A multidisciplinary study and palaeoenvironmental interpretation of middle Miocene Keles lignite (Harmancık Basin, NW Turkey), with emphasis on syngenetic zeolite formation

The Harmancık Basin, in the north-easternmost Miocene graben in western Anatolia, hosts a 12.6-m-thick coal seam located in the Keles coalfield, in which coals are being exploited by open-cast mining methods. Syngenetic clinoptilolite/heulandite-type zeolite formation in the upper part of coal seam and carbonaceous clayey diatomite as a roof rock have been identified for the first time, and the palaeoenviromental reconstruction of the coal seam was conducted using a multidisciplinary approach. The coal facies and palynological data show that vegetation and depositional changes took place during the middle Miocene, which resulted in vertical variations in elemental and mineralogical compositions. During the initial stages of mire development woody vegetation (e.g. pollen with affinity to Cupressaceae) prevailed, telmatic conditions were common, and preservation of organic matter was high due to anoxic conditions. Thus, relatively low-ash yield was observed in the lower and middle parts of the coal seam. Furthermore, the presence of kaolinite and smectite-type clay mineral aggregates in these parts of the coal seam suggests that alteration of synchronous volcanic inputs took place under weak acidic to neutral conditions. In contrast, limnotelmatic conditions prevailed during the late stages of peat-accumulation, and macrophytes coinciding with herbaceous peat-forming vegetation (e.g. Osmundaceae, Polypodiaceae, and Nymphaeaceae) were dominant. The elevated Gelification Index (GI) values in the uppermost parts of the coal seam could be related to development of alkaline conditions in the palaeomires, which also caused formation of syngenetic clinoptilolite/heulandite-type zeolite from the alkaline activations of synchronous volcanic inputs. Even though palynological data points to the prevalence of freshwater conditions during peat-accumulation, B enrichments along Sr/Ba ratio higher than 1.0 could point to possible marine influence; however, no Neogene marine deposits have been identified in the Harmacik Basin. Nevertheless, the SEM-EDX data show the presence of traceable Ba and Sr in clinoptilolite/heulandite grains, and Sr-bearing barite around feldspar grains in the studied samples from the upper parts of the coal seam. This implies K-feldspars and K-rich alkali-feldspars, derived from synchronous volcanic ash fall, altered under alkaline conditions. Moreover, alginite proportions increased towards the upper parts of the seam, while relatively high Hydrogen Index (HI) values were reached in the uppermost part of coal seam and carbonaceous clayey diatomite roof-rock sample. Furthermore, palynolgical data imply that vegetation changes towards the roof of the coal seam reflect the progressive development of more humid conditions and nutrient-rich surface waters, which favoured increased algal activity.