Depositional processes of plant fragment-concentrated sandstones in turbiditic sequences recorded by plant biomarkers (Miocene Kawabata Formation, Japan)

Abstract Occasionally, distinctive plant fragment (PF)-concentrated sandstone beds are found in the turbiditic sequence of the Miocene Kawabata Formation (Yubari, central Hokkaido, Japan) deposited in a foredeep basin. Sedimentological and organic geochemical analyses were carried out on the PF-concentrated sandstone bed to investigate the transport and depositional processes of the abundant PFs in the basin. The PF-concentrated sandstone bed was subdivided into lower and upper PF-concentrated layers based on their sedimentary structures. The lower layer contains chaotically dispersed PFs, and the upper layer is characterized by PF laminae. The dispersed PFs in the lower layer were likely deposited by rapid depositional processes such as freezing or rapid fall-out of suspended particles. The huge amount of PF deposition was confirmed by biomarker data such as sterane distributions and pristane/phytane ratios. The compositions of n-alkanes and diterpenoids in the PF-concentrated sandstone bed are distinct from those in the other sandstone and mudstone beds. These results imply that the terrigenous particles originated from specific terrestrial higher plants that were effectively transported and deposited, resulting in formation of the PF-concentrated sandstone bed. The distinctive sedimentary structures observed in the sequences could be attributed to a large flood discharge and the huge volume of PFs might have been transported directly from an upper reach of river and then reached the deep-sea basin by density flows, such as relatively concentrated hyperpycnal flows.