5:15 PM - 6:30 PM
[BCG04-P14] Paleoenvironments of the Pliocene Kuwae Formation in the Kitakanbara region, Niigata prefecture
Keywords:paleoenvironment, Niigata prefecture, Pliocene Kuwae Formation, ostracods
It is known that the global climatic changes containing the mid-Pliocene warm period (MPWP) and the northern hemisphere glaciation (NHG) occurred during the Pliocene. The paleoceanography in the Japan Sea between 3.5 and 2.7 Ma differed from those in the recent, as cold water had flowed from the Pacific Ocean into the Japan Sea through the Tsugaru Strait, and a few warm currents flowed through the Tsushima Straits that was shallower than those of today (Itaki, 2016). Some cooling events relating to NHG were identified in the Japan Sea (eg., Sato et al., 2002). However, paleoceanographic shifts in the Japan Sea corresponding to the glacial- and interglacial-cycles are ambiguous yet. The Pliocene Kuwae Formation continuously crops along the Tainai River (Natsui Section) in the Niigata Prefecture. Investigations of microfossils and chemical composition of the sediments in this section found paleoenvironmental shifts corresponding to the glacial- and interglacial-cycles (Miwa et al., 2004; Yamada et al., 2005; Irizuki et al., 2007). However, the oceanic changes in the Japan Sea have been discussed by the results from only one section. Thus, our aims are to reveal the distribution of the Kuwae Formation around the Natsui section, and to reconstruct the spatial and temporal paleoenvironments corresponding to the glacial- and interglacial-cycles during the Pliocene based on fossil ostracods.
The study area was 7 km from north to south and 4 km from east to west. The Tainai River is in the central part of the study area. The Uchisugawa, the Kuwae, and Shimoterauchi formations are found upward. In addition, the Kuwae Formation containing shell fragments in some horizons, was distributed in the northeastern and southwestern parts of the area. It is divided into three parts based on the facies; Unitⅰ, massive mudstone, Unitⅱ, bioturbated mudstone, Unitⅲ, mudstone-dominated alternative beds of mudstone and sandstone. It is interpreted that the sediments in the three facies were deposited in the outer shelf, and each facies reflects the difference in the water depth within the outer shelf. Thirty-five samples from the left bank route along the Tainai River, 5 samples from the Sakai route, and 14 samples from the Natsui route were examined of fossil ostracod assemblages. As a result, at least 130 species belonging to 58 genera of ostracods were recognized. The most dominant taxa are Acanthocythereis dunelmensis s.l., Robertsonites tabukii, and Krithe spp.. Since these taxa are found from the bottom sediments deeper than the water depth of 150 m in the recent shelf and the slope, it is inferred that the Kuwae Formation was deposited in the shelf environment. The facies and ostracod assemblage results suggest that the Kuwae Formation was deposited under the influence of the Japan Sea Intermediate-Proper Water. Furthermore, at least two cycles in paleo-water depth change and relatively warm water mass existence in the southwestern and central parts of the study area were observed.
The study area was 7 km from north to south and 4 km from east to west. The Tainai River is in the central part of the study area. The Uchisugawa, the Kuwae, and Shimoterauchi formations are found upward. In addition, the Kuwae Formation containing shell fragments in some horizons, was distributed in the northeastern and southwestern parts of the area. It is divided into three parts based on the facies; Unitⅰ, massive mudstone, Unitⅱ, bioturbated mudstone, Unitⅲ, mudstone-dominated alternative beds of mudstone and sandstone. It is interpreted that the sediments in the three facies were deposited in the outer shelf, and each facies reflects the difference in the water depth within the outer shelf. Thirty-five samples from the left bank route along the Tainai River, 5 samples from the Sakai route, and 14 samples from the Natsui route were examined of fossil ostracod assemblages. As a result, at least 130 species belonging to 58 genera of ostracods were recognized. The most dominant taxa are Acanthocythereis dunelmensis s.l., Robertsonites tabukii, and Krithe spp.. Since these taxa are found from the bottom sediments deeper than the water depth of 150 m in the recent shelf and the slope, it is inferred that the Kuwae Formation was deposited in the shelf environment. The facies and ostracod assemblage results suggest that the Kuwae Formation was deposited under the influence of the Japan Sea Intermediate-Proper Water. Furthermore, at least two cycles in paleo-water depth change and relatively warm water mass existence in the southwestern and central parts of the study area were observed.