抄録
Distinctive plant fragment-concentrated sandstones are found in a turbiditic sequence deposited under deep sea environment in the Miocene Kawabata Formation distributed along the Higashiyama-gawa River, Yubari, central Hokkaido, Japan. We observed microscopically the plant fragments and analyzed terrestrial higher plant biomarkers in the sediments to evaluate sedimentological and hydrodynamic behaviors of plant particles by gravity flow depositional processes. Biomarker compositions such as pristane/phytane and regular sterane ratios indicate a large contribution of terrestrial organic matter, especially woody fragments, in the sandstone layer. The relative abundances of terrestrial higher plant terpenoids (pentacyclic triterpenoids, des-A-triterpenoids and diterpenoids) are high in all parts of the sandstone layer, while long-chain n-alkane concentrations are remarkably higher only at the uppermost part. Also, leaf cuticle fragments are mainly found in the uppermost part. These results suggest that plant leaves are mainly deposited in the uppermost part of the sandstone layer. Thus, it is concluded that the variations in plant fragment types (tissues) deposited are thought to be resulted from various hydrodynamic behavior in gravity flow.
