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(1)271. GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP IN WESTERN SHIKOKU, JAPAN. TOSHIHARU NISHIMURA. (Received November 15, 1983). INTRODUCTION. The Upper.Cretaceous Izumi Group crops out in a narrow and long belt from the. Izumi Mountain Range in Kii Peninsula. lnfYe▼ヱOne. to Matsuyama in western Shikoku through the Yuzuruha Mountain Range in southern Awaii Island and the Sanuki Mountain Range in eastern Shikoku (Fig.1). In the geological sence, the Izumi Group is distributed on the north side of and along the Median Tectonic Line between the Ryoke and the Sanbagawa Metamorphic Belts. Much work on the Izumi Group has been accumulated since the stratigraphic succession in the. Izumi Mountain: Range was proposed by KOBAYASHI (1931). The Izumi Group Figハ1.. Locality. has a thickness of 5 to 7 kilometers. map.. A:Awaji Island S:Sumoto H:Hiketa Irlkeda M:Matsuyama M.T.L.rMedian Tectonic Line kShiroyama 2:Dogo-himezuka 3:Yokogawara 4:Kawakami 5:Onda. in each area of the Izumi Mountain Range, Awaji Island, the Sanuki Mountain Range, and the Matsuyamadistrict CSA-. 6:Soedani :Umega-toge 8:Ainotam 9:Naru 10:Kawamshi ll :Kimi. SAI.1936: NAKANO, 1953 ; HARADA,. 12:Kaminada 13:Konokawa 14:Mukaibara. 1965 ; TANAKA.1965; HAYASHI, 1969. 15:InuyoseHoge 16:Tanigami-yama 17:Otomo-yama. 18:Kamimura-j′ama. 20:Hanayama 21:Bandagamori. MS∴NISHIMURA , 1976 ; TAKAHASHI, 19:Sako. 1977 ). As the Izumi Group forms. 22:Shiogamon 23:Okusodadani 24:Toiya a syncllnal structure wlth an axis. Geoscience Institute, the Hyogo University of Teacher Education.

(2) 212. plunging eastwards, the apparent total thickness attains to about 100 kilometers. Paleontological evidences such as ammonites and inoceramids indicate that the Izumi Group. belongs. to. the. Campanian. to. Maestrichtian. age(SUYARI,. 1973;MATSUMO′rO,. 1977). Sandstones of the Izumi Group are composed mainly of fragments of acidic volcanic and granitic rocks, and the grain composition is homogeneous. PaJeocurrent directions based on sole markings of sandy turbidites indicate that sediments of. the Izumi Group were transported from east or northeast in every district (HARADA.1965; TANAKA,1965; SUyARI, 1966; SuYARI et al, 1968; HAYASHI, 1969 MS;NISHIMURA ,1976). As mentioned above, the Izumi Group is a pecullar sedimentary body ; it was deposited in a narrow and long sedimentary basin of. the E - W trend, and has a very large thickness in spite of the shortness of the time of deposition, showing a comparatively monotonous lithofacies. Many acidic vitric tuffs are intercalated with the Izumi Group. The glass shards. are. altered. 1962;. NAKAJIMA. to. and. zeolites. TANAKA. ,. and. alkali. 1967;. feldspars. N主SHIMURA,. (NAKAJIMA. 1973MS. et. ). alリ. IITIMA. etal. (1974) and NISHIMUHA et al. (1980) clarified the zeolitic zones of the lzumi Group in Shikoku and Awaji Islands, analysed, the Izumi sedimentary basin based on the zeohtic burial diagenesis, and presumed a significant fault between the northern and southern Matsuyama districts in western Shikoku. At that locality, the presence of the active Kawakami Fault is pointed out by UKADA (1972). According to my recent investigations, however, the Kawakami Fault is only a small rejuvenated displacement along a big and old fault. Therefore, this old fault is called the Shigenobugawa Fault in this paper. This paper aims at confirming the presence of the Shigenobugawa Fault and clarifying its geological significances, and analysing the Izumi sedimentary basin in detail. I describe the geology, the zeolitic zones, and mudstone porosity of the. Izumi Group in the Matsuyama district and its environs in western Shikoku. Then, I estimate the magnitude of the Fault from the burial depth of the Izumi Group, and discuss the Izumi sedimentary basin. This paper is a portion of the doctoral dissertation which has been submitted to the University of Tokyo ACKNOWLEDGEMENTS : I wish to express my sincere gratitude to Professor Azuma IIJIMA of the University of Tokyo for his guidance and encouト agements throughout this study and critical reading of the manuscript. I also wish to thank Professor Akira TOKUYAMA of the Hyogo University of Teacher Education for his encouragements and reading of the manuscript. I am indebted to Associate Professor Minoru UTADA of the University of Tokyo for his precious.

