The Tateyama district is situated in the southernmost part of the Bōsō Peninsula in central Japan and occupies a quadrangle area of lat. 34°50′ to 35°0′ and long. 139°45′ to 140°0′ (lat. 34°50′12″1 to 35°0′12″0 and long. 139°44′48″4 to 139°59′48″3 referred to the International Terrestrial Reference Frame, ITRF). The eastern and southern parts of the district face the Pacific Ocean. Topography of the terrestrial area is characterized by a highly dissected hill encompassed by an uplifted Holocene lowland. The hill that corresponds to the southern part of the Awa Hill is composed of Late Miocene to Middle Pleistocene sedimentary rocks of the Nishizaki Formation and the Minamibōsō , Chikura, and Toyofusa Groups (Fig. 1). The Holocene lowland filled with alluvium is mainly divided into four districts: Tateyama, Chikura, Heisaura and around the Tomoe River. Stepwise-emerged beach ridges are distributed in the Tateyama and Chikura Lowlands. Most of the Heisaura Lowland is covered by dune sand. Many steps of Holocene rock strath marine terrace recording the history of subduction zone earthquakes of the Sagami Trough are developed in a width of about 1 km along the south-southeast and west coasts.
Fig. 1 Geologic map of the Tateyama district
Fig. 2 Geologic cross sections
NEOGENE and QUATERNARY
The Neogene sequence is composed of the late Miocene to early Pliocene Nishizaki Formation, early Pliocene Minamibōsō Group, and late Pliocene to early Pleistocene Chikura Group (Fig. 3). The Quaternary sequence is composed of the early to middle Pleistocene Toyofusa Group, late Pleistocene terrace deposits, Kanto loam, alluvium, and Holocene terrace deposits (Fig. 3)．
Fig. 3 Summary of the Tateyama district
Nishizaki Formation
The Nishizaki Formation is distributed in the southwestern part of Tateyama City and Uda and Minamiasai areas of Minamibōsō City and has an apparent thickness of at least 400 meters. It mainly consists of tuffaceous mudstone and is characterized by mesoscopic disrupted and folded structures in the field.
Minamibōsō Group (newly defined)
The Minamibōsō Group, newly defined, is composed of middle Miocene to lower Pliocene alternating beds of sandstone and siltstone with interbedded scoria beds. The Hedate and Kagamigaura Formations are distributed in this area, and other formations of the Minamibōsō Group are distributed in the northern part of this quadrangle.
Hedate Formation
This formation is distributed in the southern part of the Hedate area, Minamibōsō City. It mainly consists of sandy alternations of sandstone and siltstone with disturbed beds and has at least 350 meters in thickness.
Kagamigaura Formation
This formation is distributed at the edge of the Kagana plain and southern part of the Tateyama metropolitan area. It mainly consists of scoriaceous sandy alternations of sandstone and siltstone and is at least 300 meters thick. It is unconformable with the Nishizaki Formation at the edge of Kagana plain.
Chikura Group
The Chikura Group is extensively in the southeastern part of Minamibōsō City. It is made up of a thick sequence of marine sediments deposited on the middle to upper bathyal environment. It consists of an alternation of conglomerate, sandstone, and siltstone. A variety of tuff beds are intercalated in the Chikura Group.
Shirahama Formation
This formation is distributed in the coastal area between the Shiramazu and Kawashimo areas, and northern part of Mera Harbor. It mainly consists of mahogany colored volcaniclastic sandstone with interbedded mudstone layer, and it has at least 100 meters in thickness. This formation is interbedded by the Nojimazaki Conglomerate Member, which is composed of pebble-to cobble-sized conglomerate beds.
Shiramazu Formation
This formation is distributed in the southern part of the Shirahama area, Minamibōsō City. It mainly consists of sandy alternations of sandstone and tuffaceous siltstone, and exhibits an overall fining- and thinning-upward trend. This formation has 200 to 400 meters in thickness, which is thicker in the eastern part and thinner in the western part, and is conformable with the Shirahama Formation.
Mera Formation
This formation is extensively distributed in the southern part of Minamibōsō City and conformable with the Shiramazu Formation. It mainly consists of tuffaceous siltstone and is interbedded by the Rendaiji Conglomerate Member, which consists of rip-up clasts. This formation is 200 to 300 meters thick south of the Chikura Fault and 300 meters thick north of this fault. It is unconformable with the Hedate and Nishizaki Formations in the Minamiasai area.
