Keywords: tephra, refractive index, Tama Loam, Kiyokawa Formation, Shimosa Group, Middle Pleistocene

Transcription

1 Tsutomu akazawa, Hiroomi akazato, Taku Komatsubara, and Hitoshi Tsukamoto (23) Reexamination of correlation between Middle Pleistocene marker tephras TB-8 and Ky3, Kanto, central Japan. Bull. Geol. Surv. Japan, vol. 4(9/), p.341-3, figs. Abstract: Marker tephras TB-8 and Ky3 of Middle Pleistocene age are redescribed in this paper. TB- 8 and Ky3 are conspicuous key beds indicating the middle part of the Tama Loam and the lower part of the Kiyokawa Formation of the Shimosa Group, respectively. These tephras have been thought to be correlated with each other as they both include orthopyroxene crystal grains exhibiting high refractive indices. However, it has been also pointed out that they are largely different in the range of the refractive indices. Detailed observation of these tephras revealed that TB-8 is composed of six fall units which are different with each other in the refractive indices of orthopyroxene crystal grains. Ky3 is considered to be a mixed layer of some units of TB-8. Moreover, marker tephras TB-9 and, which are settled above TB-8 and Ky3, respectively, both include hornblende crystal grains exhibiting the same refractive indices. Therefore, the combination of TB-8 and TB-9 is correlated with that of Ky3 and. TB-8 (Ky3) widely occurs in the southern and central parts of the Kanto Plain including a metropolitan area. Only a few widespread marker tephras are known in the subsurface of the middle part of the Kanto Plain. This tephra is, therefore, important as a key bed for a study of a subsurface geologic structure in such an urban area. Keywords: tephra, refractive index, Tama Loam, Kiyokawa Formation, Shimosa Group, Middle Pleistocene

2 多摩ローム層 上杉 (1976) 七国峠ローム層 早田ローム層 藤沢ローム層 大磯丘陵 町田ほか (1974) TB TAl テフラテフラテフラ Tm-21 TB-9 Tm-19 TB-8 Ky3 Tm-18 TB-7 Ky2 Tm-6 Tm-2 TCu TCl TCu-4 TCu-1 Km2 房総半島 徳橋 遠藤 (1984) 清川層 上泉層 藪層 下総層群 Fig. 1 Correlation of tephras between Oiso (left) and Boso (right). Solid lines indicate correlation by Arai et al. (1977), Sugihara et al. (1978), and akazato (21), and broken lines show correlation by Kanto Ash Layers Research Group and Volcanic Ash Research Subgroup of Subsurface Research Group of the Port of Tokyo (2).

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4 Loc.A Loc.7 Loc.B 36 Loc.1 Loc km Loc.3 Loc.4 Loc.7 Loc. 1km 1km 原図は国土地理院発行 2 万 千分の 1 地形図 木更津 原図は国土地理院発行 2 万 千分の 1 地形図 岩槻 1km Loc.1 Loc.2 Loc.6 1km 原図は国土地理院発行 2 万 千分の 1 地形図 小田原北部 原図は国土地理院発行 2 万 千分の 1 地形図 木更津 1km 原図は国土地理院発行 2 万 千分の 1 地形図 上総横田 Fig. 2 Map showing locations of TB-8 and Ky3. Locs.1-7 are examined in this study. Locs. A and B are locations of cores in which Ky3 was identified by Saitama Prefecture (1996) and Kuroda et al. (198), respectively. Black dots without numbers indicate locations of Ky3 reported by akazato (1993) and Sato (1993).

5 凡例 Loc.4 烏田造成地 泥極細粒砂細粒砂中粒砂粗粒砂極粗粒砂砂礫 貝化石生痕化石 Macaronichnus 巣穴化石植物根化石斜交層理 Ko1 木下層 火山灰土, 凝灰質泥 水平あるいは低角斜交層理 腐植層 テフラ Loc.1 大磯菊川 Loc.2 東坂田 新期段丘堆積物及び新期関東ローム層 Loc.3 大久保 Loc. 上烏田 TB-9 TB-8 Ky3 清川層 Ky3 早田ローム層 TCu-4 m 上泉層 Fig.3 Columnar sections of successions intercalating examined tephras. See Fig. 2 for locations of columnar sections. Loc.6 打越 Ky3 Loc.7 春日部 GS-KB-1 C 層 Ky3 深度 32.9m

6 大磯丘陵 ( 大井町菊川 :Loc.1)TB-8 ユニット区分 層厚粒径主要構成物質重鉱物組成 ユニット 6 2cm -4mm 岩片, スコリア, 黄白 - 褐色軽石 opx, cpx, mg ( 重鉱物少ない ) 1 ユニット 4cm -mm 黄白色軽石岩片, スコリア opx, cpx, mg ( 重鉱物少ない ) ユニット 4 ユニット 3 ユニット 2 ユニット 1 6cm cm 7cm 8cm -mm -mm.-1mm 1-8mm 岩片, スコリア, 軽石 黄白色軽石, 岩片, スコリア スコリア, 軽石 白色軽石 ( スコリア含む ) opx > mg > cpx opx > cpx > ho, mg ho > cpx > opx >mg >> bi mg > ho, opx > cpx ho (n2) ( ) ( , 1.69) (1.68, 1.688) 木更津台地 ( 木更津市大久保 :Loc.3)Ky3-2cm 1-mm 黄白色軽石スコリア, 岩片 cpx > opx > mg 木更津台地 ( 木更津市上烏田 :Loc.)Ky cm 1-mm 黄白色軽石岩片, スコリア cpx > opx > mg 袖ヶ浦台地 ( 袖ヶ浦市打越 :Loc.6) Ky3 cm 1-mm 黄白色軽石スコリア, 岩片 cpx > opx > mg >> ho ho (n2) (1.68, ) 大宮台地 ( 春日部市坊荒句 :Loc.7) Ky3 cm.-2mm 黄白色軽石スコリア, 岩片 cpx > opx > mg cm 軽石 スコリア, 岩片 泥層 火山灰土, 凝灰質泥層 Fig. 4 Characteristics of TB-8 and Ky3. See Figs. 2 and 3 for locations of tephras. opx: orthophyroxene, cpx: clinopyroxene, ho: hornblende, mg: magnetite

7 1 角閃石の屈折率 (n2) 大磯菊川 (Loc.1) TCu-4 ho > mg 検出されず 1 大磯菊川 (Loc.1) TB-9 ho > opx > cpx > mg 上烏田 (Loc.) 1 ho > opx > cpx > mg 2 東坂田 (Loc.2) ho > opx > cpx > mg 烏田造成地 (Loc.4) ho > opx > cpx > mg Fig. Refractive indices of TCu-4, TB-9, and. See Figs. 2 and 3 for locations of tephras. opx: orthophyroxene, cpx: clinopyroxene, ho:hornblende, mg: magnetite

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