Impact of basaltic intrusion on organic-rich clays: organic geochemistry and rock magnetic perspective

We studied the impact of the basaltic intrusion on the Miocene organic-rich clay level found in the Grabiszyce quarry, Lower Silesia, Poland. The clays were deposited under terrestrial conditions, and fragments of fossil wood and small plant debris are often present within them. The total organic carbon (TOC) values range from 4 to 12% depending on the distance from the intrusion and the organic debris content. The vitrinite reflectance values increase gradually from 0.15% to 1.4% close to the intrusion. Gas chromatography–mass spectrometry (GC-MS) results show significant changes with distance from the intrusion. Carbon preference index (CPI) values change from c.a. 1 for samples near the basalt to > 2 for samples about 0.5 m and more from the intrusion. As the distance from the basalt decreases, some biomarker groups disappear, including tricyclic and tetracyclic diterpanes, des-lupanes and des-oleananes, hopenes, ββ-hopanes, oleanenes and sterenes. Moreover, such polar compounds as sitosterol, stigmastanol, α- and β-amirin and fridelan-3-one are only present in samples far from the basalt. In contrast, there is an increase in the content of unsubstituted and methyl derivatives of PAHs as well as stable derivatives of hopanes and steranes near the intrusion. Notable differences caused by the intrusion are documented also by rock magnetic methods. Both in-phase and out-of-phase magnetic susceptibility decreases with the distance from the basalt and with the measured vitrinite reflectance. In addition, magnetic susceptibility values correspond very well with the second applied maturity parameter based on the benzo[e]pyrene/(benzo[e]pyrene +perylene) ratio. Moreover, the elevated temperatures resulted in the formation of fine-grained superparamagnetic magnetite, which is documented by frequency-dependent susceptibility, hysteresis curves and their parameters. The thermal impact of the intrusion on the magnetic mineralogy diminishes at around 20 cm distance from the basalt.