Early Diagenesis Recorded in Glendonites of Wandrawandian Siltstone of the Sydney Basin, Australia

Stephanie Thomas¹, Tracy Frank², and Christopher Fielding^2
1 Southern Methodist University, Dallas, TX
² University of Nebraska-Lincoln, Lincoln, NE

The mid-Permian Wandrawandian Siltstone represents a marine highstand that can be correlated throughout the Sydney-Gunnedah-Bowen Basin System. The Wandrawandian Siltstone shows evidence for relative sea level fluctuations throughout the formation. These fluctuations may have been caused, at least in part, by glacio-eustasy, and, in conjunction with cold climate indicators such as ice-rafted debris, may reflect glacial conditions.

One unusual feature of the Wandrawandian Siltstone is the presence of discrete horizons of glendonites, mineral pseudomorphs after ikaite (CaCO3•6H2O). Ikaite forms during early diagenesis in mud-prone, organic-rich sediments where temperatures are near 0ºC. As ikaite transforms to calcite, it experiences a significant loss in volume, creating a porous pseudomorph. In the Wandrawandian Siltstone, subsequent carbonate phases have filled much of the porosity resulting from this transition, creating a record of diagenesis from early to late stage. Stable isotope geochemistry suggests that carbon was derived from organic matter. Pyrite, in association with carbonate, suggests a coupling of sulfate-reduction and methane oxidation. Textures within the glendonites appear similar to carbonate textures found near methane seeps. Glendonite formation seems to be strongly related to the presence of organic matter. An inhibited weathering regime associated with cold climate could have contributed to increased transportation and deposition of terrestrial organic matter. However, based on C:N ratios, much of the carbon appears to have been derived from marine sources. Enhanced marine productivity may have been promoted by either upwelling conditions or release of nutrients associated with melting ice.