The Kōnosu district is located in the eastern part of Saitama Prefecture and the western part of Ibaraki Prefecture, in the central Kantō Plain in NE Japan. The quadrangle occupies the area of long. 139°29′48″4-139°44′48″3 E and lat. 36°0′11″5-36°10′11″4 in WGS 84. The topography of this district consists of uplands and alluvial lowlands. The uplands are divided into the Sashima and Ōmiya Uplands by the Nakagawa Lowland, which distributes along the Watarasegawa and Nakagawa Rivers. While the Ōmiya Upland is distributed widely around the district, the Sashima Upland is situated in its northeastern corner. The Kazo and Arakawa Lowlands lie on the north and west sides of the Ōmiya Upland, respectively. The Arakawa Lowland is situated in a narrow part of the district's southwestern corner.
GEOLOGY
The Kōnosu district is situated within the central part of the Kantō Sedimentary Basin and is underlain by thick Cenozoic successions. Shallower than about 1500 m, the subsurface belongs to the Lower Miocene Hiki Group, Middle to Upper Miocene Tokigawa Group, Lower to Middle Pleistocene Kazusa Group, and Middle to Upper Pleistocene Shimōsa Group, with distributed younger terrace deposits and younger loam and Alluvium.
HIKI AND TOKIGAWA GROUPS
The Lower Miocene Hiki Group and the Middle to Upper Miocene Tokigawa Group are distributed deeper than 1000 m. The Niwaya Unconformity, which divides these two groups, is estimated to be at an elevation of about -1434 m in Otone town. The Hiki Group is characterized by marine sediments (mudstone and muddy sandstone). The Tokigawa Group can be divided to a lower and an upper part. The lower part is characterized by marine sediments mainly composed of sandstone and sandy mudstone, and the upper part consists of non-marine sediments (sandstone and mudstone containing lignite). A stratigraphic framework of these groups has not been established. The ages of the Hiki and Tokigawa Groups are estimated from biostratigraphy as being older than about 15 Ma and about 15-10 Ma, respectively.
KAZUSA GROUP
The Lower to Middle Pleistocene Kazusa Group commonly occurs at elevations deeper than -150 m. The Kurotaki Unconformity, which defines the base of the Kazusa group, is estimated to lie at elevations between -1000 to -1500 m. The boundary between this group and the upper Shimōsa Group is defined by the base of the Jizōdō Formation, which correlates to MIS 12. The Kazusa Group consists of depositional cycles of non-marine and marine sediments in the Kōnosu district, but its stratigraphic framework has not been established.
SHIMŌSA GROUP
The Middle to Upper Pleistocene Shimōsa Group is distributed in the subsurface shallower than an elevation of approximately -150 m in the Kōnosu district. This group is divided into six formations and a bed.
Fig. 1 Stratigraphic summary in the Kōnosu district
Jizōdō Formation
The lower and middle parts are dominated by mud and/or sandy mud. The middle part contains molluscan fossils. The upper part consists of sand and/or gravelly sand. It is less than 33 m thick. This formation was deposited at MIS 12-11．
Yabu Formation
The lower part is composed of sand and gravel, sand and/or mud. The middle part consists of muddy sand and/or mud containing molluscan fossils. The upper part contains cross-stratified, well sorted sand and/or gravelly sand. It is less than 55 m thick. This formation developed at MIS 10-9．
Kamiizumi Formation
The lower part consists of gravelly sand, sand and mud contacting fossil plant roots. The middle part is composed of muddy sand and sandy mud, and parts of it contain molluscan fossils. The upper part is characterized by well-sorted sand, and occasionally includes mud and peaty mud. It is less than 35 m thick. This formation formed at MIS 8-7.5．
Kiyokawa Formation
This formation is gravely sand, sand, mud containing fossil plant roots and peat. The thickness is less than 38 m. It accumulated at MIS 7.4-7.3．
Kioroshi Formation
This formation consists of sand, mud, and alternating beds of sand and mud. The basal part of this formation includes abundant mulluscan fossils in the east part of the district. It is less than 20 m thick, and is deposited at MIS 6-5.5．
Ōmiya Formation
This formation consists of gravel, cross-stratified, poorly sorted sand and mud. The depositional surface of this formation corresponds to the Ōmiya Surface. This formation is less than 20 m thick and formed at MIS 5.4-5.3．
Jōsō Clay
This bed is composed mainly of tuffaceous mud and tuffaceous silty sand, and locally intercalates "Kata-zuna", a well-sorted solid sand. The thickness is less than 6 m. This bed comformably overlies the Ōmiya Formation.
