Evidence for Large Amplitude Base Level Fluctuations in Fine-Grained Strata of the Mid Permian Wandrawandian Siltstone in the Southern Sydney Basin, Australia

Christopher R. Fielding, Stephanie G. Thomas, and Tracy D. Frank
University of Nebraska-Lincoln, Lincoln, NE

The mid Permian Wandrawandian Siltstone in the southern Sydney Basin of New South Wales, Australia forms a relatively fine-grained interval enclosed by coarser-grained, sandstone-dominated formations. The unit preserves a variety of facies ranging from clay-rich, bioturbated siltstones, siltstones and sandy siltstones, through bioturbated sandy siltstones and admixed silt/fine sandstones, to cleaner but again heavily bioturbated fine- to medium-grained sandstones. Additional features of the unit include 1. an interval characterized by extensive synsedimentary slump deformation, 2. deeply incised, sandstone-filled channels cut into fine-grained, heavily bioturbated facies and filled by sparsely bioturbated conglomerate and sandstone, and 3. reworked volcanic fallout beds. Trace fossil assemblages are interpreted to reflect a restricted Skolithos ichnofacies in the larger channel bodies through various expressions of the Cruziana ichnofacies in finer-grained facies to a Zoophycos ichnofacies in the finest-grained mudstones. The unit is interpreted as recording an array of open marine depositional environments ranging from shelfal to middle shoreface (~100m+ to ~25 m), with the slumped interval attributed to coseismic sea floor failure. Channels incise fine-grained, offshore to shelfal mudrocks but are filled by clean sandstones of lower to middle shoreface origin, and are interpreted to record regionally extensive sequence boundaries. As such, these provide evidence of drops in relative sea-level of <75 m, consistent with glacioeustatic forcing. Thus, the mud-dominated Wandrawandian Siltstone preserves evidence both of major tectonic events that shaped the Sydney Basin and of a possible glacial episode at the close of the late Paleozoic Gondwanan Ice Age.

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