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[HCG21-P06] Paleoenvironment indicated by fossil forests and paleosols in the Hitoyoshi Formation of the Pliocene
Keywords:Pliocene, Sedimentary environment, Paleosol, Hitoyoshi Formation
Introduction
The Kuma River bed in Hitoyoshi City, Kumamoto Prefecture, contains a fossil forest composed of numerous tree-trunk fossils, and paleosols spread around it. In the Japanese Islands, plant fossils show that warm and humid forests declined, and cool-temperate trees increased after 2.9 Ma as the climate became colder (Momohara, 2016). In this study, we aim to reconstruct the terrestrial environment of that time using the paleosols developed in the Hitoyoshi Formation, which is distributed in the Kyushu region.
Geological setting
The Hitoyoshi Formation is divided into an upper part dominated by tuffaceous mudstone and a lower part dominated by conglomerate and sandstone of Shimanto Belt and Hizakata Volcanic Group origin (Tamura et al., 1962). Sedimentary facies analysis of the lower part of the Hitoyoshi Formation was conducted by Hayashi et al., 2007, and it was interpreted as fan deltaic sediments that developed around a lake. Fossils indicating a lake or marsh environment, such as water chestnut fossils (Imanishi and Miyahara, 1972), freshwater shell fossils (Tamura et al., 1962), and freshwater sponge fossils (Matsuoka et al., 2006), have been reported from the upper part of the Hitoyoshi Formation. The sedimentary age of the Hitoyoshi Formation has been investigated in detail by measuring the K-Ar age of tuff. The lowest part of the Hitoyoshi Formation, the Funado Tuff Member, is 2.72±0.25Ma, and the uppermost part of the Hitoyoshi Formation, the Yamada Tuff, is 2.58±0.08Ma (Torii et al., 1999).
Sedimentary environment
Field observations were carried out in the western part of the Hitoyoshi Basin, and columnar sections were created for the riverbed of the Kuma River and the slope of the Kuma Village Sports Park. In. As a result, the lower part of the Hitoyoshi Formation is composed of Lower Fan sediments, in which gravelly braided river deposits predominate, and Upper Fan sediments, in which debris flow deposits predominate, and it is thought that the lower part of the Hitoyoshi Formation formed an alluvial fan. On the other hand, the upper part of the Hitoyoshi Formation is composed of unstratified siltstone intercalated with muddy turbidite layers, and is thought to be lacustrine.
Paleosols of fossil forests
The lower part of the Hitoyoshi Formation contains a fossil forest layer that is rich in tree trunk fossils and corresponds to the floodplain facies. This layer is located near the boundary between the lower part of the Hitoyoshi Formation (alluvial fan sediments) and the upper part of the Hitoyoshi Formation (lacustrine sediments), and it is thought that the lakeside forest at the time is preserved. The paleosols of the fossil forest are clearly differentiated into three soil horizons: A, which is rich in black organic matter; B, which is white in color; and C, which retains its sedimentary structure. In particular, A is fine-grained, and slicken lines that cross the bedding plane are well developed. In addition, the Hitoyoshi Formation universally contains abundant pumice and volcanic glass, and the parent material of the soil is volcanic ash.
Discussion
Volcanic ash soils are considered to be easily pedogenized because they contain a large amount of volcanic glass (Shoji et al., 1993). Furthermore, the fact that the paleosols are concentrated in the lower part of the Hitoyoshi Formation (alluvial fan environment) and are hardly found in the upper part of the Hitoyoshi Formation (lacustrine formation), and the fact that strata containing abundant tree-trunk fossils have been identified in the floodplain near the lake margin, suggest that environmental changes around the lake may have had a significant impact on soil formation and vegetation. In addition, the slickens lines that develop in the paleosol are thought to have been formed by the generation of clay minerals and their seasonal swelling and shrinking, indicating that the paleosol of the fossil forest is strongly pedogenic and a mature soil.
The Kuma River bed in Hitoyoshi City, Kumamoto Prefecture, contains a fossil forest composed of numerous tree-trunk fossils, and paleosols spread around it. In the Japanese Islands, plant fossils show that warm and humid forests declined, and cool-temperate trees increased after 2.9 Ma as the climate became colder (Momohara, 2016). In this study, we aim to reconstruct the terrestrial environment of that time using the paleosols developed in the Hitoyoshi Formation, which is distributed in the Kyushu region.
Geological setting
The Hitoyoshi Formation is divided into an upper part dominated by tuffaceous mudstone and a lower part dominated by conglomerate and sandstone of Shimanto Belt and Hizakata Volcanic Group origin (Tamura et al., 1962). Sedimentary facies analysis of the lower part of the Hitoyoshi Formation was conducted by Hayashi et al., 2007, and it was interpreted as fan deltaic sediments that developed around a lake. Fossils indicating a lake or marsh environment, such as water chestnut fossils (Imanishi and Miyahara, 1972), freshwater shell fossils (Tamura et al., 1962), and freshwater sponge fossils (Matsuoka et al., 2006), have been reported from the upper part of the Hitoyoshi Formation. The sedimentary age of the Hitoyoshi Formation has been investigated in detail by measuring the K-Ar age of tuff. The lowest part of the Hitoyoshi Formation, the Funado Tuff Member, is 2.72±0.25Ma, and the uppermost part of the Hitoyoshi Formation, the Yamada Tuff, is 2.58±0.08Ma (Torii et al., 1999).
Sedimentary environment
Field observations were carried out in the western part of the Hitoyoshi Basin, and columnar sections were created for the riverbed of the Kuma River and the slope of the Kuma Village Sports Park. In. As a result, the lower part of the Hitoyoshi Formation is composed of Lower Fan sediments, in which gravelly braided river deposits predominate, and Upper Fan sediments, in which debris flow deposits predominate, and it is thought that the lower part of the Hitoyoshi Formation formed an alluvial fan. On the other hand, the upper part of the Hitoyoshi Formation is composed of unstratified siltstone intercalated with muddy turbidite layers, and is thought to be lacustrine.
Paleosols of fossil forests
The lower part of the Hitoyoshi Formation contains a fossil forest layer that is rich in tree trunk fossils and corresponds to the floodplain facies. This layer is located near the boundary between the lower part of the Hitoyoshi Formation (alluvial fan sediments) and the upper part of the Hitoyoshi Formation (lacustrine sediments), and it is thought that the lakeside forest at the time is preserved. The paleosols of the fossil forest are clearly differentiated into three soil horizons: A, which is rich in black organic matter; B, which is white in color; and C, which retains its sedimentary structure. In particular, A is fine-grained, and slicken lines that cross the bedding plane are well developed. In addition, the Hitoyoshi Formation universally contains abundant pumice and volcanic glass, and the parent material of the soil is volcanic ash.
Discussion
Volcanic ash soils are considered to be easily pedogenized because they contain a large amount of volcanic glass (Shoji et al., 1993). Furthermore, the fact that the paleosols are concentrated in the lower part of the Hitoyoshi Formation (alluvial fan environment) and are hardly found in the upper part of the Hitoyoshi Formation (lacustrine formation), and the fact that strata containing abundant tree-trunk fossils have been identified in the floodplain near the lake margin, suggest that environmental changes around the lake may have had a significant impact on soil formation and vegetation. In addition, the slickens lines that develop in the paleosol are thought to have been formed by the generation of clay minerals and their seasonal swelling and shrinking, indicating that the paleosol of the fossil forest is strongly pedogenic and a mature soil.