Influence of floral assemblage, facies and diagenesis on petrography and organic geochemistry of the Eocene Bourgas coal and the Miocene Maritza-East lignite (Bulgaria)

Abstract

Numerous differences in the petrography and organic geochemistry of two Tertiary, low rank coal deposits of Bulgaria, formed within comparable depositional environments, are outlined. Different floral assemblages are indicated as the main sources of organic matter. For the Eocene Bourgas sub-bituminous coals a warm climate plant community dominated by angiosperms is reflected in the biomarker composition, whereas a coniferous flora was the main source of the resinous organic matter of the Miocene Maritza-East lignite. The results are in agreement with palaeobotanical data. These differences in the peat forming vegetation of the Paleogene compared to the Neogene of Bulgaria are attributed to decreasing temperature during the Tertiary. The abundance of resinous compounds relative to lipids from plant waxes (long chain n-alkanes), as well as the preservation of plant tissue, are mainly controlled by gymnosperm/angiosperm ratios. Pristane/phytane and diasterenes/sterenes ratios reflect variations in redox conditions and the pH of the depositional environments, caused by eustatic sea level changes, freshwater inflow, or varying (ground)water table. Changing Eh and pH values are associated with differences in microbial activity controlling the extent of gelification of plant tissue. Bacteria most probably contributed to the aromatisation of triterpenoids in the mire. Enhanced thermal maturation of organic matter during diagenesis is reflected in higher contents of n-alkanes and the presence of alkyl naphthalenes in the Bourgas coals. Alkyl naphthalenes are thought to have been formed from pentacyclic triterpenoids rather than from resinous compounds.