The provenance of sediments in Moreton Bay, Australia: a synthesis of major, trace element and Sr-Nd-Pb isotopic geochemistry, modelling and landscape analysis

Abstract

Moreton Bay (MB) is a large semi-enclosed coastal embayment, located on the east coast of Australia adjacent to the Queensland capital city of Brisbane. The MB catchment is ca. 22 000 km2 while the bay itself is ca. 1500 km2. This represents a catchment:bay ratio of ca. 15:1. Within the MB catchment there are over 85 rocktypes with the most abundant, the Marburg Formation, constituting only ca. 12% of the catchment area. Detailed landscape analysis and a reconnaissance soil sampling program in combination with major and trace element and Sr, Nd and Pb isotope geochemistry were used to identify the major sources of sediment delivered to MB. A Bayesian linear mixing model allowed the proportion of sediment sources to be estimated. Model estimates suggest that there are substantial differences in the proportions of sediment being delivered into MB by the two major tributaries, the Brisbane and Logan Rivers despite both rivers containing a similar suite of major rocktypes. Over 50% of the sediment delivered to MB is derived from soils developed on the Marburg Formation. This equates to a catchment area/bay sediment deposition ratio of ca. 5. Basaltic soils of the Main Range Volcanics (Brisbane River catchment) and Lamington Group (Logan River catchment) and sediments from the Walloon Subgroup (present in both catchments) are on average also enriched in MB relative to the catchment by a factor of ca. 2.