Strontium–isotope stratigraphy of the Lower Cretaceous of Australia

The first comprehensive 87Sr/86Sr analysis of the Lower Cretaceous of Australia is presented, derived from a suite of well-preserved Barremian–Cenomanian belemnite rostra. Samples were obtained from the east Australian platform (northern Eromanga Basin, with additional material from selected Aptian and Albian stratigraphic horizons in the southern Eromanga, Carpentaria and Laura basins) and the Carnarvon Basin. Strontium-isotope ratios plotted for the Carnarvon Basin and a composite curve for the east Australian platform permit close correlation of their lithostratigraphic successions. Comparison to the global strontium-isotope reference curve confirms stage assignments and significant disconformities in both successions as recognised from biostratigraphy. Adjustments are required to the ages of some dinocyst biozones of importance in the Australian early Cretaceous time scale. On the basis of the strontium-isotope data the Muderongia australis/Odontochitina operculata boundary is Barremian, and the Pseudoceratium ludbrookiae Zone is longer ranging, extending across the late Albian and into the earliest Cenomanian. A small but persistent offset in Albian strontium-isotope ratio values for samples from the east Australian platform relative to those from the Carnarvon Basin might be attributed to the local influence of siliceous volcanism along the east Australian margin. Eustatic trends for the epeiric sea, which related to the east Australian platform succession and the west Australian continental shelf represented by the Carnarvon Basin succession, are closely matched in time. An Aptian excursion to less radiogenic strontium-isotope values is correlative with transgressive marine inundation of the Australian continent, consistent with the enhanced global generation of new ocean floor as the driving mechanism for both sea-level rise and the isotopic trend. An excursion towards more radiogenic values in the Albian, also associated with transgression, is attributed to global warming and an enhanced continental strontium flux as a result of weathering.