Mid-Ediacaran incised valleys in the Johnnie Formation of eastern California (the Johnnie valleys) and the Wonoka Formation of South Australia (the Wonoka canyons) are of interest for their unusually large scale and broad time concordance with the largest negative carbon-isotope anomaly in Earth history (the Shuram excursion) and the emergence of multicellular life (the Ediacara fauna). The Johnnie valleys and Wonoka canyons have been widely accepted as originating in a submarine setting at a continental margin. My new data suggest an alternative scenario: that both features were cut subaerially concomitant with sea-level lowering in excess of 200 m, and were subsequently drowned and filled by marine sediments.
Critical evidence includes 1) the presence in the basal fill of both valley systems of polymictic conglomerate/breccia with a quartz sand matrix that is locally associated with stratified quartz sandstone, suggesting both local and far-traveled fill components; 2) multiple upward-fining, polymictic conglomerate-based cycles in the basal Wonoka canyon fill; 3) beds and blocks of giant ooid packstone-grainstone indicative of shallow marine sedimentation during the early stages of Johnnie valley filling; 4) the observed transition in the direction of paleoflow in the Wonoka from stratified boulder conglomerate to sandstone and siltstone event beds; and 5) regional restoration of the northern Flinders Ranges indicating that several deep canyons in the Wonoka are > 20 km inboard of the paleoshelf edge. Modern submarine canyons rarely incise that far into continental shelves.
My new carbon isotopic data demonstrate negative carbon-13 (δ13C) values in the basal Johnnie valley fill, indicating that like the Wonoka canyons, the Johnnie valleys are bracketed by the Shuram excursion. Additionally, in South Australia, regional allochthonous salt breakout is observed at the same stratigraphic level as the canyon-cutting unconformity, with no evidence for triggering by regional crustal shortening or deep marine non-deposition. Clasts from diapiric breccia and the basal Wonoka canyon fill share sedimentologic, petrographic, and geochemical characteristics indicating the presence of diapiric contributions to the canyon fill, and that allochthonous salt and the canyons interacted dynamically at the Earth’s surface during the Ediacaran.
Each of these observations is more consistent with the expectations of a subaerial rather than submarine setting. I hypothesize that the Johnnie valleys and Wonoka canyons were cut by a combination of fluvial incision and subaerial mass wasting, before being drowned. Sea-level lowering is thought to have been triggered by the ~580 Ma Gaskiers glaciation. My interpretation is based on high-resolution physical stratigraphic mapping supported by sub-meter scale 3-D drone imagery, geochemical analysis (δ13C, δ18O, δ26Mg, Mg/Ca), structural restoration, as well as sedimentologic and petrographic analysis. The overall interpretation has several implications for connections between mid-Ediacaran phenomena globally. Given that the Johnnie valleys and Wonoka canyons are stratigraphically bracketed by negative δ13C values putatively correlated with the Shuram excursion, my data suggest that the Shuram excursion may encompass rather than postdate the Gaskiers glaciation in eastern California and South Australia, and that the onset of the excursion may be diachronous at a global scale.
My interpretation presents the first outcrop evidence for subaerial erosion and non-deposition as a mechanism capable of triggering appreciable salt breakout. The suggested occurrence of regional isolation and rapid environmental change closely precedes the emergence of the Ediacara fauna, and presents new context for the organisms and the sediments in which they are recorded.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/4x51-ka85 |
| Date | January 2024 |
| Creators | Giles, Sarah |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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