5:15 PM - 6:45 PM
[HQR05-P01] Developments and ages of marine deposits and dissected valley filling with organic sediments at the sea cliff in Menokoshi, Noheji-cho, Aomori
Keywords:Pleistocene, Noheji, marine sediments, valley-fill deposits
This study reports on the stratigraphy and age of marine deposits found in the sea cliff facing Mutsu Bay in the southern part of the Shimokita Peninsula, as well as on the formation mechanism of valley-fill deposits. The study area is generally called Menokoshi, Noheji-cho as an agricultural settlement area (administrative name; Aza Mukaida, Noheji-cho, Kamikita-gun, Aomori Prefecture). The flat surface (altitude 16-40 m) that slopes gently toward Mutsu Bay in the west is classified as Noheji terrace, which is marine terrace formed during MIS5e (Miura, 1968). The terrace cliffs of the Noheji terrace facies have been eroded by sea erosion, and the marine stratum that forms the base of the terraces can be observed along a total length of about 1.3 km down to the beach. On poster session, based on the results of geochemical analyses of tephra layers and radiometric dating of valley-fill sediments discovered during the exploration of the sea cliff, the stratigraphic relationships will be explained by showing a composite orthophoto image of the sea cliffs taken by a drone flying over the sea.
The K-Ar age of biotite included in the upper tephra layer of two layers in marine deposits is determined as 1.14 ± 0.06 Ma, therefore marine strata in this area have deposited during early Pleistocene. When it is based on this result, this marine sediment has been correlated with the upper part of the Hamada Formation, the Kattchi Formation, the Shimizume Formation, and their equivalents as the Pliocene to early Pleistocene formations shown in 1:200,000 geological map "Noheji", 2nd edition (Kudo et al., 2021). However, Kamata and Saito (2004) pointed out that Quaternary stratigraphic studies in this area, especially between older than the terrace components such as the Noheji terrace and their basement sediments in the Kamikita area and Shimokita Peninsula, have long been confused since the 1950s, by the lack of stratigraphic correlation using key layers and definite geologic ages for chronological flamework. Therefore, the stratigraphic correlations should be done with caution in this study.
Marine formation in this study is overlain by terrace components of angular unconformity, and organic sediments that filled dissected valleys can be observed at several locations. It is estimated that the valleys have been dissected the terrace deposits and marine formations after the terrace surfaces of the MIS5e stage were formed. Furthermore, among the valley-fill deposits that can be observed at several sites, deposit at one site was interbedded with Toya tephra (109±ca.3 ka; Tomiya and Miyagi, 2020), as known as a widespread tephra. On the other hand, Ishimura and Hiramine (2020) reported interbedded Towada-Chuseri tephra (5986-5899 cal. BP; Mclean et al., 2018) in another valley-fill deposit. We also obtained Holocene age data by 14C dating. Based on these results, we suggest that the marine terrace surface during MIS5e in this area have been dissected according to each of the repeated low sea-level periods, eroding the terrace components and/or the underlying marine strata, then valleys were filled up by organic sediments as the sea level rose.
References
Miura, O. (1968) Annals of the Tohoku Geographical Association; Kudo, T. et al., (2021) Geological Map of Japan 1:200,000, Noheji (2nd Edition); Kamata, K., and Saito, N. (2004) Regional Studies, Doctoral Course, Graduate School of Hirosaki University; Tomiya, A. and Miyagi, I. (2020) Bulletin of the volcanological society of Japan; Ishimura and Hiramine (2020) Journal of Quaternary Science; Mclean et al. (2018) Quaternary Science Reviews
The K-Ar age of biotite included in the upper tephra layer of two layers in marine deposits is determined as 1.14 ± 0.06 Ma, therefore marine strata in this area have deposited during early Pleistocene. When it is based on this result, this marine sediment has been correlated with the upper part of the Hamada Formation, the Kattchi Formation, the Shimizume Formation, and their equivalents as the Pliocene to early Pleistocene formations shown in 1:200,000 geological map "Noheji", 2nd edition (Kudo et al., 2021). However, Kamata and Saito (2004) pointed out that Quaternary stratigraphic studies in this area, especially between older than the terrace components such as the Noheji terrace and their basement sediments in the Kamikita area and Shimokita Peninsula, have long been confused since the 1950s, by the lack of stratigraphic correlation using key layers and definite geologic ages for chronological flamework. Therefore, the stratigraphic correlations should be done with caution in this study.
Marine formation in this study is overlain by terrace components of angular unconformity, and organic sediments that filled dissected valleys can be observed at several locations. It is estimated that the valleys have been dissected the terrace deposits and marine formations after the terrace surfaces of the MIS5e stage were formed. Furthermore, among the valley-fill deposits that can be observed at several sites, deposit at one site was interbedded with Toya tephra (109±ca.3 ka; Tomiya and Miyagi, 2020), as known as a widespread tephra. On the other hand, Ishimura and Hiramine (2020) reported interbedded Towada-Chuseri tephra (5986-5899 cal. BP; Mclean et al., 2018) in another valley-fill deposit. We also obtained Holocene age data by 14C dating. Based on these results, we suggest that the marine terrace surface during MIS5e in this area have been dissected according to each of the repeated low sea-level periods, eroding the terrace components and/or the underlying marine strata, then valleys were filled up by organic sediments as the sea level rose.
References
Miura, O. (1968) Annals of the Tohoku Geographical Association; Kudo, T. et al., (2021) Geological Map of Japan 1:200,000, Noheji (2nd Edition); Kamata, K., and Saito, N. (2004) Regional Studies, Doctoral Course, Graduate School of Hirosaki University; Tomiya, A. and Miyagi, I. (2020) Bulletin of the volcanological society of Japan; Ishimura and Hiramine (2020) Journal of Quaternary Science; Mclean et al. (2018) Quaternary Science Reviews