Palaeoenvironmental and climatic changes in Australia during the early Cretaceous

Abstract

Biochronostratigraphy for the Tethyan and Boreal mid-Cretaceous (Barremian-Albian) is traditionally based on ammonites. Because of the lack of useful ammonites in the Australian mid-Cretaceous, and the existence of strong faunal and floral latitudinal contrasts, to date chronostratigraphic correlation between the different realms is not straightforward. In the present study dinoflagellate cyst events (first and last occurrences) combined with changes in organic-carbon-isotope (δ13Corg) stratigraphy are demonstrated to be good indicators for assessing the mid-Cretaceous Stage and Substage boundaries in Australia. The Australian dinocyst information was compared to ammonite calibrated Tethyan and Boreal successions. This resulted in recognition of global dinocyst events as well as in age assessment for Australian dinocyst zones of the traditional dinocyst zonal scheme. The established stratigraphic framework further allowed comparison of the δ13Corg records with time equivalent isotope records from other areas. Correlation of the selected results reveals similar patterns and responses in the δ13Corg records for the Tethyan, Boreal and Austral realms, and that two periods of oceanic anoxic events (OAE) representing OAE 1a and 1c, can be recognized in Australia as well. In Australia the inferred relatively warm and wet conditions prior to the presumed OAE 1a change to cooler and drier conditions at the start of the event. In the marine environment this temperature shift was coeval with a maximum sea-level rise of the Ap3 sequence; possibly coinciding with the development of a circum-Antarctic-Australian connection in the Early Aptian bringing cold waters to northeastern Australia. The Toolebuc Formation in Australia correlates to OAE 1c and palaeoenvironmental reconstructions indicate relatively cooler and drier conditions at the onset of the Toolebuc in comparison to the later part, when conditions became warmer and more humid. Within the Toolebuc a threefold division in sea-level can be made, e.g., a rise in the lower part; a fall in the middle and correlative to sequence boundary Al7; followed by a rise again in the upper part. In agreement with the climatic changes inferred for the Early Cretaceous, the succession of events in the Toolebuc Fm shows a fluctuating pattern toward increasingly warmer environmental conditions.