Hata Formation
This formation is also extensively distributed in the southern part of Minamibōsō City and southeastern part of Tateyama City. It mainly consists of alternations of tuffaceous sandstone and tuffaceous siltstone, and interfingers with the Nagaogawa Sand Member, which consists of sandy alternations of sandstone and tuffaceous sandy siltstone. The formation thickness is 300 meters and 300 to 400 meters north and south of the Chikura Fault, respectively. It is conformable with the Mera Formation and unconformable with the Nishizaki Formation.
Toyofusa Group
The Toyofusa Group extensively occurs in the northwest part of this area. It is made up of a thick sequence of marine sediments deposited on the uppermost bathyal to shallow environment. It consists of an alternation of conglomerate, sandstone, and siltstone. A variety of tuff beds are also intercalated in the Toyofusa Group.
Kamo Formation
This formation is distributed in the Uda area north of Mera Harbor. It mainly consists of sandy alternations of sandstone and tuffaceous siltstone, which occur as shell fragments. It has 200 to 300 meters of thickness and is unconformable with the Hata, Mera and Nishizaki Formations.
Higashinagata Formation
This formation is distributed in the Yamahagi area to the Sano area, Tateyama City. It mainly consists of silty alternations of sandstone and tuffaceous siltstone. Its thickness is 400 to 500 meters, and it is unconformable with the Kamo Formation.
Takigawa Formation
This formation is distributed from the Komoguchi area to the Yatsufujiwara area and around Okinoshima Island. It consists of conglomerate, sandstone, and sandy siltstone, which frequently occur as shallow to brackish fossil fragments. It is at least 150 to 300 meters thick and interfingered with the Higashinagata Formation. In addition, it is unconformable with the Nishizaki Formation in the Yatsufujiwara area.
Late Pleistocene terrace deposits and Kanto loam
Late Pleistocene terraces, distributed along the Nagao River, are divided into six levels of Nagao-gawa I to VI in descending order. Most of the terraces are composed of fluvial gravel, sand and flood loam that are 1-3 m thick, but only the Nagao-gawa I is possibly marine origin related to the marine isotope stage 5a (MIS 5a). These terraces and gentle surface of the hill top around Kanamaribatake are covered by volcanic ash soil of the Kanto Loam.
Alluvium and Holocene terrace deposits
The alluvium composed of gravel, sand and mud is defined as post-glacial transgression deposits from the end of the Pleistocene to the middle Holocene in this report. Maximum thickness of the alluvium is at least 45 m in the Tateyama Lowland. The upper part of the alluvium characterized by shallow bay mud and beach sand can be observed in the outcrops. Along the middle to lower Tomoe River, 10-80cm thick tsunami deposits containing shell and plant fragments are repeatedly intercalated into shallow bay mud of the early Holocene. The fossils of Numa coral composed of 90 species of coral reef are yielded from small dissected valleys in the western part of Awa Hill.
Holocene marine terraces, which are classified into rock strath terrace and fill strath terrace, have been formed after the peak of post-glacial transgression. Terrace deposit of rock strath terrace is characterized by veneered gravel on an emerged shore platform or wave cut bench. Fill strath terrace accompanied with beach ridge is composed of gravel, sand and silt that were deposited in a beach environment between the shoreface and backshore.
GEOLOGICAL STRUCTURE
The formations south of the Mineoka Tectonic Belt are distributed from north to south with a southward younging trend. These strata are structurally controlled by some faults, e.g. the Uda, and Chikura Faults. There is three major faults in this reported area as follows from the north: the Uda, Chikura, and Kawaguchi Faults. These faults formed ENE-WSW trend syn-sedimentary folds, e.g. the Mizuoka Anticline, Sakuna Anticline, and Kanamari Syncline. These folds controlled the depositions of the Chikura and Toyofusa Groups. The Uda and Chikura Faults also formed shorter wavelength folds in several hundreds of meters only south of these faults, except south of the Onigase Anticline. In addition, there is the NW-SE to E-W curvature folds belt in the Nishizaki Formation in the Sunosaki area.
QUATERNARY TECTONICS
The southern part of the Bōsō Peninsula was uplifted during two major historical earthquakes of the 1703 Genroku and 1923 Kanto, and thus marine terraces were formed intermittently. Because the amount of uplift of the 1703 Genroku earthquake (3-6 m) was two or three times larger than that of the 1923 Kanto earthquake (1-2 m), the width of its terrace appears as a wide surface whereas the terrace resulting from the Kanto earthquake is a narrow step. Holocene marine terraces are mainly divided into four levels of wide surfaces named the Numa I to IV in descending order, but each boundary of the Numa series can be subdivided into several narrow steps. This geometry indicates the recurrence of two types of uplift events as the 1703 and 1923 earthquake. Based on the evaluation of emergence ages of each terrace, the subduction zone earthquake along the Sagami Trough occurred every 400 years usually as a 1923 type, but it becomes a large uplift like the 1703 type in one of several events (recurrence interval of 2,000-2,700 years)．
Since the height of the highest Holocene marine terrace (Numa I) emerged at 7,150 cal yBP reaches 30 m above sea level, the Holocene uplift rate is estimated to be about 4.0 mm/year that is the most rapid in Japan. However, if the Nagao-gawa I is correlated to the MIS 5a marine terrace, the uplift rate since 80 ka can be estimated to be 1.3-1.4 mm/year.
