Palaeontological and geochemical facies in the Gastrioceras subcrenatum marine-band and associated rocks from the North Crop of the South Wales Coalfield

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Abstract
Vertical sections through the Gastrioceras subcrenatum marine-band and associated shales exposed at three different localities on the North Crop of the South Wales Coalfield have been studied palaeontologically and geochemically. On the basis of quantitative palaeontology four palaeoecological phases are distinguished which are related to marine transgression and regression. Lingula can be divided into two types: those associated with goniatites and bivalves are considered to be marine, and the other type, without goniatites or bivalves, brackish. Differences in the chemical composition of the shales, after correcting for detrital quartz, can be correlated with the palaeoecological phases. Offshore sediments are characterized by higher amounts of organic carbon, carbonate, sulphur, phosphorus and uranium, together with smaller amounts of quartz. Most of the uranium is contained in organic material and collophane, while thorium occurs mainly in the clay fraction. Boron exibits a poor correlation with the palaeosalinities based on palaeontological criteria. The reasons for this are discussed. The amounts of detrital quartz, organic carbon, and the ratios of quartz to carbon, thorium to uranium, and sodium to potassium are shown to provide useful indicators of relative sedimentation rates. The quantitative distribution and type of fauna is mainly controlled by the sedimentation rate. Marine bivalve populations, like their modern representatives, exhibit a close relationship to the organic carbon content of the associated bottom sediments. While absolute values of redox potential during deposition of the shales cannot be determined, present variations in the ferric to ferrous iron and thorium to uranium ratios give some indication of relative differences. The depositional and post-depositional geochemical evolution of the shales has been controlled to a large extent by their organic carbon content.