Late Cryogenian glaciation in South Australia: Fluctuating ice margin and no extreme or rapid post-glacial sea-level rise

Abstract

Postulated extreme sea-level rise of up to 1–1.5 km with the late Cryogenian Ghaub deglaciation in Namibia is contentious, as is the great rapidity (<104 yr) of the sea-level rise. Such extreme glacioeustatic events, if real, would have been global and affected all continents. In South Australia, up to six glacial advances and retreats during the late Cryogenian Elatina glaciation indicate a fluctuating ice margin. The latter stage of the Elatina glaciation and the immediate post-glacial environment are examined here for evidence of extreme and rapid sea-level rise. In the central Adelaide Rift Complex, diamictite with faceted and striated clasts occurs at the top of the Elatina Formation <1–2 m beneath the early Ediacaran Nuccaleena Formation ‘cap carbonate’. One hundred kilometres to the south, ∼30 m of siltstone and sandstone followed by ∼6 m of clast-poor diamictite with clasts 10 + cm long occur between tidal rhythmites and the cap carbonate. Three hundred kilometres further south, ∼70 m of siltstone, dolomitic siltstone and minor dolomite separate tidal rhythmites and early Ediacaran strata. Hence the rhythmites were deposited during a high stand (interstadial or interglacial), not during post-glacial sea-level rise. Storm-generated erosional surfaces within tidal rhythmites at Warren Gorge indicate intermittent rhythmite deposition, and water depth and other palaeoenvironmental factors are uncertain, casting doubt on a published estimate of rapid sea-level rise during rhythmite deposition. The lack of late Cryogenian deeply incised valleys and thick valley-fill deposits in South Australia and central Australia argues against extreme sea-level variations. A hiatus occurred between Elatina deglaciation and deposition of the Nuccaleena cap carbonate, and three palaeomagnetic polarity chrons identified in the cap carbonate imply slow deposition spanning 105–106 yr. This is supported by independent evidence from magnetic chronostratigraphy for Ediacaran strata in South Australia and California, and by stratigraphic and sedimentological arguments for condensed deposition of cap carbonates. It is concluded that neither extreme nor rapid sea-level rise was associated with late Cryogenian deglaciation in South Australia.