass extinction is based mainly on northern hemisphere microfloral assemblages and southern hemisphere Gondwanan macrofloral collections. It is well established that taphonomic filters control the ultimate collectable fossil assemblage in any depositional regime. Recognition and comparison of isotaphonomic assemblages is critical before conclusions can be drawn about evolutionary trends over time. Such an approach has been taken in the investigation of pre-boundary, trans-boundary, and post-boundary plant-fossil assemblages in the Karoo Basin.

Fourteen stratigraphic sections were evaulated in the Balfour and Normandien Fms. (Lower Beaufort Group), Katberg Fm., and overlying Burgersdorp Fm. (Upper Beaufort Group). These include previously published (e.g., Bulwer, Bethulie, Carlton Heights, Wapadsberg, Commando Drift) as well as newly discovered localities (e.g., Clouston Farm), and span the Late Permian to Middle Triassic. Fossiliferous intervals were characterized with respect to their sedimentology and plant taphonomy, and bulk collections were made at several stratigraphic levels for future evaluation of floristic and plant-insect associational trends.

The depositional regimes and plant taphonomic character of assemblages change through time. Much of the Lower Beaufort Group is characterized by parautochthonous assemblages within oxbow-lake channel fills. Just below the P/Tr boundary these are replaced by allochthonous assemblages preserved in lateral accretion deposits and barforms of relatively shallow fluvial nature. Allochthonous assemblages within the same fluvial context continue across the boundary into the earliest Triassic, and also typify the Middle Triassic where scour-and-fill structures preserve plant debris. Based on the literature, parautochthonous assemblages reappear in the Upper Triassic Molteno Fm. Hence, the change in taphonomic regime to allochthonous assemblages at the critical interval across the P/Tr extinction event requires extreme caution when interpreting global patterns from these data.