Cap carbonates are continuous layers of limestone (CaCO₃) and/or dolostone (Ca_0.55Mg_0.45CO₃) that sharply overlie Neoproterozoic glacial deposits, or sub-glacial erosion surfaces where the glacial deposits are absent. They are typically 3-30 m thick and occur on platforms, shelves and slopes world-wide, even in regions otherwise lacking carbonate strata. Sturtian (~700 Ma) and Marinoan (635 Ma) cap carbonates are lithologically distinct, and both have unusual traits (e.g., thick sea-floor cements, giant wave ripples, microbial mounds with vertical tubular structure, primary and early diagenetic barite, BaSO₄) that distinguish them from standard carbonates. Marinoan cap carbonates are transgressive (i.e., inferred water depths increase with stratigraphic height) and most workers associate them with the flooding of continental shelves and platforms as ice sheets melted. Most Sturtian cap carbonates were not deposited until after post-glacial flooding had taken place. The preservation of cap carbonates and related highstand deposits after isostatic adjustment (or post-glacial “rebound”) implies substantial erosion and/or tectonic subsidence during the glacial period. Post-Marinoan (Ediacaran and Phanerozoic) glaciations lack cap carbonates, or they are poorly developed.

Characteristically cap carbonates are moderately depleted in ¹³C (i.e., δ¹³C<0‰ V-PDB) and this has figured prominently in discussions concerning their origin, which revolve around the source(s) of alkalinity needed to drive carbonate sediment production. An important constraint is the time-scale of post-glacial flooding, generally assumed to be that of ice-sheet meltdown. Quaternary meltdowns lasted 10,000 years at the outside, and climate modeling suggests that a snowball earth meltdown under strong radiative forcing could occur in as little as 2,000 years. In contrast, the occurrence of magnetic polarity reversals in Marinoan cap dolostones implies a minimum time scale 100 times longer. The postulated sources of alkalinity include (1) carbonate and silicate weathering during and after glaciation (Fairchild, 1993; Higgins & Schrag, 2003), (2) breakdown of methane-rich permafrost and submarine gas hydrate (Kennedy et al., 2001b), and (3) upwelling of alkalinity-charged ocean deepwaters (Grotzinger & Knoll, 1995; Ridgwell et al., 2003). Only the first of these would be appropriate if the longer time scale is correct, but the unique sedimentological characteristics of cap dolostones are more consistent with sustained high sedimentation rates and therefore a shorter time scale. If cap carbonates were deposited on the shorter time scale, they should reflect a stable density stratification maintained by meltwater injection and surface warming. If on the longer time scale, they should represent the end-product of atmospheric CO₂ drawdown through silicate weathering. Cap carbonates remain a “hot” topic of research with 2005, highlighted by reports of ppb-level iridium spikes (interpreted as millions of years worth of cosmogenic iridium trapped in ice) at their bases in central Africa (Bodiselitsch et al., 2005) and large changes in boron isotope ratios in Namibia (Kasemann et al., 2005) denoting increased acidification of the ocean over the glacial period due to CO₂ buildup.

Historically, the characteristic sharp, smooth, flat contact between cap carbonates and subjacent glacial deposits, with no reworking, was fully recognized and discussed by Norin (1937) in his magnificent report on the Quruqtagh area, eastern Tien Shan, Xinjiang Province, China. He concluded that the contact must be disconformable (i.e., separated by a “missing” time interval of lithification and erosion). However, Australian workers in the 1960’s found the same relationship all across Australia (e.g., Dunn et al., 1971), making a causal connection more likely and Norin’s hiatal explanation less attractive. The description “flaggy pink dolostone marker capping glacial tilloid” is commonly encountered in the Australian literature of the time, and the earliest paper expressly devoted to their origin (also the first to document their carbon isotopic signature) is Williams (1979). However, the first reference to “cap” dolostone to my knowledge is in Plumb (1981). Williams (1979) and Plumb (1981) both concern the Kimberley Ranges of Western Australia, and a well-exposed contact in that area is shown in the accompanying photograph.

Recommended reading (chronological order)
Fairchild, I.J., 1993. Balmy shores and ice wastes: the paradox of carbonates associated with glacial deposits in Neoproterozoic times. Sedimentology Review1, 1-16.

Grotzinger, J.P. and Knoll, A.H., 1995. Anomalous carbonate precipitates: Is the Precambrian the key to the Permian? Palaios 10, 578-596.

Kennedy, M.J., 1996. Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial cap dolostones: deglaciation, δ¹³C excursions, and carbonate precipitation. Journal of Sedimentary Research 66, 1050-1064.

Hoffman, P.F., Kaufman, A.J., Halverson, G.P. & Schrag, D.P., 1998. A Neoproterozoic snowball Earth. Science 281, 1342-46.

Kennedy, M.J., Runnegar, B., Prave, A.R., Hoffmann, K.-H. &Arthur, M.A., 1998. Two or four Neoproterozoic glaciations? Geology 26, 1059-1063.

James, N.P., Narbonne, G.M. & Kyser, T.K., 2001. Late Neoproterozoic cap carbonates: Mackenzie Mountains, northwestern Canada: precipitation and global glacial meltdown. Canadian Journal of Earth Science 38, 1229-1262.

Kennedy, M.J., Christie-Blick, N. & Sohl, L.E., 2001. Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals? Geology 29, 443-446.

Hoffman, P.F. & Schrag, D.P., 2002. The snowball Earth hypothesis: testing the limits of global change. Terra Nova 14, 129-155.

Sumner, D.Y., 2002. Decimetre-thick encrustations of calcite and aragonite on the sea-floor and implications for Neoarchaean and Neoproterozoic ocean chemistry. Special Publications of the International Association of Sedimentologists 33, 107-120.

Higgins, J.A. & Schrag, D.P., 2003. Aftermath of a snowball Earth. Geophysics, Geochemistry, Geosystems 4, 10.1029/2002GC000403.

Ridgwell, A.J., Kennedy, M.J. Caldeira, K., 2003. Carbonate deposition, climate stability, and Neoproterozoic ice ages. Science 302, 859-862.

Trindade, R.I.F., Font, E., D’Agrella-Filho, Nogueira, A.C.R. & Riccomini, C., 2003. Low-latitude and multiple geomagnetic reversals in the Neoproterozoic Puga cap carbonate, Amazon craton. Terra Nova 15, 441-446, doi: 10.1046/j.1365-3121.2003.00510.x.

Allen, P.A. and Hoffman, P.F., 2005. Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature 433, 123-127.

Bodiselitsch, B., Koeberl, C., Master, S., and Reimold, W.U., 2005. Estimating duration and intensity of Neoproterozoic snowball glaciations from Ir anomalies. Science 308. 239-242.

Corsetti, F.A. and Grotzinger, J.P., 2005. Origin and significance of tube structures in Neoproterozoic post-glacial cap carbonates: example from Noonday Dolomite, Death Valley, United States. Palaios 20, 348-363.

Kasemann, S.A., Hawkesworth, C.J., Prave, A.R., Fallick, A.E., and Pearson, P.N., 2005. Boron and calcium isotope composition in Neoproterozoic carbonate rocks from Namibia: evidence for extreme environmental change. Earth and Planetary Science Letters 231, 73-86.

Shields, G.A., 2005. Neoproterozoic cap carbonates: a critical appraisal of existing models and the plumeworld hypothesis. Terra Nova 17, 299-310.