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

Highlights

• Synsedimentary marine diagenesis in the Tonian Beck Spring Dolomite examined.

• Marine dolomitization and cementation is ubiquitous.

• Chemistry of marine cements indicates anoxic and euxinic conditions.

Abstract

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.

Introduction

The huge volume of dolomite in many Precambrian marine carbonate successions compared to Phanerozoic carbonates is difficult to explain, because a large source of magnesium-rich fluids is required for such extensive dolomitization. To add to this problem, many Precambrian dolomites have a mimetic character and are quite unlike the fabric-destructive dolomite so common in Phanerozoic successions (Tucker, 1982a; Wallace, 1990; Sibley, 1991; Hood et al., 2011; Hood and Wallace, 2018). An obvious answer to these problems is to invoke seawater—arguably the most voluminous fluid at the Earth's surface—as the dolomitizing agent. But in modern oceans, marine dolomite precipitation is uncommon and restricted to specific near-shore environments and island settings (Baker and Kastner, 1981). However, there is increasing evidence that Precambrian seawater was unlike modern seawater in its chemical composition and redox state, perhaps promoting more widespread dolomite precipitation (Tucker, 1982a; Burns et al., 2000; Hood et al., 2011; Hood and Wallace, 2012, Hood and Wallace, 2014, Hood and Wallace, 2015; Wood et al., 2017). Primary marine dolomite precipitation in the Precambrian was first proposed from a study on the early Neoproterozoic Beck Spring Dolomite of California, USA (Tucker, 1982a). Tucker controversially suggested that the Beck Spring Dolomite was entirely precipitated as primary dolomite (depositional constituents and marine cements) directly from seawater. Significant debate followed this proposal, with the general consensus emerging that Precambrian carbonates were not originally precipitated as dolomite (Ricketts, 1982; Zenger, 1982; Tucker, 1983; Zempolich et al., 1988; Marian and Osborne, 1992).

However, since this debate, new evidence has emerged suggesting that while Precambrian depositional constituents may not have been precipitated as dolomite, some Precambrian marine cements probably were originally composed of dolomite (Hood et al., 2011; Hood and Wallace, 2012; Harwood and Sumner, 2012; Wood et al., 2017). It is therefore timely to re-examine the classic Beck Spring Dolomite succession on which this original marine dolomite controversy was centred with new sedimentological and geochemical techniques.

This study uses petrology and geochemistry (major and trace elements) to constrain the early marine diagenetic history of the Beck Spring Dolomite, and to infer marine conditions during the Tonian. The Beck Spring Dolomite is then compared to the Cryogenian Balcanoona Formation in South Australia, a carbonate platform with a similar marine diagenetic history, consisting mostly of dolomite which precipitated from anoxic, ferruginous seawater (Hood and Wallace, 2015). In both platforms, only minor limestone is preserved; in allochthonous blocks in basinal debris flows of the Balcanoona Formation, and rarely within the Beck Spring Dolomite (Saratoga Springs) (Zempolich, 1989; Hood and Wallace, 2012, Hood and Wallace, 2014). Although the Balcanoona Formation is almost entirely dolomite, depositional components originally precipitated as aragonite and high-Mg calcite, and abundant fibrous marine cements in the Balcanoona Formation are primary marine dolomite precipitates (Hood et al., 2011; Hood and Wallace, 2012). We suggest that while the depositional components of the Beck Spring Dolomite were likely precipitated as aragonite, some marine cements within the unit were precipitated as marine dolomite phases, similar to the Balcanoona Formation. We then use the chemical composition of these dolomite components to discuss the redox state of shallow Tonian seawater.