Alteration of Cenozoic cool-water carbonates to low-Mg calcite in marine waters, Gambier Embayment, South Australia

Abstract

The Oligo-Miocene Gambier Limestone is a poorly lithified, fine-grained, bryozoan-rich grainstone/wackestone and marl. Allochems in these rocks, which are typical of many modern and ancient cool-water carbonates, were originally calcite (LMC), Mg-calcite (IMC and HMC) and minor aragonite, but most are now neomorphosed to LMC. With the exception of calcretes, most limestones have < 5% cement, δ 13C and δ 18O values from 0 to +1.5%, and preserved 87Sr/ 86Sr 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 grainstones are significantly lower than in the original sediments, whereas a less porous marl has higher Sr and Mg contents. The grainstones have about 300 ppm Sr and 6000 ppm Mg, consistent with slow abiotic precipitation of calcite under near-equilibrium conditions with seawater, which is the thermodynamic drive for this diagenesis. The higher Sr and Mg content of the marl results from variable preservation of original allochems of biotic calcite. The results from this study and others on modern cool-water carbonates indicate that the extent of diagenesis varies with lithology, but much neomorphism occurred in seawater, with minimal effects from meteoric waters. As such, the chemical characteristics of seawater may be effectively preserved in some ancient cool-water carbonates.