Basalt meets shale: The role of lava flow in the alteration of organic-rich host rocks

Abstract

Organic-rich Miocene mudrocks from the Grabiszyce quarry (southwest Poland) provide a rare natural laboratory for investigating the short-term thermal effects of a basaltic lava flow on immature organic matter (OM) and mineral phases. Samples were collected along a decimeter to meter-scale transect from the basalt-sediment contact and analyzed using organic geochemistry, Rock-Eval, vitrinite reflectance, rock magnetism, and detailed clay mineralogy. Total organic carbon values are the lowest close to the lava contact (similar to 5 wt%) and increase to >10 wt% at distances greater than c.a. 0.3-1 m, consistent with thermal degradation of OM. Vitrinite reflectance increases sharply towards the basalt, from similar to 0.20% Rr to 1.38% Rr within 2-5 cm of the contact, whereas T-max values (425-430 degrees C) remain low or could not be determined near the contact due to very low S2 yields. Hydrogen Indices decrease from 180 mg HC g(-1) TOC in immature samples to as low as 6 mg HC g(-1) TOC at the contact. Molecular compositions show consistent loss of thermally labile compounds and enrichment in stable polycyclic aromatic hydrocarbons towards the lava flow, while unusually unstable biomarkers (e.g., beta beta-hopanes) persist in thermally altered samples, reflecting kinetic inhibition during rapid heating. The preservation limit of tocopherols is constrained to similar to 0.3-0.4% Rr. Rock magnetic parameters show changes consistent with distance towards the lava flow. Enhanced formation of superparamagnetic magnetite within similar to 35 cm of the basalt is documented, which correlates well with PAH concentrations and biomarker maturity ratios. Furthermore, clay mineralogy reveals no illitization of smectite, despite elevated organic maturity, indicating a fluid-deficient, short-lived thermal pulse that is insufficient to trigger potassium-driven mineral reactions. These results demonstrate that OM and magnetic mineralogy respond more rapidly than clay minerals to brief, high-temperature events, highlighting a fundamental decoupling between organic and inorganic maturity indicators under contact metamorphic conditions. Show more