(3) E^^^K^^^Ki ◆. ◆. ◆. ◆. ・・・・々。●● "^・. Izuml Group Northern Area. l. Matsuyama. ■* s W^^m^^^^^^^Kt. Southern Area. t : : j. IOcD. River Shi9enobu竺wajく. I/ r AL.A .●. 1 : Alluvial Deposits 2 : Pleistocene Deposits 3 : Gunchu Formation 4 : Andesite 5-6 : Ishizuchi Group (5 ; Andesite, 6 ; Andesite dyke). 56. 7-8 : Kuma Group (7; Myojin Formation, 8 ; Nimyo Formation). Nl : Lower member. (conglomerate-bearing sandstone-predominant alternation). N2 : Middle member (mudstone-predominant alternation) N3 : Upper member. (sandstone-predominant alternation ). Sl : Lower member. (sandstone-predominant alternation ). S2 : Middle member. (mudstone-predominant alternation ). S3 : Upper member. (sandstone-predominant alternation). Circled number shows the locality of the column of Fig.4. X: outcrop of the Median Tectonic Line. m. 15: Sanbagawa Crystalline Schists 16: fault. ′. 9-12 : Izumi Group (9 ; tuff, 10; conglomerate, ll ; shear zone, 12; slump deposits) 13: Ryoke metamorphic rocks 14: Ryoke granites. 234. Fig-2. Geologic map of the Matsuyama district and its environs.. LJJrJ.irrl. I. 図m¥BU¥. 5000m I.....

(4) GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 275. advice and encouragements. Thanks are due to Drs. Ryo MATSUMOTO and Ryuji TADA of the University of Tokyo for their advice on the method of mudstone porosity measurement- This research was partly supported by Grant-in-Aid for Fundamental Scientific Research from the Ministry of Education of Japan (Grant No.474364).. GEOLOGIC SETTING. The Izumi Group in the Matsuyama district and its environs lies unconformably on the Ryoke granites or the Ryoke metamorphic rocks on the north side. On the south side, the Izumi Group bounds on the Sanbagawa Crystalline Schists or the Eocene Kuma Group by the Median Tectonic Line of the Tobe phase. Andesitic rocks of the Miocene Ishizuchi Group intrude at some places along the Median Tectonic Line. The lzumi Group in the east of Tobe is unconformably overlain partly by the Kuma and the Ishizuchi Groups. Geologic map and profiles of the Matsuyama district and its environs are shown in Figs. 2 and 3 respectively. In this article, geology of the Matsuyama district and its environs is described briefly. For details, refer to NISHIMURA (1984). Shigenobugawa. Fig・3. Geologic profiles of the Matsuyama district and its environs. Legend is shown in Fig.2.

(5) 276. 1. Stratigraphy. A. Izumi Group In the Matsuyama district and its environs, stratigraphic succession of the工zumi. Group on the north side of the Shigenobugawa Fault is different from that on the south side. Columnar sections of the Izumi Group are shown in Fig- 4 South of the Shigenobugawa Fault. North of the Shigenobugawa Fault. Shigenobugawa Fault. Fig.4. Columnar sections of the lzumi Group in the Matsuyama district and its environs.. Legend is shown in Fig.2.. Sandstones of the Izumi Group are gray or yellowish gray to greenish gray, medium- to fine-grained, and contain abundant hthic fragments. Sandstone beds of. sandstone-predominant alternation often show graded bedding. Mudstones of the Izumi Group are black to gray, and often contain sand grains heterogeneously.. The Izumi Group on the north side of the Shigenobugawa Fault covers unconformably the Ryoke granites or the Ryoke metamorphic rocks. The Izumi Group is subdivided into three members ; Nl , N2, and N3 members in ascendingorder. The Nl member is composed of conglomerate-bearing sandstone-predominant alternation. The N2 and N3 members are mudstone-predominant and sandstone-predominant alternations respectively- The total thickness of the Izumi Group of the northern.

(6) GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 277 block attains to about 5,000 met占rs. Two thick acidic tuff layers are persistent; Tuff NA and Tuff NB in ascending order. Two tuff layers are made of alternated beds of vitric tuff, some centimeters thick, and shale, some millimeters thick. The distribution of the Izumi Group on the south side of the Shigenobugawa Fault is restricted southwards by the Median Tectonic Line. The Izumi Group of thesouthern block is also subdivided into three members ; SI , S2, and S3 members in ascending order. The Sl and S3 members are sandstone-predominant alternations, while the S2 member is mudstone-predominant alternation. The Izumi Group of the southern block is about 6,500 meters thick. Two thick layers of acidic tuffs are traceable in the S2 member ; Tuff SA and Tuff SB in ascending order. The Izumi Group on the north side of the Shigenobugawa Fault yields Inoceramus. schmidti. Ml。HAEL,. /・balticus. BOEHM,. and. Gaudryceras. striatum. (JIMBO) which belong to the Campanian age (MATSUMOTO, 1977). On the other hand, the Izumi Group on the south side of the Fault has been barren of reliable index fossils. Though mudstones of the Izumi Group on both sides of the Fault contain radiolarian fossils, these are not effective for age determination. Consequently, it is impossible to correlate the Izumi Group on both sides of the Shigenobugawa Fault b,∫ fossil evidences. It is also impossible to correlate the Group lithologically: Sandstone-predominant alternation prevails in the north block whereas mudstone-predominant alternation dominates in the south block; the horizons of marker tuff layers in mudstone-predominant alternation are different between both blocks.. B. Kuma and Ishizuchi Groups The Eocene Kuma Group lies unconformably on the Sanbagawa Crystalline Schists and is distributed in the area mainly to the south of the Median Tectonic Line. The、 total thickness of the Kuma Group is about 700 meters. The Kuma Group is divided into the Nimyo and the Myojin Formations in ascending order. The former is overlain unconformably by the latter. Most of the Nimyo Formation comprises conglomerate beds, whose pebbles and cobbles are very angular and are. derived mostly from the Sanbagawa Crystalline Schists. The Myojin Formation consists mainly of conglomerates interbedded with arkosic sandstones. Pebbles and cobbles of these conglomerates are well rounded, and are derived mainly from the Izumi sandstones and granitic rocks and subordinately from the Izumi mudstones and crystalline schists.. Andesitic rocks of the Miocene Ishizuchi Group ( NAGAI, 1956) are distributed.

