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Trace Elements in Devonian Carbonates of the Canning Basin, Western Australia


The Canning Basin Chronostratigraphy Project (CBCP) aims at developing alternative methods for high-resolution chronostratigraphic correlation, addressing problems such as subsurface data limitations and poor biostratigraphic resolution. One type of constraint is ‘Elemental Chemostratigraphy’, which has been widely applied successfully in siliciclastic settings and is a developing practice in its application to carbonate settings. Elemental data have been obtained for ca. 50 elements on over 1800 hand samples collected from the Canning Basin, Western Australia, including platform-top, reef core and slope environments. The samples range from Givetian to Famennian (Middle-Upper Devonian) in age and are taken from outcrop localities near Windjana Gorge and the South Oscars Range, and a mineral core from the Horseshoe Range. The study incorporates both attached and detached reefal platform configurations. High resolution stratigraphic frameworks are already defined for these sections through facies analysis, rock magnetics and stable isotope analysis. Thus, we were able to compare and demonstrate how elemental signatures are expressed in carbonate settings as a function of depositional environment, paleogeographic configuration, and long-term platform evolution. Results from platform-top settings of the Windjana Gorge area show high-frequency trends in Cr/Al2O3, and K2O/Al2O3, which infer changes in heavy mineral composition, and clay mineralogy respectively. The Zr/Al2O3 ratios serve as a proxy for heavy mineral and clastic input, and can be linked to sea level fluctuations through time. The South Oscars section shows high frequency trends in Zr/Al2O3 and K2O/Al2O3 ratios, however the signature recovered in the Cr/Al2O3 ratios are not as intense as in those observed in the Windjana sections, indicting a change in heavy mineral composition between platform top and slope depositional environments. The Horseshoe Range section demonstrated a gradual raise in the K2O/Al2O3 ratio, indicating build up in clay mineral deposition. The Cr/Al2O3 ratios in this section are closely related to lithology, with sandstones containing greater heavy mineral content than platform derived sediments. These elemental trends allow for the inference of environmental changes in local, regional and global settings and offer correlation constraints boundaries that until now have not been recognizable in traditional carbonate sequence stratigraphy.