APPLIED GEOLOGY
Resources
The sand and gravel are used in concrete, mixed with cement and important aggregate for construction around the Kanto area. They are the most important mineral resources in the Chiba Prefecture. There are some working sand pits in this reported area. On the other hand, well-sorted fine glassy tephra beds are also important mineral resources in this area for abrasive compounds called "Hakudo" or "Boshu-Ko"．
Hot Spring
Over 30 sources of hot spring are developed in the southern part of Bōsō Peninsula. Most of them are characterized by low temperature (less than 25℃) and containing hydrogen sulfide.
Earthquake and Tsunami
During both the 1703 Genroku and 1923 Kanto earthquakes, this area was damaged by strong shaking and a large tsunami. The other historical tsunamis generated from distant sources have struck this area during the 1498 Meio, 1605 Keicho, 1677 Enpo, 1707 Hoei and 1854 Ansei earthquakes.
Landslide and mass movement
There are no typical landslides in this area, but the surfaces of slopes in dissected hills have collapsed during heavy rainfall or an earthquake. Emerged sea cliff of the boundary between Holocene rock strath marine terrace and hill has a steep slope, and there are risks of a surface collapse and debris flow from a small hanging valley.
本図幅地域の調査研究及び図面・原稿の作成にあたり，産業技術総合研究所竹内圭史主任研究員及び同研究所尾崎正紀研究グループ長には貴重なご助言を頂いた．房総半島南部における凝灰岩鍵層の追跡調査にあたり，品田芳二郎氏をはじめとする安房団体研究グループの方々には長年にわたりご教示頂いた．筑波大学の小川勇二郎教授には，川上の学生時代より房総半島の地質構造に関してご教授，叱咤激励を頂いた．静岡大学の山本由弦博士には，西岬層における貴重なデータを教えて頂いた．産業技術総合研究所活断層研究センターのThan Tin Aung博士，千葉大学大学院生の宇野知樹氏には野外調査を手伝って頂いた．株式会社土質リサーチの大里重人氏には現地を案内して頂き，また貴重なデータをご提供頂いた．筑波大学館山研修所の村上良ご夫妻及び南房総市白浜町の相正弘ご夫妻には現地での生活及び野外調査に際して様々な面でご援助を頂いた．また，千葉県在住の数多くの学校関係者や地質愛好家の方々に現地の案内をして頂いた．これらの方々に感謝と共に厚くお礼を申し上げる．
房総半島南部の地層は，北側から鴨川地溝帯北断層，鴨川地溝帯南断層，石堂断層，宇田断層，千倉断層，そして川口断層という東西～東北東―西南西方向の断層によって規制されて分布する（第2.1図）．鴨川地溝帯北断層及び鴨川地溝帯南断層は高橋（1997）によって定義され，さらに高橋（1997）では2つの断層によって挟まれた地域を嶺岡構造帯とし，そこに分布する正常堆積物（安房層群：中嶋・渡辺，2005）以外の変成岩，蛇紋岩，玄武岩，堆積岩といった多様な岩石を一括して「広い意味での」嶺岡層群とした．本報告では嶺岡構造帯及び嶺岡層群について高橋（1997）の定義に従う．また，石堂断層は，斎藤（1992a）によって定義された石堂断層のうち南房総市富浦町原岡に分布する石堂断層に相当する．宇田断層は，小竹（1988）による宇田断層及び斎藤（1992a）による真野断層に相当する．千倉断層及び川口断層は本報告により再定義した．これらの断層によって房総半島の地層は，最も古い時代の地層（嶺岡層群）が分布する嶺岡構造帯を境にして，北側には，下位から安房層群，上総層群，下総層群が分布する（中嶋・渡辺，2005）．南側には，下位から保田層群，南房総層群（新称），西岬層，千倉層群，豊房層群が分布する．なお，嶺岡層群は，中期始新世～前期中新世の堆積岩に加えて前期中新世のアルカリ玄武岩（19.62±0.90Ma：Hirano et al. (2001)）が得られている．また，保田層群下部からは，最後期漸新世の珪質微化石群集が得られている（鈴木ほか，1996）．
