A subaerial origin for the mid-Ediacaran Johnnie valleys, California and Nevada: Implications for a diachronous onset of the Shuram excursion

Abstract

Mid-Ediacaran incised valleys in the Johnnie Formation of eastern California are of interest for their broad time concordance with comparable features elsewhere (e.g., the Wonoka canyons of South Australia), and the largest negative carbon isotope anomaly in Earth history (the Shuram excursion). The Johnnie valleys have been previously interpreted as originating through mass wasting in a submarine setting at a continental margin, and perhaps as a result of syndepositional tectonism. New data suggest an alternative scenario: that the valleys were cut by a combination of fluvial incision and subaerial mass wasting concomitant with sea-level lowering in excess of 200 m, and were subsequently drowned and filled by marine sediments representing paleowater depths as great as tens of meters. Critical evidence from a series of outcrops in the southern Nopah Range includes (1) a new estimate of the erosional relief of the valleys of at least 208 m; (2) the presence in the basal fill 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; and (3) beds and blocks of giant ooid packstone-grainstone indicative of shallow marine sedimentation during the early stages of valley filling. The conglomerate/breccia is channelized and associated locally with cross-stratification. Our interpretation is based upon a total of forty-five measured sections, physical stratigraphic mapping supported by sub-meter scale resolution 3-D drone imagery, as well as carbon isotope analyses. Comparable valleys, arguably of the same age, and likewise attributed to subaerial incision in earlier studies, are present at or near the top of the Caddy Canyon Quartzite of northern Utah and southeastern Idaho (a straightline distance of ∼ 850 km). Our working hypothesis is that both sets of valleys correspond in time with Gaskiers glaciation (∼580 Ma). Given that negative δ13C values have been obtained from both below and (new data) within the basal valley fill, and correlated by most workers with the Shuram excursion, we infer that the Shuram encompasses rather than postdates the Gaskiers event in eastern California, and that the onset of the excursion may be diachronous at a global scale.