Sandstone petrology and mudstone geochemistry of the Peruc-Korycany Formation (Bohemian Cretaceous Basin, Czech Republic)

We have studied the petrography and the bulk-rock geochemistry of arenites and mudstonesof the Cenomanian Peruc–Korycany Formation to characterize their provenanceand sedimentary history, as well as the influence of weathering, hydraulic sorting, andrecycling of the source rocks. The Peruc–Korycany Formation contains sedimentaryfacies reflecting both meandering- and braided-river systems and shallow-marinesystems. Differences in the three depositional settings did not cause distinctly differentmodifications of the framework compositions of the arenites. The sand from the fluvialsystems is very mature (Qm98F0Lt2). These fluvial arenites were subsequently modifiedby shallow-marine processes; reworking produced very slight decreases in theabundance of lithic fragments and polycrystalline quartz grains. The Cenomanianstrata of the Bohemian Cretaceous Basin were derived dominantly from metasedimentaryand crystalline rocks of the Palaeozoic Teplà-Barrandian and Cadomian Moldanubianunits, respectively. Periods of low tectonic activity resulted in the deposition ofarenites with quartzose framework compositions, indicating that climatic and/or transport/depositional-environmental controls overwhelmed factors such as source-rockcompositions. Ultrastable dense minerals are useful indicators of sedimentary recyclingwithin the Peruc–Korycanytarenites. Mudstone samples are characterized byabundant kaolinite, illite, chlorite, and quartz but by negligible amounts of goethite andgypsum. Concentrations normalized to the post-Archaean Australian shale (PAAS)show that the sediments are strongly depleted of Na, K, Ca, Sr, and Ba, probablybecause of the mobility of these elements during weathering. Chemical indices of alteration(CIA, CIW, and PIA) show that the degree of weathering of the source area washigh. The data fall closer to the compositional fields of highly weathered minerals suchas kaolinite, gibbsite, and chlorite on an A-CN-K diagram. The indices of compositionalvariability of the studied samples are much less than 1, suggesting that the samples arecompositionally mature and were likely dominated by recycling. The elemental ratioscritical of provenance (La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th) are similar to fine fractionsderived from the weathering of mostly granitoids rather than mafic rocks.