Carbon isotope stratigraphy of the Late Proterozoic Wonoka Formation of the Adelaide Fold Belt: diagenetic assessment and interpretation of isotopic signature and correlations with previously measured isotopic curves

Abstract

The Wonoka Formation of the Adelaide Fold Belt represents the only well­ described example of a late Proterozoic storm dominated carbonate shelf sequence with the considerable thickness and lateral extent of the formation making it an excellent opportunity for applying the principles of isotope stratigraphy. Sequences exposed at Warraweena, which lies on the boundary between the Central and Northern Flinders Zones, were analysed for stable isotope ratios of carbon and oxygen.

Geochemical, petrographic and cathodoluminescent analysis of individual samples was used to identify those carbonates which have experienced significant diagenetic alteration. These values were not included in the interpretation of the formation's isotopic signal.

Plotting of the least altered values against stratigraphic height revealed a consistent carbon isotopic trend. This trend was divided into two sections, termed the Lower Wonoka Signal and the Upper Wonoka Signal. The Lower Wonoka Signal is defined by the extremely consistent negative signal (delta13c = -8 to -7o/oo) characteristic of the lower- to mid-Wonoka Formation. This signal is interpreted to be a product of deposition and lithification in basinal waters that contain anomalously light dissolved carbonate. The Upper Wonoka Signal comprises a shift to more positive values (a13c = -5 to +6 o/oo) and is interpreted to be a reflection of carbonate deposition in shallow surface waters, possibly in association with the formation of a partially restricted lagoon.

Strontium isotopic analysis reveals 87Sr/86Sr values that are interpreted to be of primary origin. These values, when compared to data obtained by previous authors for the equivalent time period, give an estimated age of 560-590 Ma for the Wonoka Formation.