Variability in coal facies as reflected by organic petrological and geochemical data in Cenozoic coal beds offshore Shimokita (Japan) - IODP Exp. 337

This study reports on Cenozoic coal seams recovered at Integrated Ocean Drilling Program (IODP) Site C0020 during Expedition 337. IODP Site C0020 is located in a forearc basin formed by the subduction of the Pacific plate off Shimokita Peninsula (Japan). Hole C0020A penetrated 14 coal layers between 1825 and 2466 mbsf. Eleven of them were investigated within the frame of this paper. Investigated seams show a slight maturity increase with depth from lignite to sub-bituminous coal. In order to detect temporal changes in maceral and molecular composition and to relate them to changes in vegetation and depositional environment, macro- and micropetrographic data, bulk geochemical parameters, biomarker analysis, stable isotope geochemistry, and vitrinite reflectance measurements were performed. Results were also compared with palynological data obtained from 9 coal samples. Elevated sulfur contents and high ash yields occur in the upper seams (cores 14R to 18R) whereas low sulfur contents and varying ash yields were determined for the lower coal seams (24R to 30R). The maceral composition and biomarker ratios of the uppermost seams argue for coal formation in a paralic environment and brackish, alkaline water conditions. In contrast, lignite samples from the lower part of unit III point to a limnic-fluviatile deposition. Conifers contributed significantly to peat formation in the uppermost seams (from cores 15R to 22R) and in the lowermost lignite seam. In all other samples, angiosperms are considered as the major peat-forming plants. The pollen and spore floras indicate rich angiosperm vegetation, however significant contributions from Pinaceae and Taxodiaceae are evident for all coals. Sporophytes have no dominant influence on the coal flora. Microbial activity in the peat is suggested for instance by higher concentrations of hop-17(21)-ene with increasing contents of hopanes or by low δ¹³C values of hop-17(21)-ene. The revealed changes in the environment during coal deposition highlight the importance of combined organic petrography, organic geochemistry and palynology to reconstruct palaeoenvironmental conditions.