175-7 Textured Organic Surfaces Associated with Delta Front Sedimentation in the Mid-Ediacaran Wonoka Formation, Flinders Ranges, South Australia

Session: The Neoproterozoic Earth and Life Co-evolution, Part I

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Abstract:

Textured organic surfaces (TOS) are featured over a wide range of marine paleoenvironmental settings in Ediacaran siliciclastic successions that have been studied globally. The megascopic organic record of TOS has been previously unrecognized in the Wonoka Formation, but is documented in this study to be present in the basal fill of kilometer deep paleocanyons in event layers of current-rippled siltstone and sandstone interpreted most recently as deposited at a delta front. Commonly referred to as the Wonoka canyons, these paleocanyons are stratigraphically associated with the largest negative carbon isotope anomaly in Earth history (the Shuram) and are broadly coincident with the emergence of the Ediacara fauna.

The best-preserved TOS examples in the Wonoka are in the basal fill of the Oodnapanicken canyon. Our study utilizes geologic mapping, eleven measured sections placing the TOS within a physical stratigraphic framework, and thin-section petrography of the TOS facies at Oodnapanicken canyon to show that the TOS occur in delta front facies overlying fluvial conglomerate. The TOS have an unusual irregular fabric that is not observed in the Ediacara Member, and occur as centimeter scale patches on top of bedding surfaces of calcite-cemented, very-fine grained sandstone composed primarily of angular quartz and feldspar grains with occasional calcite peloid grains. This facies is interpreted to have been transported by hyperpycnal flows down slope. Finer-grained interlayers that typically overlie the TOS are clay cemented and dominated by clay, mica, and quartz. We interpret these layers as the settling of hemipelagic material, suggesting the TOS likely formed atop hyperpycnal event layer deposits during periods with lower energy sedimentation. While previous investigations of Ediacaran TOS documented substantial TOS stratigraphic heterogeneity within the same depositional environment, our study finds a singular TOS texture within the same delta front environment. At this time, we have not observed other TOS morphologies in alternative facies types of the Wonoka. We suggest that the exclusive recognition of TOS in delta front facies of the Wonoka indicates that the unique shallower-water environment preserved at the base of the Wonoka canyons could have led to the establishment of ecosystems not observed in the deeper-water stratigraphy of the canyon walls. This conclusion provides a new constraint on the controls of paleoenvironmental setting on Ediacaran organisms closely preceding the Ediacara fauna.

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

doi: 10.1130/abs/2025AM-8495

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