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[SGL18-02] Provenence of the Lower to Middle Miocene Chikubetsu Formation in the Shumarinai-Soeushinai area, Horokanai town, Central Hokkaido
Keywords:Serpentinite, Early to Middle Miocene, Central Axial Zone of Hokkaido
Introduction
Serpentinite derived sediments are important because they provide evidence for the timing and location of exposure of serpentinite bodies [1]. Serpentinite-clast-rich sedimentary rocks [2, 3] and serpentinite blocks intercalated in sedimentary rocks [4] are reported in the Early to Middle Miocene strata in the southern part of the central axial zone of Hokkaido, and they suggest the intrusion and exposure of serpentinite during the Miocene time. In this study, we describe the occurrence of serpentinitic sedimentary rocks in the Lower to Middle Miocene Chikubetsu Formation in the Shumarinai-Soeunai area of Horokanai Town, central part of the central axial zone of Hokkaido, and discuss the paleotopography and tectonic movements based on sedimentological and petrological methods.
Stratigraphic classification of the Chikubetsu Formation in the Shumarinai-Soeunai area
The Chikubetsu Formation in the study area is divided into the Shumarinai Coal-Bearing Member, the Sanjissenzawa Fossiliferous Sandstone Member (SjFSM), and the Tengunohana Mudstone Member [5]. SjFSM consists of two types of lithology: “coarse serpentinitic sandstone and conglomerate” and “stratified sandstone intercalating thin conglomerate layers”. In this study, the latter is separated from the SjFSM and newly defined as the Shirajise Sandstone Member (SrSM).
Depositional Environment and source rock of the SjFSM
This member is composed of sandstone deposited from high-density turbidite and conglomerate deposited from debris flows, which accumulate without any specific accumulation pattern. This feature indicates that the sedimentary rocks are an accumulation of gravity flow deposits irregularly occurred on the steep slope.
Sedimentary rocks are mostly composed of serpentine-derived materials such as serpentinite fragments and Cr-spinel and have abundant carbonate matrix. Monogenic serpentinite source suggests that the mechanism of intrusion and exposure of the serpentinite body was protrusion.
Depositional Environment and Source rock of the SrSM
Sedimentary rocks of this member have sedimentary structures formed by unidirectional flow and/or combined flow and contain abundant shell fossils and carbonaceous materials. These features indicate that the sedimentary rocks are deposited in a shallow marine environment.
The sandstone is rich in volcanic glass, acidic volcanic rock fragments, and is accompanied by feldspar and quartz, sandstone and shale fragments, chert fragments, and metamorphic rock fragments. The volcanic derived materials account for up to 70% of the modal composition, suggesting that the hinterland is volcanic area. Some of the conglomerates are rich in strongly deformed chert fragments and metamorphic rock fragments, suggesting that these rocks were locally exposed.
Paleo-topography and paleogeological distribution
The serpentinitic sedimentary rock in the central part of central axial zone of Hokkaido is thought to have originated from serpentinite exposed by protrusion. On the other hand, in the southern part of the central axial zone of Hokkaido, nappe [6] or diapirs [7] are thought as the factor of serpentinite intrusion and exposure. The fact that serpentinitic sedimentary rocks are found continuously at least from southern to central part of central axial zone of Hokkaido suggests that intrusion and exposure events of serpentinite were universal phenomena in the central axial zone of Hokkaido, but the formation mechanism was not uniform.
References
[1] Arai et al., 1983, Jour. Geol. Soc. Japan, 89, 287-297. [2] Okada, 1964, Mem. Fac. Sci. Kyushu Univ. 15, 23-38. [3] Niida & Fukui, 1987, Bull. Hobetsu Mus., 4, 33-47. [4] Kato et al., 2003, Abst. 110th Annu. Meet. Geol. Soc. Japan, 66. [5] Hashimoto et al., 1965, Hokkaido Development Agency. [6] Arai & Okada, Tectonophys., 195, 65-81. [7] Kato & Gouchi, 2008, Abst. 115th Annu. Meet. Geol. Soc. Japan, 229.
