Thermal maturity and depositional environments of organic matter in the post-Variscan succession of the Holy Cross Mountains

Abstract

The results of GC-MS investigations of biomarkers in organic matter from the epicontinental Permian and Triassic strata of the Holy Cross Mts. indicate generally suboxic (Permian) and oxic (Triassic, except for the Middle Muschelkalk) depositional environments with accompanying intensive bacterial reworking of organic remains. The biomarker spectra found in the Middle Muschelkalk and in the Entolium discites Beds (Upper Muschelkalk) show the presence of gammacerane, an increased concentration of C34 homohopanes and a Pr/Ph ratio not exceeding 1. All these observations suggest a hypersaline sedimentary environment with water column stratification and poorly oxygenated conditions. Thermal maturity investigations were based on both vitrinite reflectance and biomarker indices. The organic matter in the Triassic succession is slightly more mature along the northern margin of the Holy Cross Mts. than along their southern part, due probably to a regional increase in the maximum burial depth during the Mesozoic. The maturity of the Permian strata at Gałęzice (southern Holy Cross Mts.) is similar to that of the Triassic rocks whereas Kajetanów (in the north) is characterised by values significantly higher and comparable to those found in wells further north. The difference cannot be accounted for solely by differential burial depths, and implies the existence of a positive thermal anomaly active before the Middle Triassic. Our investigations generally confirmed the low thermal maturity of the Triassic. However, contrary to earlier interpretations, we suggest that the maximum burial depths based on a reconstructed thickness of the post-Triassic deposits may account for the observed maturity levels, assuming that geothermal gradients in the Mesozoic were similar to today’s.