The depositional conditions and the palaeoenvironment of the Achlada xylite-dominated lignite in western Makedonia, Greece

Abstract

The depositional palaeoenvironment of the Neogene Achlada lignite deposit (western Makedonia, Greece) was deciphered with respect to the evaluation of the macro-petrographic characteristics, and the data resulted from coal petrographic analyses.The total thickness of the studied lignite seam is ~26m and consists of organic and inorganic alternations. The macro-petrographic observations suggest extremely high contributions of woody-originated tissue (xylite) from a single tree type, among the lignite layers, whereas, numerous in situ stems and trunks were observed. The results of the coal petrographic analysis indicate the dominance of huminite (88.3vol.%), whereas liptinite and inertinite show much lower values (11.0vol.% and 0.7vol.% respectively). High contribution of mineral matter is also observed in many cases. Ulminite types A and B are the most abundant macerals, whereas the content of ulminite B is followed by the xylite content. Facies modeling using maceral composition and maceral indices suggested the alternation of forest swamp and reed-moor environments where peat accumulated under highly-saturated and intensely-anoxic conditions. A terrestrial origin with wet forest to piedmont plain depositional conditions was suggested by the Gelification Index (GI) and Tissue Preservation Index (TPI) plot. A cross-plot of the ground water index (GWI) vs. the vegetation index (VI) suggested swamps under mesotrophic and rheotrophic hydrogeological conditions, where the dominant source for the peat was woody vegetation and vegetation high in preservation potential.The overall macro and micro characteristics of the Achlada lignite suggest a depositional palaeoenvironment that includes three independent parts: a) the main channel of a meandering river system; b) the floodplain area; and c) the oxbow lake. The establishment of telmatic/limnotelmatic conditions in this palaeoenvironment resulted in the formation of the entire Achlada lignite deposit, whereas numerous flood episodes interrupted the peat accumulation and resulted in the deposition of clay-rich sediments.