砂勝ち砂岩凝灰質シルト岩互層からなる．白間津層は上方細粒化・薄層化を示す．白間津層下部は模式地の白間津海岸（第5.1図）で観察され，数十cm～数mの層厚のスコリア質粗粒砂岩が優勢で，しばしば砂質基質支持の砂礫層を挟む．海岸地域に分布する白間津層には乱堆積物層が発達し，そこにシロウリガイ化石が産出する（間嶋ほか（1992）など）．中～上部は南房総市白浜町の林道畑線（ルート12：第5.8図）で観察され，層厚10cm～数十cmのリズミカルな等量互層（第5.3図左上）をなし，まれに層厚10mを超す厚い砂岩層を挟在し，次第に上方細粒化・薄層化して布良層に漸移する．本層下部ほど土石流堆積物からなる厚層砂層が発達し，中～上位では古典的ブーマ・シーケンスが観察される混濁流堆積物（turbidity current product: Shanmugam and Moiola, 1995）の方が頻繁に観察される．砂岩層は黒～暗褐色を示し，シルト岩層は明灰色～青灰色を示す．また，全層準を通じてシルト岩中には生痕化石が普遍的に観察される（小竹（1995）など）．白間津層中に観察される砂岩層の斜交層理の示す古流向は南から北に向かうものが多く観察される（第5.3図下）．東側ほど層厚が厚く最大で200m程（ルート14：第5.9図）になり，西側ほど薄く100m程になる（ルート7：第5.8図）．
本層に挟在されるYK鍵層の前後の層準（林道畑線（第5.8図）の西側の沢，長尾川向斜軸部，千倉断層の北側）では，層厚1m～数mのスコリア質砂岩層が発達する．また，根本向斜南翼から林道畑線にかけてのYK鍵層層準は，泥質基質からなる乱堆積層を発達させる．なお，林道畑線のYK鍵層は乱堆積層中のブロックとして観察され，その露頭はIto and Sugiyama (1989)の「Loc.H（Fig. 5）」として記載されており，YK鍵層を含むブロックは「Brecciation Zone (Fig. 5)」中に観察される．
小竹ほか（1995）及び藤岡・小竹（2003）は，温谷地域の千倉・豊房層群の不整合境界直下（本報告のNY鍵層の上位）において大型の石灰質ナノ化石Gephrocapasa属の絶滅層準（1.21Ma：亀尾・佐藤，1999）を報告している．また，小竹ほか（1995）では，本層を不整合に覆う東長田層下部の火山灰鍵層Hn-7（本報告のCR鍵層）とHn-8（本報告のIH鍵層）層準の間に松山逆磁極期とブリュンヌ正磁極期との境界（0.78：Ogg and Smith，2004）の存在を報告している．したがって，加茂層の年代は1.21Ma～0.78Maのどこかに相当すると考えられる．
小竹ほか（1995）では，東長田層下部の火山灰鍵層Hn-7（本報告のCR鍵層）とHn-8（本報告のIH鍵層）層準の間に松山逆磁極期とブリュンヌ正磁極期との境界（0.78Ma：Ogg and Smith, 2004）を報告している．また，卜部（1997）は，東長田層上部の凝灰岩鍵層を上総層群の鍵層（Ks11（0.57±0.17Ma）：徳橋ほか，1983）と対比している．したがって，東長田層の年代は中期更新世のどこかに相当すると考えられる．
なお，館山地域の基盤をなす西岬層には，洲崎海岸を中心にして北西―南東～東西性の北方に向かって折れ曲がったような褶曲群が存在する（第9.1図）．洲崎北岸から波左間にかけての地域では，200～500mの間隔で背斜・向斜が繰り返し観察される．この構造は，本地域を含む房総半島南部においても異質な構造であり，Yamamoto and Kawakami (2005)によると，伊豆弧の衝突によって形成された構造であると考えられている．
歴史上，本地域を大きく揺るがした地震は，前述の大正地震（1923年）と元禄地震（1703年）が知られている．いずれの地震も相模トラフ沿いで生じているが，震源断層面は，本地域直下にあり（宍倉，2003；Nyst et al., 2006），沖積層の厚い館山低地では，震度7の揺れが生じたと推定されている（宇佐美，2003；武村，2003）．また，これらの地震に伴う津波の高さは，大正地震では布良で6m，千倉で2m，館山で1.8mと記録されている（羽鳥ほか，1973）．元禄地震では，古文書の記録などから，布良で10m，千倉で8.8m，館山で5.6mと推定されている（羽鳥，1976）．
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