Post-Sturtian (717-659 Ma)
Otavi Group , Northern Namibia, Congo Craton: Rasthof Formation (Sturtian)
The Snowball Earth Hypothesis is supported by evidence of two severe glaciation periods with glacial ice sheets extending to the equator during the Neoproterozoic era (Stern and Miller, 2019), known as the Sturtian and Marinoan glaciations. The Sturtian glacial epoch has been dated approximately from 716 Ma to 659 Ma (Bosak et al., 2012) (Stern and Miller, 2019), while the Marinoan glacial epoch has been dated approximately from 645Ma to 635 Ma (Prave et al., 2016; Stern and Miller, 2019). Geologic evidence for these glaciations consists of changing seawater d13C composition, glacial deposits (dropstones and diamictites) on different paleocontinents, the Great Unconformity, recurrence of banded iron formation, and most importantly for this project post-glacial cap carbonates (Stern and Miller, 2019). There is no known cause for these Cryogenian glacial episodes, but hypotheses including extraterrestrial, geodynamic, oceanographic, biotic, and tectonic controls (Stern and Miller, 2019).
In the Otavi Group of northern Namibia, the carbonate Rasthof formation caps the glacial Chuos formation (Bosak et al., 2012). The ~720-660 Ma Chuos formation consists of massive diamictites, sandstones, siltstones, and shales, and is considered to be a Sturtian glacial tillite (Lechte et al., 2019). The overlying ~658-646 Ma Rasthof formation cap carbonate was deposited under low-energy conditions that may have included intermittent or stagnant anoxia close to the water sediment interface (Bosak et al., 2012). Surprisingly, the Rasthof formation contains small tubular microfossils (possible early foraminifera), demonstrating that organisms were able to survive the Sturtian glaciation and contributed to Neoproterozoic carbon cycling before the rise of complex animals (Bosak et al., 2012). These fossils are evidence of eukaryotic diversification, and evolutionary innovations that drove the evolution of complex life (Bosak et al., 2012).
Anoxic ocean conditions most likely caused by the ice cover which would be a barrier to biological survival (Johnson et al., 2017). During the Sturtian glaciation there is some evidence that suggests that foraminifera was able to thrive even with intermittent or permanent anoxia (Bosak et al., 2012). Conversely, during the Marinoan glaciation (see Keilberg Member), evidence suggests regions of open marine water and active biologic productivity (Johnson et al., 2017).
Scout Mnt Member, Pocatello Formation (Idaho, USA)
The Pocatello Formation comprises an area of southeastern Idaho expanding eastward from the American Falls Reservoir. The formation is made up of sedimentary marine Precambrian and Cambrian rocks. Ludlam (1942) defined the Pocatello Formation as including all the sedimentary rocks bookended by the Bannock Volcanic Formation from below and the Blackrock Canyon Limestone above. Later investigations deemed the Bannock Volcanic Formation to be a local thick lens in the lower part of the Pocatello Formation. The formation consists of four distinct members. The first is an unnamed lower member, the second as a “tillite series” (later to be known as the Scout Mnt Member), the third is the Bannock Volcanic Member, and an upper member, known as the “varved slate series” by Ludlum. The thickness of this formation is not well known but estimates put it at no less than 2,400 m.
The Scout Mountain Member is characterized by having large layers of non-stratified, unsorted metadiamictite that was likely marine tillite (Trimble, 1976). The member also contains interstratified quartzite and conglomerate with small amounts of argillite and siltite. The formation also has beds of limestone and dolomite and has an estimated thickness of about 1500 meters or more. Because the metadiamictite shares many features with the Mineral Fork Tillite in Wasatch, Utah, such as a similar mineralogy, lack of sorting and stratification, and the presence of large quartzite boulders, (Crittenden et al., 1952) researchers have come to the conclusion that metadiamictite is a submarine tillite equivalent to the Utah Tillite but was deposited from shelf ice into a marine environment (Frakes and Crowell, 1967).
