Abstract
Mudstones of the Shiiya Formation distributed along the Gomoto River, Niigata Pref. are classified into turbidite (Et) and hemipelagic (Ep) mudstones of a Bouma sequence. However, both Et and Ep kerogens from the Shiiya Formation consist mainly of terrigenous macerals (vitrinite and non-fluorescent amorphous organic matter: NFA), i.e., no evident differences are found in the maceral composition between Et and Ep, whereas the differences are obvious in the Teradomari Formation (Watanabe and Akiyama, 1996). On the other hand, the biomarker analysis of the Shiiya Formation indicates that Et mudstones contain abundant terrigenous triterpenoids, while those triterpenoids are absent or quite scarce in Ep mudstones. This characteristic biomarker composition suggests that Et mudstones contain more organic compounds of terrestrial origin than Ep, and that the kerogen macerals of marine origin in Ep should have been selectively decomposed during deposition. The difference between the Teradomari and Shiiya Formations in the organic matter composition is thought to be due to the different sedimentary environments: the Shiiya Formation was deposited at the more proximal, shallower and more oxic submarine fan environment than the Teradomari Formation.
