Geochemistry of Middle Jurassic mudstones (Kraków-Częstochowa area, southern Poland): interpretation of the depositional redox conditions

Abstract

Middle Bathonian iron-bearing dark grey mudstones and claystones from the Kraków-Częstochowa Upland (southern Poland) were subjected to geochemical and mineralogical study in order to evaluate palaeo-redox conditions of their deposition and diagenesis. They are mainly composed of kaolinite, with smaller amounts of other clay minerals and detrital quartz. Organic matter is composed of type III and/or IV kerogen and its d13 C is between -23 and -24, suggesting a mainly terrigenous source, with a possible admixture of marine input. Most geochemical parameters: (total organic carbon) TOC/S, U/Th, Ni/Co, V/Cr, (Cu+Mo)/Zn ratios, authigenic uranium content, and Fe-TOC-S relationship, indicate deposition under oxygenated bottom water conditions. By contrast, DOP (degree of pyritization) and V/V+Ni indices suggest a dysoxic environment. However, DOP probably reflects redox conditions in the sediment during diagenesis rather than in the overlaying water column. The composition of the organic matter and the domination of pyrite euhedra over framboids indicates that the V/V+Ni ratio may not be reliable for determination of redox conditions in the rocks studied. There is no significant difference between the geochemical indices of host rocks with nodules and those without them.