PALEOENVIRONMENTAL IMPLICATIONS OF EARLY CRETACEOUS MARINE INFLUENCED LACUSTRINE SEDIMENTS IN THE GIPPSLAND BASIN, SOUTHEASTERN AUSTRALIA

In this study geochemical and palynological analyses were performed on Early Cretaceous sedimentary rocks from the Wonthaggi Formation of the Gippsland Basin, Victoria, southeastern Australia. Previously interpreted as being wholly non-marine, the Wonthaggi Formation is demonstrated to contain a record of marine influence in the form of various geochemical parameters and the presence of pyrite, bioturbated horizons, marine palynomorphs, and records of plesiosaur remains. The marine influence is compatible with: conceptual models for the development of accommodation space by rifting between Early Cretaceous Australia and Antarctica and high sea levels globally at that time.

Carbon/Sulfur ratios, a rough measure of relative paleosalinity in sedimentary rocks, and modified van Krevelen diagrams, a means for assessing organic matter type, were constructed to evaluate the paleoenvironment of deposition. The geochemical analyses revealed that although a majority of the C/S values fall within the interpreted freshwater region of the plots (C/S > 5), some values fall within the marine region. These data, combined with the presence of marine algal organic matter and observations of dinoflagellates, provide a strong argument for marine influence in the Wonthaggi Formation.

Marine influence on the sedimentary system has several implications. First, if the marine-influenced horizons can be satisfactorily dated and correlated around the basin, then they could provide better age control, and perhaps marker horizons for understanding the accumulation of non-marine strata in the Gippsland and Strezlecki Basins. Second, the presence of marine waters in the basin at this time may suggest circulation between paleo-Pacific and Indian oceans began earlier than the Paleogene. Lastly, the influx of marine water may have profound implications to the interpretations of coeval dinosaur habitat and lifestyle. For example, circulation of marine water from the global ocean could have had an ameliorating effect on the nearby climate, such as warmer winters that did not require dinosaurs to be adapted to survive cold polar winters.