(7) 278. in the southern Matsuyama district. There are two types of andesitic rocks m the study area ; one, intrusive bodies along the Median Tectonic Line, and the other, lava flows which cover the Izumi and the Kuma Groups locally.. 2. Geologic structure. A. Folding Folded structures of the Izumi Group in the Matsuyama district and its environs are discontinuous with the Shigenobugawa Fault between. The Izumi Group on the north side of the Shigenobugawa Fault represents the north wing and axial parts of a large scale synclinal structure with an axis of ENEWSW trend. A small scale pair of anticline and syncline can be recognized near the Median Tectonic Line and the Shigenobugawa Fault. Judging from graded bedding of sandstones, overturned anticlinal structure is recognized to the northeast of Hirai Town. In the lzumi Group to the south of the Shigenobugawa Fault, a repetition of synclinal and anticlinal structures with axes of ENE-WSW trend is recognized. The axis of main synclinal structure plunges eastwards as well as that on the north side of the Fault.. B.Fault i) Median Tectonic Line The Izumi Group thrusts over the Myoiin Formation of the Kuma Group as observed on the river floor of Tobe River, where is the type locality of the activity of the Median Tectonic Line of the Tobe phase (KOBAYASHI, 1950). The fault plane strikes EW and dips 34 north. While mudstone-predominant alternation of the Izumi Group is sheared considerably, only small scale dislocations parallel to the Median Tectonic Line・ are observed in pebbles and sandy matrix of the Myojin Formation. To the west of Tobe, black schists of the Sanbagawa Crystalline Schists are m fault contact with the Miocene andesite, which is also in fault contact with the Izumi Group_ The Izumi Group has a shear zone of about 1 meter width. The strike line of this fault bends southwards on the northeast of the Inuyose Pass, bends again westwards on the west of the Pass, and runs straightly to Kaminada, judging from the distribution of the sheared Izumi Group and the Sanbagawa Crystalline Schists..

(8) Geologic. map. of. the. area. along. the. Shigenobugawa. Fauユt.. 1 : Pleistocene Deposits 2 : Andesite dyke 3-8 : IzumiGroup (3;S3 member, 4 ;S2 member, 5; N3 member, 6 ;N2member, 7 ; tuff, 8, shearzone) 9 : Sanbagawa Crystalline Schists. GEOLOGYANDBASINANALYSISOFTHEUPPERCRETACEOUSIZUMIGROUP. Fig.5..

(9) 280. To the east of Tobe, sandstone-predominant alternation of the Izumi Group may be in fault contact with the Nimyo Formation of the Kuma Group, which can be inferred from shearing of the Nimyo Formation. In the Tanbara area, sandstones and mudstones of the Izumi Group are sheared about 500 meters wide along the Median Tectonic Line. ii) Shigenobugawa Fault The Shigenobugawa Fault branches.off from the Median Tectonic Line at a locality about 1 kilometer south of Naru, Tanbara Town, and runs westwards to Onda through Ainotani and Umegatoge Pass. Geologic map and profiles along the Shigenobugawa Fault are shown in Figs. 5 and 6 respectively. Lithofacies and geologic structure are discontinuous with the Shigenobugawa Fault between. Sandstonepredominant alternation dipping south prevails on the north side of the Shigenobugawa Fault, and mudstone-predominant alternation dipping north on the south side. The Izumi Group from Ainotam to Shigenobugawa Fault. Umegatoge Pass is subjected to strong shearing, forming a shear zone of the maximum width of 500 meters. The width of shear zone, however, dies out westwards abruptly, and to the west of Soedani, the. S.L.. Izumi Group is sheared only locally. C. The Miocene andesite shows no evidence of shearing- Localities where the Shigenobugawa Fault runs can be inferred from the existence of the strongest shearing of the Izumi Group, though the outcrop of the Shigenobugawa Fault itself has not been found. In a little distance from the Fault, dip and strike of bedding and shear planes can be measured. Shear planes are nearly parallel to bedding planes, and they. Fig.6. Geologic profiles across the Shigenobugawa Fault. Legend is shown in Fig.5.. strike N70 E and dip south steeply on the north side of the Fault anddip north on the south side.. Farther west from Kawakami, the Shigenobugawa Fault is covered by Recent alluvial deposits along the Shigenobu River. However, it is certain that the Shigenobugawa Fault exists under the Matsuyama Alluvial Plain, judging from.

(10) GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 281. such facts as the impossibility of stratigraphic correlation, the discontinuity of folded structures and the difference of zeolite zones as mentioned in the next chapter.. DISTRIBUTION OF ZEOLITE ZONES. Three zeolite zones formed by burial diagenesis were recognized in acidic vitric tuffs of the Izumi Group in Shikoku and Awaji Islands (IIJIMA et al-, 1974;. NISHIMURA. et. al一,. 1980;. NISHIMURA,. 1984).. In. this. chapter,. the distribution of zeolite zones in the Matsuyama district and its environs is described briefly. For more detailed descriptions, see NISHIMURA et al. (1980) and NISHIMURA (1984). Fig. 7 shows the distribution of authigemc analcime and albite in acidic tuffs. in the Matsuyama district and its environs. Authigenic albite occurs only on the north side of the Shigenobugawa Fault, while authigenic analcime only on the south side. This distribution of authigenic minerals is concordant with the fact that stratigraphic succession and geologic structure are different with the Shigenobugawa Fault between.. Fig.7. Distribution of authigenic analcime (▲) and albite (・) in silicic vitric tuffs in the Izumi Group in west Shikoku. 1 : Kuma and Ishizuchi Groups 2 : Ryoke granites and Ryoke metamorphic rocks 3 : Sanbagawa Crystalline Schists 4 : Tuff 5 : Boundary betweenmembers Ⅲ: Zonel IV: ZoneW - M.T.L.: Median Tectonic Line.

