Abstract
Sturtian cap carbonate deposits, previously unknown in South China, are documented in this study from the base of the Datangpo Formation in the Nanhua Basin (Guizhou Province). The age of these deposits is constrained by an LA-ICP-MS U-Pb zircon age of 662.7±6.2Ma from the overlying Mn-shale layer, which is approximately coeval with the termination of the Sturtian glaciation globally. The newly discovered cap carbonates are composed of massive, laminated, pisolitic, and sandy dolomites, and formed subtidally on the summits of horsts within the Nanhua Basin. Two δ13Ccarb profiles (from the JJS-1 and ZK01 sections) show positive shifts of 4–5‰ from the base to the top of the cap carbonates with a ∼2‰ offset between the profiles. These features suggest both a large global perturbation as well as local influences on carbonate carbon isotopes during the Sturtian deglaciation, which is consistent with aspects of both the Snowball Earth and gas hydrate destabilization hypotheses for the origin of cap carbonates.The newly discovered cap carbonates are inferred to be correlative with rhodochrosite (Mn-carbonate)-bearing deposits of the basal Datangpo Formation in adjacent grabens of the Nanhua Basin. This correlation is based on (1) the equivalent stratigraphic positions of the cap carbonates and Mn-deposits, both of which are found at the base of the Datangpo Formation, and (2) consistent U-Pb dates of 667–663Ma for tuff layers interbedded with these deposits at two locales. The Mn-deposits show significantly higher TOC content (1–3% vs <0.7%) and more negative δ13Ccarb values (−10.5 to −5.5‰ vs −4.0 to +2.5‰) than the cap carbonates. The upsection trend toward higher δ13Ccarb in the cap carbonates may reflect increases in marine productivity and the burial rate of organic carbon following the Sturtian glaciation. Despite coeval formation, the dolomitic cap carbonates deposited on horsts and the Mn-carbonate sediments deposited in grabens are heterogeneous facies with different formation mechanisms.
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