Carbon isotope chemostratigraphy and long-term sedimentary cyclicity of the mid-Cryogenian non-glacial interlude, Stuart Shelf, South Australia

Abstract

The Cryogenian non-glacial interlude on the Stuart Shelf, South Australia comprises offshore to supratidal sedimentary successions deposited after the Sturtian glaciation at ∼660 Ma. This study's results present new data on fine-tuning carbon and oxygen isotope excursions in the Cryogenian, using material from five Stuart Shelf drillholes. The carbon isotope signature of the succession shows a broadly reproducible increase from −4.2 to +4 ‰ δ¹³C_carb from the base of the Tapley Hill Formation (including Sturtian cap carbonates) to the Brighton Limestone. This isotopic trend reflects changes in the marine dissolved inorganic carbon reservoir in the aftermath of the Sturtian glaciation, during an initial rapid transgression followed by an overall regressive phase. Most importantly, the observed isotopic trend is independent of the thickness of the different studied successions. Isotopic variations and sedimentary sequences are interpreted to be mainly controlled by change in the marine dissolved inorganic carbon reservoir and eustatic sea-level, both driven by global tectonic reconfigurations. Thickness variations relate to local or regional variations in subsidence rate and palaeo-topography. The secular trend within the non-glacial Cryogenian succession is interpreted to correlate with the Twitya carbon isotope excursion reported globally within a ∼10 Myr transgressive-regressive sedimentary cycle.