(11) 蝣JSI. The distribution areas of the authigenic analcime and albite correspond to respective zone HI (analcime zone) and zone IV (albite zone) of zeolitic burial diagenesis. by IIJIMA and UfADA (1965) andtlJIMA (1978 a,b,1980). Zone W on the north side of the Shigenobugawa Fault represents the westernmost part of the zeolite zones in the Izumi Group, which appear progressively from zoneII (clinoptilolite-mordenite zone) in the east of Awaii Island to zoneIV in western Shikoku through zoneM in the east of the Sanuki Mountain Range (NISHIMURA et al.,1980).. EFFECTIVE POROSITY OF MUDSTONES OF THE IZUMI GROUP. Effective porosity of muclstones of the lzumi Group was measured in order to estimate the maximum burial depth of the Izumi Group in the Matsuyama district and its environs.. Fig. 8 shows sampling localities of the mudstones collected for porosity measurement. At fresh outcrops, samples were collected from massive, homogeneous, clayey mudstones lacking cracks and veins. No samples were collected from the axial part of folding and from near faults. Thirty-one samples were collected on the north side of the Shigenobugawa Fault, and twenty-seven on the south side. For the comparison with the Matsuyama district, thirty-seven samples were collected from the Sumoto district (zone II), thirty-five from the Hiketa district (middle part of zone HI), and thirty-one from the Ikeda district (upper part of zone IV). Because mudstones. of. the. Izumi. Group,. especia一ly. of. the. Sumoto. and. southern. Matsuyama. districts, are so weathered as to exfoliate easily, fresh mudstone samples can be collected o・nly from river floor and quarry. The mudstone specimens sampled were cut into a rectangular shape of 12× 12×. 45 milimeters. After dried at 110 。C for 48 hours, they were weighted. T<Jtal volume of a sample (V) was measured by the use of mercury. Pore volume of a sample (Vp) was measured by carbon tetrachloride method (MuLLER, 1967), Effective porosity ( め e) was calculated according to the equation:. ・e-普×100 Number of samples, range and average value of porosity, and estimated burial depth are shown in Table 1. Samples replaced and cemented by calcite are excluded. As already stated, lower horizons of the Izumi Group crop out westwards, and the total thickness of the Group attains to 100 kilometers apparently. It is obvious that apparent thickness of the lzumi Group does not mean the burial.

(12) GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 283. V. i ≡;二二二コ. : -rll. 「. i.______lJ. Ikeda. 二.∴ Fig. 8-1. Sampling localities in the northern and southern Matsuyama districts to measure mudstone porosity.. ☆ shows the sample of the maximum. Fig. 8-2. Sampling localities in the Ikeda district to measure mudstone porosity. 蝣&. shows. the. sample. of. the. maximum. value of porosity in the Ikeda district.. value of porosity in each district.. Fig. 8-3. Sampling localities in the Hiketa district to measure mudstone porosity.. ☆ shows the sample of the maximum value of porosity in the Hiketa district.. Fig- 8-4. Sampling localities in the Sumoto district to measure mudstone porosity. it shows the sample of the maximu- value of porosity in the Sumoto district.. depth (NISHIMURA et al-, 1980). Therefore, it is impossible to estimate the burial depth in each district stratigraphically. Then, setting the sampling locality of the uppermost horizon in a given district to be the standard, the relationship between the stratigraphic interval (i.e..thickness) from that standard horizon and the porosity was investigated, and the result is shown in Fig.9. There is no tendency that porosity decreases gradually with the increase of burial depth. There is a tendency that the average value of porosity decreases gradually from the Sumoto district to the northern Matsuyama district, among four districts on the north side of the Median Tectonic Line and the Shigenobugawa Fault (Table 1).This fact is not inconsistent with the fact that the zeolite zones form-.

(13) 284. North即・n Matsuyama. Sou them Matsuyama e. lO・/.. Ikeda. Oe. O* 'i.. Hiketa. Oe. Sumoto. Oe ・. 10. JOAJ31UIDmdDj61)DLIS. 2. l°. 3. Fig. 9. Effective porosity - stratigraphic interval diagram of mudstones of the Izumi Group in the Sumoto district, Awaji Island and four districts, Shikoku. ed by burial diagenesis are progressive westwards. Furthermore, the average value of mudstone porosity in the southern Matsuyama district is larger than that in the northern Matsuyama district, which also coincides with the arrangement of zeolite zones, that is, Zone皿in the south and Zone IV in the north. Table 1. Effective porosity of mudstones, estimated burial depth, and zeolite zone in five districts of the Izumi Group. N UM BE R D I S T R IC T. O F SA M PL E. RA NG E %. AV ERA GE OF %. PO R OS ITY. PO RO SIT Y. O F. BU R IA L. D EP TH km. ZEO L ITE. (り. (2 ). (3 ). 5 .9. 4 .9. 2 .9. SUM OT O. 2 3. 4 .5 - 2 .1. 2 .9. H IKE TA. 2 5. 5 . 3 - 0 .8. 1 .9. > 6. 4 .6. IK ED A. 23. 3 . 2 - 1 .3. 2 .2. > 6. 5 .6. 4 .3. 26. 3 . 6 - 0 .5. 1 .4. =・ 6. 5 .5. 6 .3. 2 2. 9 .7 - 1 .9. 2 .9. NO RTH ER N n A T S U Y A 椎A SOU TH ERN M A T S U Y A 九A. 5 .9. 3 .6. 2 .9 - 4 .〕. 2 .9 - 4 .3. ( 1 ) estimated from the average value of porosity (2 ) est与mated from the maximum value of porosity ( 3 ) estimated from zeolite zone. ZON E. lo w er. IH. m idd le. IV. up pe r. IV. ⅡI.

