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[HCG21-P04] Decomposition process of sandstone in a subtropical environment: An approach from physical properties and clay mineralogy
Keywords:rock weathering, subtropical island, Shimanto belt, sandstone, clay minerals, Okinawa-jima
Introduction
Rock weathering is broadly classified into physical and chemical weathering, with the latter influenced by climatic conditions such as temperature and precipitation (e.g., Strakhov, 1967). In northern Okinawa-jima, which has a subtropical climate, red to yellow weathered soils known as Kunigami Mahji are widely distributed. Their parent materials include terrace deposits (Kunigami Formation) and basement rocks such as phyllite, sandstone, and greenstone.
Thought the process of sandstone weathering has been reported in temperate regions like Kyushu, Japan (e.g., Nishiyama & Matsukura, 2002), it remains limited in subtropical islands. This study investigates a sandstone of the Nago Formation in northern Okinawa-jima, corelative to the Shimanto Belt, to characterize weathering progression from fresh rock to weathered soil based on physical properties and mineral composition.
Geology
In northern Okinawa-jima, the Upper Cretaceous Nago Formation and Paleogene Kayo Formation, both part of the Shimanto Belt, are distributed. The Nago Formation consists of phyllite, greenstone, and sandstone, though its distribution and nomenclature vary among researchers (Miyagi et al., 2013). While fresh portions appear dark gray, they develop thick red to yellow weathering crusts near the surface.
This study focuses on a highly weathered sandstone outcrop in southern Ogimi Village, considered part of the Nago Formation. The area is situated in hilly terrain at 100–170 m elevation, lacking high mountain masses compared to adjacent areas. Consequently, steep river development is limited, resulting in lower erosion rates and better preservation of the weathering crust.
Methods
The study outcrop is a cut slope with a relative height of approximately 10 m. To quantitatively evaluate the degree of weathering in the field, soil hardness was measured at 1-meter intervals along the slope using a Yamanaka-type soil hardness meter. Additionally, samples were collected from the outcrop for laboratory analysis, including microscopic observation, mineral identification using X-ray diffraction analysis, and density measuring. Fresh rock samples were collected from Kunigami Village in northern Okinawa-jima, while moderately weathered samples were obtained from Higashi Village. These samples were used to analyze the progression of weathering from fresh rock to residual soil.
Results
At the Ogimi Village outcrop, strongly weathered medium-grained sandstone with thin intercalated tuffaceous layers exhibits red to yellow color. The sandstone is predominantly orange from the top to ~3.5 m, transitioning to reddish-brown below. Soil hardness is ~25 mm at the top, increasing to ~30 mm at the bottom.
Microscopic observations show that fresh (Kunigami Village) and moderately weathered (Higashi Village) samples contain quartz, rock fragments, plagioclase, alkali feldspar, muscovite, and volcanic and sedimentary rock (originated from chert) fragments. In the Ogimi outcrop, plagioclase was absent in the lower part, though alkali feldspar remained. In the middle and upper parts, alkali feldspar also disappeared. Volcanic fragments altered into mica clay minerals. Quartz content remained stable at 30–40% from fresh to highly weathered zones.
X-ray diffraction identified quartz, muscovite, and kaolinite in the lower part of the outcrop. Muscovite decreased upward and disappeared near the top, where illite/smectite mixed-layer minerals appeared. These results suggest that the most weathered top portion consists mainly of quartz, chert-derived fragments, kaolinite, and illite/smectite mixed-layer minerals.
Conclusion
The sandstone of the Nago Formation in northern Okinawa-jima eventually undergoes transformation into the weathered soil known as Kunigami Mahji due to the alteration of primary feldspar and volcanic rock fragments into clay minerals. Future research is required additional analyses such as microscopic observation, X-ray diffraction analysis, X-ray fluorescence analysis, and physical testing to further clarify the sandstone weathering process.
Reference
Miyagi et al., 2013,Whole rock chemical composition of the pre-Neogene basement rocks and detritus garnet composition in the Okinawa-jima and neighbor islands. Jour: Geol. Soc.Japan,119,665-678.
Nishiyama and Matsukura,2002,Sequential changes in rock texture of Shimanto sandstone due to weathering in south Kyushu.Jour:Geol.Soc.Japan,108,410-413.
