Shallow-Marine Sedimentary Rhythms from the Onshore Canning Basin, Western Australia: A Mid-Permian Record of Glacio-Eustacy

Rhonda Adkins Welch
ChevronTexaco, Houston, TX

Earth's climate during the latter part of the Permian has long been the subject of speculation and debate. Today, the exact timing of Permo-Carboniferous deglaciation remains unclear. This study endeavors to address this issue by focusing on the Artinskian Tuckfield Member of the Poole Sandstone, where it outcrops around the periphery of the St George and Poole Ranges of the Fitzroy Trough (onshore Canning Basin, Western Australia). In this area, the Tuckfield Member consists of a series of laterally extensive, coarsening- and thickening-upward cycles that were deposited predominantly in a shorezone environment. In outcrop, these cycles are typically 5 to 12 m thick. Gamma-ray and grainsize data, collected from 15 outcrops located throughout the study area, were analyzed to better define cyclicity. Spectral analysis identified several orders of meter- to decameter-scale cycles recorded in the Tuckfield Member. The average periodicities of the identified cycles occur in the ratio of ~36:10:4:2 m. This ratio correlates strongly with the known mid-Permian orbital periodicities of elongated eccentricity, eccentricity, obliquity, and precession (~410:100:40:20 thousand years). This favorable comparison indicates that Milankovitch-forcing and glacio-eustacy probably controlled the formation of depositional cyclicity in the Tuckfield Member. The 5 to 12 m cycle, predominant in outcrop, suggests that eccentricity and possibly obliquity were the main influences on global sea-level fluctuation during this time. This recognition of Milankovitch-band cyclicity within the Tuckfield Member provides evidence that Permo-Carboniferous glaciation persisted into at least the Artinskian stage of the Permian.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005