The biostratigraphic and palaeoenvironmental significance of the late middle jurassic-early late cretaceous palynology of the Great Australian Bight Region

Abstract

The late Middle Jurassic to early Late Cretaceous non-marine and marine sediments in the Great Australian Bight region were deposited during initial rifting between Australia and Antarctica. These sediments have been drilled and logged in Echidna-1 and Platypus-1 in the Duntroon Embayment. Potoroo-1 on the northernmost margin of the Great Australian Bight Basin, and Jerboa-1 in the Eyre Sub-basin. In these logged wells is recorded a conformable sequence through the Murospora florida, Retitriletes watherooensis, Cicatricosisporites australiensis, Foraminisporis wonthaggiensis and Cyclosporites hughesii spore-pollen Zones. The Coptospora paradoxa spore-pollen Zone is present in Platypus-1 and Potoroo-1, and three wells(Jerboa-1, Platypus-1, Potoroo-1) include the Phimopollenites pannosus and Appendicisporites distocarinatus spore-pollen Zones. In these three zones, dinoflagellates represent the Canninginopsis denticulata, Pseudoceratium ludbrookiae and Diconodinium multispinum Zones. The palynological zonation of the wells in this study assumes that the first appearance of certain spore-pollen species in the Great Australian Bight region were reliable, in the sense that they corresponded with those elsewhere in the continent as summarised in Helby et al. (1987). The ages of these first appearance datums have been critically re-evaluated to allow correlation of the wells with the standard geological time scale. The age control established by using palynological information suggests that depositional history of the Great Australian Bight region commenced in the Callovian, and this sequence of non-marine sedimentation continued without interference through until the early Aptian. Albian marine sedimentation (recorded in Jerboa-1 and Potoroo-1) most probably relates to a major marine transgression in the Eucla Basin. In Platypus-1 age equivalent sediments are non-marine. Cenomanian marine sediments indicate a marine transgression that probably encroached from the west of the continent following the separation of Australia and Antarctica. The establishment of reliable spore-pollen first appearance datums showed that the ranges of other species are at variance with those elsewhere on the continent. For some species, a west-east variation in the ranges suggested a migration path across the Great Australian Bight region in the Callovian to early Aptian, which corresponded to a movement of flora from high to low latitudes. This migration path from the west to the east appears to have been maintained until the Cenomanian, even though it no longer involved a latitudinal gradient. This seems to imply that the unique, unstable rift environment was the chief vehicle of floral channeling, perhaps reinforced by the encroaching marine environment from the west into the region during the younger interval. During the Callovian to Berriasian vegetation in the Great Australian Bight region was dominated by the gymnosperm family Araucariaceae. A possible increase in precipitation during the late Berriasian saw a decrease in the Araucariaceae, with podocarps taking over the dominant role. Albian and Cenomanian vegetation in the Great Australian Bight region was also dominated by podocarpaceous gymnosperms, with the fern family Gleicheniaceae being the major component of the local vegetation. The dinoflagellate assemblages at this time are typified by taxa of the Heterosphaeridium Superzone of Helby et al. (1987).