2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 1
Presentation Time: 9:00 AM-6:00 PM


KNIGHT, Cassi1, GASTALDO, Robert A.1 and NEVELING, Johann2, (1)Department of Geology, Colby College, 5807 Mayflower Hill Drive, Waterville, ME 04901, (2)Council for Geosciences, Private Bag x112, Pretoria, 0001, South Africa, knight.cassi@gmail.com

Beaufort Group paleosols from the Karoo Basin, South Africa, record the paleoenvironmental conditions that existed prior to and after the Permian-Triassic extinction event. Paleosol exposures from Wapadsberg Pass, Eastern Cape Province, represent a well preserved forest-floor litter overlying an interpreted inceptisol, a condition unique to the basin. Vegetation that colonized this landscape included a canopy of the gymnosperm Glossopteris and an understory of sphenopsids (Phyllotheca and Tryzygia). Wapadsberg Pass paleosol sites were sampled for petrographical and geochemical analyses to constrain interpretations of paleoenvironmental conditions that existed ~ 70 m below the Permian-Triassic Boundary (PTB). This project focuses on determining paleosol-nutrient quality to test a hypothesis that plant toxicity may be responsible for the reported decrease in Glossopteris-leaf size prior to the PTB event.

The greenish-grey (5GY 6/1) paleosol is an iron-stained siltstone with intervals of bedded very-fine sand to silt. The paleosol has a maximum thickness of 70 cm, with a coarser interval at ~30 cm depth. The litter horizon is concentrated in the upper 20 cm of the profile, and includes remnants of poorly preserved Glossopteris leaves and Vertebraria roots. Rooting structures penetrate to ~70 cm depth. Tuffite is interspersed in and caps the paleosol.

Primary structures in petrographic section include ripples, parallel bedding, and small-scale soft-sediment deformation. These are partially destroyed due to phytoturbation and bioturbation. All paleosol sites examined contain an identifiable tuffite, characterized as a mix of well-rounded to angular, transparent clasts. These are distributed as irregular pockets cross-cutting bedding and as dispersed isolated clasts within the fine matrix. TOC:TON, in addition to geochemical data obtained using ICP, are pending. Analytical results will be presented and discussed.