Genesis and diagenesis of Santonian to Early Campanian (Cretaceous) phosphatic chalks of the Anglo-Paris Basin
The phosphatic chalks of the Anglo-Paris Basin are granular phosphorites of Santonian to early Campanian age. They were deposited in erosional cuvettes up to 1 km long, 250 m wide and 30 m deep, incised into white chalks. Most are situated in the Picardy region of northern France. Cuvettes are floored by strongly indurated and mineralized basal hardgrounds developed in intraclastic sediments. The hardgrounds are penetrated by prominent phosphatic-chalk filled Thalassinoides burrows and overlain by intraclast-pebble lags containing 'Terebella' phosphatica Leriche, Diblasus arborescens Parent and Lopha semiplana (J. Sowerby). Lithification occurred a few centimetres below the sediment/water interface in a sediment of increased permeability, and is geochemically discernible ~90 cm below the hardground surface. Glauconitization was restricted to replacement of clay minerals during the early development of the hardground, later phosphatization replacing carbonate. Actinocamax verus Miller occurs in basal phosphatic chalks and a bed of Gonioteuthis quadrata quadrata (Blainville) occurs at the summit. The Gonioteuthis Bed is commonly underlain by, but separated from, a bed of Offaster pilula (Lamarck). The proportion of faecal pellets, phosphatic ooliths, echinoderm fragments and benthonic foraminiferans decline above the basal hardgrounds as the phosphorites become finer-grained and less phosphatic. Inoceramid prisms or pelagic foraminiferans are the dominant component of poorer phosphatic chalks. Intraformational slump folds and hardground mélanges occur at the base of some successions. The palaeobiology of belemnites is considered in detail and it is concluded that the Gonioteuthis Beds were the products of massmortalities accompanying reproduction. Pelletal phosphate was formed by the replacement of carbonate during early diagenesis in an organic-rich, anoxic environment and water depths of <150 m. Current activity and subsurface anoxia were intermittent; colonization, bioturbation and winnowing alternating with quiescent, anoxic phases of mineralization. Phosphatic chalk cuvettes were eroded by a proto-Gulf Stream during a eustatic regression. Upwelling of this current was the main source of phosphate in the phosphatic chalks.