(14) GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 285. Mudstone Porosity(%) I. 20. 30. 40. The maximum burial depth of the 50. lzumi Group in each district was estimated by using the porosity-burial depth diagram of mudstones of Neogene. (∈こ至dan│Dun血. oil. fields. of. Japan. (.MATSUMO′fO. and. IIJIMA, 1981 ) (Fig.10). Valuesof porosity corresponding to the depth. ./. more than 5 kilometers were extrapo-. lated. Tablel gives burial depth㌔ estimated from the average value of. .H . ltNF. porosity. The values of burial depth have no areal variation, and are larger than those estimated from zeolite. Fig- 10. Maximum burial depth of the mudstones of the Izumi Group with lar等eSt value of effective porosity in each district. The original porosity - burial depth diagram is after MATSUMOTO and llJIMA (1981). M: northern Matsuyama district T : southern Matsuyama district I蝣Ikeda. zones (NISHIMURA et al., 1980 ) Taking this fact into consideration with the already stated fact that there is no tendency of porosity decrease with the increase of burial depth in each. district. H; Hiketa district S : Sumoto district. district, it may be thought that mudstones of the Izumi Group were. more compacted than the Neogene mudstones and have less porosity at the same depth of burial. This fact may be caused by progressive cementation or by tectonic pressure.. Burial depths estimate"rom the maximum value of mudstone porosity in each district are in good concordance with those estimated from the zeolite zones (Table 1). This fact is interpreted that these mudstones escaped the influences of regional cementation or tectonic movements, and that they were influenced only by burial diagenesis. In the Matsuyama district and its environs, there is a significant differenα in burial depth between both sides of the Shigenobugawa Fault. The difference is estimated to be about 2 kilometers.. DISCUSSION. 辛 : These values were read on the center line of diagram of MA.TSUMOrFO and. IIJIMA (1981)..

(15) :8fi 1. Geological significance of the Shigenobugawa Fault The Shigenobugawa Fault matches the active, right-lateral Kawakami Fault (UKADA, 1972) in the northeast of Kawakami, Kawauchi Town. However, the Shigenobugawa Fault is older and has a much larger geological significance than the Kawakami Fault. Consequently, a new name of "the Shigenobugawa Fault" is proposed in this paper. Most parts of the Shigenobugawa Fault is covered by the alluvial deposits along the Shigenobu River. In the Matsuyama district and its environs, however, all of the stratigraphic succession, geologic structure, and zeolite zone of the Izumi Group are discontinuous with the Shigenobugawa Fault and its westward extention between. It is impossible to estimate the throw of the Shigenobugawa Fault stratigraphically, as stated in the chapter of geologic setting. As already stated, on the north side of the Shigenobugawa Fault, distributed is Zone IV formed under a higher-temperature condition, and on the south, Zone m under a lower-temperature condition. Zone HI on the south side of the Shigenobugawa Fault should have been formed at shallower depth than Zone IV on the north side of the Fault. The throw of the Shigenobugawa Fault is estimated. to be 1 t0 2 kilometers from the zeolite zones(NISHIMURA et al., 1980). Excepting the case that the variation in the original thickness of a given bed between the axial and limb parts of a fold has occurred by the folding, it can be said that primary porosity of mudstones reflects their maximum burial depth in general. (ATHY, 1930). As the Izumi Group forms a comparatively open folded structure, burial depth of mudstones may be estimated from their primary porosity- As mentioned in the chapter of mudstone porosity in detail, a distinct difference of primary porosity of mudstones can be recognized between both sides of the Shigenobugawa Fault, and mudstones belonging to Zone IH have larger porosity than those belonging to zone IV. Based on the burial depth - porosity relationship of mudstones of Neogene oil fields, burial depth of the Izumi Group is estimated to be 5・5 kilometers on the north side of the Shigenobugawa Fault and 3-6 kilometers on the south. The difference of burial depth between both sides of the Fault is about 2 kilometers, which coincides well with the difference estimated from zeolite zone. The main part of the Izumi Group of both sides consists of sandy turbiditepredominant alternation and mudstone-predominant alternation, and sole markings of these turbidites indicate the southwestward axial current and southward lateral current (Fig.ユ1). Sandstones on each side of the Fault resemble in heavy mineral. association as well as in grain composition, and the sand grains were supplied from acidic volcanic rocks, Ryoke granitic rocks, and Ryoke metamorphic rocks dis-.

