The Ōmuta district (a quadrangle of lat. 33°0′12″1- 33°10′12″0 N, and long. 130°14′51″7-130°29′51″6 E) is located in Kyushu, southwest Japan. The mapped district consists of Late Triassic to Middle Jurassic Suō Metamorphic Complex, Cretaceous low-pressure and high-temperature polymetamorphosed area, Cretaceous plutonic rocks, Paleogene sedimentary rocks, and Neogene and Quaternary deposits. The geology of the Ōmuta district is summarized in Fig. 1．
Figure 1． Summary of the geology of the Ōmuta district.
LATE TRIASSIC TO MIDDLE JURASSIC SUŌ METAMORPHIC COMPLEX
The Late Triassic to Middle Jurassic high-pressure and low-temperature Suō Metamorphic Complex is distributed in the Chikuhi Mountains in this district. The Suō Metamorphic Complex is composed mainly of pelitic schist. These metamorphic rocks have suffered high-pressure pumpellyite-actinolite subfacies metamrphism or transition zone metamorphism between the epidote blueschist facies and greenschist facies.
CRETACEOUS HIGH-TEMPERATURE AND LOW-PRESSURE POLYMETAMORPHOSED AREA
The Cretaceous low-pressure and high-temperature metamorphism has overprinted over the Suō Metamorphic Complex in this district. The polymetamorphosed area is composed mainly of metamudstone. Using mineral assemblage of the metamudstone, the polymetamorphosed area can be divided into the chlorite-biotite, muscovite-andalusite and K-feldspar-sillimanite zones. Pelitic schist of the Suō Metamorphic Complex gradually changes to migmatitic pelitic gneiss with increasing degree of the polymetamorphism. The highest-grade part of the polymetamorphosed area has suffered amphibolite facies metamorphism.
CRETACEOUS PLUTONIC ROCKS
Cretaceous plutonic rocks are the Tamana Granodiorite and allied small intrusives exposed in the southeastern margin of the district. The Tamana Granodiorite is a medium- to coarse-grained hornblende-biotite granodiorite partly foliated. The granodiorite is dated at 106 Ma by the hornblende K-Ar method and at 99.7 Ma by the biotite K-Ar method. Small intrusives are scattered on the Tamana Granodiorite's north margin, and include fine- to medium-grained biotite granodiorite to granite with garnet-muscovite-biotite granodiorite to granite.
PALEOGENE
The Paleogene strata occur in the southeastern portion of the district along the east coast of the Ariake Kai (Bay). Their sediments are mainly sandstone with small amounts of mudstone, conglomerate and coal seam. The Paleogene strata of the district are famous not only for the Miike Coal-field, but also for their Eocene molluscan fossils. The Paleogene of the district are divided into three groups, the Akasaki, Ōmuta and Manda Groups in ascending order.
The Akasaki Group is only composed of the Ginsui Formation (30-70 meters in thickness) and characterized by the presence of reddish beds. Similar beds are known as the basal member of the Paleogene formations in the west to northwest Kyushu.
The Ōmuta Group includes the coal-bearing formations and is divided into three formations, the Komenoyama, Tōka and Nanaura Formations in ascending order. The Komenoyama Formation (60-70m) is fine-grained muddy sandstone, mudstone, and alternation of sandstone and mudstone, and yields abundant molluscan fossils. The Tōka (70-100m) and Nanaura (100-130m) Formations are medium- to coarse-grained arkosic sandstone with thin mudstone. A few molluscan fossils are also collected from these two formations. The Ōmuta Group is mainly characterized by occurrence of Pseudoliva japonica and Colpospira (Acutospira) okadai.
The Manda Group is composed of mostly marine deposits and divided into two formations, the Kattachi and Yotsuyama Formations in ascending order. The Kattachi Formation (100-130m) consists of medium- to coarse-grained glauconite sandstone, fine-grained muddy sandstone and alternation of sandstone and mudstone. This formation yields abundant gastropod and bivalve fossils accompanied by some accessory elements including cephalopods, brachiopods, echinoids, decapods, bird and plants. The Yotsuyama Formation (200-800m) consists of medium- to fine-grained muddy sandstone, mudstone and alternation of sandstone and mudstone with tuffaceous mudstone. The molluscan fauna of the Manda Group is characterized by Venericardia (Venericor) nipponica and V. (V.) mandaica is clearly different from that of the Ōmuta Group. The latter is correlated with the Lower Eocene of the Amakusa-kamishima, slightly south of the Ariake Kai. The former is correlated with the Middle Eocene of the Nagasaki Prefecture in west and northwest Kyushu. The Upper part of the Yotsuyama Formation is correlated with the Upper Eocene by nannofossils from drilled samples under the Ariake Kai.
