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

By
Paul F. Hoffman
Paul F. Hoffman
Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA, 02138, USASchool of Earth and Ocean Sciences, University of Victoria, Box 1700, Victoria, BC V6W 2Y2, Canada
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Francis A. Macdonald
Francis A. Macdonald
Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA, 02138, USA
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Galen P. Halverson
Galen P. Halverson
School of Earth and Environmental Sciences, The University of Adelaide, North Terrace, Adelaide, SA 5005, AustraliaPresent address: Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montréal, PQ H3A 2K6, Canada
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Published:
January 01, 2011

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.

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Contents

Geological Society, London, Memoirs

The Geological Record of Neoproterozoic Glaciations

Emmanuelle Arnaud
Emmanuelle Arnaud
University of Guelph, Canada
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Galen P. Halverson
Galen P. Halverson
McGill University, Canada
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Graham Shields-Zhou
Graham Shields-Zhou
University College London, UK
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Geological Society of London
Volume
36
ISBN electronic:
9781862394117
Publication date:
January 01, 2011

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