(16) GEOLOGY AND I∋ASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 287. Fig. ll. Paleocurrent directions of flute casts at the bottom of turbidite of the Izumi Group in west Shikoku. The arrow shows the direction of groove cast. One scale, half of radius of the circle. represents one measuremerit.. /. A: north of the Shigenobugawa Fault B: south of the Shigenobugawa Fault. tributed in the Inner Zoneof SouthwestJapanC NISHIMURA,1984 ). There is no tendency that gram size and thickness of sandy turbidite beds abruptly change between both sides of the Fault. However, a small but significant difference in the heavy mineral association can be recognized between both sides Amphiboles and greenish tourmaline occur only in the north side. This difference is considered to be due to the difference in stratigraphic horizonsthe strata of the southern Zone M are much higher than those of the northern Zone IV.The Median Tectonic Line is the boundary between the Sanbagawa and the Ryoke Metamorphic Belts in eastern Kii Peninsula and in centralーHonshu where the Izumi Group is missing- Consequently, it is likely that the Sanbagawa Crystalline Schists bounds the Ryoke metamorphic rocks and granites below the Izumi Group with the Shigenobugawa Fault which branches off from the Median Tectonic Line in western Shikoku, as shown in Fig. 12.. Fig・ 12. An N-S profile of the Matsuyama district (Slightly modified 1 IJIMA. mlo°oO. ヽ-. 、. IBfi^SSS :.トl. +、二L二◆ ◆. +たコ++++++ ,k避++++++ ++++++ 1Km. et al., 1974). :. Ryoke. granites. 2 : Sanbagawa Crystalline Schists 3: KumaGroup 4 : Ishizuchi Group 5 : Andesitedyke 6 : Boundary between ZoneDI and Zone IV. 、巨∃ 2E] ,[∃.[∃ 5E堊∃ 6[∃. Andesite belonging to the Miocene Ishizuchi Group (NAGAI, 1956) is distributed widely near the junction of the Shigenobugawa Fault and the Median Tectonic Line. As already stated, the andesite shows no evidence of shearing even.

(17) 288. at Umegatoge Pass nearest to the Shigenobugawa Fault while the Izumi Group is intensely sheared. Accordingly, it is concluded that the Shigenobugawa Fault was in activity mainly in the Paleogene time, after the Izumi Group had been deposited and folded, before the intrusion of the Miocene andesite dyke- In the Paleogene, the Eocene Kuma Group was deposited mainly to the south of the Median Tectonic Line in the Matsuyama district and its environs. The basal Nimyo Formation of the Kuma Group filled up the rugged surface of the Sanbagawa Crystalline Schists with clinounconformity, and consists mostly of rubbles of the crystal-. line schists. The lower part of the Myojin Formation of the group is made up of conglomerates consisting'mainly of well-rounded pebbles and cobbles of sandstones of the Izumi Group and granitic rocks of the Ryoke Belt; the upper part of the formation is composed mainly of terrestrial to shallow marine, arkosic sandstones (NAGAI, 1968). These facts may be considered that the Izumi Group and Ryoke granitic rocks on the north side of the Shigenobugawa Fault were uplifted and eroded by the activity of the Fault, and their detritus were transported southwards to form conglomerates of the lower part of the Myojin Formation.. 2. Analysts of the lzumi sedimentary basin The Izufni Group in Shikoku and Awaji Islands, as a whole, forms a synclinalstructure with an axis plunging eastwards. The south wing part of the synclinal structure bounds on the Sanbagawa Crystalline Schists by the Median Tectonic Line, while the north wing lies unconformably on the basement rocks belonging to the Ryoke Metamorphic Belt. The Izumi Group is distributed in a narrow and long belt at present, but how far spread the Izumi sedimentary basin?. In the Izumi Group on the north side of the Shigenobugawa Fault and the Median Tectonic Line, thickness between given two marker tuff beds decreases toward. the north (HAYASHI, 1969MS ; NISHIMURA, 1973MS, 1976). Therefore, it is difficult. to. consider. that. the. Izumi・sedimentary. basin. spread. far. north. beyond. the northern limit of the present-day distribution of the Izumi Group. On the south side of the Median Tectonic Line in the southern Matsuyama district,the Sanbagawa Crystalline Schists is overlain directly by the Eocene Kuma Group, at the base of which is observed red beds showing lateritization at early Tertiary (IIJIMA, 1963). In addition, the middle and upper subgroups of the Onogawa Group belonging to the Santonian age contain rubbles of the Sanbagawa Crystalline Schists in eastern Kyushu (TERAOKA, 1970). On the contrary, clastic grains derived from the Sanbagawa Crystalline Schists can not be noticed in the Izumi.

(18) GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 289. .亘2 3* 4二=5二二5三 Fig. 13. Schematic N-S profiles of the Matsuyama district. (A) Deposition of the Izumi Group (late Cretaceous) (B) Deposition of the Nimyo Formation (middle Eocene) after the activity of the Median Tectonic Line of the Ichinokawa phase (C) Deposition of deltaic facies of the Myojin Formation (D) Activity of the Shigenobugawa Fault and deposition of the Myojin Formation Gate Eocene) (E) Activity of the Median Tectonic Line of the Tobe phase (F) Deposition of the Ishizuchi Group (Miocene) 1 : Ryoke granites 2 : Sanbagawa Crystalline Schists 3 : Nimyo Formation 4 : Deltaic facies of the Myojin Formation 5 : Myojin Formation 6 : Ishizuchi Group.

