Middle Jurassic black shale deposits from the Pieniny Klippen Belt, Western Carpathians: Insights into organic matter composition, thermal maturity, depositional, and palaeoenvironmental variations

Abstract

An integrated multi-method research study, employing maceral identification and analysis, microphotometry, fluorescence microspectrometry, palynofacies analysis, Rock-Eval pyrolysis and biomarker geochemistry, and was applied to determine sources of variation of organic matter (OM) type, its provenance, thermal maturity, as well as hydrocarbon potential of the Middle Jurassic Szlachtowa Formation in the Pieniny Klippen Belt of the Western Carpathians in Poland and Slovakia. In addition, a reconstruction of paleoenvironmental conditions and dinoflagellate cyst biochronology were performed. The presence of first-cycle vitrinite was well-discerned from phyllovitrinite and a reworked vitrinite population. It allowed for determination of thermal maturity at 0.74 % VR_o (0.04–0.10 % SD). Although a comparison between measured and calculated λ_max and Q_650/500 for sporinite and alginite yielded a marked discrepancy, both spectral fluorescence parameters illustrate a clear similarity between lamalginite and dinoflagellate cysts. The samples contain predominantly thermally mature organic-lean Type III kerogen of terrigenous-derived macerals of first-cycle vitrinite, phyllovitrinite, reworked vitrinite, and liptinite (sporinite) with admixture of Type II kerogen associated with lamalginite. This is in line with Rock-Eval and biomarker data as well as the palynofacies composition, the latter indicating predominance of phytoclasts kerogen groups over palynomorphs and amorphous components. The occurrence of reworked, oxidized, and fragmented terrigenous OM is attributed to weathered and eroded sedimentary rocks from the Upper Carboniferous to Triassic and Lower Jurassic periods as well as from the lowermost Middle Jurassic cover. Together with predominant primarily indigenous terrigenous OM, the reworked sedimentary strata served as a source for siliciclastic sedimentation and formation of turbiditic deposits in marine slope to abyssal plain settings. In the extensional active tectonic setting, sediment flux and runout in turbidity currents were derived from overstepped margin slopes, fault-bounded tilt blocks, and the Czorsztyn Ridge. In addition, organic contributions to pelagic deposits included planktonic microorganisms such as dinoflagellate cysts, foraminiferal test linings, and, to a lesser extent, prasinophytes and acritarchs. It is likely that the deposition of Middle Jurassic black shales occurred in a predominantly heterolithic-oxic shelf and dysoxic-suboxic shelf to basin transition. The Szlachtowa Formation yielded dinoflagellate cysts indicative of the Middle Jurassic Aalenian to Bajocian stages, with a few species also appearing in the Toarcian and Bathonian stages. Significant changes in OM preservation influenced hydrocarbon generative potential of studied black shales, accounting for its low to fair source rock generative potential (avg. ∼0.6 wt% TOC; Hydrogen Index avg. ∼100).