Paleoenvironmental settings of the Soﬁa lignite basin: Insights from coal petrography and molecular indicators Maya Stefanova a, ⁎, Jordan Kortenski b , Alexander Zdravkov b , Stefan Marinov a a Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Soﬁa, Bulgaria b Department of Economic Geology, University of Mining and Geology, 1700 Soﬁa, Bulgaria abstract article info Article history: Received 11 May 2012 Received in revised form 19 September 2012 Accepted 19 September 2012 Available online 13 October 2012 Keywords: Lignite Petrography Biomarkers TMAH thermochemolysis Paleoenvironment Compositions of Balsha lignite, Soﬁa coal-bearing basin, Bulgaria and carbonaceous shales are studied by pet- rological 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 typ- ical 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 spectrom- etry. 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 s.l., Taxodiaceae as the most probable vegetation. For one of the proﬁles ap- preciable 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-one/friedelin 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 contri- bution 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. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Lignites are the most abundant coal resources in Bulgaria (www. euracoal.be/pages/layout1sp.php?idpage=69). They constitute more than 85% of the total coal reserves of the country (Šiškov, 1997). The lig- nite deposits are accumulated within multiple collapse structures of various dimensions. They are formed during the Miocene and Pliocene due to the Late Alpine post-orogenic extensional tectonic on the territo- ry of Bulgaria. Šiškov (1997) combines these basins into two large coal provinces, i.e. Thracian and Soﬁa ones. The present study focuses on the Soﬁa Basin, the largest lignite basin within the Soﬁa coal bearing province (Fig. 1). It is nearly 80 km long and more than 20 km wide. The lignite seam is very extensively studied in the North part of the basin, where resources of more than 1.2 billion tons of low-grade coal are established. In previous studies, source speciﬁc biomarkers in Bulgarian Neogene coals and their diagenetic transformations during matura- tion are appreciated (Bechtel et al., 2005; Stefanova et al., 1995, 2005a, 2005b, 2008; Zdravkov et al., 2011). Later on investigations focused on the total biomarker assemblage of macroscopically iden- tiﬁed macrofossils (Stefanova, 2004; Stefanova and Simoneit, 2008) with the purpose to determine source speciﬁc biomarkers in the basins and to relate them to the peat-forming vegetation. Paleobo- tanical methods and geochemical techniques are used to appraise plant contribution and paleoenvironment for the Каrlоvо Basin (Stefanova et al., 2010). The intention is to relate paleovegetation change and climate oscillation based on pollen and statistical analy- sis with organic geochemical proxies for a Late Miocene lacustrine to paludal sedimentary succession (Stefanova et al., 2011). The petro- graphic composition of the lignite from the Soﬁa province basins are the focus of investigations by Kortenski and Sotirov (2004), Sotirov and Kortenski (2003), Zdravkov and Kortenski (2004), Zdravkov et al. (2006), and Zdravkov and Kortenski (2007). International Journal of Coal Geology 107 (2013) 45–61 ⁎ Corresponding author. E-mail address: maia@orgchm.bas.bg (M. Stefanova). 0166-5162/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coal.2012.09.014 Contents lists available at SciVerse ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo