Geochemical signatures of chert concretions and bedded cherts from the Kraków-Częstochowa Upland (Poland), the southeastern Franconian Alb, and the Bayerwald (Germany)

Abstract

The Upper Jurassic bedded limestones developed within the microbial-sponge megafacies along the northern margin of the Tethys Ocean host, among other things, chert concretions and bedded cherts. This megafacies has been identified in a number of exposures located in the Kraków-Częstochowa Upland (Poland) as well as in the southeastern Franconian Alb and the Bayerwald (Germany), where it revealed mineralogical, petrographic and geochemical similarities. Hence, siliceous rocks formed by the replacement of primary calcium carbonate with silica also showed significant resemblance. The major chemical component of the analyzed siliceous rocks is SiO2, accompanied by minor amounts of Al2O3, TiO2, Fe2O3, and trace elements. The field observations combined with the results of geochemical analyses have indicated that the silica forming the chert concretions and the bedded cherts was primarily supplied by hydrothermal solutions rather than being supplied by the skeletons of siliceous sponges and radiolarians. The results of geochemical analyses document the formation of siliceous rocks during multistage diagenetic processes strongly influenced by hydrothermal solutions. The negative Ce anomalies observed in samples from all studied locations suggest oxic conditions during the silicification processes whereas the positive Eu anomalies indicate the local contribution of hydrothermal solutions under partially anoxic conditions. The differences in Eu anomalies found between particular locations are the result of the varying temperatures and intensity of hydrothermal solutions influx during the replacement processes, thus revealing the spatial variability of hydrothermal activity in the studied regions of the northern Tethys margin.