Paleovegetation and environment during deposition of the Late Oligocene sub-bituminous coal in the Bobov Dol Basin (SW Bulgaria) as deduced from petrographic and geochemical characteristics

Abstract

Coal and carbonaceous shale samples were taken from the Late Oligocene Bobov Dol Basin, SW Bulgaria. Seams I^a, I, II^a+b, III, IV and V from the coal-bearing Bobov Dol Fm. were sampled to provide insights into the peat forming vegetation and depositional environment based on detailed organic petrological and geochemical study.

The petrographic composition indicates that both coals and carbonaceous shales contain predominantly terrestrial organic matter with abundant huminite macerals and locally enriched liptinite macerals. Inertinite is rare or absent. Good tissue preservation in seams I, IV and V indicates deposition under wet sedimentary environment, whereas predominance of detrohuminite and lower Tissue Preservation Index (TPI) in seams I^a, II^a+b and III are linked to enhanced humification of the plant remains due to changes in the hydrological regime. The low percent of inertinite and the overall high Gelification Index (GI), however, argue for peat formation under wetter climatic settings, whereas low to moderate ash yields of coal samples indicate organic matter deposition in oligo- to mesotrophic topogenous mires. Reconstructed evolution of the environmental settings during peat formation denotes differences which might be caused by changes in the hydrological regime and tectonic activity.

The biomarker composition shows prevalence of gymnosperm-derived sesqui- and diterpenoids, followed by lower concentration of n-alkanes (apart from the carbonaceous shale samples from seam I^a), whereas angiosperm-derived non-hopanoid triterpenoids, acyclic isoprenoids, steroids and hopanoids, are generally present in minor amounts. Diterpenoid composition indicates dominant contribution from Cupressaceae and Pinaceae conifers. Minor amounts of predominantly lupane-type non-hopanoid triterpenoids denote limited development of angiosperm vegetation, presumably mostly related to Betulaceae family. Relative abundance of mid-chain (n-C_21–25) n-alkanes, associated with elevated proxy ratio (Paq) furthermore, suggest possible contribution from aquatic macrophytes to peat formation. Very low hopane concentrations and low/moderate ββ/(ββ + αβ) hopane ratios imply minor aerobic bacterial activity under mild acidic conditions. The presence of inorganic carbon suggests that environmental acidity might be controlled by the occasional input of neutral to slightly alkaline ground waters. Moderate sulfur contents and the occurrence of hopenes denote anaerobic bacterial activity after organic matter burial.

Low maturity of the organic matter and limited hydrocarbon generation potential is indicated by the low huminite reflectance, hopane (C₃₁ 22S/(22S + 22R)) and sterane (αααC₂₉ 20S/(20S + 20R)) isomerization ratios, as well as Rock-Eval parameters, T_max and Production Index (PI).