Localities
Described below are the geologic formations from which our samples originate. The formations are grouped based off of the Neoproterozoic glacial episode that they formed after. These glacial episodes from oldest to youngest these are Pre-Sturtian, Sturtian, and Marinoan.
Tonian (1000-720 Ma)
The following Tonian Samples are not necessarily cap carbonates themselves but rather, are indicative of pre-Cryogenian changes in ocean chemistry. Whether or not these carbonates are true cap carbonates remains a topic debate.
Beck Spring Dolomite (Death Valley, USA)
The Beck Spring Dolomite Member of the Pahrump Group was deposited between 770-740 Ma (Macdonald et al. 2013) on the rifted western margin of Laurentia and stratigraphically rests between the older Crystal Spring and younger Kingston peak members. This cap carbonate is characterized by cherty grey dolostone, occasionally brecciated with roll-up structures, oncoids, pisolites, ooids, and very common stromatolite structures throughout (Marian and Osborne, 1992; Corsetti and Kaufman, 2003). No underlying diamictite occurs; however, the eroded surface of the upper Crystal Spring Member suggests rapid erosion prior to the deposition of the Beck Spring Dolomite(Mbuyi and Prave, 1993), plausibly caused by glacial meltwater floods. Substantial evidence for a glaciogenic origin is mostly δ13C excursions that trend from -2 ‰ to 5‰ from basal to mid-section and then down to -4% in the uppermost section (Corsetti and Kaufman, 2003). A negative to positive δ13C excursion, as well as the Beck Spring Dolomite’s stratigraphic position and color is broadly consistent with other cap carbonates deposited following the Sturtian Neoproterozoic glacial episode (Corsetti and Lorentz, 2006).
Assem Limestone, Tambien Group, Northern Ethiopia
The Assem Limestone lies within the Tambien Group, a 5-km thick, “mixed carbonate-siliciclastic succession” (Swanson-Hysell et al., 2015). The limestone overlies the Werii Slate, the oldest of the Tambien Group. This carbonate unit lies in the Mai Kenetal synclinorium in northern Ethiopia which maintains overlying contact with the Tsaliet Group. The regional stratigraphy is unresolved between Mai Kenetal and synclinoria out farther east where diamictite at the top of the Tambien Group has been preserved. The base layers of the Tambien group are slates that lie above Tsaliet volcanic arc deposits. The timing and number of Cryogenian glaciations was previously unresolved between 2 and 4 panglacial events. The oldest is typically referred to as the Kaigas Glaciation with a relatively poorly constrained age range of ~775-755 Ma within the 1000-717 Ma Tonian Period. Due to this along with the sudden contact of the Assem Limestone above the lowest slates in the Tambien group, coupled with the negative d13 C data indicated that the limestone could potentially be a cap carbonate.
According to work conducted by Beyth et al., (2003), the Negash synclinorium is determined by diamictite with extrabasinal clasts interpreted to correlate with diamictites of the Sturtian glaciation constrained to 717–662 Ma. Additionally, the Assem formation contains “abundant stromatolite and microbialaminite lithofacies interbedded with lenses of grainstone and intraclast breccia” in the westernmost exposure while to the east, in the Tsedia and Chehmit synclinoria, the Assem Formation is dominated by wavy to parallel-laminated limestone micrite (Swanson-Hysell et al., 2015) Given the depositional context coupled with chemical properties and textural qualities indicate the rock is a transgressive carbonate sequence that contain negative d13Ccarb intervals that correspond to a recovery from “an earlier pre-Sturtian cooling event, perhaps related to the Kaigas glacial interval”. However, subsequent global studies have not supported the existence of a “Kaigas” panglacial event. The negative d13C excursion has been correlated with the Bitter Springs negative CIE which is constrained to 825.29±0.32 and 778.72±0.24 Ma for dates within the underlying Werii Slate and overlying Tsedia Slate (Swanson-Hysell et al., 2015; Park et al., 2019). Accordingly, Assem Limestone deposition occurred ~80 myr before the Sturtian Glaciation. If such findings are correct, the Assem Limestone would be one of the oldest carbonates in the Arabian-Nubian Shield (Miller et al., 2009).
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).
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).