Late Cretaceous turmoil in the southern high latitudes: a story of environmental stress, basin restriction and  deltaic sedimentation from IODP Site U1512, Bight Basin, Australia

Carmine C. Wainman1 and Peter J. McCabe1

1Australian School of Petroleum and Energy Resources, University of Adelaide, Adelaide, Australia

The Bight Basin is a relatively poorly explored basin that developed during the break-up of Australia and Antarctica. The basin hosts a 15 km thick deltaic succession deposited during the height of the Cretaceous Greenhouse. Although there have been extensive seismic investigations and a few provenance studies from cuttings in the region, there is little well data and core material to undertake detailed investigations through this sequence. At Site U1512 offshore of southern Australia, IODP Expedition 369 recovered a 690 m thick succession of lower Turonian to upper Santonian silty claystone with only a few thin beds of glauconitic and sideritic sandstone (<32 cm thick). This core provides the most comprehensive record of depositional and paleoenvironmental events obtained from the Bight Basin. Clay-rich sedimentary facies and a dominance of agglutinated benthic foraminifera suggest the succession was rapidly deposited by hyperpycnal and hypopycnal flows in a marine prodelta setting, presumably in response to high terrestrial runoff. Organic geochemical and palynofacies data indicate sustained fluvial input into the basin with increases in delivery of plant-derived material. Marine-algal input slowly increases from the Turonian into the Santonian as indicated from C27/C29 steranes ratios between 1 and 1.5. Pristane/phytane ratios (1.73 to 0.13) in conjunction with common glauconite and pyrite reveal the basin became more dysoxic and restricted during Turonian and Coniacian times before slowly opening up during the Santonian. This was possibly due to the combined effect of a eustatic lowstand during the late Turonian, unstable patterns in oceanic circulation and increasing continental influence. Provenance studies from analysis of detrital zircons from the Coniacian part of the succession show that sediment was derived from similar sources determined for the Santonian to Maastrichtian succession in the nearby Gnarlyknots 1A well. Sediment was not only derived from the Whitsunday Large Igneous Province (~125–95 Ma) and the New England Fold Belt (~300–200 Ma) to the northeast, but also from the Albany-Fraser Orogen (~1.3–1.0 Ga) to the west. The gradual switch in sedimentary provenance during the Late Cretaceous was likely related to exhumation and erosion of these regions during the rifting of Zealandia off southeastern Gondwana. Evidence from Site U1512 suggests the basin was fed by two or more large transcontinental rivers that entered the restricted basin that were present before the break-up of Australia and Antarctica. These records offer a new understanding of Gondwanan paleogeography and our understanding of marginal marine settings in the southern high latitudes during the Cretaceous Greenhouse.


Biography

I am a Visiting Research Fellow at the University of Adelaide. I completed my PhD at the same university and received an MSci in Geology from the University of Southampton. My research focuses on Permian and Jurassic coal-bearing strata in Australia, and the Mesozoic evolution of Australia’s southern margin.

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