NEOGENE
Intrusive rocks and subconsolidated sandstone and mudstone formation, which occur underground, are estimated as the Neogene of the district. The intrusive rocks are thought to be dolerite sill intruding into the Paleogene strata under the Ariake Kai. The age of the dolerite sill is not known, but probably corresponds to the Miocene Kitamatsuura Basalts.
The subconsolidated sandstone and mudstone formation is unconformably covered by the Quaternary, and these subconsolidated rocks may be correlative with the Pliocene Kurume and Kuroki Formations. But details of their ages and distribution are not known.
QUATERNARY
The Quaternary of the district mainly spread in the northern portion of the district along the north and east coast of the Ariake Kai. The Quaternary of the district is divided into the Middle Pleistocene, Late Pleistocene and Holocene.
The Middle Pleistocene is subdivided into the Tateishi and Kawazoe Formations. The Tateishi Formation includes higher terrace deposits and older talus deposits. The Kawazoe Formation only exists beneath the surface in the district. The formation intercalates 0.6 Ma Setaka Tephra.
The Late Pleistocene includes the Nakabaru Formation, Aso-3 Pyroclastic Flow Deposits, Takagise Formation, Aso-4 Pyroclastic Flow Deposits and Mitagawa Formation in ascending order, but the Aso-3 Pyroclastic Flow Deposits and Takagise Formation are not exposed in the district. The Nakabaru Formation consists of gravel, sand, silt and fresh-water clay. It intercalates the Aso-3 Pyroclastic Flow Deposits and Takagise Formation. The Aso-3 Pyroclastic Flow Deposits (130 ka) is a wide spread tephra in central Kyusyu. The Takagise Formation consists of silt, marine clay and shell, and was formed during the last interglacial stage (MIS 5e). The Aso-4 tephra (90 ka) is one of the most areal standard tephras in Japan. The Aso-4 tephra occurs in this district as a pyroclastic flow deposit (Aso-4 Pyroclastic Flow Deposits) with welded pumice lapilli tuff. The Mitagawa Formation is composed of gravel, sand and mud, with basal lahar deposits derived from the Aso-4 Pyroclastic Flow Deposits. The Mitagawa Formation is covered by yellowish loess including glass shards of the Aira-Tn tephra (25 ka)．
The Holocene consists of the Hasuike and Ariake Clay Formations. The Hasuike Formation consists of fresh- to brackish-water clay, silt, sand and gravel. The Ariake Clay Formation is composed of marine clay, silt, sand and shell, and its sedimentation continues on the sea floor of the Ariake Kai.
ECONOMIC GEOLOGY
Coal
Miike Colliery was the largest colliery in Japan and produced 6.6 million tons of clean coal in 1970. The Komenoyama, Tōka, Nanaura and Kattachi Formations of the Paleogene strata intercalate coal seams called the Eighth Seam (1 m± in thickness), Miike Main Seam (2.4-6.0 m), Miike Upper Seam (1.9-5.6 m) and Second Upper Seam (1.3-4.7 m), respectively. The coal quality of the Miike Colliery is a high volatile bituminous coking coal. The colliery was closed in 1997.
Building stones
The Aso-4 Pyroclastic Flow Deposits and Paleogene sandstone were quarried for building stone in the district. The welded part of the Aso-4 Pyroclastic Flow Deposits was used for stone bridges. Paleogene sandstone of the Nanaura Formation was used as building material of the Miike port and stone wall.
Hot spring
A hot spring (50.1 degrees centigrade temperature) used for bathing is in Yanagawa city, northwest of the district.
