256-2 A Deep Cut: Submarine Canyons of the Ediacara Member of South Australia

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology, Part I

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The rise and diversification of metazoans in the late Ediacaran have been linked to the global sea-level changes, driving oxygenation of shallow-marine environments and perturbations to the carbon cycle. However, the regional record of these changes remains ambiguous. Here, we revisit the record of sea-level fluctuations in the Flinders Ranges in South Australia, focusing on the fossiliferous Ediacara Member of the Rawnsley Quartzite. The base of the unit is marked by an erosional unconformity incising into the intertidal Chace Member and the shallow-marine to fluvial Bonney Sandstone. The unconformity is traceable throughout the Flinders Ranges, creating 2-10 m depressions and southeast-oriented deep valleys reaching 250 m in depth and more than 2 km in width. Their infill varies depending on the position relative to the paleocoastline. In more proximal settings, the lower part of the infill is characterized by chaotic rip-up clast breccia, with 1-7 cm rip-ups in feldspathic matrix, commonly with granules of well-rounded quartz, reworked from the underlying fluvial conglomerate. In a more distal setting, the base is marked by thick-bedded sandstone with chaotic rip-up clasts at bed bases, overlain by thick-bedded and massive sandstone with loading structures, interbedded with planar siltstone. The 2-10 m depressions are filled by either channelized sandstones or planar siltstones. The highly irregular topography formed on top of the intertidal Chace Member suggests erosion during subaerial exposure, with many shallow valleys formed during the relative sea-level fall. Submarine erosion is unlikely to create this complex erosive surface without any deposition across the shelf. However, the lack of fluvial deposits and depths exceeding 100 m suggests further incision during marine transgression, transforming incised valleys into shelf-incising submarine canyons. We interpret the infill sequence of these canyons and drowned valleys as marine, deposited largely below wave base. Although the basin was tectonically active at the time, the sea-level fluctuations recorded in the Rawnsley Quartzite need to be at least partially eustatic. The erosion at the base of the Ediacara Member overlaps well with the suggested global lowstand at ca. 550 Ma, followed by a rapid transgression linked to deglaciation of high- to mid-latitude ice sheets. Testing connections between the late Ediacaran climate state, sea-level fluctuations, and diversification of metazoans will require further integration of regional and global record.

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

doi: 10.1130/abs/2025AM-7990

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