Depositional settings of the Eocene Suhostrel bituminous coal, SW Bulgaria, inferred from organic petrology and molecular proxies

Abstract

The paper reports results from organic petrological and geochemical characterization of Suhostrel bituminous coal deposit, SW Bulgaria. Three high ash (21–47 wt%) and thin (up to 0.6 m) coal seams, representing splits from a locally compact coal bed (∼1.8 m thick), were formed around the southern and south-western rim of the Padesh Graben during its Eocene evolution. High total organic carbon contents (avg. ∼86 wt%, daf), vitrinite reflectance (avg. Ro = 1.05%) and Tmax (avg. 457 °C) argue for high vol. A/medium vol. bituminous coalification rank. Maceral composition is dominated by vitrinite (avg. ∼ 95 vol%), accompanied by small amounts of liptinite (avg. ∼ 5 vol%). Apart from few occurrences of funginite, inertinite is not present. The organic matter is mostly represented by detrovitrinite and collotelinite, the latter generally occurring in the form of gelified leaf-derived tissues. Liptinite is mainly represented by cutinite and microsporinite and display no evidence for organic matter contribution from algae. Low TPI and VI indices indicate peat formation from vegetation with poor preservation potential. Insignificant contents of gelovitrinite, coupled with high GI indices denote greater importance of the geochemical processes for the gelification of the organic matter. Striking predominance of mid- and long-chain n-alkanes argues for peat formation from a mixture of aquatic macrophytes and terrestrial vegetational communities. Presence of oleanane, albeit in low concentration, provides robust argument for contribution from angiosperm plants, whereas absence of resin-derived sesqui- and diterpenoid hydrocarbons denotes insignificant role of conifers during peat accumulation. Reconstructed depositional settings indicate organic matter accumulation under marginal aquatic, marsh/fen-type peatland with meso- to rheotrophic characteristics (GWI > 0.5). High mineral matter contents, coupled with the abundance of siliciclastic grains, imply hydrologically active environment. Plant remains deposited under oxidizing conditions (Pr/Ph = 1.5–2.5), but because of high water table were likely subjected to frequent (perhaps seasonal) changes in Eh settings. The resultant limited aerobic biodegradation of the plant remains is evidenced by low concentrations of hopanoid biomarkers and their diagenetic degradation products – drimane-type sesquiterpenoids. Post-depositional marine influence and downward infiltrating sulfate-rich waters is considered responsible for the presence of dibenzothiophene derivatives in Suhostrel coal.All molecular maturity-related ratios (e.g. CPI, Ts/(Ts + Tm), βα/(αβ + βα) C30 hopanes, ββ/(ββ + αα) C29 steranes, MPI-1, etc.) are fully compatible with the bulk geochemical parameters (e.g., TOC, Tmax, Ro) and confirm the bituminous coalification rank of the coal. Based on this data, temperatures of ∼130–140 °C were likely reached during burial of the sediments in a zone, characterized by high thermal influx (geothermal gradient ∼43–47 °C/km).