The Tonian Beck Spring Dolomite: Marine dolomitization in a shallow, anoxic sea

The reason for the abundance of dolomite lithologies in Earth's early geological record compared to modern environments remains contentious. This study provides new insight into this Precambrian “dolomite problem” by revisiting one of the most controversial dolomite localities, the Beck Spring Dolomite, of Death Valley, USA. Consistent with some previous studies, petrographic evidence indicates that although the Beck Spring Dolomite now consists almost entirely of dolomite, it was originally precipitated largely as aragonite and high-Mg calcite. Depositional constituents (microbialites and ooids) were likely originally aragonitic, and early marine length-fast cements (now dolomite) are suggested to have precipitated as high-Mg calcite then replaced syntaxially by dolomite. Based on petrographic and geochemical evidence, we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit, and for the most part, involved syntaxial and mimetic replacement. Further, a length-slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement. This is indicated by the length-slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation. This new evidence for Tonian marine dolomite precipitation reinforces the idea of Precambrian marine environmental conditions, including the chemical composition of seawater, promoting dolomite formation at this time. The trace metal geochemical composition of well-preserved marine components, especially dolomite marine cements, reveals information about redox conditions in this Tonian shallow seawater. In terms of rare earth element geochemistry, the Beck Spring Dolomite has no significant Ce anomaly, and a ubiquitous positive Eu anomaly, consistent with widespread oceanic anoxia during deposition. Furthermore, the relatively low levels of iron and chalcophile elements Co, Cu, Pb and Zn in marine components compared to other Neoproterozoic carbonates suggest euxinic conditions (both anoxic and sulphidic) prevailed during deposition of the Beck Spring Dolomite.