Strakhov, N.M.,1967,Principles of Lithogenesis,vol.1,Oliver &Boyd, London.
Rock weathering is broadly classified into physical and chemical weathering, with the latter influenced by climatic conditions such as temperature and precipitation (e.g., Strakhov, 1967). In northern Okinawa-jima, which has a subtropical climate, red to yellow weathered soils known as Kunigami Mahji are widely distributed. Their parent materials include terrace deposits (Kunigami Formation) and basement rocks such as phyllite, sandstone, and greenstone.
Thought the process of sandstone weathering has been reported in temperate regions like Kyushu, Japan (e.g., Nishiyama & Matsukura, 2002), it remains limited in subtropical islands. This study investigates a sandstone of the Nago Formation in northern Okinawa-jima, corelative to the Shimanto Belt, to characterize weathering progression from fresh rock to weathered soil based on physical properties and mineral composition.
Geology
In northern Okinawa-jima, the Upper Cretaceous Nago Formation and Paleogene Kayo Formation, both part of the Shimanto Belt, are distributed. The Nago Formation consists of phyllite, greenstone, and sandstone, though its distribution and nomenclature vary among researchers (Miyagi et al., 2013). While fresh portions appear dark gray, they develop thick red to yellow weathering crusts near the surface.
This study focuses on a highly weathered sandstone outcrop in southern Ogimi Village, considered part of the Nago Formation. The area is situated in hilly terrain at 100–170 m elevation, lacking high mountain masses compared to adjacent areas. Consequently, steep river development is limited, resulting in lower erosion rates and better preservation of the weathering crust.
Methods
The study outcrop is a cut slope with a relative height of approximately 10 m. To quantitatively evaluate the degree of weathering in the field, soil hardness was measured at 1-meter intervals along the slope using a Yamanaka-type soil hardness meter. Additionally, samples were collected from the outcrop for laboratory analysis, including microscopic observation, mineral identification using X-ray diffraction analysis, and density measuring. Fresh rock samples were collected from Kunigami Village in northern Okinawa-jima, while moderately weathered samples were obtained from Higashi Village. These samples were used to analyze the progression of weathering from fresh rock to residual soil.
Results
At the Ogimi Village outcrop, strongly weathered medium-grained sandstone with thin intercalated tuffaceous layers exhibits red to yellow color. The sandstone is predominantly orange from the top to ~3.5 m, transitioning to reddish-brown below. Soil hardness is ~25 mm at the top, increasing to ~30 mm at the bottom.
Microscopic observations show that fresh (Kunigami Village) and moderately weathered (Higashi Village) samples contain quartz, rock fragments, plagioclase, alkali feldspar, muscovite, and volcanic and sedimentary rock (originated from chert) fragments. In the Ogimi outcrop, plagioclase was absent in the lower part, though alkali feldspar remained. In the middle and upper parts, alkali feldspar also disappeared. Volcanic fragments altered into mica clay minerals. Quartz content remained stable at 30–40% from fresh to highly weathered zones.
X-ray diffraction identified quartz, muscovite, and kaolinite in the lower part of the outcrop. Muscovite decreased upward and disappeared near the top, where illite/smectite mixed-layer minerals appeared. These results suggest that the most weathered top portion consists mainly of quartz, chert-derived fragments, kaolinite, and illite/smectite mixed-layer minerals.
Conclusion
The sandstone of the Nago Formation in northern Okinawa-jima eventually undergoes transformation into the weathered soil known as Kunigami Mahji due to the alteration of primary feldspar and volcanic rock fragments into clay minerals. Future research is required additional analyses such as microscopic observation, X-ray diffraction analysis, X-ray fluorescence analysis, and physical testing to further clarify the sandstone weathering process.
Reference
Miyagi et al., 2013,Whole rock chemical composition of the pre-Neogene basement rocks and detritus garnet composition in the Okinawa-jima and neighbor islands. Jour: Geol. Soc.Japan,119,665-678.
Nishiyama and Matsukura,2002,Sequential changes in rock texture of Shimanto sandstone due to weathering in south Kyushu.Jour:Geol.Soc.Japan,108,410-413.
Strakhov, N.M.,1967,Principles of Lithogenesis,vol.1,Oliver &Boyd, London.