Tossing Neoproterozoic snowballs between Death Valley, USA, and Namibia; a comparison of unique post-glacial facies and carbon isotope trends

TitleTossing Neoproterozoic snowballs between Death Valley, USA, and Namibia; a comparison of unique post-glacial facies and carbon isotope trends
Publication TypeConference Proceedings
Year of Conference1999
AuthorsCorsetti FA, Kaufman AJ
Conference NameGeological Society of America, 1999 annual meeting
PublisherGeological Society of America (GSA) Boulder CO United States.
Conference LocationDenver, CO USA
ISBN Number(print) 0016-7592
Accession Number2001-053981
Keywords12 Stratigraphy, Africa, Basin and Range Province, Beck Spring dolomite, C-13/C-12, California, carbon, carbonate rocks, chemostratigraphy, Chuos Formation, clastic rocks, Death Valley, diamictite, Ghaub Diamictite, glacial environment, glaciation, grainstone, Great Basin, isotope ratios, isotopes, lithofacies, Maieberg Formation, Namibia, Neoproterozoic, Noonday Dolomite, North America, Otavi Group, paleoclimatology, Precambrian, Proterozoic, Rasthof Formation, sedimentary rocks, shallow-water environment, Southern Africa, stable isotopes, United States, upper Precambrian

Determining the uniqueness of post-glacial sedimentary facies and carbon isotope trends bracketing glacial diamictites in the Otavi Group (northwestern Namibia) is a critical test of the Neoproterozoic snowball Earth hypothesis. The unusual cap carbonates atop the Chuos and Ghaub diamictites in the Otavi Group, the Rasthof and Maieberg formations, respectively, find exact facies matches in the Great Basin. Strikingly similar to the Rasthof Formation, the base of the Beck Spring Dolomite in Death Valley contains unique microbially-laminated and organic-rich structures, although no diamictites have yet been reported beneath the Beck Spring. Mimicking the marked negative-to-positive carbon isotope trend in the Rasthof, preliminary data from 10 meters above the base of the Beck Spring reveal a rapid transition from values near 0 to +6 per mil. In Namibia, the shift from strongly positive-to-negative carbon isotope compositions in the shallow water carbonates of the Ombaatjie Formation (immediately beneath the Ghaub diamictite) is one of the key geochemical trends supporting the snowball Earth hypothesis. At the top of the Beck Spring Dolomite (in shallow water facies containing grainstones and microbial laminites), carbon isotope values as enriched as +6 per mil decline monotonically to as low as -4 per mil beneath Kingston Peak glacial deposits in two investigated localities. Further comparison indicates that the carbonate facies of the post-Ghaub Maieberg Formation in Namibia contains distinctive "tubes" and unique cement fabrics that are indistinguishable from similar structures in the Noonday Dolomite overlying the Kingston Peak in Death Valley. These comparisons reveal strong similarities in post-glacial facies, as well as carbon isotopic trends bracketing Neoproterozoic glacial diamictites, although their equivalence should not be assumed. However, if the new carbon isotopic data from the lower Beck Spring Dolomite is confirmed to be a post-glacial phenomenon, Neoproterozoic strata of the Great Basin may record as many as four discrete glacial intervals.