Palynostratigraphic and U-Pb CA-IDTIMS isotopic dating of the upper Orallo Formation, northern Surat Basin, Queensland

Abstract

The Jurassic–Cretaceous transition in the Surat Basin, Queensland, has not been previously subjected to detailed palynological study and biostratigraphic assessment. The associated lithostratigraphic succession, that is represented by the upper Westbourne Formation, the unconformably overlying Gubberamunda Sandstone, and the succeeding Orallo Formation and Mooga Sandstone, represents the most complete record of deposition for the interval in Queensland. Several important aquifers and aquitards occur in this succession, and a refined palynological biostratigraphy may assist in providing more accurate correlations and improved conceptual models for its deposition and spatial variability. Preliminary results of the investigation suggest that the existing pan-Australian zonal scheme and the informal alphanumeric scheme widely applied in eastern Australia can be modified to more accurately reflect the stratigraphic distribution of palynomorph species present in, and the overall palynofloral composition of, these uppermost Jurassic–lowermost Cretaceous rocks, not only in the Surat Basin but also, by extrapolation, their equivalents in other eastern Australian basins. This will thus facilitate more accurate intra- and inter-basinal correlations. In the Orallo Formation material studied to date, zone-index fossils—though present—have proved to be scarce, but this is relative to the extreme richness and the diversity of taxa in the assemblages, the composition of which appears to differ significantly from that described in the literature, and reflects the temperate climate in the environment of deposition. On an introductory basis—as the study is in its early stages—a number of new species have been provisionally identified, and several existing species are seemingly in need of taxonomic emendation.