2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 15
Presentation Time: 8:00 AM-12:00 PM


GRAY, Samuel W., Department of Geology, Colby College, 5807 Mayflower Hill Drive, Waterville, ME 04901, NEVELING, Johann, Collections Management, Council for Geosciences, 280 Pretoria Street, Silverton, Pretoria, South Africa and GASTALDO, Robert A., Geology Department, Colby College, Waterville, ME 04901, swgray@colby.edu

The Permian-Triassic boundary marks the greatest extinction event known in Earth history with biodiversity loss in terrestrial ecosystems reported to parallel that in the marine realm. The Karoo basin of South Africa is an ideal place to research the boundary event and its effects on land because it contains a complete terrestrial sedimentary record spanning the Late Carboniferous to the Early Jurassic. Many papers have detailed the general stratigraphy across this critical boundary interval, but microstratigraphic analyses of the pre-, trans-, and post-boundary rocks have not been presented. This study focuses on several boundary sections in the Karoo, including Wapadsberg Pass, Carlton Heights, Commando Drift, and the type section near Bethulie, and tests the unique character assigned to these rocks by several authors.

The boundary rocks are in the Balfour Fm. of the Beaufort Group in which the Palingkoop Mbr. is assigned to the Upper Permian and Lower Triassic. The base of the Palingkoop Mbr. is identified by the presence of green mudrocks whereas the P/Tr boundary is marked by the transition to red mudrocks. The transition occurs within a few tens of meters, making it stratigraphically difficult to place exactly the P/Tr boundary.

Some authors recognize the boundary to be above the highest paleosol, the End Permian Paleosol (EPP). Above the EPP is a feature termed “event beds” or “laminites” that are interpreted as a “dead zone.” These consist of laminated red gray and green gray beds that have been correlated across the basin with the boundary interval. The lithologic nature of the event beds, though, varies throughout the basin. At Bethulie they consist of fine sandstone and siltstone couplets on a cm-dm scale, whereas the section at Carlton Heights displays 0.5 m red siltstones with occasional interbedded gray sandstones. Identifying the changing conditions surrounding the P/Tr boundary is an important piece of unraveling what happened during the extinction event. Using the microstratigraphy and petrographic character of the “laminite” interval we aim to describe and characterize the depositional systems and environments surrounding the terrestrial P/Tr boundary conditions.