Clay minerals as palaeoenvironmental indicators in the Bathonian (Middle Jurassic) ore-bearing clays from Gnaszyn, Kraków-Silesia Homocline

Abstract

This paper reports the results of X-ray diffraction quantitative mineralogical studies of the clay-rich Middle Jurassic sedimentary rocks from Gnaszyn, central Poland and their palaeoenvironmental interpretation. The palaeoenvironmental interpretation is aided by the fact that the sediments have not been significantly altered by diagenesis. The mineral composition is uniform throughout the succession: quartz, K-feldspar, plagioclase, calcite, gypsum, anhydrite, pyrite, illite, kaolinite, chlorite, and glauconite. The clay assemblage is dominated by illite, which alone accounts for about 20 wt% of the total mineral content. Kaolinite amounts usually <10 wt% and chlorite and glauconite occur in subordinate quantities. The clay mineral assemblage is largely of detrital origin and indicates rather cool and/or dry climatic conditions favouring mechanical erosion of the source rocks.