Sedimentology, geochemistry and origin of phosphatic chalks: the Upper Cretaceous deposits of NW Europe

Abstract

Santonian‐Lower Campanian and Lower Maastrichtian phosphatic chalks in northern France, southern England and Belgium are Europe's largest sedimentary phosphatc deposits. The stratigraphy, sediment‐ology, petrography, mineralogy and geochemistry of the lithofacies are reviewed and new data presented. Depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high‐ and low‐productivity phosphogenic systems. It is concluded that phosphatic chalks were deposited in well‐oxygenated, current‐swept environments. Phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates, reduced bulk sediment accumulation rate and an enhanced rate of bioturbation. Precipitation of carbonate‐fluorapatite (francolite) accompanied the bacterially mediated decomposition of organic matter, occurring within centimetres of the sediment‐seawater interface, and taking place preferentially within microbial bodies and coatings. In addition to the organically derived component, pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction. Mineralization was probably a dominantly post‐oxic process, but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation. Phosphogenesis occurred predominantly on the NE margins of the Anglo‐Paris Basin where shallower sea floors and suitable palaeoceanographic conditions prevailed. Phosphogenic episodes were limited by sea level fluctuations'which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity.

Anatomy of an unconformity on mid‐Oligocene Amuri Limestone, Canterbury, New Zealand

“…The depth of submergence is unknown, but striking glauconite and phosphate replacement of lithified carbonate clasts and uppermost surfaces, together with sparse epifaunas, are characteristics of relatively deep water, shelfedge hardgrounds in European chalks (Kennedy & Garrison 1975;Jarvis 1992).…”

Tethyan Phosphates and Bioproductites

“…On the widespread drowned continental surfaces, shallow oxygenated waters were less supplied with clastics, despite the humid climate that favored a developed continental vegetation in the North Atlantic area. In these shallow waters, plankton with carbonate and siliceous tests is dominant, explaining the formation of chalk widely known north of the Tethys Ocean, from Ukraine to the Paris Basin (Bushinskii, 1954;Jarvis, 1992). Glauconitic in some places, mainly at the base of the formation, this chalk almost always contains siliceous nodules or banks (chert, silex, trepel, opoka, etc.).…”

Glauconite and Phosphate Peloids in Mesozoic Carbonate Sediments (Eastern Subbetic Zone, Betic Cordilleras, SE Spain)

“…The existence of an abundant phosphatic phase in most of the studied glauconitic peloids agrees with the hypothesis proposed by Stille & Clauer (1994) and reveals a wide extension of the sea-water effect. The presence of dissolution features on carbonate bioclast frameworks indicates lower pH conditions in these substrata before or during the phospate precipitation (Jarvis, 1992). Pr6v6t & Lucas (1986) have shown in experimental phosphatization processes of foraminifera, that apatite replaces carbonate shells and later, neoformed apatitic fingers extend from the walls growing towards the interior of the chambers and progressively filling up these chambers.…”