南関地域の周防変成コンプレックスは白亜紀玉名花崗閃緑岩との接触部から約3～5kmの範囲に黒雲母が生じている．また，玉名花崗閃緑岩から500m以内の接触部付近ではカリ長石・紅柱石・菫青石を生じている（井上，1963，渡辺・藤本，1993）．熊本県玉名東方の木の葉変成岩類（藤本・橋本，1960）は，筑後変成岩類（本報告での周防変成コンプレックス）が高温低圧型重複変成作用を被って生じた岩石であると考えられている（唐木田・古田，1994）．木の葉変成岩類は全域で高温低圧型の変成作用を被っており，泥質変成岩に黒雲母・紅柱石を生じている（Hashimoto and Fujimoto，1962）．玉名花崗閃緑岩との接触部付近では泥質片麻岩にカリ長石＋菫青石＋珪線石が生じている（唐木田・古田，1994）．木の葉変成岩類の泥質片岩の黒雲母K-Ar年代は103±8，95±6,102±8Ma（Miller et al.，1963）である．
本帯の変成泥岩は黒色―銀灰色を呈し，片理が発達する泥質片岩である．波長数cmの微褶曲が発達する．本帯の泥質片岩は緑泥石含有斜長石石英黒雲母白雲母片岩である．ただし，主要構成鉱物の相対的量比は試料により変化し，IUGSの変成岩命名法（Schmid et al.，2007）に従うと記載岩石学的な名称も異なってくる．これらの泥質片岩はいずれも少量の石墨，スフェン，ジルコン，燐灰石，電気石，不透明鉱物を含む．また，径1mm以下のざくろ石の斑状変晶を含み，ざくろ石緑泥石含有斜長石石英黒雲母白雲母片岩の部分もある．いずれの泥質片岩も白雲母と緑泥石の形態定向配列による片理の発達が顕著である．黒雲母は白雲母や緑泥石と片理を形成するものと，片理を切って成長するものの両方がある．
古生物学的研究は大塚（1901）の時代には第三紀と判断する以上の化石を得られなかった．その後Yokoyama（1911）は大牟田市戸田の竪坑より採集された動植物化石標本15種のうち10種を新種として古生物学的記載を行い，二枚貝，オウムガイ化石よりその地質時代を古第三紀暁新世―始新世とした．この時記載されたVenericardia nipponicaはその後始新世の化石帯名に使用されて，天草や長崎県高島を含む九州北西部の古第三系の層序・古生物の研究の端緒となった．長尾（1923）は天草よりNummulitesを発見し，対比される九州北西部の地層を始新世とした．その後長尾（1924，1925a，1926a，b，c，d，1928）とNagao（1928a，b，c）の九州古第三紀層の層序の一連の研究があり，その一部として大牟田周辺の軟体動物化石も記載報告された．松下（1949）は北部九州の古第三系の報文の中で長尾の記載・報告以外の化石について若干の種類を挙げている．以後Kuroda and Urata（1964）による新種Perotrochus eocenicusの記載，鎌田（1980）によるOrthaulax japonicusを現生のPseudoliva属に帰属すべきとする研究がある．このほか地元の化石採集家によってコケ虫，底生有孔虫化石（野田，1989）や，鯨の椎骨などが発見されている．本報告では富田ほか（1992）の軟体動物化石を再検討し，若干の種については種名の変更を行い，そのリストを第6.2表に示す．この表は古生物の分類を概ねOyama et al.（1960）のそれに準じている．微化石については陸域の露頭ではほとんど微化石を産しないため研究例が少ない．木原（1960）は主に小型有孔虫化石を検出し，海成の勝立層・四ッ山層は挟炭層たる稲荷層・七浦層と同時異相の堆積物と考えたが，ほかにこれを支持する有力な証拠は見出されていない（富田ほか，1992）．Okada（1992）は有明海の試錐試料によって石灰質微化石群集を検討し，四ッ山層を古第三紀微化石帯のCP14b～CP15b（中期始新世末―後期始新世末）相当として天草炭田の二江層上部から鬼池層に対比した．
