Inorganic geochemistry of the type Caradoc series (Sandbian to middle Katian, Upper Ordovician), Onny valley, Shropshire, UK

Abstract

AbstractThe geochemistry and petrology of the type section of the Caradoc Series in the Onny valley indicate that it was deposited on a marginal basin continental shelf similar to the western side of the present Sea of Japan. The lower beds form a transgressive–regressive sequence in which the rocks become less mature upwards. All the coarser sediments above the basal quartzites and conglomerates are greywackes in which the apparent muddy and ferrous matrix is due to the breakdown of unstable minerals and particles. Higher values of Na2O and Na/K ratios are found in the coarser shallow-water sandstones of the Horderley Sandstone Formation and decrease markedly in the succeeding beds, accompanied by an increase in K2O. Higher values of carbonate-corrected (and hence related) other major and minor elements like SiO2, CaO, P2O5, MnO and most trace elements correlate with the transgressive systems and maximum flooding surfaces of the three sequences recognized where they are related to condensation at those horizons. Chemical Indices of Alteration (CIA) suggest that the Horderley Sandstone Formation underwent greater predepositional physical weathering than lower and higher beds, which is compatible with the petrography, and were deposited during a cool phase within overall warm Sandbian–Katian times. Trace element ratios suggest an oxic to suboxic depositional environment.