168-8 New insights on the Gaskiers timing, volcanic rifted margin setting and orbitally forced cyclicity of the mid-Ediacaran Wonoka canyons of South Australia

Session: Laurentia Without Borders: Pre-Pangea Intercontinental Connections

Presenting Author:

Authors:

Abstract:

Kilometer-deep paleocanyons in the mid-Ediacaran Wonoka Formation of South Australia are remarkable for their unusually large relief and their association with δ¹³C values as low as –12‰ in calcite-cemented siliciclastic rocks (the Shuram excursion). Though widely viewed as comparable to the submarine canyons of contemporary continental margins, stratigraphic and sedimentological data from the northern Flinders Ranges reinforce an alternative interpretation: that the canyons were cut by rivers during a short-lived Messinian-style drawdown of sea level in a temporarily isolated marine embayment. The timing of incision is ascribed to the development of a volcanic rifted margin at ~600-580 Ma and sea-level lowering triggered by the onset of Gaskiers glaciation at ~580 Ma. Renewed rifting at an existing ~660 Ma passive margin is significant not merely in terms of timing, but because rapidly emplaced volcanic rocks may, briefly at least, have built to or above sea level (seaward-dipping reflector sequences). The formation of a volcanic rifted margin provides a novel explanation for closure of the hypothesized embayment, and is in turn reinforced by the drawdown model for the canyons. The inferred timing also challenges the generally accepted synchrony of the Shuram excursion (~574.0-567.3 Ma). We hypothesize that quasi-periodic sea-level changes responsible for the development of polymictic conglomerate-based cycles in the basal canyon fill at Umberatana syncline were due to orbitally forced (precession) variations in precipitation/evaporation within the embayment and its catchment. Climatic precession periods at 580 Ma are estimated to have clustered at ~16.0 kyr and ~18.7 kyr (an average of ~17.35 kyr). The implied total span of the cyclic interval (~175 kyr) is thus consistent with the short estimated duration of Gaskiers glaciation (as brief as ~340 kyr). Flooding of the canyons and a renewed connection with the global ocean would have occurred immediately after deposition of the uppermost conglomerate.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4440

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