(19) 290. Group in Shikoku (NISHIMURA, 1984). Consequently, the Sanbagawa terrane in Shikoku ought to have been so low as not to supply detritus or submerged beneath. the sea inthe Campanianto Maestrichtian age when the Izumi Group was deposited. In the Outer Zone of Shikoku, contemporaneous strata with the Izumi Group are distributed : the shallow marine Sotoizumi Group of late Albian to Campanian. age. (OGAWA.. 1971;. MATSUMOTO,. 1977;. MIYAMO′TO,. 1980),. the. shallow marine Uwajima Groupof the Turonian to Campanian age (TANABE , 1972 MAl、SUMO′TO. ,. 1977;. YANAI,. 1981),. and. the. Shimanto. Group. of. the. Albian. to Maestrichtian age (NAKASEKO, 1979). Most sandstones of these Groups as well as those of the Izumi Group are characterized by the presence of abundant fragments of acidic volcanic rocks distributed in the Inner Zone (TERAOKA, 1977b, 1979). Because it may be thought, as already stated, that the Izumi Group on the south side of the Shigenobugawa Fault lies on the Sanbagawa Crystalline Schists and that it is as thick as the Group on the north side, we can have the conclusion that the Izumi sedimentary basin at least in Shikoku spread southwards beyond the submarine barrier of the Sanbagawa Crystalline Schists. Based on the above-mentioned consideration, change of the Izumi sedimentary basin. is shown in Fig 13. Fig- 13-A shows the N-S profile of the Matsuyama district at the time of deposition of the Izumi Group- The Izumi sedimentary basin spread a little north beyond the present limit, while the basin spread far south.. After the Sanbagawa terrane was upheaved and a normal fault, the Median Tectonic Line of the Ichmokawa phase, took place m the terrane at middle Eocene (Fig. 13-B), the Izumi Group resting on the Sanbagawa Crystalline Schists was eroded out, and the Nimyo Formation of the Kuma Group was deposited on a rugged surface of bedrock. At next stage when the Sanbagawa terrane began to subside and the Inner Zone was uplifted, the Izumi Group began to be eroded, followed by the accumulation of basal deltaic facies of the Myoiin Formation (NAGAI, 1968) overlying the Sanbagawa Crystalline Schists and the Nimyo Formation. Further more, after the activity of the Shigenobugawa Fault, upheaved block of the north side was eroded to form the main part of the Myojin Formation on the subsided Izumi Group of the southern block, the Sanbagawa Crystalline Schists, and the Nimyo Formation (Fig- 13-D). The Median Tectonic Line of the Tobe phase thrust the Izumi Group over the Myojin Formation (Fig- 13-E). In the Miocene, volcanic rocks of the Ishizuchi Group erupted (Fig. 1㌢-F)..

(20) GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 291 REFERENCES. ATHY, L. F., 1930 : Density, Porosity, and Compaction of Sedimentary Rocks. Amer.Asso.Petrol.Geologists Bull., 14, 1 -24. HARADA, M-, 1965 : The Izumi Group in the Kawauchi-Shigenobu Area to the. East. of. Matsuyama.. Sci-Rep-Fac.Sci-. Kyushu. Univリ8,. 137. -. 155.(J.E.) HAYASHI,. M‥. 1969MS. :. Studies. on. the. Izumi. Group. in. the. central. Asan. range,. with special references to analyses of sedimentary structures and lateral. change of lithofacies.地ster Thesis of Geol. Inst. Unw- Tokyo. IIJIMA, A., 1963 : "Red beds" in the Paleogene coal fields of Japan. Jour.Geol.Soc.Japan, 69, 315. (J.) 一一1-I-. 1978a:. Zeolitic. diagenesis.. Mem.. Geol.. Soc.. Japan,. No.15,. 135-. 150.(J.E.) --, 1978b : Geological occurrences of zeolite in marine environments In SAND, J.B. and MUMPTON, F.A. (ed) Natural Zeolites, Occurrence, Properties, Use. Pergamon Press, Oxford, 175-198. -ll----, 1980 : Geology of natural zeolites and zeolitic rocks. Pure and Appl. Chem., 52, 2115-2130. -I-and UTADA, M., 1965 : Zeolites in sedimentary rocks, with Reト erence to the Depositional environments and Zonal distribution. Jour. Geol. Soc. Japan, 71, 56-65, 138-147. (J.E.) 一一一-一一and. NISHIMURA,. T.,. 1974. :. Analysis. of. the. Izumi sedimentary basin by zeolite zones. GDP circular, II- i -d), Structural geology, No.2 , 51-55. (J.) KOBAYASHI, T., 1931 : On the Izumi Sandstone Series in the Izumi Mountain Range. Jour. Geol. Soc,Japan, 38, 629-640. (J.E.) 1950 : Regional geology of Japan ; Shikoku Province. Asakura, 266p. (J.). MATSUMOTO, R. and IIJIMA, A., 1981 : Origin and diagenetic evolution of Ca-Mg-Fe carbonates in some coal fields of Japan.良dimentology, 28, 239-259.. MATSUMOTO, T., 1977 : Zonal correlation of the Upper Cretaceous in Japan. Palaeont- Soc. Japan, Special Paper No.21, 65-74. MIYAMOTO, T., 1980 : Stratigraphical and sedimentological studies of the Cretaceous system in the Chichibu terrain of the Outer Zone of Southwest Japan.. Geol.. Rep.. Hiroshima. Univ一,. 23,. 1. -138.. (J.E.). MULLER, G., 1967 : Methods in Sedimentary Petrology. Hafner, New York. 283p.. NAGAI, K -, 1956 : Geological Age of Ishizuchiyama Tertiary and Paleogene Paleogeography of West Japan.施m.Ehime Univ., Sect,n , 2 ,ユ45154.(J.E.).

