12 - Isotopic signatures of carbonates associated with Sturtian (Neoproterozoic) glacial facies, central Flinders Ranges, South Australia

Summary

Abstract

The Appila Tillite in the southwest Flinders Ranges outcrops as a narrow, continuous unit, near the western edge of the Adelaide Geosyncline, resting disconformably on dolomitic Burra Group sediments. Interpretation of measured sections suggests the Appila Tillite represents two glacial advances, abruptly overlain by a post-glacial transgressive shale. These sections of the Tillite show condensed and commonly reworked sediments when compared with sections in the Northern Flinders Ranges, suggesting deposition close to the margin of the basin. Most clasts within this formation are of Burra Group origin with only a few erratics from the western Gawler Craton.

Dolomite is common in the Appila Tillite, either as dolostone clasts or as a diamictite matrix. Isotope analysis gives an average of – 6.26 permil δ¹⁸ O PDB and 1.34 permil δ¹³C PDB for the matrix, different to dolomitic clasts (0.28 δ¹⁸O and 4.39 δ¹³C). The dolomitic matrix is likely to be either detrital rock flour altered after deposition by isotopically light pore fluids, or a carbonate precursor that has undergone penecontemporaneous dolomitization. The former is favoured.

Geochemical results suggest the light δ¹³C values of the Appila matrix were caused by input of biologically derived carbon. The variation in isotope values between sampled material from different formations (Appila Tillite Fin., Tapley Hill Fm., and Burra Group) indicate that metamorphic equilibrium has not occurred.

Introduction

The Appila Tillite (defined by Mirams, in Thompson et al., 1964) in the Southern Flinders Ranges, is a local equivalent of extensive Sturtian glacial deposits found throughout the Adelaide Geosyncline (Preiss, 1987). Ten sections through this formation were measured along the Emeroo Range in the southwestern Flinders Ranges (Fig. 12.1).