Chapter 5 Chemical sediments associated with Neoproterozoic glaciation: iron formation, cap carbonate, barite and phosphorite

Abstract

Orthochemical sediments associated with Neoproterozoic glaciation have prominence beyond their volumetric proportions because of the insights they provide on the nature of glaciation and the records they hold of the environment in which they were precipitated. Synglacial Fe formations are mineralogically simple (haematite jaspilite), and their trace element spectra resemble modern seawater, with a weaker hydrothermal signature than Archaean-Palaeoproterozoic Fe formations. Lithofacies associations implicate subglacial meltwater plumes as the agents of Fe(II) oxidation, and temporal oscillations in the plume flux as the cause of alternating Fe-and Mn-oxide deposits. Most if not all Neoproterozoic examples belong to the older Cryogenian (Sturtian) glaciation. Older and younger Cryogenian (Marinoan) cap carbonates are distinct. Only the younger have well-developed transgressive cap dolostones, which were laid down during the rise in global mean sea level resulting from ice-sheet meltdown. Marinoan cap dolostones have a suite of unusual sedimentary structures, indicating abnormal palaeoenvironmental conditions during their deposition. Assuming the meltdown of ice-sheets was rapid, cap dolostones were deposited from surface waters dominated by buoyant glacial meltwater, within and beneath which microbial activity probably catalysed dolomite nucleation. Former aragonite seafloor cement (crystal fans) found in deeper water limestone above Marinoan cap dolostones indicates carbonate oversaturation at depth, implying extreme concentrations of dissolved inorganic carbon. Barite is associated with a number of Marinoan cap dolostones, either as digitate seafloor cement associated with Fe-dolomite at the top of the cap dolostone, or as early diagenetic void-filling cement associated with tepee or tepee-like breccias. Seafloor barite marks a redoxcline in the water column across which euxinic Ba-rich waters upwelled, causing simultaneous barite titration and Fe(III) reduction. Phosphatic stromatolites, shrub-like structures and coated grains are associated with a glacioisostatically induced exposure surface on a cap dolostone in the NE of the West African craton, but this appears to be a singular occurrence of phosphorite formed during a Neoproterozoic deglaciation.

Microbial activity records in Marinoan Snowball Earth postglacial transition layers connecting diamictite with cap carbonate (Otavi Group, NW-Namibia)

“…Pyrite and chlorite provided the nutrient source for the growth of filamentous rusty bacteria. The Gallionella-like microfossils occur in the erosion rim of chlorite and in degradation cavities, which provide the iron-bearing minerals as a source of nutrients in C2a (Narachaamspos sample), similar to what was proposed by Hoffman et al (1998bHoffman et al ( , 2011.…”

Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

“…In this scenario, the cap carbonate precipitated from supersaturated seawater with DIC mainly derived from atmospheric CO 2 , explaining the negative carbon isotope signals (Fig. 2) and widespread isopachous cements and aragonite fans in the cap carbonate (7,44,45).…”

Neoproterozoic iron formation: An evaluation of its temporal, environmental and tectonic significance

“…The iron is derived from glacial scouring and dissolution of bedrock under relatively low pH conditions (pH = 6.2), and stable isotopic data indicate a prominent role for dissimilatory iron reduction in the cycling of iron in this system. Mikucki et al (2009) argued that Blood Falls may be an analog for NIF, and Hoffman et al (2011) raised the possibility that like the waters flowing to Blood Falls, snowball seawater could have been anoxic and sulfate-rich. However, because Blood Falls is the only documented ferrous glacial outwash system, it seems unlikely that it can account for the voluminous IF found, for example, in the Rapitan Group.…”