5:15 PM - 6:45 PM
[MIS08-P01] Organic geochemical analysis and reconstruction of the depositional environment of Oma Formation in Oma-zaki area, Shimokita Geopark.
Keywords:Geopark, Shimokita Geopark, Miocene, Organic geochemistry
Shimokita region is composed of four geological features that form Northeast Japan Arc.
Especially, Hardshale and volcanic rocks deposited during Miocene are exposed in northern Shimokita Peninsular. These hardened rocks form a topographic rise extending to the seabed and contribute to the formation of rich fishing grounds off the coast of Oma.
The geological features distributed in western Shimokita Peninsula have been considered to be an area recorded expansion of Japan sea. However, the geological history of Shimokira Geopark has been changing. Hotokegaura, conventionally regarded as a green tuff deposited during Miocene, was reported to be the ejecta of a submarine caldera eruption in early Pliocene (Ueda et al.,2023).
In addition, few studies about sedimentological and paleoceanographic have been conducted in Shimokita Peninsular. There have been few examples of studies on the Oma Formation, and even those studies have focused on geological structures. In this study, we conducted a geological survey and organic geochemical analysis of Oma Formation and reconstruct the sedimentary environment of the northern Shimokita Peninsula during Miocene.
The hardshale distributed around Oma-zaki are described as Oma Formation. Oma Formation consists of alternating layers of siliceous hardshale and tuffaceous sandstone. The age of Oma Formation is estimated to be Miocene because it interbedded tuff that is thought to be of the same source as the Ikokuma andesites of the Upper Miocene.Oma formation is compared with the Kodomari Formation in the northern Tsugaru Peninsula and the Yakumo Formation in southern Hokkaido. In this study, hardshales and tuffaceous sandstone sections were sampled from the river exposed in Oma Formation. Organic geochemical analysis was carried out by extracting free form components from the sediments of Oma Formation, which were measured and analyzed by GC-MS.
The concentration of Biomarker in Oma Formation is very low, n-alkane concentration of 120ng/g-sedi.
HBI alkanes and HBI thiophenes and C28 steranes were detected in the hard shales, strongly suggesting a diatom contribution. On the other hand, diatom biomarkers were detected in the altered tuffaceous sandstone. These results suggest that the Oma Formation is an environment in which diatom-dominated sediments are deposited, and that alternating layers were formed by transport and deposition of organic-poor volcanic debris from surrounding volcanos.
The marine/terrestrial ratio (C27/C27+C29 sterane ratio) shows 0.4-0.63, it indicates the marine algal component, suggesting that the western Shimokita region was a pelagic depositional environment at that time. On the other hand, components derived from terrestrial higher plants were also detected. Especially Friedeline and degraded triterpenoids derived from angiosperm were detected in all samples. It has been pointed out that submarine volcanos were activated from the western Shimokita Peninsula to southern Hokkaido at that time, suggesting terrestrial materials may have been transported from the volcanic islands.
Pristane/Phytane ratio, which indicates the redox index shows highly anoxic condition (Pr/Ph=0.74 - 0.46). On the other hand, the homohopane index was slightly oxidative condition (HHI=0.05). Previous studies in Akita basin have suggested that the difference in trends between the HHI and Pr/Ph is due to supply of α-tocopherol, a precursor of pristane and mainly included in terrestrial higher plants. Pr/Ph may shift towards more reducing values, especially in a pelagic environment due to low supply of α-tocopherol (Asahina et al.,2021).
ased on the low contribution of terrestrial plants and the low amount of organic matter in the hardshales, we interpreted that the depositional environment of northern Shimokita Peninsula was pelagic and slightly oxic condition. Kudo et al. (2020) reported that the eastern Shimokita Peninsula deposited pelagic mudstones during the Middle Miocene, suggesting that a pelagic environment prevailed widely in Miocene.
Especially, Hardshale and volcanic rocks deposited during Miocene are exposed in northern Shimokita Peninsular. These hardened rocks form a topographic rise extending to the seabed and contribute to the formation of rich fishing grounds off the coast of Oma.
The geological features distributed in western Shimokita Peninsula have been considered to be an area recorded expansion of Japan sea. However, the geological history of Shimokira Geopark has been changing. Hotokegaura, conventionally regarded as a green tuff deposited during Miocene, was reported to be the ejecta of a submarine caldera eruption in early Pliocene (Ueda et al.,2023).
In addition, few studies about sedimentological and paleoceanographic have been conducted in Shimokita Peninsular. There have been few examples of studies on the Oma Formation, and even those studies have focused on geological structures. In this study, we conducted a geological survey and organic geochemical analysis of Oma Formation and reconstruct the sedimentary environment of the northern Shimokita Peninsula during Miocene.
The hardshale distributed around Oma-zaki are described as Oma Formation. Oma Formation consists of alternating layers of siliceous hardshale and tuffaceous sandstone. The age of Oma Formation is estimated to be Miocene because it interbedded tuff that is thought to be of the same source as the Ikokuma andesites of the Upper Miocene.Oma formation is compared with the Kodomari Formation in the northern Tsugaru Peninsula and the Yakumo Formation in southern Hokkaido. In this study, hardshales and tuffaceous sandstone sections were sampled from the river exposed in Oma Formation. Organic geochemical analysis was carried out by extracting free form components from the sediments of Oma Formation, which were measured and analyzed by GC-MS.
The concentration of Biomarker in Oma Formation is very low, n-alkane concentration of 120ng/g-sedi.
HBI alkanes and HBI thiophenes and C28 steranes were detected in the hard shales, strongly suggesting a diatom contribution. On the other hand, diatom biomarkers were detected in the altered tuffaceous sandstone. These results suggest that the Oma Formation is an environment in which diatom-dominated sediments are deposited, and that alternating layers were formed by transport and deposition of organic-poor volcanic debris from surrounding volcanos.
The marine/terrestrial ratio (C27/C27+C29 sterane ratio) shows 0.4-0.63, it indicates the marine algal component, suggesting that the western Shimokita region was a pelagic depositional environment at that time. On the other hand, components derived from terrestrial higher plants were also detected. Especially Friedeline and degraded triterpenoids derived from angiosperm were detected in all samples. It has been pointed out that submarine volcanos were activated from the western Shimokita Peninsula to southern Hokkaido at that time, suggesting terrestrial materials may have been transported from the volcanic islands.
Pristane/Phytane ratio, which indicates the redox index shows highly anoxic condition (Pr/Ph=0.74 - 0.46). On the other hand, the homohopane index was slightly oxidative condition (HHI=0.05). Previous studies in Akita basin have suggested that the difference in trends between the HHI and Pr/Ph is due to supply of α-tocopherol, a precursor of pristane and mainly included in terrestrial higher plants. Pr/Ph may shift towards more reducing values, especially in a pelagic environment due to low supply of α-tocopherol (Asahina et al.,2021).
ased on the low contribution of terrestrial plants and the low amount of organic matter in the hardshales, we interpreted that the depositional environment of northern Shimokita Peninsula was pelagic and slightly oxic condition. Kudo et al. (2020) reported that the eastern Shimokita Peninsula deposited pelagic mudstones during the Middle Miocene, suggesting that a pelagic environment prevailed widely in Miocene.