本地域の古第三系の重鉱物組成については小原（1961），佐藤（1963）の研究がある．古第三系を通して10数種類の重鉱物が確認され，小原（1961）はそれらの組み合わせ・産出頻度・特徴的重鉱物の出現などによってMI-1～MI-6の6重鉱物帯を識別した．最下部の銀水層（MI-1帯）は露出地域ごとの基盤岩の重鉱物組成を反映し，結晶片岩を不整合に被覆する古第三系分布域北部の甘木山では褐色電気石・ざくろ石が多く，ジルコンは最上部を除くと全般に少ない．また特異な鉱物として紅れん石を含む．一方玉名花崗閃緑岩を基盤とする大間山の銀水層は褐色電気石，ざくろ石，ジルコン，緑れん石に富む．更に南の小岱山花崗岩を基盤とする南隣「荒尾」地域内の府本ではジルコン・電気石・モナズ石に富み，ざくろ石に乏しく，特に基底部ではモナズ石が50～60%を占めるモナズ石砂岩と言うべき特異な砂岩が分布する（小原，1961；Miki and Matsueda，1985）．このような重鉱物組成は，砕屑物供給源としての近接後背地の影響を受け易い，古第三系堆積開始期の地質状況を反映したものと推定される．銀水層砂岩の物理的性質では帯磁率がきわめて低い点が特徴的である（第6.3表）．
赤色の原因物質は石英・長石などの砕屑粒子間の基質部を充填する微粒の赤鉄鉱で，その存在以外には赤色岩層内で互いに隣接する赤色部と非赤色部との間に鉱物組成・組織上の差異はないことが，透過・反射顕微鏡，X線回析，EPMAによる検討で確かめられている（Miki and Matsueda，1985）．砕屑性と考えられる赤鉄鉱・磁鉄鉱・いわゆる褐鉄鉱などの比較的粗粒の鉄鉱物粒子は両色彩部に共に存在する．粘土鉱物としてはイライト・緑泥石・イライト／スメクタイト混合層鉱物・カオリン鉱物が含まれる．銀水層砂岩・泥岩の重鉱物組成は，色彩に関係なく各分布地の基盤岩（北部の結晶片岩・南部の玉名花崗閃緑岩）のそれと類似しており（Miki and Matsueda，1985；小原，1961），かつ赤色岩層を含まない米ノ山層よりも上位の地層とは異なっている（小原，1961）．これらのことから上述の微粒赤鉄鉱は，近接した後背地に露出する基盤岩類から鉄鉱物・含鉄珪酸塩鉱物として堆積盆地へ供給された鉄分に起源を持つものと推定される．
赤色岩のFe2O3含有量は赤色部で4.62～6.98wt.%，あい伴う非赤色部で1.80～2.37wt.%の範囲にあり，銀水層に対比される本地域南方の熊本県天草地域の赤崎層に比べてやや低い値を示す（三木・松枝，1974；Miki and Matsueda，1985）．FeO含有量は0.39～0.82wt.%で，色彩による顕著な差は認められない（第6.4図；Miki and Matsueda，1985）．Fe2O3は赤鉄鉱・磁鉄鉱・針鉄鉱―鱗鉄鉱（いわゆる褐鉄鉱）・緑れん石などに，FeOは磁鉄鉱・緑泥石・黒雲母などの鉱物中に存在するものと考えられる．世界各地に産する赤色岩層の成因については，乾燥気候・高温湿潤気候・熱帯多雨気候下でのラテライト化作用などの関与を考える例が多く，風成層・蒸発岩・古土壌などが赤色岩層に伴って産することも多いことから，古気候の指標としての赤色岩層の意義が主張されている．しかし銀水層の赤色岩層については，色彩境界が少なくとも5万分の1地質図に表現できる程度では地層境界と斜交しておらず，特別の古気候条件を示唆する資料も得られていない．したがって銀水層堆積盆地への後背地からの鉄分の供給と，供給された鉄を3価状態に維持する酸化的な堆積環境とが，赤色岩層形成にとって重要な役割を果たしたものと考えられる．なお三木（1991）は米ノ山断層中の銀水層から炭質物に伴う黄鉄鉱を検出し，nucula? sp．化石の存在とともに，銀水層堆積時に既に海水の浸入があったことを示唆するものと考えた．
新勝立町ひばりヶ丘団地東側の崖から採取した粗粒砂岩中の海緑石は，EPMA分析によればかなりのばらつきはあるものの，概ね15～22wt.%程度のΣFeと7～8wt.%程度のK2Oを含み（三木・福岡，1983），Odin and Matter（1981）によるglauconitic mineralsの組成領域に入る（第6.16図）．このような化学組成，特に鉄含有量には，天草炭田一町田層（天草炭田地質図内），福岡炭田姪浜層（「福岡」地域内），諫早炭田侍石層など他地域産の海緑石との間に差異が認められる（三木・福岡，1983）．
石炭層を置き換えたようにシル―ラコリスが貫入する理由として，三池炭田の石炭が揮発性物質に富み強粘結性があるため，350度以上に熱せられると軟化して流動化する性質が考えられる．地下の石炭層が火成岩の貫入によって流動化する現象は筑豊炭田でも観察されており（Takahasi and Tomita，1985），溶融した貫入岩の高熱によって，地層中の石炭層が流動化して周辺の割れ目を通じて逸失したために石炭層を置換して上下の地層中に胚胎したと推定される．
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