Barite-bearing cap dolostones of the Taoudeni Basin, northwest Africa: Sedimentary and isotopic evidence for methane seepage after a Neoproterozoic glaciation

The Taoudeni Basin of the West African craton contains one of the few genuine terrestrial records of a Neoproterozoic ice age. In the Adrar region of Mauritania, an extensive permafrost landscape, lithified moraines (tillites) and striated pavements are draped by a thin, generally < 5-m thick dolostone, which is lithologically and isotopically similar to other basal Ediacaran cap dolostones worldwide. In Adrar, the cap carbonate unit exhibits a complex depositional history with significant lateral facies variation related to the irregular post-glacial, topographic relief and the complex interplay between glacioeustasy and isostatic rebound. The Adrar cap carbonate package consists of one or two dolostone units, with an intervening siliciclastic package of up to 40 m thickness, and a laterally extensive, thin limestone bed that disconformably overlies the uppermost dolostone. The cap dolostone comprises mechanically laminated beds that are disrupted by fitted-brecciation, sheet cracking, tepee formation, karstic dissolution and chaotic vein networks of silica calcite and barite. The overlying thin bed of limestone breccia comprises volcaniclastic and detrital debris and authigenic barite crystals, cemented and commonly replaced by marine calcite. The close association between relative sea-level changes, glacier retreat and cap dolostone deposition across the Taoudeni Basin implies that cap dolostones formed largely over a period of similar to 10(4) years, the maximum interval over which isostatic rebound is likely to operate. Barite has been reported from identical stratigraphic levels overlying terrestrial glacial deposits throughout NW Africa and formed locally during fluid mixing on, and in cavities beneath the seafloor during late stages of the post-glacial marine transgression. Samples of barite were collected from two distant localities in the Taoudeni Basin, in Mauritania and Mali. (87)Sr/(86)Sr ratios exhibit an unusually narrow range for barite that closely matches contemporaneous seawater (87)Sr/(86)Sr (0.7077-8). Barite delta(34)S values range widely between 20 parts per thousand and 45 parts per thousand CDT, which indicates that sulphate derived from seawater and was subsequently modified by microbially mediated sulphate reduction. The consistent stratigraphic level and irregular distribution of barite deposits are consistent with a sedimentary exhalative origin for the barite, whereby Ba-rich fluids from shallow locations within the rock pile interacted with sulphate-bearing seawater; a hydrothermal origin for these fluids can be excluded. Isotopic constraints and the association of barite with terrestrial glacial deposits across the West African craton suggest that methane seepage from underlying permafrost may be one possible mechanism for Ba sequestration. The occurrence of seafloor barite precipitates at the contact between cap dolostones and overlying post-glacial limestones worldwide implies that changes in ocean composition, in particular increases in the sulphate content of ambient seawater provided an overriding control on barite mineralisation. (c) 2006 Elsevier B.V. All rights reserved.