Giles, Sarah M.1, Giles, Katherine A.2, Rowan, Mark G.3, Christie-Blick, Nicholas1, Lankford-Bravo, David F.2
1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA, 2University of Texas, Department of Geological Sciences, El Paso, TX 79968, USA, 3Rowan Consulting, Inc., Boulder, CO 80302, USA
Passive salt diapirs can be triggered to spread subhorizontally (allochthonous flow) as a result of contractional tectonics, a marked decrease or cessation in sediment accumulation over the diapir, or from the removal of significant overburden. In the Flinders Ranges of South Australia, salt diapirs sourced from the ~802-777 Ma Callanna Group have risen passively through much of the later Neoproterozoic to Cambrian interval, and spread allochthonously at discrete stratigraphic levels. One such level of appreciable allochthonous salt breakout has previously been placed near the contact between the Ediacaran-age Bunyeroo and Wonoka formations. However, recent chemostratigraphic results suggest that the level of allochthonous breakout could be more accurately placed within the lower Wonoka. Constrained near that same stratigraphic level is a regional unconformity associated with incision of ~ 1 km deep paleocanyons (the Wonoka canyons), which have an origin debated between deep-water or subaerial incision. Research in progress continues to favor subaerial erosion hypothesized to have resulted from an evaporative drawdown event corresponding with the onset of the Gaskiers glaciation. In the absence of clear-cut tectonic and accumulation-related explanations for allochthonous salt breakout across the Flinders Ranges, we offer an alternative: that breakout may have been triggered by the sea-level drawdown thought to have been responsible for subaerial incision of the Wonoka canyons due to the removal of overburden. There is evidence for tens of meters of stratigraphy having been removed regionally at canyon shoulders.
Our timing constraints are based upon preliminary carbon isotope chemostratigraphy and geologic mapping that indicate allochthonous salt flow and canyon incision initiated near the same stratigraphic level. We plan to collect a more robust chemostratigraphic dataset and to develop a physical stratigraphic framework for the Patawarta, Pinda, and Beltana allochthonous salt sheets, in order to constrain the level of allochthonous salt breakout with respect to the paleocanyons at Umberatana syncline, Oodnapanicken, and Beltana. The breakout of allochthonous salt possessing sedimentary inclusions and associated with carbonate caprock could have facilitated a short-lived pulse of organic carbon into the seaway in which the Wonoka Formation was deposited, potentially bearing on the local expression of the Shuram carbon isotope anomaly and the emergence of the Ediacara biota.
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.