Geochemical characteristics of cool-temperate carbonates, Tasmania, Australia

Abstract

Modern carbonates predominate on the cool-temperate Tasmanian shelf. Bryozoa are the dominant fauna found in all samples with some mollusca, foraminifera, echinoderms, sponge spicules, algae and coccoliths. These are often encrusted and show borings, with some being ironstained. Individual grains of bryozoan sand consist predominantly of high Mg-calcites with variable amounts of low Mg-calcite and aragonite. Cementation of calcite and aragonite ranges up to 90% of the bryozoan sand. The Mg content ranges from 0.1 to 2.7% (using AAS) or 0 to 3.5% (based on X-ray speaks) which indicates <3 to 11° or 15°C ambient water temperatures. Sr-Mg data points lie both above and below the bryozoa field and extend up to aragonite and calcite fields because of appreciable calcite and aragonite cementation. Na-Mg values are mostly below the bryozoa field, due to cementation, with a few above the bryozoa field which indicates higher than normal biochemical fractionation or Na entrapment. Mn and Fe are positively correlated with Mg because of the dominance of marine diagenesis. Mn and Sr are randomly distributed, unlike the products of meteoric diagenesis. Mn and Na are positively correlated because of the marine origin of Mn. Sr and Na are positively correlated and their values are much higher than those in meteorically altered limestones. The Sr-Na data points are widely scattered away from the average bryozoa values because of cementation and biochemical fractionation of Na or due to entrapment of Na. Sr/Na ratios are ∼1, much lower than the ratios observed warm-water carbonates. Theδ18O and δ13C field is distinctly different from the warm-marine field and cool-temperate meteoric diagenetic calcite values. The sea-floor diagenesis line that corresponds to positive correlation of δ18O and δ13C passes through the Tasmanian carbonate isotopic field. The Sr and Na values are uniform throughout the range of δ18O. The Sr is positively correlated to δ13C and Na varies uniformly with δ13C. Both Fe and Mn are randomly related to δ18O and to δ13C. All these trends are indicative of marine diagenesis and are unlike meteoric diagenetic trends. The above chemical characteristics present cool-temperate marine diagenetic models, which might be used to differentiate similar ancient temperate carbonates from warm-water carbonates, and to understand meteoric and burial diagenesis common in ancient temperate carbonates.