Coal characteristics, palynology, and palaeoenvironmental interpretation of the Yenikoy coal of Late Oligocene age in the Thrace Basin (NW Turkey)

Abstract

The Yenikoy coal field is located in a narrow area between the Black Sea coast and the Istranca (Standrja) Mountain in the Thrace Basin. The aims of this study are to characterize coal petrographical, mineralogical, palynological, geochemical, and sedimentological properties of the Yenikoy coal deposit and to reconstruct palaeoenvironmental conditions during peat accumulation. A total of 9 coal samples collected from the coal seam section are almost entirely composed of xylite-rich lithotype, and are characterized by low ash yield (7.9-21.1%, average 13.6%; on dry basis) and high total S contents (1.0-3.7%, average 2.2%; on dry basis). The macroscopic features, maceral composition, and coal fades suggest that the peat was accumulated under mesotrophic anoxic conditions, and the peat-forming vegetation consisted mainly of arboreal species (e.g. Cupressaceae, Ulmus, Carya, Zelkova, Alnus, Pterocarya and Quercus). The low HI values of the analysed samples are presumably related to presence of xylite-rich lithotype rather than the dominance of herbaceous plant and/or low preservation of organic matter in the palaeomire. Even though, relatively B-enrichments, high total S contents, and weak anomalies of some REY (e.g. Y, Ce, and Gd) in the analysed samples could refer possible sea water influence into palaeomire in the study area during Late Oligocene; the sedimentological and palynological (freshwater algaes: Pediastrurn, Spirogyra, and Mougeotia) data that indicate the palaeomire was located within a delta plain environment where small freshwater lakes and/or ponds developed. The statistical analysis and SEM-EDX data show the vast majority of elements in the analysed coal samples have inorganic affinity. The major elements Al, K, and enriched elements like B, V, Cr, Hg, and U seem to be affiliated with aluminosilicate minerals, whereas Fe, Ni, and As have probably sulphide affinity. The detrital presences of apatite crystals as detected by SEM-EDX analysis indicate that REY have mainly phosphate affinity in the analysed samples. Vertical distributions of the elements and ash yields suggest that the detrital input ratio was changeable and controlled some elemental enrichment during peat accumulation. These variations could be related to development of and fluvial conditions in the study area which interrupted peat-accumulation several times during Late Oligocene.