Petrography, trace elements and oxygen and carbon isotopes of Gordon Group carbonates (Ordovician), Florentine Valley, Tasmania, Australia

Abstract

Gordon Group carbonates are a thick (∼ 2 km), stratigraphically continuous Ordovician sequence that ranges in age from the Arenig to the Ashgill. They were deposited in subtidal, intertidal, supratidal and channel depositional environments at a paleolatitude of about 10°N. The biota is similar to modern tropical chlorozoan assemblages with abundant corals, oncolites, calcareous algae and stromatolites. Non-skeletal grains are abundant and are predominantly intraclasts and pellets with some oolites and aggregates. Micrite is abundant with some sparry calcite cement. Dolomite, which commonly replaces micrite, extensive burrows and mud-cracks, formed mostly during early diagenesis by marine to mixed-marine waters. The Gordon Group limestones are characterized by high Sr/Na ratios (2.6–4.6) similar to modern aragonitic carbonates. The stratigraphic variations of Sr, Na and Mn are related to depositional environments, diagenesis and dolomitization. The low Mn concentrations indicate oxidizing peritidal depositional environments with high rates of carbonate deposition. Na varies with salinity and the Sr varies with facies. The largest Sr concentrations are in corals and micrites. The Sr, Na and Mn concentrations have been affected by two-stage meteoric diagenetic stabilization. The first stage involves marked depletion of Sr and Na with no appreciable gain of Mn, due to inversion of aragonite to low-Mg calcite. The second stage involves no significant change in Sr and Na with appreciable gain of Mn, which is due to recrystallization of low-Mg calcites and addition of spar cement in a semi-closed diagenetic system. Sr concentrations decrease whereas Mn and Fe concentrations increase with increasing Mg concentrations due to dolomitization in predominantly marine waters with some influx of meteoric waters. The δ 18O and δ 13C isotopic fields of calcites and dolomites partly overlap and the dolomites are enriched relative to co-existing calcites by 2 to 3‰ in δ 18O and by about 1% in δ 13C similar to modem marine and mixed-marine dolomites. Low Mn and Fe concentrations, indicative of least meteoric diagenetic change, correspond to − 5.0±0.5‰ δ 18 O and +1.0±05‰ δ 13 C suggesting that these values were the original Late Ordovician marine calcite values in Tasmania. These isotopic values are similar to the least altered Late Ordovician brachiopods and marine calcite cements from other areas. The Middle Ordovician δ 18O and δ 13C values of marine cements are ∼ − 7‰ and ∼ 0‰ respectively and these values match the marine cement values of the same age from other areas. This shift towards heavier isotopic values from Middle to Late Ordovician is probably due to both glaciation in the higher latitudes of Gondwana and a slight drop in temperature (< 4° C). Depletion in δ 13C, δ 18O, Sr and Na and enrichment in Mn in the lower and upper most part of the Late Ordovician Tasmanian limestones indicative of pronounced meteoric diagenesis coincide with a major Australia-wide marine regressions.