The mineralogy and petrology of the ‘Coprolite Bed’ phosphorite deposit (Lower Cretaceous) at Speeton, North Yorkshire

Abstract

SUMMARY The ‘Coprolite’ Bed (Basal Bed or E Bed of the Speeton Clay, Berriasian) rests on Kimmeridge Clay. It is made up of phosphatic pebbles, some of which are internal moulds of bivalves, brachiopods and fragmented ammonites. Recognisable vertebrate remains are almost entirely restricted to occasional microscopic teeth and bone fragments. The pebbles are made of francolite (72–85% carbonate fluorapatite) intimately mixed with microcrystalline quartz, clay minerals, pyrite and organic matter. Some pebbles have been bored and re-phosphatised. Some francolite (with a peloidal structure), glauconite and detrital quartz occur in the matrix, but partially phosphatised calcite is the main interstitial phase. The pebbles are probably derived from already lithified and phosphatised Upper Kimmeridgian material, which may have been present as individual nodules or phosphatised hardground. The unweathered bed contains 21.6 to 25.2% P 2 O 5 , whereas individual pebbles have 26.2 to 30.5% P 2 O 5 . The carbonate fluorapatite has minor Na and Mg substitution for Ca, and approximately one sixth substitution of CO 3 for PO 4 . Approximately 1.7 atoms of F are also present in the structure. The glauconite has an overall ordered structure. The bed is very thin and, although its extent is unknown, the nature of the Kimmeridge Clay/Lower Cretaceous unconformity in this area makes it unlikely that a thicker and economically workable deposit occurs.