Australia: The Land Where Time Began

A biography of the Australian continent 

Nuccaleena Formation, South Australia - Testing Models for Post glacial Deposition of 'Cap Dolostone'

The glacial deposits that are associated with the younger Cryogenian ice age are consistently draped over by carbonate sequences that are sedimentologically and geochemically distinctive. The authors¹  suggest at least the lower portion of the cap sequence records deglaciation, as is implied by ice-rafted debris in the basal dolostone. Cap dolostones have been suggested by an isochronous model to have been synchronously deposited around the world, whatever the depth of the water, while it is suggested by a diachronous model that the deposition of cap dolostones tracked glacioeustatic flooding at times of deglaciation. In the Adelaide Rift Complex (ARC), South Australia, the Nuccaleena Formation has many sedimentary unique features that have been seen in other cap dolostones from around the world from the younger Cryogenian. Oscillatory flow that is driven by the wind has produced some bedforms, which constrains the deposition of dolostone of less than 400 m to post-glacial times. It is suggested by the lack of these sedimentary features in the deepest basinal facies of the northern ARC, that this region was beneath the base of storm waves, even at the lowstand of the glacial sea level. There is a distinct relationship between the variability of the lateral facies in the pre-, syn-, and post-glacial sediments and the axes of the 50 m scale structural folds. Basinal facies characterise the limbs of the northern fold, and upper fold facies are associated with the southern limbs. The abrupt transition of the facies have been interpreted by the authors¹ to reflect breaks from the lower slope to upper slope/shelf across a series of linked, south-facing half-grabens. Monotonic declines in δ¹³C of less than 2‰, with starting points between -0.5 and -3.5‰ are shown in the carbon isotope records of the majority of cap dolostones. An about 3‰ lateral gradient from platform to lower slope, that varies dramatically on a short spatial scale and with palaeobathymety, non-systematically, is implied by an isochronous model. The values of  δ¹³C become progressively lighter over time, if palaeobathymety is inferred by the use of pre- and syn-glacial facies, and there is diachronous deposition of cap dolostones as the sea level rises at times of deglaciation, it implies that the deposition of cap dolostone tracked the glacioeustatic rise in sea level over a series of half-grabens opening to the north. Uniquely isochronous or diachronous models cannot be ruled out by the carbon isotope dataset, and the temperature cannot be the dominant control on the cap dolostone isotopic variability.

Sources & Further reading

1. Rose, Catherine V., and Adam C. Maloof. "Testing Models for Post-Glacial ‘Cap Dolostone’ Deposition: Nuccaleena Formation, South Australia." Earth and Planetary Science Letters 296, no. 3–4 (8/1/ 2010): 165-80.