T76 Trace Fossils, Mass Extinctions, and Event Boundaries: Endobenthic and Fossorial Responses to Terrestrial and Extraterrestrial Perturbations

Monday, 29 October 2007
8:00 AM - 12:00 PM

TRACE FOSSILS AND ENVIRONMENTS ACROSS THE PERMIAN-TRIASSIC BOUNDARY IN THE TRANSANTARCTIC MOUNTAINS

Paper number: T76-3
Program: Technical Sessions
Time: 8:30 AM – 8:50 AM

Authors

Abstract

Permian and Triassic siliciclastic rocks in the Transantarctic Mountains (TAM) comprise a > 2 km thick sequence of southern polar paleolatitude continental deposits that record changes in climate, hydrology, and ecosystems across the Permian-Triassic boundary and through the longer-term icehouse to greenhouse transition. After glacial conditions, earliest Permian lakes were filled by deposits delivered by braided streams; by mid-late Permian the landscapes were dotted with forests, swamps, and seasonally ice-covered lakes. Glossopteris leaves accumulated in lakes, where near-surface traces such as Cochlichnus were produced; in the Beardmore Glacier area (TAM) bedding plane bioturbation is common but not pervasive. In channel-margin sands, including crevasse splays and abandoned channels, a single arthropod (?insect) burrower produced a compound trace fossil consisting of Skolithos, Cruziana, and Palaeophycus components, but it did not colonize bars in the channels.

Major climate changes between the Permian and Triassic are tracked by trace fossils. The burrower in channel-margin sands of Permian age produced traces within-channel bars in the Triassic, while maintaining its dominance in floodplain sands. Colonization of within-channel bars required periods of very low flow; rapid flow would have carried a small arthropod away. The facies distribution of this compound trace fossil thus indicates highly seasonal flow, with an extended dry season. This interpretation is supported by soil types and by the Early Triassic appearance in the Beardmore and Shackleton Glacier areas (TAM) of vertebrate burrows that extend decimeters into the substrate, indicating significant lowering of water tables from Permian levels.

Biogenic structures in the Allan Hills (TAM) confirm that climate change occurred near the Permian Triassic boundary. Bedding planes with co-occurring Glossopteris and Mermia ichnofacies trace fossils record Permian deposition in a lake on a swampy floodplain. Overlying (~ 10 m above) Triassic channel-fill sandstones yield the compound trace indicative of low stream flow. This sequence shows no sign of significant erosion. If deposition was indeed continuous, the evidence indicates rapid climate change and hydrologic and ecosystem response, although the timing is poorly constrained.