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Event deposits are deposits formed by tsunamis and floods, and are known as the only proxies recorded depositional processes of past disasters. Sedimentological analysis has been traditionally used to study event deposits, contributing to the estimation of tsunami inundation areas and the probability of occurrence. On the other hand, some event deposits are destroyed or lost after deposition, and have been difficult to identify event deposits. So the development of new research methods that could contrast and complement sedimentological methods has recently attracted attention. In the present study, I conducted organic geochemical analysis of tsunami and event deposits with focus on degraded organic molecules to evaluate the sedimentological process of event deposits and establish proxy could be identified past-event deposits.Tsunami deposit is mainly characterized by mixture of marine sand and sedimentary structures formed by huge tsunami flow. In many cases, however, sedimentological information recorded as the structure of tsunami deposit was erased by erosion and disturbance during the post-depositional processes. In the present study, we performed organic geochemical analyses using biomarker and kerogen of the peat sediment cores from the Akkeshi and Kiritappu areas, eastern Hokkaido, Japan to evaluate the sedimentological and geochemical features of the paleo-tsunami deposits. In kerogen analysis, terrestrial plant fragments are mainly observed in all sand layers, indicating the redeposition of peat-derived matter by tsunami. However, the inner organic lining of foraminifera and the marine dinocyst could be identified in the tsunami layers. These reults can be direct evidences of paleo-tsunami. In steroid biomarkers, stanols could be detected in all samples of the Akkeshi and Kiritappu cores. The stanol conversion reaction from biosterol is thought to occur by microbial reduction in the sediment–water interface and anoxic water column of marine and lacustrine environments. Therefore, the stanol/sterol ratio can be used as redox indicator. We found that the C27 stanol/sterol (cholestanol/cholesterol) ratios are clearly higher than those of C29 in the sand layers of both Akkeshi and Kiritappu cores. The higher C27 stanol/sterol ratios suggest the contribution of marine compounds deposited under more reduced condition in the tsunami layer. However, some sand layers do not show this trend, and they can be considered event deposits of other origin.In the middle Miocene of central Hokkaido, island-arc collision resulted in the formation of narrow foreland basins, Ishikari and Hidaka basins, extending about 400 km north-south and several tens of kilometers wide. Previous studies (Kawakami, 2013) have shown that a number of turbidite beds, Kawabata Formation, containing large amounts of terrestrial organic matter derived from higher plants have been identified. This suggested that a sedimentary system in which terrestrial organic matter was directly transported from the land to the deepseafloor was prevalent in the Ishikari Basin. In the present study, we conducted sedimentological and organic geochemical analyses on turbidites of the Abetsu Formation deposited in the Hidaka Basin to reconstruct sedimentary processes that occurred in the Hidaka Basin in the middle Miocene. In the Hobetsu area of Mukawa-city, south-central Hokkaido, sand and mud alternations deposited in the Hidaka Basin during the middle to late Miocene are widely distributed. The upper part of the Abetsu Formation (coarse-grained and sandstone/mudstone alternation) and the Nibutani Formation (fine-grained sandstone alternation) are exposed at the Horokanbe-sawa River. We analyzed two thin, fine-grained turbiditic sequences collected from the upper part of the Abetsu Formation. These are mainly composed of three units from the lower part: massive sandstone part, organic lamination part, and mudstone part. For organic geochemical analysis, the turbiditic sequences were divided into units about 1 cm thick, focusing on the changes in sedimentary structure. Diterpenoids gymnosperm components), triterpenoids (angiosperm components), and des-A terpenoids (angiosperm components generated by microbial degradation under anoxic conditions) were detected as major biomarkers in the turbidites from the Abetsu Formation. The concentrations of diterpenoids and des-A terpenoids nearly constant throughout the sequences, whereas the triterpenoids significantly vary, and especially, concentrated in the from the top of the massive sandstone to the lamination section. These results suggests that the turbulent flow-forming turbidites in the Abetsu Formation contained large amounts of components originated from angiosperms. In the Kawabata Formation, the des-A terpenoids were remarkably abundant rather than triterpenoids and diterpenoids in the turbidites (Furota et al., 2014). These differences indicate that the terrigenous matter in the turbidites of the Abetsu Formation was transported from different type land area and/or different source vegetation to the deep sea. The terrestrial/marine organic matter ratio using steroid (sterane) composition was evaluated in the turbidite sequence. The sterane index shows a high proportion of terrestrial organic matter over the massive sandstone and lamination part, with a rapid shift to marine organic matter in the mudstone part. This trend is observed in the both Abetsu turbidites, but some sequences having a higher contribution of marine organic matter throughout the sequence. Such trends were also confirmed by the pristane/phytane index (Pr/Ph), which indicates the redox condition and contribution of terrestrial organic matter.Aliphatic and aromatic degraded triterpenoids (TTs) including des-A and des-E TTs were investigated in the turbidite and hemipelagic mudstones from the Miocene Kawabata Formation (Ishikari basin) and Abetsu and Nibutani formations (Hodaka basin), south-central Hokkaido, Japan. The des-A TTs, having the carbon skeletons of oleanane, ursane, and lupane, are derived from angiosperm, and the des-E TTs, having the carbon skeletons of hopane, are bacterial origin. These compounds are thought to be formed by the microbial degradation under dysoxic and anoxic conditions. We found that the concentrations and relative abundances per TOC of total degraded TTs, especially des-A TTs, were remarkably higher in the Kawabata Formation, and significantly contained in the Abetsu and Nibutani formations. These results clearly indicate that the huge amounts of the des-A TTs were possibly transported and deposited in the Ishikari basin during the deposition of the Kawabata Formation of the late Miocene. The degraded TTs/TOC ratios are correlated with the values of the aquatic macrophyte n-alkane proxy (Paq) in the Kawabata and Abetsu formations. The higher Paq values are interpreted to be high contribution of aquatic and submerge/floating macrophytes, and moreover, were commonly observed in lake and pond environments. Thus, large amounts of the degraded TTs were possibly produced by biodegradation of the transported angiospermous TTs in the dysoxic or anoxic environments such as ponds and wetlands. Furthermore, it is presumed that the organic matter deposited in the Ishikari basin was transported from wetland or marsh areas of the paleo-Hidaka Island. The class distributions of the aliphatic and aromatic degraded TTs varied in the samples from these formations. In the Kawabata Formation, the des-A lupanes are detected as major compounds, but the des-A oleananes are the most major compounds in aliphatic degraded TTs in the Abetsu and Nibutani formations. The higher relative abundances of the des-A lupanes in the only Ishikari basin (Kawabata Formation) suggest that the TTs had been supplied from mountain forest areas, where lupenoid-producing woody plant taxa might be spread. Meanwhile, the des-A lupanes are less abundant in the Abetsu and Nibutani formations, suggesting no or less supply of TTs from the mountain forest areas in the Hidaka basin.According to these studies, we believe that the degraded organic molecules formed under anoxic environments are important markers. The anoxic environments are distributed in only a limited range of natural environments and the composition of degraded organic molecules vary by location to setting. It could be pointed out that degraded organic molecules are highly useful in event deposit studies, especially in estimating the sources of event flows and reconstructing transport processes.
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