Geochemical compositions and depositional conditions of Upper Jurassic and Lower Cretaceous Yorkshire clays, England

Abstract

Fifty-three samples from the Oxford, Kimmeridge and Speeton Clays of the Yorkshire area have been analysed petrographically and geochemically, by X-ray diffraction, X-ray fluorescence, atomic absorption and combustion (TOC). Major elements and the following trace elements have been determined: Rb, Sr, Ba, Cr, V, Co, Ni, Cu, Zn and Mo. P2O5, Rb, Ba, Cr and V show positive correlation to the clastic fraction (quartz, feldspar and clay minerals). CaO, MnO, and Sr and to a lesser extent Ni, Cu and Zn seem to be positively correlated to calcite, while Fe2O5, MnO, MgO and Co are linked to siderite. Stratigraphical variations of element/element ratios have been used to obtain sedimentological information from the petrographical and geochemical data.The Oxford Clay, with its coarse and immature clays, was deposited in a regressive development. An upwards shallowing with ventilated deposition of more mature and altered clastic material took place throughout the unit. During deposition of the Kimmeridge Clay more reducing conditions existed and finer grained, smectite-rich sediments were formed. The fine grained Speeton Clay was deposited during changing depositional conditions, varying ventilation in a generally regressive development. Several sedimentological breaks characterize the unit.