Print this pageMultistage Neogene Mixed-Water Nucleation and Marine Growth of Sucrosic Dolomites in Cool-water Limestones, South Australia
JAMES, NOEL P.; T. KURTIS KYSER; YVONNE BONE
The Oligo-Miocene Gambier Limestone, a fine-grained bryozoan calcarenite of deep-shelf origin is replaced by lenticular bodies of sucrosic, medium crystalline dolomite up to 30m in thickness. Fortuitous Pleistocene, fabric-specific dedolomitization permits delta{18}O, delta{13}C and {87}Sr/{86}Sr analysis of crystal cores and rims separately. Results are interpreted to indicate a history of multistage crystal growth in different, shallow diagenetic environments.
Silt-size cores of, luminescent, inclusion-rich dolomite with micro- domains of calcite, high-Mg calcite and protodolomite have delta{18}O values compatible with precipitation from mixed marine and fresh waters. Much larger rims to the sand-size crystals are essentially non-luminescent, inclusion-poor and have delta{18}O values interpreted to result from precipitation by marine fluids. {87}Sr/{86}Sr values suggest several distinct periods of dolomite formation. Correlation with known sealevel excursions implies that crystals nucleated in pre-existing sediments from mixed groundwater and seawater during transgressions and grew during highstands as the sediments were filled with marine porewaters. The inclusion-rich cores are unstable and can be altered in a variety of diagenetic environments, while the rims retain their marine signature. Since the bulk of the crystal is the rim, this leads to the mistaken assumption that the whole crystal formed in a marine setting.