3:45 PM - 4:00 PM
[HCG32-06] Evaluation of plant-fragment concentration mechanism in the turbiditic sediments in the Bouma Sequence by biomarker analysis in the Kawabata Formation, Yubari area, central Hokkaido, Japan.
Keywords:Turbidite, Biomarker, Hokkaido
Kawabata Formation distributed in Yubari area, central Hokkaido, is formed by basin-filled and slope-apron turbidite systems, and sandstone and mudstone are mainly predominated as turbiditic sequence. Distinctive plant-fragment concentrated sandstones were found in the Kawabata Formation, and their sedimentological and biomarker analyses suggested that these sediments were deposited by hyperpycnal flow (Furota et al., 2014). However, such thin layer concentrated plant fragments are also formed in the typical turbidite characterized as the Bouma sequence. In the present study, we conducted biomarker analysis of sandstone and mudstone characterized as the Bouma sequence from the Kawabata Formation to evaluate sedimentological processes and to compare plant-fragment concentrated sandstones in hyperpycnite.
The sandstones and mudstones analyzed were collected from the route of the Soumokumai-zawa River, Yubari, central Hokkaido, during 2017. For the biomarker analysis, lipids were extracted with dichloromethane/methanol, and separated to aliphatic, aromatic and polar fractions. These lipids were identified and quantified by GC-MS and GC-FID.
The turbiditic sequence analyzed is about 15cm thick, and corresponds to the Bouma sequence, but no containing Td unit. Sedimentary layer equivalent to Ta units consists of medium-fine sands with tiny black plant-fragments. Tb unit consists of fine sands with parallel laminations containing the concentrated plant fragments. Tc unit also consists of fine sands about 1-2cm thick without cross laminations, and the concentrated plant fragments are observed as many black bands in this unit. Te unit is massive sediment that consists of very fine sands to silty muds without plant fragments.
Concentrations of n-alkane in the turbiditic sequence are the minimal values in the Ta units. The concetrations increase in Tb and Tc, and then decrease in Te units. This trend is also shown in variations in the concentrations of plant-derived terpenoids. Pristane/Phytane ratios are the highest value (ca. 5) in Tb and Tc units, indicating much higher contribution of terrestrial plant-derived particles in these sedimentary phases. The ratios of C27/C29 sterane, which are indicators of marine / terrigenous material ratios as source of organic matter, tend to decrease from Ta to Tc units, and are especially about 0 at Tc unit. On the other hand, the C27/C29 sterane ratio of the Te unit shows the highest value in the turbiditic sequence. These results imply that terrigenous components were separated and concentrated in specific layers by turbidity flow, which differs from continuous transport of terrigenous organic matter by hyperpycnal flow. Triterpenoids and diterpenoids are useful as angiosperm and gymnosperm (conifer) biomarkers, respectively. The diterpenoid compositions and HPP, which is an indicator of coniferous vegetation, are hardly changed throughout Ta to Te units. Triterpenoid compositions are also almost constant within the turbiditic sequence. These results suggest that the terrigenous materials were homogenized in sediments during transporting and reworking by turbidity current. In addition, A-ring degraded triterpenoid (des-A triterpenoids) were detected as major components in all sediment units. The des-A triterpenoids are thought to be generated by microbial degradation during early diagenesis. The des-A triterpenoid compositions are also nearly constant as the results of diterpenoids and non-degraded triterpenoids. However, the ratios of des-A triterpenoid/pentacyclic (non-degraded) triterpenoid are higher throughout the turbiditic sequence, especially Tb and Tc units. Furota et al. (2014) reported that the des-A/pentacyclic triterpenoid ratios in sandstone units were clearly lower than mudstone units in the hyperpycnite, suggesting that more fresh terrigenous organic matter is rapidly transported by the hyperpycnal flow. On the other hand, our results imply that terrigenous matter is not rapidly deposited and reworked by turbidity current forming a typical Bouma sequence, and might be diagenetically altered in these processes. Thus, it is suggested that organic geochemical features are clearly different between the hyperpycnal and typical turbidity flows, and the hyperpycnal flow system occurs the effective transport and deposition of fresh terrigenous organic matter compared to those by turbidity current.
The sandstones and mudstones analyzed were collected from the route of the Soumokumai-zawa River, Yubari, central Hokkaido, during 2017. For the biomarker analysis, lipids were extracted with dichloromethane/methanol, and separated to aliphatic, aromatic and polar fractions. These lipids were identified and quantified by GC-MS and GC-FID.
The turbiditic sequence analyzed is about 15cm thick, and corresponds to the Bouma sequence, but no containing Td unit. Sedimentary layer equivalent to Ta units consists of medium-fine sands with tiny black plant-fragments. Tb unit consists of fine sands with parallel laminations containing the concentrated plant fragments. Tc unit also consists of fine sands about 1-2cm thick without cross laminations, and the concentrated plant fragments are observed as many black bands in this unit. Te unit is massive sediment that consists of very fine sands to silty muds without plant fragments.
Concentrations of n-alkane in the turbiditic sequence are the minimal values in the Ta units. The concetrations increase in Tb and Tc, and then decrease in Te units. This trend is also shown in variations in the concentrations of plant-derived terpenoids. Pristane/Phytane ratios are the highest value (ca. 5) in Tb and Tc units, indicating much higher contribution of terrestrial plant-derived particles in these sedimentary phases. The ratios of C27/C29 sterane, which are indicators of marine / terrigenous material ratios as source of organic matter, tend to decrease from Ta to Tc units, and are especially about 0 at Tc unit. On the other hand, the C27/C29 sterane ratio of the Te unit shows the highest value in the turbiditic sequence. These results imply that terrigenous components were separated and concentrated in specific layers by turbidity flow, which differs from continuous transport of terrigenous organic matter by hyperpycnal flow. Triterpenoids and diterpenoids are useful as angiosperm and gymnosperm (conifer) biomarkers, respectively. The diterpenoid compositions and HPP, which is an indicator of coniferous vegetation, are hardly changed throughout Ta to Te units. Triterpenoid compositions are also almost constant within the turbiditic sequence. These results suggest that the terrigenous materials were homogenized in sediments during transporting and reworking by turbidity current. In addition, A-ring degraded triterpenoid (des-A triterpenoids) were detected as major components in all sediment units. The des-A triterpenoids are thought to be generated by microbial degradation during early diagenesis. The des-A triterpenoid compositions are also nearly constant as the results of diterpenoids and non-degraded triterpenoids. However, the ratios of des-A triterpenoid/pentacyclic (non-degraded) triterpenoid are higher throughout the turbiditic sequence, especially Tb and Tc units. Furota et al. (2014) reported that the des-A/pentacyclic triterpenoid ratios in sandstone units were clearly lower than mudstone units in the hyperpycnite, suggesting that more fresh terrigenous organic matter is rapidly transported by the hyperpycnal flow. On the other hand, our results imply that terrigenous matter is not rapidly deposited and reworked by turbidity current forming a typical Bouma sequence, and might be diagenetically altered in these processes. Thus, it is suggested that organic geochemical features are clearly different between the hyperpycnal and typical turbidity flows, and the hyperpycnal flow system occurs the effective transport and deposition of fresh terrigenous organic matter compared to those by turbidity current.