Jarvis, Ian (2006) The Santonian – Campanian phosphatic chalks of England and France. Proceedings of the Geologists’ Association, 117(2), pp. 219-237. ISSN (print) 0016-7878

Abstract

Phosphatic chalks were deposited during the mid-Santonian to early Campanian between 85 Ma and 83 Ma in NE France and southern England. The beds contain abundant brown granular phosphate, largely consisting of phosphate-filled and coated foraminiferal tests, phosphatized macrofossil fragments, faecal pellets, phosphatized intraclasts and vertebrate remains. Phosphate contents typically range from 5–20% P2O5. Beds 1–20 m thick occur within elongate 1 km long, 250 m wide and 30 m deep erosional basins (‘cuvettes’), cut into the underlying Coniacian–Santonian white chalks. Several superimposed cuvettes with consistent orientations may occur. Cuvettes are floored by phosphatized, bored and encrusted hardgrounds, and contain unique faunal assemblages that include ahermatypic colonial corals, and encrusting oyster and worm patches. Belemnites, fish and reptile remains are common. Extensive reworking by currents and biota occurred during the deposition of the phosphate. Gravity flow led to the dislocation and redeposition of basal hardground blocks and the formation of synsedimentary slump folds. Phosphatic chalks accumulated in water depths of between 70 m and a few hundred metres in oxic, current-swept areas of an open epicontinental sea of normal salinity. High surface productivity, moderate organic-carbon fluxes, reduced sedimentation rates and high rates of sediment mixing led to the precipitation of phosphate within hardground surfaces and carbonate grains. Bacterial decomposition of organic matter raised phosphate contents in pore waters and promoted the geochemical conditions necessary for phosphate precipitation. Phosphate precipitates, now transformed to carbonate fluorapatite, include μm-sized ovoid bodies and massive coatings interpreted to be of microbial origin. Phosphatization was a rapid process occurring under mixed aerobic/anaerobic redox conditions in the upper decimetre of the sediment column, close to the sediment–water interface. Phosphatization took place predominantly within the confines of the cuvettes, where phosphate grains became concentrated by current winnowing and transport. Phosphatic chalk sedimentation occurred on the NE and northern margins of the Anglo-Paris Basin following regressive events that enabled bottom currents to erode the cuvettes and maintain productivity and the other sedimentary conditions necessary for phosphate accumulation.

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