Deposition and chemical composition of Early Cambrian salt in the eastern Officer Basin, South Australia

Abstract

Lower Cambrian salt deposits in the eastern Officer Basin occur in troughs along the western and northwestern margin of the Gawler Craton, and are interpreted to have been deposited in marginal to restricted marine environments. The salt horizons in two drillholes [Manya 6 (373554E, 6938478N: GDA94) and Wilkinson 1 (263545E, 6693435N: GDA94)] were studied utilising multiple techniques including mineralogical investigation, fluid inclusion and isotopic analysis. New data on Br values of 19 salt samples provide evidence of marine origin for the salt. Abundant two-phase (gas – liquid) inclusions and poor preservation of primary inclusions indicate intensive post-depositional recrystallisation. Homogenisation studies of fluid inclusions in halite from Manya 6 suggest that the upper part of the saltbearing strata (Ouldburra Formation) has been heated to 808C, whereas the lower part reached even higher temperatures. This heating event may be related to the Devonian – Carboniferous Alice Springs Orogeny. The chemical composition of fluid inclusions in Manya 6 and Wilkinson 1 samples indicates that parent brines were Ca-rich and SO4-poor, whereas modern seawater is SO4-rich and Ca-poor. Such a composition is similar to that recorded in the Lower Cambrian halite from the East Siberia Basin.