T153 Permian-Triassic Changes and Extinction Event: New Insights from Sedimentary, Geochemical, and Paleobiological Records and Modeling Approaches I

Tuesday, 11 October 2011
8:00 AM - 12:00 PM
Room 200FG

CHRONOMETRIC CONSTRAINTS ON THE END-PERMIAN CRISIS IN THE KAROO BASIN, SOUTH AFRICA

Paper number: T153-8
Program: Technical Sessions
Time: 9:50 AM – 10:05 AM

Authors

Abstract

The exposures at Wapadsberg Pass in the Karoo Basin, Eastern Cape Province, South Africa, play a key role in our understanding of the terrestrial ecosystem response to the end-Permian mass extinction. The Permian-Triassic Boundary (PTB), and its correlation with events in the marine realm, is based on vertebrate biostratigraphy. A concretionary nodule horizon exposed near the top of Old Wapadsberg Pass is reported to mark the loss of Dicynodon and concomitant demise of the Glossopteris flora in the southern hemisphere. The current prevailing hypothesis is that the demise of the genus Dicynodon is coincident with the zenith of marine extinctions at ~252 Ma. To date, few chronometric constraints are known for this part of the Karoo Supergroup, and those reported in the literature consist of maximum detrital zircon ages.

Recently, a fossilized plant-litter horizon above an aggradational, wetland paleosol was described from a stratigraphic position ~70 m below the PTB at Wapadsberg Pass. The low diversity flora consists of a glossopterid canopy underlain by a sphenophyll ground cover in which an array of invertebrates are identified, along with a palynological assemblage assigned to a late Changhsingian age. The assemblage is preserved in a tuffaceous siltstone, and several beds of tuff exist in the burial sequence.

Zircon grains recovered from tuffite units were pretreated by chemical abrasion to remove both internal and external alteration zones. U and Pb were analyzed by isotope dilution thermal ionization mass spectrometry (ID-TIMS) methods using the EARTHTIME (ET535) tracer (spike). U-Pb zircon ages indicate components that cluster in the NeoProterozoic (~1.0 Ga and ~0.6 Ga), Early Cambrian (534 to 527 Ma), Middle-to-Late Ordovician (456 Ma), Late Carboniferous (307 Ma), Permian (275 to 253 Ma), and latest Permian. Ongoing sampling will better constrain the maximum age of the fossil assemblage, allowing for the first chronometric constraint on the duration of the Glossopterisflora in this part of the southern hemisphere.