Evidence that Early Carboniferous ostracods colonised coastal flood plain brackish water environments (original) (raw)
2006, Palaeogeography, Palaeoclimatology, Palaeoecology
A study of the stable isotope composition (d 18 O, d 13 C) of biogenic (ostracod, mollusc) and authigenic carbonates in the Ballagan Formation, Lower Carboniferous of Scotland, coupled with evidence from sedimentology and associated fossil fauna and flora, supports the argument that this formation was deposited in a coastal flood plain setting, in brackish (0.5 b 30x NaCl) and hypersaline (N 40x NaCl) waters, but in the absence of persistent normal marine conditions. The oxygen isotope data from the Ballagan Formation divide into three clusters: a diagenetic field defined by low d 18 O (b À 11x VPDB); an intermediary field (d 18 O À11x to À 9x) composed of a mixture of known primary and secondary (diagenetic) carbonates; and samples within the range of À 9x to À 4x which, as far as we can ascertain, are largely unaltered. No samples give typical Early Carboniferous d 18 O marine values. Average marine carbonates from Europe have d 18 O between À 4x to À 3x. The Ballagan Formation carbonates were probably deposited in evaporated freshwater and/or brackish water. This conclusion is supported by the presence of evaporites (gypsum, anhydrite, halite pseudomorphs) and common desiccation-cracked mudstone surfaces throughout the Ballagan Formation, suggesting conditions of fluctuating salinity in ephemeral bodies of water. The stable isotope data support the notion that the ostracod assemblages of the Ballagan Formation were colonising brackish water and hypersaline ecologies on a coastal flood plain during the Early Carboniferous, a stage of development that may have encouraged their colonisation of fully non-marine (limnetic) environments during the later Carboniferous. The ostracods include cyther-0031-0182/$ -see front matter D (M. Williams).
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