17:15 〜 18:45
[HCG23-P08] 亜熱帯環境下での砕屑物生産における岩石の鉱物学的・組成的変容
キーワード：国頭マージ、風化、土壌生成
Introduction: Okinawa Prefecture, located in the southwestern part of the Japanese islands, belongs to the subtropical climate zone, and the hot and rainy environment also affects the present soil formation. The red and yellow soils derived from non-calcareous substrates such as clastic rocks are widely distributed in the mountains, hills, and terraces of the Southwestern Islands and are called "Kunigami Mahji". However, the process by which characteristic soils such as the Kunigami Mahji are formed is not clear. In this study, we try to clarify the process by which metamorphic rocks and sediments that have not been mentioned are transformed into Kunigami Mahji, and describe the diversity of Kunigami Mahji with the parent material and formation process.
Shirahama outcrop: An outcrop located on a west-facing slope at an altitude of 159 m in the Shirahama District of Ogimi Village. The outcrop is 3~4m high and 4m wide, and has been excavated by recent upland development, and it consists of the coarse-grained sand layer of the Kunigami gravel layer in the upper 40~150cm, and unconformably overlie the Nago Formation mudstone.
Research Method: Two lateral sections were established in the outcrop (outcrops ① and ②), and the sedimentary structure, particle size, and color of the phyllite of the Nago Formation and the conglomerate sand layer of the Kunigami gravel layer. The changes of the rock texture and mineral assemblage by the weathering were anlysysed by X-ray diffraction analysis and X-ray fluorescence analysis. In addition, samples of fresh Nago Formation muddy rocks were collected in the Tsuha District of Ogimi Village and compared with the sediments of the Shirahama outcrop.
Results: Chlorite, calcite, and plagioclase contained in the fresh muddy Nago Formation were not observed in the entire outcrop, and muscovite and potash feldspar were also contained in the samples near the surface. X-ray diffraction analysis showed goethite in the sediment layer above the unconformity, and gibbsite in the sample near the surface. In addition, the outcrop samples were richer in Al2O3, Fe2O3, and TiO2 than in fresh Nago Formation muddies, and were less rich in Na2O, CaO, MgO, MnO, and P2O5.
Discussion (1): Mineral and elemental compositions: The mineral changes caused by weathering can be concluded as a following processes: 1) disappearance of chlorite, calcite, and plagioclase, 2) disappearance of muscovite and K-feldspar, and 3) formation of gibbsite. Since X-ray diffraction did not sufficiently recognize the increase in minerals produced by weathering, it is thought that many persistent elements are fixed as amorphous substances.
Discussion (2): Diversity of Kunigami Mahji: Comparison of the phyllite layer and sediment layer of the Shirahama outcrop shows clear differences in characteristics in microscopic observation and laboratory analysis. In addition, minerals such as chlorite and chlorite-vermiculite intermediate products, which have been reported to be included in the Kunigami Mahji by previous studies [2] and [3], were not found in this outcrop. Our result suggests wide variety of the soil in Kunigami Mahji due to differences in the parent material, the degree of weathering, and the water permeability depending on the topography. In the subtropical environment, sediments with completely different mineral and chemical compositions from the parent rocks may be produced. Paleoclimatic changes have a significant impact on sedimentary records, and multifaceted approaches are required for estimation of supply sources.
References: [1] Yuji Maejima, 2016, Pedologist, Vol. 60, No. 1.[2] Shigeru Araki, 1993, Pedologist, Voi. 37, No. 2. [3] Yoshihiro Tokashiki et al, 1982, Academic Report of the Faculty of Agriculture, University of the Ryukyus, No. 29.
Shirahama outcrop: An outcrop located on a west-facing slope at an altitude of 159 m in the Shirahama District of Ogimi Village. The outcrop is 3~4m high and 4m wide, and has been excavated by recent upland development, and it consists of the coarse-grained sand layer of the Kunigami gravel layer in the upper 40~150cm, and unconformably overlie the Nago Formation mudstone.
Research Method: Two lateral sections were established in the outcrop (outcrops ① and ②), and the sedimentary structure, particle size, and color of the phyllite of the Nago Formation and the conglomerate sand layer of the Kunigami gravel layer. The changes of the rock texture and mineral assemblage by the weathering were anlysysed by X-ray diffraction analysis and X-ray fluorescence analysis. In addition, samples of fresh Nago Formation muddy rocks were collected in the Tsuha District of Ogimi Village and compared with the sediments of the Shirahama outcrop.
Results: Chlorite, calcite, and plagioclase contained in the fresh muddy Nago Formation were not observed in the entire outcrop, and muscovite and potash feldspar were also contained in the samples near the surface. X-ray diffraction analysis showed goethite in the sediment layer above the unconformity, and gibbsite in the sample near the surface. In addition, the outcrop samples were richer in Al2O3, Fe2O3, and TiO2 than in fresh Nago Formation muddies, and were less rich in Na2O, CaO, MgO, MnO, and P2O5.
Discussion (1): Mineral and elemental compositions: The mineral changes caused by weathering can be concluded as a following processes: 1) disappearance of chlorite, calcite, and plagioclase, 2) disappearance of muscovite and K-feldspar, and 3) formation of gibbsite. Since X-ray diffraction did not sufficiently recognize the increase in minerals produced by weathering, it is thought that many persistent elements are fixed as amorphous substances.
Discussion (2): Diversity of Kunigami Mahji: Comparison of the phyllite layer and sediment layer of the Shirahama outcrop shows clear differences in characteristics in microscopic observation and laboratory analysis. In addition, minerals such as chlorite and chlorite-vermiculite intermediate products, which have been reported to be included in the Kunigami Mahji by previous studies [2] and [3], were not found in this outcrop. Our result suggests wide variety of the soil in Kunigami Mahji due to differences in the parent material, the degree of weathering, and the water permeability depending on the topography. In the subtropical environment, sediments with completely different mineral and chemical compositions from the parent rocks may be produced. Paleoclimatic changes have a significant impact on sedimentary records, and multifaceted approaches are required for estimation of supply sources.
References: [1] Yuji Maejima, 2016, Pedologist, Vol. 60, No. 1.[2] Shigeru Araki, 1993, Pedologist, Voi. 37, No. 2. [3] Yoshihiro Tokashiki et al, 1982, Academic Report of the Faculty of Agriculture, University of the Ryukyus, No. 29.