The ecology and preservation of the Oxford Clay fauna at Woodham, Buckinghamshire

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Abstract
The exposure of Oxford Clay at the Woodham Brick Company's Pit, near Akeman Street Station, Buckinghamshire, is described in detail. Macrofaunal bed-by-bed collections reveal that throughout the deposition of the Oxford Clay at this locality a wide range of ecological niches was inhabited. There are no major ecological differences between the faunas of the main lithological units (the Spinosum Clays, the Lamberti Limestone, and the Mariae Clays). Such differences as do exist between the clays and the limestone are probably related mainly to differing rates of deposition. The authors have found no evidence to support the contentions of Rutten (1956) that waterblooms periodically killed off the clay faunas, nor that the limestone is a littoral deposit. Many Woodham fossils are preserved as internal, pyritic moulds. There is abundant evidence that pyrite deposition occurred only in small, enclosed shell chambers which had not been invaded by mud. Hence the nuclei of ammonites are preserved as internal pyritic moulds, but beyond a diameter of about 20 mm the remaining whorls (with consequently larger chambers) are preserved as clay moulds. Other once-aragonitic shells not offering small, enclosed voids are preserved as composite clay moulds, whereas calcitic shells remain preserved as such, often with detailed micro-structure. Sulphates dissolved in sea-water are considered to be the source of sulphide in pyrite formation, and from comparison with recent sediments the authors envisage pyritization as beginning beneath the mud-sea-water interface, possibly at a depth of several centimetres. Forms of preservation not recorded at Woodham are briefly mentioned as occurring at other Oxford Clay localities. It is concluded that the Oxford Clay sea-bottom at Woodham was not stagnant and anaerobic, and that there is no evidence of dwarfed faunas. The relationship between original shell mineralogy and structure, and mode of preservation is stressed.