Palaeoenvironmental implications of coal formation in Dobrudzha Basin, Bulgaria: Insights from organic petrological and geochemical properties

Abstract

The Dobrudzha Basin in northeast Bulgaria contains 86 coal seams distributed within three coal-bearing formations of Late Carboniferous age. This study presents the organic petrological and geochemical characteristics of coals from the Makedonka and Krupen Formations, containing some of the most prominent coal seams. These were characterized by means of proximate and Rock-Eval analyses, vitrinite reflectance measurements, organic petrography and GCMS analysis in order to establish variations in vegetation and depositional settings. Based on random vitrinite reflectance (avg. 0.7% Rr) and TOC contents (avg. ~80%, daf) the coals can be classified as low rank bituminous (high volatile B-C). Low Tmax values (avg. 430°C) further indicate a low degree of organic matter maturity. The low to moderately high ash yields, together with the presence of shaly partings, and generally low sulfur contents are consistent with peat formation in a fluviatile sedimentary environment. Petrographic analysis reveals complex maceral composition with abundant liptinite and inertinite. The predominance of detrovitrinite and the almost complete lack of strongly gelified macerals argue for enhanced humification of the plant tissues and peat formation in relatively dry environments (above water table). The distributions of n-alkanes, diterpenoids, and steranes indicate that the organic matter originated from terrestrial vascular plants. The high pristane/phytane ratios point to peat deposition under oxic conditions. Low concentrations of hopanes argue for limited bacterial degradation of the organic matter, possibly due to low pH conditions during peatification. The presence of tetracyclic diterpanes and their aromatic counterparts is attributed to pre-gymnosperm (cordaitean) input. The existence of cordaiteans, although as minor component of the palaeovegetational communities, is further evidenced by the presence of trace amounts of aromatic arborane/fernane derivatives in almost all studied samples. The occurrence of combustion-derived PAHs in all studied samples is consistent with the abundant inertinite and indicates frequent forest fires during peat formation.