ALTERATION OF CENOZOIC COOL-WATER CARBONATES TO LOW-MG CALCITE IN MARINE WATERS, GAMBIER EMBAYMENT, SOUTH AUSTRALIA

The Oligo Miocene Gambler Limestone is a poorly lithibed, fine-grained , bryozoan-rich grainstone/wackestone and marl, Allochems in these roc ks, which are typical of many modern and ancient cool-water carbonates , were originally calcite (LMC), Mg calcite (IMC and HMC) and minor ar agonite, but most are now neomorphosed to LR IC. With the exception of calcretes, most limestones have < 5% cement, delta(13)C and delta(18) O values from 0 to +1.5 parts per thousand, and preserved Sr-87/Sr-86 ratios that indicate that neomorphism occurred in seawater rather than meteoric or evolved pore waters, Marine diagenesis of the limestones was facilitated by their slow accumulation rates, which were ca, 15 mm /ky based on Sr-isotope stratigraphy, relatively fine grain size, and high porosities which locally reach 50%. The Sr and Mg contents of gra instones are significantly lower than in the original sediments, where as a less porous marl has higher Sr and Mg contents, The grainstones h ave about 300 ppm Sr and 6000 ppm Mg, consistent with slow abiotic pre cipitation of calcite under near equilibrium conditions with seawater, which is the thermodynamic drive for this diagenesis, The higher Sr a nd Mg content of the marl results from variable preservation of origin al allochems of biotic calcite. The results from this study and others on modern coal-water carbonates indicate that the extent of diagenesi s varies with lithology, but much neomorphism occurred in seawater, wi th minimal effects from meteoric waters. As such, the chemical charact eristics of seawater mag be effectively preserved in some ancient coal -water carbonates.