Early Diagenesis of Carbonates on a Cool-Water Carbonate Shelf, Southern Australia

Abstract

Abstract The southern Australian continental margin is an immense cool-water carbonate factory. The high-energy open shelf is blanketed by heterozoan carbonates of both Holocene and late Pleistocene age. Three distinct grain types have been identified and radiocarbon dated. These include (1) relict grains, highly abraded iron-stained intraclasts formed during the intermediate sea-level stands of Marine Isotope Stages 3 and 4, (2) stranded grains, gray and buff-colored abraded biofragments and intraclasts marooned during the sea-level rise associated with Marine Isotope Stage 2, and (3) Holocene grains, unaltered biofragments formed during Marine Isotope Stage 1. Optical, mineralogical, and chemical characterization of grains of each age has revealed pathways of early diagenesis in this realm. Both calcitic and aragonite biogenic grains undergo dissolution. While dissolution of calcitic components is minor and incomplete (relict sediments are predominantly Mg calcite), aragonitic skeletons undergo significant dissolution within ~ 30,000 years. In contrast to early dissolution of aragonite, micritic calcite cement, with ~ 12 mol% MgCO3, is a significant diagenetic phase in these sediments, infilling many stranded and most relict skeletons. Similar cement precipitates between grains, forming small numbers of cemented grain aggregates in most samples. Microprobe and ICP-MS analysis demonstrates Fe oxides, Mn oxides, and associated clay minerals discolor stranded and relict grains, by coating grain surfaces and infesting micropores. Such coatings begin to form on Holocene grains during or soon after formation. The oxides scavenge trace elements including V, Ti, Cr, and REE, altering the elemental compositions of biogenic carbonate.