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Gault Clay Formation

The Gault Clay Formation covered northern Europe during the Middle-Late Albian stage of the Early Cretaceous (around 105 to 95 million years ago). This was a time when the world was experiencing global warming, sea levels were rising rapidly, culminating with record high levels during the deposition of the Chalk. The base of the Gault Clay often directly overlies the eroded, unconformable surface of older sediments. In Buckinghamshire, borehole evidence shows that it sits directly on Upper to Middle Jurassic towards the south of the county. (Beneath London, the Gault Clay rests directly upon the old, Devonian, land surface. The Gault Clay is a grey clay, particularly fossiliferous near its base, but becoming more monotonous in its higher part. The sediment was probably derived from Jurassic Clay formations being eroded from low-lying surrounding land areas (such as the London Platform; the Armorican area - Devon, Cornwall; Northern England)

Fossil shells are often preserved in their original material, albeit very fragile and crushed as the clay has compressed. Others attracted a phosphatic cementation shortly after burial. These are now preserved as hard, fully 3-dimensional casts which are very robust. Some concentrations of phosphatic fossil casts occur in discrete bands and represent a time of ocean deepening, and the subsequent period when the sediment did not reach this offshore area of the sea-floor.

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Typical block of pale grey Gault Clay, with fossil shell material. Artist's impression of Belemnites and other creatures, in the Cretaceous sea. An ammonite preserved in a phosphate nodule. Ammonite, Prohysteroceras (Goodhallites) with a bivalve, Birostrina sulcata, embedded in the centre. Ammonite, Euhoplites Ammonite from the Lower Gault Clay, Hoplites bonarelli. The original white shell encloses a black, phosphatised clay internal cast. Shallow-burrowing bivalve, Nucula pectinata. Only the phosphatised internal cast remains Bivalve, Birostrina concentricus, characteristic of the Lower Gault Clay Bivalve, Birostrina sulcata, an Upper Gault Clay form, which is believed to have evolved from the concentrically-ribbed species, as some intermediate forms have been discovered. This Scaphopod Mollusc, Dentalium, was a calcareous tube inhabited by a small animal with a filter feeding arm which could protrude from the shell. Belemnites, mainly Neohibolites, were abundant in the Gault sea. These remains are part of the internal skeleton of squid like molluscs. Although generally preferring to grow free from mud, these solitary corals existed in the Gault sea. Fish teeth and bones, like this vertebra, are common, particularly in the more phosphate-rich layers.

 
BEHG Contact : Mike Palmer (mpalmer@buckscc.gov.uk)

page last updated: 13th August 2010

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