LATE PLEISTOCENE TERRACE DEPOSITS AND YOUNGER LOAM
Terrace deposits (Ayasegawa terrace deposits) younger than the Shimosa Group are found along the Ayase-gawa and Moto-Arakawa Rivers. These deposits consist of gravelly sand, sand and mud, which is deposited mainly at MIS 5.3-5.1．
The younger Loam (Kantō Loam) bed is mainly composed of brownish volcanic ash soil. The loam bed in the Kōnosu district is intercalated with marker tephra layers of Hk-TP and AT.
ALLUVIUM
The alluvium distributes in the Nakagawa, Kazo, Arakawa Lowlands and in small valleys. A thin layer of alluvium overlies the younger loam bed on the northern side of the Ōmiya Upland in the Kōnosu district.
The alluvium in the Nakagawa and Kazo Lowlands can be divided into lower, middle and upper parts. The lower part is composed of gravelly sand, sand and peaty mud and/or peat. The middle part mainly consists of mud. In the Nakagawa Lowland this mud layer contains molluscan fossils. The upper part is composed of sand and mud. The alluvium is less than 40 m thick in the Nakagawa Lowlands and 20 m thick in the Kazo Lowlands.
GEOLOGIC STRUCTURE
The Middle to Upper Pleistocene and Holocene successions are almost horizontally distributed in the Kōnosu district except in the eastern part. The Middle to Upper Pleistocene successions are deformed on the west side of the active Ayasegawa fault, which has a NW-SE trend extending from the north Kōnosu to north Kitamoto area.
GROUNDWATER
Groundwater occurs in: 1) the alluvium and/or uppermost parts of the Shimōsa Group at depths of 4 to 30 m from the ground surface, 2) the Middle-Late Pleistocene Shimōsa Group at depths of 90 to 150 m, and 3) the Late Pliocene-Middle Pleistocene Kazusa Group at depths from 180 to 400 m. The spatial variation in chloride (Cl-) concentration indicates that in the Kazo Lowlands, groundwater with high Cl- concentrations (up to 216 mg/l) occurs in the Kazusa Group. This Cl- rich groundwater is also characterized by low δD, low δ18O, and low 14C values. The area with the Cl- rich groundwater, spreading from NW to SE, is bounded by the Ayasegawa Fault on its SW side and corresponds to a fold structure (syncline) of the Kazusa Group formed in association with the Ayasegawa Fault movement. This unique geologic setting has put groundwater in the Kazo Lowland in an isolated hydrologic environment for a long period of time, resulting in the occurrence of groundwater with anomalous Na-Cl hydrochemistry. With regard to the origin of isotopically depleted groundwater, a potential source is assumed to be precipitation and influent seepage from large rivers in a cooler climate than the present. An admixture of connate seawater originally contained in the marine sediments of the Kazusa Group or downward movement of Cl-rich groundwater recharged in the period of the Middle-Late Pleistocene Shimōsa Group deposition is likely to account for the elevated Cl- concentration of the groundwater in the Kazo Lowland area.
APPLIED GEOLOGY
The Kantō Earthquake in 1923 severely damaged wooden houses in the Nakagagwa Lowlands. The damage frequency was especially high in areas where the Alluvium was thick.
