Oxygen and carbon isotope composition of Gordon Group carbonates (Ordovician), Florentine Valley, Tasmania, Australia

The Gordon Group carbonates consist of biota of the Chlorozoan assemblage, diverse non-skeletal grains and abundant micrite and dolomite, similar to those of modern warm water carbonates. Cathodoluminescence studies indicate marine, meteoric and some burial cements. Dolomites replacing burrows, mudcracks and micrite formed during early diagenesis. δ18O values of the non-luminescent fauna and marine cement are lighter than those of modern counterparts but are similar to those existing within low latitudes during the Ordovician because of the light δ18O values of Ordovician seawater. The δ18O difference between marine and meteoric calcite indicate that during the Early Ordovician there were high atmospheric CO2 levels than at present but during the Middle and Late Ordovician they became comparable with the present because of a change from 'Greenhouse' to glacial conditions. δ18O values of Late Ordovician seawater were heavier than in the Middle Ordovician mainly because of glaciation. It is thus concluded that dolomitization took place in marine to mixed-marine waters while the original calcium carbonate was undergoing marine to meteoric diagenesis. 41 refs., 1 tab., 9 figs