MARINOAN GLACIAL TO POST-GLACIAL TRANSITION AT THE EDIACARAN GSSP, SOUTH AUSTRALIA

A robust conclusion from diverse lines of reasoning is that emergence from Earth's greatest ice age (~650-635 Ma) was episodic and protracted (10⁵ to 10⁶ years), and not at a timescale as short as a few hundred years. Both deglaciation and the deposition of cap carbonates typically only a few meters thick are inferred to have been diachronous, with carbonate sediments accumulating initially at low latitude while ice sheets continued to exist in continental interiors and at higher latitude. Post-glacial flooding associated with the Nuccaleena Formation (Marinoan cap) in South Australia (a low-paleolatitude site) appears to have lagged local glacio-isostatic rebound, resulting locally in up to several tens of meters of erosional relief at the sub-cap unconformity, though < 1 m in the vicinity of the Ediacaran Global Stratotype Section and Point (GSSP). In this scenario, and harking back to an idea popular in the 1970s and 1980s, cap carbonates are fundamentally due to condensation, but with methane release from mostly marine gas hydrates as the ocean warmed continuing to represent the most plausible explanation for the pronounced negative carbon isotopic excursion in these rocks. Prominent meter-scale tepee-like structures in the Nuccaleena are primarily deep-water constructional features, oriented parallel to unidirectional underflows (obstacle marks and ripples), and it is hypothesized that their spacing of up to several meters scales with the dimensions of helical vortices in those flows. Given that the time required for the growth of such structures is likely to have been substantially shorter than the timescale for cap carbonate deposition (~ 10⁵ years and perhaps longer at any location), accumulation of the carbonate is inferred to have been less continuous than generally assumed.