Chemostratigraphy of Cambrian carbonates in the Amadeus Basin: implications for paleo-depositional environments and marine redox

Abstract

The Paleozoic sedimentary sequences from the Amadeus Basin in the Northern Territory (NT) have been a subject of several studies due to their economic and scientific significance. However, the applications of trace element and/or stable and radiogenic isotope proxies (e.g., REE, C and Sr isotopes) to Cambrian carbonate records from the Amadeus Basin are quite limited. Strontium isotope ratio (87Sr/86Sr) and the major and trace elemental concentration data (including REE, and paleo-redox Ce anomaly) were used in this study to constrain the paleo-depositional environment and the redox conditions in the NE parts of the Amadeus Basin during the middle Cambrian, based on analysis of carbonates from two drill cores (Alice 1 and Dingo2). Importantly, acquired data from these two remote cores are consistent with a presumably regional and perhaps basin-wide signal. In detail, this study shows that the lower or older successions (Giles Creek Dolomite mainly) have 87Sr/86Sr record that is consistent with the expected coeval middle Cambrian paleo-seawater Sr isotope composition, and REE data point to more oxic conditions. This in turn suggests either a limited restriction of the basin from the Cambrian global ocean during this time (~515 to 505 Ma), or alternatively an existence of “evaporitic seaway” setting. Considering the evidence for major evaporitic cycles in the studied records, the latter scenario is more plausible. In contrast, the upper and younger successions have a 87Sr/86Sr ratio that is non-marine and much more radiogenic, plus REE data indicate more reducing conditions. These proxy data thus indicate a substantial restriction of the basin during this time (~505 to 495 Ma) and an overall increase in the input of continental Sr flux into the basin via weathering, likely due to more humid climate. Overall, this study argues that the older middle Cambrian sedimentary record from the Amadeus Basin was deposited under generally oxic conditions while still being connected at some level to the open ocean during times of hotter or greenhouse climate, the latter reflected as repetitive evaporitic cycles. The younger Cambrian record, during the positive C isotope (SPICE) event, was in contrast deposited under more reducing redox conditions and enhanced continent-derived weathering fluxes, driven by climate change (arid to humid), amplified by the equatorial position of the Amadeus Basin during the Cambrian times.