The origin and evolution of Tertiary phosphorites from eastern England

Abstract

Remanié deposits of nodular phosphorite are found at the base of the Coralline Crag (Pliocene) and the Red Crag (Plio-Pleistocene) of Suffolk and Essex. The phosphatic components include phosphorite concretions (nodules), phosphatized vertebrate teeth and bones, and cobbles of sandstone cemented by a phosphatic matrix ('boxstones'). The phosphorite mineral is carbonate fluorapatite (francolite). The source of the phosphorite concretions was the London Clay (Eocene). They formed in unlithified anoxic sediment around sites of organic decay. The source of the 'boxstones' was a Miocene mud-rich sand formation tentatively correlated with the Belgian Anversian. They were formed by concretionary growth in anoxic sediment around sites of organic decay by a similar mechanism to that producing the London Clay concretions. Miocene teeth and bones with a low organic content were phosphatized by an apatite replacement process. This replacement process also enriched the phosphate content of derived phosphatic fossils and phosphorite concretions from the London Clay. Phosphorite formation in eastern England was linked with periods of transgression, while periods of regression were important in concentrating dense phosphatic material as a remanié deposit.