Spatial and temporal variability in vegetation and coal facies as reflected by organic petrological and geochemical data in the Middle Miocene Çayirhan coal field (Turkey)

Abstract

Coal channel samples of Middle Miocene age are collected from the first (Tv) and second (Tb) seams from underground mines and from exploration boreholes within the Çayirhan coal field in the Beypazari Basin (Turkey). They are investigated in order to detect spatial and temporal changes in maceral and molecular composition of coal and to relate them to changes in vegetation and depositional environment.The mean random reflectance values of ulminite (0.40% Rr) indicate a lignite to subbituminous-C coal in rank. Maceral composition and biomarker ratios of the samples from both seams at the Çayirhan deposit argue for coal formation in a limno-telmatic environment under dysoxic to anoxic conditions. Alkaline surface waters of changing pH-values and a high and unstable water level at the palaeomire are evidenced by moderate to high gelification index (GI) and ground water influence (GWI) values, as well as high sulphur contents. Variations in tissue preservation index (TPI) and vegetation index (VI) values point to minor variations in the palaeovegetation during peat formation. Herbaceous plants dominated in both Tb and Tv palaeomires (low TPI and VI values), whereas in the surroundings arboreal vegetation were predominant.The decreasing trends in pristane/phytane ratios and carbon preference index (CPI) values towards the NE are suggested to reflect oxygen deficient conditions during peat formation due to a higher (ground)water level in this part of the basin. The occurrence of C29 diasterenes in low abundances provides evidence for periods of lower pH in the mire. The borehole samples from the NE show slightly enhanced contributions of n-alkanes from algal and microbial sources. The terpenoid hydrocarbons present in the lignite argue for a major contribution of angiosperms to peat formation and slightly enhanced proportions of gymnosperms in the Tv palaeomire in the NE. Based on the high concentrations of lupane-type triterpenoids in the coal seams from the underground mine, a higher density of Betulaceae in the arboreal vegetation in the SW is indicated.