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Mineralogy and geochemistry of modern temperate carbonates from King Island, Tasmania, Australia

Research paper by C. Prasada Rao, M. P. J. Jayawardane

Indexed on: 01 Sep '93Published on: 01 Sep '93Published in: Carbonates and Evaporites



Abstract

Shelf sediments around King Island range from siliciclastics to mixed carbonates and to pure carbonates. Carbonates consist mainly reworked calcitic fauna, such as bryozoans, foraminifera, echinoderms and red algae with minor intragranular CaCO3 cements. Gastropods are the main aragonitic fauna and these are rare. Bulk sediments are analyzed by both X-ray diffraction, atomic absorption spectrophotometry, X-ray fluorescence and mass spectrometry. Minerals detected by XRD are mainly high-Mg calcite, quartz and aragonite with minor low-Mg calcite. The Ca, Mg and SiO2 contents confirm the occurrence of siliciclastics to mixed carbonates and to pure carbonates. The concentrations of Sr and Na vary with carbonate mineralogy and skeletal content. The high Fe and Mn contents in calcite are due to sedimentation in reducing marine environments.The δ18O and δ13C field of bulk sediments overlaps isotope fields of bryozoans, foraminifera and brachiopods. All these isotope fields are dissected by both trendlines of seafloor diagenesis and upwelling water because sediments and fauna are in equilibrium with marine waters. The ambient water temperatures determined from δ18O values range from about 10 to 15°C, which are about 3°C lower than measured surface water temperatures.Originally calcitic ancient carbonates are abundant in stratigraphic sequences and their geochemistry can be better understood by comparison with baseline geochemical data of modern temperate calcitic carbonates rather than with modern tropical aragonitic carbonates. Many of these ancient originally calcitic bryozoan, foraminifera and echinoderm carbonates are of nontropical origin as coeval tropical aragonitic carbonates occur elsewhere.