Giles, Sarah M.1, Christie-Blick, Nicholas 1
1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA
Mid-Ediacaran (~580 Ma) paleocanyons as much as 1 km deep in the Wonoka Formation of South Australia are associated with the largest carbon-isotope excursion in Earth history, the Shuram anomaly. Widely interpreted as submarine, the canyons are thought by many to be comparable to those found at modern continental margins. New data from the northern Flinders Ranges reinforce an alternative hypothesis: that the Wonoka canyons were subaerially incised as a result of evaporative drawdown in a temporarily isolated marine embayment at the onset of the Gaskiers glaciation. Critical supporting evidence has emerged in the canyon-fill at Umberatana syncline, where four oblique cross-sections interpreted as a single sinuous canyon are being independently analyzed. The three incisions studied so far are characterized at the base by ~ 30-m-thick conglomerate-based cycles that are bounded by laterally persistent erosion surfaces and fine upwards into sandstone, siltstone, and minor carbonate (canyon-marginal tongues). Nine such cycles are confidently correlated between the first two incisions (Fortress Hill and Mt. Curtis), and at least plausibly related to seven cycles identified in the third incision (Muccabaloona south) on the basis of 168 measured sections and high-resolution physical stratigraphic mapping. Our correlation of cycles is based on similarities in facies stacking and stratigraphic position. Greater variability in cycle thickness at Muccabaloona south is attributed to facies changes within some cycles in the direction of sediment transport from channelized boulder conglomerate to pervasively rippled sheet-like sandstone and siltstone event layers, as well as a northward increase in structural complexity. The local erosional relief at cycle boundaries is comparable across the three outcrops: 3-6 m at Fortress Hill, 5-10 m at Mt. Curtis, and 3-7 m at Muccabaloona south. The observed stratigraphic organization and details of the facies lead us to interpret the diffusely stratified, channelized conglomerates as fluvial, and the prevalently rippled and laminated tabular sandstones/siltstones as deltaic. The lack of dish structures and nested channels in the sandstone facies, as well as the apparent absence of classical turbidites interstratified with hemipelagic mudstone further supports our fluvial-deltaic interpretation rather than a deep marine interpretation. An upward transition to primarily siltstone event layers interstratified with cm-scale carbonate couplets and monolithologic breccias is interpreted to represent canyon drowning. New field work is planned at the fourth incision (Muccabaloona north) to further test our model. We expect fewer conglomerate-based cycles, and additional fining overall. The correlation of stratigraphic cycles across all four incisions will allow us to evaluate the length scale of downstream facies transitions, which we expect to be much more abrupt in the subaerial incision model than the deep marine canyon interpretation.
Sarah Giles is a Ph.D. student at Columbia University in New York City, USA. Sarah’s Ph.D. research integrates geologic mapping, sedimentology, stratigraphy, isotope geochemistry, and geochronology to evaluate the origins, timing, and stratigraphic context of the mid-Ediacaran Shuram carbon isotope excursion in South Australia and eastern California.