Two episodes of Neoproterozoic glaciation have been reported on the Pocatello Formation. The first is found within the Scout Mountain Member in the form of diamictites with glacially striated clasts (Link, 1983). Data contrived from SHRIMP U-Pb zircon ages (younger than 667 ± 5 Ma) indicate that the SMM was deposited during the late phase of the Sturtian glaciation (Fanning and Link, 2003). The second proposed episode of glaciation can be found in incised valleys of the Caddy Canyon Quartzite ~2000 m above the Pocatello Formation (Lorentz, 2004) where the incision and infill were interpreted to be evidence of a Marinoan glaciation. This led academics to postulate two Neoproterozoic glaciation events correlating with Sturtian and Marinoan intervals respectively. However, if a more rigid criteria are applied, it becomes clear that there is only the Sturtian glaciation is associated with cap carbonates. The second proposed episode, which is a carbonate and marble unit, appears to be a cap-like carbonate that was deposited independent of post-glacial processes, likely the result of oceanic overturn and is associated with the Marinoan glaciation (Lorentz, 2004).
In Lorentz’s 2004 article, carbon isotopes collected from three distinct stratigraphic sections (including the cap-like Marinoan Caddy Canyon Quartzite) in the SMM. This cap-like carbonate had δ13C values ranging from -2.9 to -6.9% showing a sinusoidal trend. However, the proceeding two sections analyzed had more unwavering δ13C values of -4.3 to -5.3% and -4.3 to -6% respectively, verifying earlier findings by Smith et al. (1994). Both of these sections of the SMM corroborate Neoproterozoic glaciation events. The first stratigraphic section, the carbonate and marble cap-like bed, also lends some credence to this, however, it does not lie directly over a known glacial unit and correlations with the nearby Edwardsburg Formation show that there may be a discontinuity of almost 20 million years between the glaciation and it’s deposition thus precluding the top most carbonate from being a true cap carbonate.
Tindelpina Member of the Tapley Hill Formation (Australia)
Tindelpina Shale member is a ~15 m thick basal cap carbonate found at the base of Tapley Hill Formation, Australia, and above glaciogenic rocks of the Sturtian Merinjina Tillite (Giddings, 2009). Deglaciation resulted in deposition of a shale blanket, which constitutes Tindelpina Member (Heron, 2011). The Tindelpina Shale member is composed mainly of laminated shale beds interbedded with carbonaceous, dolomitic and pyritic sediments, and the basal cap carbonate is comprised of interbedded shales and dolostones and limestones (Giddings, 2009). Recent studies on U/Pb zircon age by Cox et al. 2018 further indicate the termination of the Sturtian glaciation at 663.03 Ma, with an estimated age of 643 ± 2.4 Ma for the Tindelpina Shale member itself (Kendall et al., 2006, Giddings, 2009).
δ13C gradient heavying upwards obtained from carbon isotope studies indicate that the Tindelpina Shale member was deposited in a heavily stratified ocean during deglaciation. Such a trend indicates that shallower facies (peloidal dolomite)had an average δ13C difference of ~3.6% compared to the deeper laminated limestone and calcareous shale (Giddings, 2009). Post-Sturtian deposition of Tindelpina Member contrasts with Marinoan cap carbonates given that a decreasing δ13C trend indicates a combination of stratification and mixing oceanic conditions after deglaciation. Such indications of stratification present in oceans during the Neoproterozoic era further support the theory that CO2 buildup in the deep ocean due to organic matter decomposition. Additional evidence for highly anoxic environments in the deep ocean comes from the production of pyrite (FeS2) via biological activity wherein bacterial sulfate production released H2S and HCO3–. HCO3– accumulated as portions of H2S were removed by metal sulfide precipitation to be buried in sediment as opposed to being neutralized by atmospheric interactions in the upper ocean(Giddings, 2009).