MOSS, GRAHAM¹ and BRIAN McGOWRAN^2
1Institute for Geophysics, University of Texas at Austin, Austin TX ([email protected])
²Dept. of Geology and Geophysics, University of Adelaide, South Australia

Abstract: Sequence Biostratigraphy in 'Greenhouse' and 'Icehouse' Worlds: A Valid Proxy for Sea Level change?

Sequence stratigraphic packages and bounding surfaces on the southern Australian margin are easier to detect in deposits characteristic of the warm and sluggish ocean of the late Eocene than in the cooler and well-ventilated early Oligocene. There is a two-tier response by neritic (<200 m paleowater depth) foraminiferal faunas to environmental change:

1. At timescales of 3-5 m.y., the first is categorized by nonreversible faunal turnover at the Eocene-Oligocene boundary, at the late early Oligocene regression, and later by a late Oligocene transgressive phase. These appear to signal more regional climate changes that are forced by glaciations followed by warming and are marked by prominent shifts in the deep-sea oxygen isotope curve and local chemofacies.

2. The second, at higher-frequency (<3 m.y.) fluctuations abundance of benthic taxa, is highlighted by rapid alternations of infauna and epifauna. These appear to chronicle reversible 'shortterm' paleoenvironmental shifts reflected by microhabitat preferences of taxa, that are obscure in Oligocene compared to latest Eocene facies.

The late early Oligocene glaciation (Oi2a) shows a weaker impact on the regional foraminiferal succession in the more open-neritic Otway Basin compared to Oi1 in the earliest Oligocene. Results argue that sequence biostratigraphic patterns reflect the relative sensitivity of assemblages configured by climate. Our conclusion is that significant turnover (last plus first appearances) was driven by climate change, and perhaps resource supply (e.g. oxygen, food) associated with development of the Southern Ocean, rather than fluctuations in sea level.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas