Lignite and carbonate deposition in Middle Lignite succession of the Soma Formation, Soma coalfield, western Turkey

Abstract

The middle Miocene carbonate-dominated units that are called the Middle Lignite succession of the Soma Formation from the Soma coalfield of western Turkey were analyzed and two facies associations containing eight facies were determined from the detailed measured sections of the opencast lignite mines. The sedimentological, stratigraphical, and palaeobotanical data, and regional geology indicate alluvial and/or floodplain, shallow freshwater carbonate lakes, and lake margin environments, including forest mountain swamps in the intramontane basin which was developed on the collision-related basement rocks. The Middle Lignite succession displays cyclic depositional nature consisting of alternating fine-grained siliciclastic rock, lignite and freshwater carbonate rock. Some diagenetic features and algal laminations/structures in carbonate rocks, as well as paleosol relics and geological evidence of the lignite-forming paludal (swampy) conditions suggest alternating wet and dry periods in depositional system. The lignite deposition occurred in wetlands that formed during deposition of the Middle Lignite succession in alluvial plain and shallow carbonate lake-margins. The paleobotanical/palynological considerations, sedimentological features and depositional setting of the Middle Lignite succession indicate a seasonal, warm and semi-arid or dry climate for lignite deposition. The subsidence, sediment supply, seasonal climate, topography, and pre-Miocene carbonate basement may suggest main factors for lignite formation and preservation.