Palaeoenvironmental controls on the uranium distribution in an Upper Carboniferous black shale (Gastrioceras listeri Marine Band) and associated strata; England
Introduction
It has long been recognised that certain Westphalian marine mudrock horizons are enriched in uranium (e.g. Edwards, 1951, Ponsford, 1995, Knowles, 1964, Spears, 1964, Bloxam and Thomas, 1969). Indeed, the recognition of marine bands by the presence of a peak on gamma-ray spectra often allows exceedingly good correlation to be made of sub-surface successions (e.g. Whittaker et al., 1985, Leeder et al., 1990). Research interest has focused on the controls of uranium content in marine bands, and the identification of which minerals/phases concentrate this element.
Bloxam and Thomas (1969) conducted careful palaeontological and geochemical analysis, which showed that uranium is concentrated in the most organic-rich parts of marine bands that were proposed to have been deposited in a relatively deep water environment (Bloxam and Thomas, 1969). It is now also known that, in addition to recording the peak salinity conditions within cycles that include freshwater deposition, marine bands record a range of oxygenation conditions. The rapid vertical variations of fossil content associated with marine bands are therefore thought to be the product of changing salinity and oxygenation regimes Calver, 1968a, Wignall, 1987. It has often been speculated that uranium enrichment of the marine bands reflects both the slow rate of sediment deposition and the presence of reducing conditions close to the sediment–water interface (e.g. Knowles, 1964, Spears, 1964, Bloxam and Thomas, 1969).
Recent advances have been made in palaeontological and geochemical indicators of ancient bottom water oxygenation levels, which potentially provide more detailed information on the controls of uranium incorporation into black shales. For example, the oxygen-restricted biofacies (ORB) scheme utilises a species diversity gradient as an indication of the oxygen content of bottom waters (Wignall and Hallam, 1991). Indicators such as degree of pyritisation (DOP) (Raiswell et al., 1988), C/S ratios (Berner and Raiswell, 1984), and framboidal pyrite size distribution Wilkin et al., 1996, Wignall and Newton, 1998 provide measurements that can be used to assess the oxygen-level of bottom waters during the deposition of ancient sediments. It is the aim of this paper to use the new palaeoenvironmental tools to more fully elucidate the controls on the uranium content of a Carboniferous black shale succession and associated strata.
Section snippets
Study sites
Material for this study was collected from the Gastrioceras listeri (G. listeri) Marine Band and associated strata at Middlecliff Quarry, near Sheffield (SE 199038; Fig. 1). This horizon marks a major Westphalian A transgression within the NW European succession (Calver, 1968a). At Middlecliff, this is recorded as an upward transition from rootlet-rich siltstones and bleached sandstones (a palaeosol horizon) into a coal and then a fissile black shale of the G. listeri Marine Band (Fig. 1).
Sampling methodology
The mudrocks immediately overlying the Halifax Hard Bed Coal (the G. listeri Marine Band) vary in terms of their colour, fabric and macrofauna, from horizon to horizon on a centimetre scale; for this reason the entire marine band was sampled at intervals of 0.5–2.5 cm throughout the lower 66 cm of mudrock overlying the coal. Above this level, changes in both the appearance and palaeontology of the mudrock are more uniform so specimens were collected at ∼0.5-m intervals. Any obvious lithological
Fossil distribution within the Hepworth mudrocks
The 66-cm-thick G. listeri Marine Band contains a very abundant but low diversity marine fauna dominated by bivalves and goniatites. Black, fissile (paper) shale horizons tended to only contain pelagic taxa (goniatites) and were assigned to ORB 2 (Fig. 2). A few levels of this facies contained very rare benthic bivalves and were labelled ORB 3 accordingly. However, levels with abundant benthos, including bedding planes covered in large examples of Dunbarella papyracea, characterise the dark and
Depositional environments of the Hepworth succession
The palaeoecological, geochemical and petrographic data set from the Hepworth succession records a consistent story of rapidly changing depositional environments. Freshwater coal swamp conditions of the Halifax Hard Bed Coal are sharply overlain by the anaerobic and dysaerobic biofacies of the G. listeri Marine Band. Intense oxygen restriction is indicated by the impoverished marine macrofauna as well as the high values of DOP, S/C ratios and authigenic U (see below). Pyrite populations
Conclusions
The Hepworth mudrock succession indicates the following about U distribution:
(i) As noted in many previous studies, high concentrations of authigenic U are only encountered in marine sediments deposited beneath oxygen-restricted bottom waters.
(ii) The close correspondence between the abundance of authigenic U and francolite strongly suggests that former is scavenged by the latter.
(iii) The highest U values are closely associated with the most oxygen-restricted biofacies (ORBs 2 and 3), but the
Acknowledgements
Much of the work here was carried out with the financial support of a NERC studentship to QJF (Grant No. GT4/89/GS/49) at Leeds University. We thank the owners of Middlecliff Quarry, Hepworth Industrial Building Products, for access to their site. Robert Anderson and Kevin Taylor are thanked for providing thoughtful reviews.
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