Land subsidence has been observed since the 1970's in the Kōnosu district. The northeast part of the Kōnosu district was the center of land subsidence during that time. Although the subsidence rate has decreased since the regulation of water usage, it still continues.
The lowlands in the Kōnosu district have suffered frequent flood damage in historical times. The flood disaster that was caused by Typhoon Kathleen in 1947 caused the most serious damage to the area in recent years.
納谷ほか（2013）は岩相ユニットA～Dから石灰質ナンノ化石を報告した（第3.3図）．岩相ユニットA及び岩相ユニットBの最下部からは，石灰質ナンノ化石帯区分（Okada and Bukry, 1980）のCN4帯に相当する群集，岩相ユニットBからはCN5a亜帯に相当する群集，岩相ユニットBの最上部及び岩相ユニットCからはCN5b亜帯に相当する群集，岩相ユニットDからはCN6-11帯に相当する群集がそれぞれ産出した．
SP-GDの342.5～342.45mにはガラス質のテフラが挟在する．ガラスの屈折率（1.506-1.507）が良く一致するとともに（第4.2表），火山ガラスの主成分化学組成も，FeOが1.7%程度，CaOが1.8%程度と高く，K2Oが2.3%程度と低いという特徴が一致することから，房総半島梅ヶ瀬層に挟在するU8テフラに対比されている（水野ほか，2006）．佐藤ほか（1999），藤岡・小竹（2003）によれば，房総半島においてU8テフラはSato et al. (2009)の石灰質ナンノ化石基準面⑤（0.853Ma）と⑥（0.987Ma）の間に位置し，Jaramillo亜磁極帯より上位のMatuyama磁極帯に挟在するとされる．
鴻巣地域の上総層群相当層の古地磁気層序は，植木ほか（2009）がGS-SB-1コアについて，植木ほか（2012）がSP-GD，SP-WMとGS-SB-1コアの最下部についてそれぞれ検討している．それらによると，Brunhes-Matuyama磁極帯境界は，GS-SB-1コアで深度266.755～276.9mの間に，SP-GDでは深度280m付近に，SP-WMでは深度250m付近にそれぞれ認められる．Jaramillo亜磁極帯は，GS-SB-1コアの深度350.0～350.02m，SP-WMの300～325mにそれぞれ認められる（第4.2図）．SP-GD及びSP-WMの最下部はMatuyama逆磁極帯に相当し，Gauss正磁極帯までは到達していない．なお，Brunhes-Matuyama磁極帯境界の年代は0.78Maとされ（Gradstein et al., 2004），房総半島では上総層群国本層中に認められる（新妻，1976）．また，Jaramillo亜磁極帯の年代は0.99-1.07Maとされ（Gradstein et al., 2004），房総半島では上総層群梅ヶ瀬層下部から太田代層上部にまたがって認められる（第4.4図）（新妻，1976；辻ほか，2005）．
第4.4図 鴻巣地域の上総層群相当層の年代対比
4.2.4 化石
有孔虫化石
木野（1965）は久喜市で掘削された深度400mのボーリング試料の，深度290m付近と360m付近から，Ammonia papillosa, A. beccarii, Nonion manpukujiense, Pseudononion japonicumなど，底生有孔虫の産出を報告した．
GS-KZ-1コアのKZ-M6層準からは海生珪藻化石が産出する（第4.3図）．この層準では，Tryblionella granulata, T. cocconeiformisなどの海生付着性種が卓越し，この層準の上部（深度228～234m）では汽水生種であるPseudopodosira kosugiiが卓越して産出する．
