Post Marinoan (645-635 Ma)
Noonday Dolomite (Death Valley, USA)
The Noonday Dolomite has long been considered a classic example of a post-glacial cap carbonate due to similarities with other glaciogenic cap carbonates worldwide. The most convincing evidence of the Noonday Dolomite’s glaciogenic petrogenesis is the preponderance of “Tube forming” stromatolite mounds whose matrices are dominated by carbonate minerals, rather than non-carbonate detrital sediment (Corsetti and Grotzinger, 2005). These unique stromatolite mounds occur in the pink dolomites of both the basal Sentinel Member and overlying Mahogany Flats Member, which are separated by the predominantly detrital clastic Radcliff Member (Petterson et al., 2011). Pervasive negative δ13C values down to -6‰ occur at all stratigraphic levels in most sections (Mahogany Flats has positive δ13C in some sections) (Petterson et al., 2011). In addition to low organic carbon concentrations, the occurrence of “sheet cracks” and the uncomformable relationship with the underlying KP4 and KP3 diamictite units of the Kingston Peak Formation, the “tube structures” and δ13C values of the Noonday Dolomite plausibly correlate with other cap carbonates from the hypothesized Marinoan Neoproterozoic glacial episode (Corsetti and Lorentz, 2006). This glaciogenic association is not without controversy. Creveling et al. (2016) suggest that the occurrence of the tube forming stromatolites separated temporally by subaerial facies (mostly in the Radcliff Member) is inconsistent with rapid deposition following glacial melting. Instead, Creveling et al. (2016) prefers a depositional model where regional extension and glacial rebound operate in conjunction to create a transgression-regression-transgression cycle following glacial retreat. Critically, no rapid deposition of the Noonday Dolomite would be required following this model, thus challenging the importance of deglaciation in the origin of the unique sedimentary structures observed in the Noonday Dolomite.
Mirassol D’Oeste Formation (Brazil)
Mirassol D’Oeste formation is composed of 45 m thick Marinoan cap dolostones deposited above the Puga Formation diamictites in the Amazon Craton (Nogueira et al, 2003).The underlying Puga formation consists of diamictites, and the overlying Guia formation is composed of a lime mudstone with thin shale laminations, according to Nogueira et al (2003). This geologic sequence is consistent with other typical glacial deposits, where the diamictites and tillities are overlain by cap dolostones, and limestones (Hoffman and Schrag, 2002). The Puga cap depleted δ13C values, along with features of soft-sediment deformation at the base of the cap overlying glacial deposits, show similarities with other Neoproterozoic cap carbonates.
Mirassol d’Oeste age can be correlated to the Maieberg and Elantoek Formations in Namibia, as a post-Marinoan deposit based on their similar δ13C isotope composition (Nogueria et al. 2007). Based on negative δ13C compositions, Font et al. (2006) suggest these dolostones formed as a result of sulfate-reducing microbial activity in dyoxic to anoxic environments. Moreover, tube-like structures reported in this study further suggest microbial activity over methane escape mechanism given that the isotopic composition of such structures indicate very similar composition to the dolomitic matrix (Font et al, 2006).
Keilberg Member of the Maieberg Formation, Namibia
The Otavi Group contains evidence of the younger (~645-635 Ma) Marinoan glacial episode (Prave et al., 2016). The Ghaub formation grades upward from laminated quartz siltstone, with authigenic pyrite, into bedded lamination with frequent glacial debris until it becomes a massive carbonate diamictite (Johnson et al., 2017). Pyrite occurrence suggests anoxic depositional conditions (Gyollai et al., 2017). Lack of evidence for wave action supports that the Ghaub formation glacial deposition occurred in deep water (Johnson et al., 2017) as glaciers advanced and retreated (Prave et al., 2016). Overlying Ghaub diamictites is the Maieberg Formation cap carbonate succession consisting of limestones and dolomites at the base, and overlying dark colored dolomite shale (Gyollai et al., 2017). The basal member of the Maieberg Formation (Keilberg member) marks deglacial sedimentation associated with sea level rise following the Marinoan glaciation (Gyollai et al., 2017).
The Keilberg Member consists of pale-grey to pale pink dolostone. The pale color of the formation suggests exceptionally low organic content. The Keilberg member is conformably superimposed by deep-water marly limestone rhythmites of the lower Maieberg Formation. Features in the Keilberg member include, sorted peloid, low angle cross-bedding, tubestone stromatolite and giant wave ripples. These structures indicate shallow water sedimentary deposition. (Hoffman, 2011).