Paleoenvironmental settings of the Sofia lignite basin: Insights from coal petrography and molecular indicators

Compositions of Balsha lignite, Sofia coal-bearing basin, Bulgaria and carbonaceous shales are studied by petrological and geochemical proxies. The macropetrographic characteristic of profiles is relatively constant. The lignite is generally composed of dull matrix coal containing significant amounts of xylite fragments. The typical microscopic constituents of the lignite are the ulminite, and less frequently textinite lenses, scattered within attrinitic groundmass. The maceral composition of the organic matter in carbonaceous shales does not differ significantly from that of the lignite samples. Biomarker assemblages of the extractable portions are determined by gas chromatography-mass spectrometry. Additional information for the compositions of residues after extractions is obtained by TMAH thermally assisted hydrolysis/methylation. Petrographic contents, lipid assemblage and TMAH thermally assisted hydrolysis/methylation data imply that the original peat bog is formed in fresh-water forest swamp preferably composed by gymnosperms. Polar diterpenoids argue for Cupressaceae Taxodiaceae as the most probable vegetation. For one of the profiles appreciable angiosperm rich herbaceous-type flora contribution is assumed. Polar oleanane triterepenoids and their aromatized derivatives argue for angiosperm input The relevance of A-norfriedel-8-en-10-onegriedelin ratio for groundwater level appraisal is discussed. It admits long period of high water level in the paleomire. High content of vanillyl structures and subordinated presence of syringyl units in products of TMAH thermally assisted hydrolysis/methylation are hint for Gymnospermae input in the paleoswamp and Angiospermae contribution to one of the profiles. Cinnamyl phenolic units, indicating non-woody angiosperm vegetation presence, are registered as well. Flavonoids in products are considered as taxonomic markers of Cupressaceae plants in the paleoswamp. (C) 2012 Elsevier B.V. All rights reserved.