菖蒲（GS-SB-1）の深度9.9～10.9mの斜交層理が発達する細～中粒砂層には，葉理に沿って細粒の白色軽石が濃集する産状のテフラが観察される（SBT-10.90：山口ほか，2009）．このテフラは中間型及び多孔型の形状をした火山ガラスを大量に含み，重鉱物としては普通輝石と角閃石を含み，火山ガラスの屈折率（n）は1.501-1.502の範囲を示す（山口ほか，2009）．火山ガラスの主成分化学組成は，TiO2が0.18%，FeOが10.8%と共に低く，K2Oが3.62%でNa2Oが3.52%と同様の値を示す．火山ガラスの屈折率と主成分化学組成の一致から，このテフラは御岳第一テフラ（On-Pm1：Kobayashi et al., 1968；町田・新井，2003）に対比されている（山口ほか，2009）．鴻巣地域では，On-Pm1に対比されるウグイス色軽石層（UP）と呼ばれるテフラ（渋谷ほか，1968）が，大宮層の上位層である常総粘土に含まれる硬砂より2.2m～7m下位に，軽石を含む黄緑色の粘土層に挟有されることが知られている（硬砂団体研究グループ，1984）．なお，本地域の南側に隣接する大宮地域の東部とその東側に隣接する野田地域では，On-Pm1は常総粘土の基底付近に挟有されることが知られている（中沢・遠藤，2002；杉原，1970）．On-Pm1テフラは南関東では小原台面の離水層準を示す指標テフラとされ（町田，1973など），降灰年代はMIS5.3に相当する約100kaと考えられている（町田・新井，2003）．
Yasuhara et al．（2007）と2008年～2012年の調査地点をまとめたもの．井戸深度90～150m，同180～400m，同1200～1400mはそれぞれ下総層群，上総層群相当層，中新統中の地下水を代表する．GS-SB-1地点ではオールコアボーリング試料に基づき詳細な層序が明らかとなっている（山口ほか，2009）．綾瀬川断層の位置は杉山ほか（1997）による．
Yasuhara et al. (2007)並びに2008年～2012年にかけて実施した地下水調査の結果に基づいて，以下に鴻巣地域の地下水の地球化学的特性を述べる．調査地点を第9.1図に示した．湧水・浅井戸（浅井戸は深度4～30m）は下総層群上部の地下水を代表する．深度90～150mの井戸については，久喜市菖蒲のGS-SB-1地点（第9.1図）における上総層群相当層と下総層群の境界が深度165～184mの間にあるとされることから（山口ほか，2009），下総層群中部～下部の被圧地下水を代表するものと考えられる．深度180～400mの井戸については，上総層群相当層中の地下水を，また，深度1200～1600mの井戸は中新統中に賦存する地下水（温泉水）を代表する．
第9.2表では，上総層群相当層中の地下水の一般水質濃度と同位体比を，綾瀬川断層の北東側の加須低地部と南西側の大宮台地部に分けて比較した．綾瀬川断層の北東側には蓮田市や白岡町等，大宮台地の一部も含まれるが，便宜上一括して加須低地と表示した．第9.2表において，大宮台地部の地下水のCl-の平均濃度27.0mg/Lに対して，加須低地部の地下水は77.4mg/Lと3倍近い高い濃度を有する．一方，Cl-とは異なり，δD値は-66‰前後と台地部と低地部の地下水の間には差がない．これは，綾瀬川断層南部地域においては，断層を挟む大宮台地部（さいたま市）と中川低地部（越谷市，松伏町，春日部市）の地下水に，Cl-濃度とδD値の両方に明瞭な差が認められる（Yasuhara et al., 2007）ことと対照的である．綾瀬川断層の北部地域と南部地域においては，水の起源や地下水システムに及ぼす綾瀬川断層の水理学的影響の程度が大きく異なる可能性がある．
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安原正也・稲村明彦・森川徳敏・高橋浩・水野清秀・高橋正明・大和田道子・風早康平・戸崎裕貴・宮越昭暢・Gingerich, S. B．・林武司・鈴木秀和・宮下雄次・藪崎志穂・鈴木裕一（2011）関東平野の被圧地下水中の塩化物イオンと水の起源に関する同位体的検討．日本地球惑星科学連合2011年大会，AHW023-P10．