Serpentinite derived sediments are important because they provide evidence for the timing and location of exposure of serpentinite bodies [1]. Serpentinite-clast-rich sedimentary rocks [2, 3] and serpentinite blocks intercalated in sedimentary rocks [4] are reported in the Early to Middle Miocene strata in the southern part of the central axial zone of Hokkaido, and they suggest the intrusion and exposure of serpentinite during the Miocene time. In this study, we describe the occurrence of serpentinitic sedimentary rocks in the Lower to Middle Miocene Chikubetsu Formation in the Shumarinai-Soeunai area of Horokanai Town, central part of the central axial zone of Hokkaido, and discuss the paleotopography and tectonic movements based on sedimentological and petrological methods.
Stratigraphic classification of the Chikubetsu Formation in the Shumarinai-Soeunai area
The Chikubetsu Formation in the study area is divided into the Shumarinai Coal-Bearing Member, the Sanjissenzawa Fossiliferous Sandstone Member (SjFSM), and the Tengunohana Mudstone Member [5]. SjFSM consists of two types of lithology: “coarse serpentinitic sandstone and conglomerate” and “stratified sandstone intercalating thin conglomerate layers”. In this study, the latter is separated from the SjFSM and newly defined as the Shirajise Sandstone Member (SrSM).
Depositional Environment and source rock of the SjFSM
This member is composed of sandstone deposited from high-density turbidite and conglomerate deposited from debris flows, which accumulate without any specific accumulation pattern. This feature indicates that the sedimentary rocks are an accumulation of gravity flow deposits irregularly occurred on the steep slope.
Sedimentary rocks are mostly composed of serpentine-derived materials such as serpentinite fragments and Cr-spinel and have abundant carbonate matrix. Monogenic serpentinite source suggests that the mechanism of intrusion and exposure of the serpentinite body was protrusion.
Depositional Environment and Source rock of the SrSM
Sedimentary rocks of this member have sedimentary structures formed by unidirectional flow and/or combined flow and contain abundant shell fossils and carbonaceous materials. These features indicate that the sedimentary rocks are deposited in a shallow marine environment.
The sandstone is rich in volcanic glass, acidic volcanic rock fragments, and is accompanied by feldspar and quartz, sandstone and shale fragments, chert fragments, and metamorphic rock fragments. The volcanic derived materials account for up to 70% of the modal composition, suggesting that the hinterland is volcanic area. Some of the conglomerates are rich in strongly deformed chert fragments and metamorphic rock fragments, suggesting that these rocks were locally exposed.
Paleo-topography and paleogeological distribution
The serpentinitic sedimentary rock in the central part of central axial zone of Hokkaido is thought to have originated from serpentinite exposed by protrusion. On the other hand, in the southern part of the central axial zone of Hokkaido, nappe [6] or diapirs [7] are thought as the factor of serpentinite intrusion and exposure. The fact that serpentinitic sedimentary rocks are found continuously at least from southern to central part of central axial zone of Hokkaido suggests that intrusion and exposure events of serpentinite were universal phenomena in the central axial zone of Hokkaido, but the formation mechanism was not uniform.
References
[1] Arai et al., 1983, Jour. Geol. Soc. Japan, 89, 287-297. [2] Okada, 1964, Mem. Fac. Sci. Kyushu Univ. 15, 23-38. [3] Niida & Fukui, 1987, Bull. Hobetsu Mus., 4, 33-47. [4] Kato et al., 2003, Abst. 110th Annu. Meet. Geol. Soc. Japan, 66. [5] Hashimoto et al., 1965, Hokkaido Development Agency. [6] Arai & Okada, Tectonophys., 195, 65-81. [7] Kato & Gouchi, 2008, Abst. 115th Annu. Meet. Geol. Soc. Japan, 229.