(21) '>ir>. 1968 : The Eocene Kuma Group.地m.Ehime Unw., Ser.D, 6 , 1-4.. NAKAJIMA, W-, KOIZUMI, M., and NAKAGAWA, C, 1962 : Discovery of zeolites from the Izumi Group. Jour.Geol一段jc.Japan, 68, 173 175.(J.) - and TANAKA, K., 1967 : Zeolite-Bearing Tuffs from the lzumi Group in the Central Part of the Izumi Mountain Range, Southwest Japan, with Reference to Mordenite-Bedring Tuffs and Laumontite Tuffs- Jour. Geol- Soc. Japan, 73, 237 - 245. (J.E.) NAKANO, M., 1953 : On the Izumi Group of the Central Part of the Sanuki Mountain Range, Japan. Geol. Rep- Hiroshima Univ., 3 , 1 -13. (J.E. NAKASEKO.K., 1979 ; On the international correlation by means of radiolarians from the Cretaceous formations in Japan. Fossils, No. 29, 27-35. (JJ NISHIMURA, T-, 1973MS : Some petrographic studies of the Izumi sandstones. in the east of the Sanuki Mountain Range, Shikoku.地ster Thesis of Geol. Inst- Univ- Tokyo. ll---- , 1976 : Petrography of the Izumi Sandstones in the East of the Sanuki Mountain Range, Shikoku, Japan. Jour-Geol.良ic.Japan, 82, 231 -240. I- ---, 1984 : Basin Analysis of the Upper Cretaceous Izumi Group in Western Shikoku, Japan. Jour. Geol. Soc, Japan, 90, 157-174. (J. E.) -I. I------蝣IIJIMA,. A-,. and. U′fADA. M.,. 1980:. Zeolitic. burialdiagenesis. and basin analysis of the Izumi Group in Shikoku and Awaji Islands, Southwest Japan. Jour.Geol.Soc.Japan, 86, 341-351. CJ. E.) OGAWA,,Y.,1971 : Geology of the Katsuuragawa district, Tokushima Prefecture, its stratigraphy and structure. Jour. Geol. Soc. Japan, 77. 617 634. (J. E.) OKADA,A-,1972 : Quaternary faulting along the Median Tectonic Line in the northwestern Shikoku. Mem.Coll.Lit.Aichi Pref. Univ. 23, 6894.(J.) SASAI, H., 1936 : The Izumi sandstones in Awaji Island. Jour. Geol- Soc. Japan, 43, 590-602. (J.) SUYARI. K., 1966 : Studies on the Izumi Group in the Eastern Asan Mountain Range, Shikoku (I ). Jour. Sci. Coll. Gen. Educ. Univ. Tokushima, 1, 9-14. (J.E.) I--. ,. 1973. ‥. On. the. Lithofacies. and. the. Correlation. of. the. Izumi. Group. of the Asan Mountain Range, Shikoku. Tohoku Univ., Sci. Rep., 2 nd ser. (Geol.), Special Volume, No.6 , 489-495. (J.E.) , Odoi y., Kume, Y-, Kondo, K., Shinoaki,S., SOFUE, K-, TERADA,T., BANDO.H., HINO, Y., HoSOI, H.,.

(22) GEOLOGY AND BASIN ANALYSIS OF THE UPPER CRETACEOUS IZUMI GROUP 293. and YA.MA.GUCHI, A., 1968 : Studies on the lzumi Group in the Eastern Asan Mountain Range, Shikoku (H ). Jour. Sci. Coll. Gen. Educ Univ. Tokushima, 2, 7-16. (J.E.) TAKAHASHI, J., 1977 : On Geology and the Median Tectonic Line in the southern part of Matsuyama City, Ehime Prefecture. Jour. Geol. Soc. Japan, 83, 325-340. (J.E.) TANABE.K., 1972 : Stratigraphy of the Cretaceous Formations in the Uwajima District, Ehime Prefecture, Shikoku. Jour. Geol. Soc. Japan, 78, 177-190. (J.EJ TANAKA, K., 1965 : Izumi Group in the Central Part of the Izumi Mountain Range, Southwest Japan, with special reference to its sedimentary facies and cyclic sedimentation. Geol. Surv- Japan Rep., No.212, 1 -34・ (J.E.) TERAOKA.Y.,. 1970. ‥. Cretaceous. Formations. in. the. Onogawa. Basin. and. its. Vicinity, Kyushu, Southwest Japan. Geol. Surv. Japan Rep., No.237, 1-87. (J. E.) -I-- , 1977b : Comparison of the Cretaceous sandstones between the Shimanto Terrane and the Median Zone of Southwest Japan, with reference to the provenance of the Shimanto geosynclinal sediments. Jour. Geol.良5c. Japan, 83, 795-810. (J. E.) -. -一一一,. 1979. :. Provenance. of. the. Shimanto. geosynclinal. sediments. inferred. from sandstone compositions. Jour. Geol- Soc.Japan, 85, 753-769. (J.E.) YANAI. S., 1981 : The stratigraphical and palaeogeographical situations of the Upper Cretaceous Uwajima Group of the shelf-facies within the Shimanto Supergroup, western Shikoku, Japan. Jour. Geol. Soc. Japan, 87, 339-352. (J.E.). (J.) : in Japanese only (J. E.) : in Japanese with English abstract.

(23) 294. 四国西部の上部白亜系和泉層群の地質と 堆積盆解析 西村年暗 (昭和58年11月15日受理). (要旨) 野外調査と室内実験に基づいて,松山地域の上部白亜系和泉層群中を中央構造線とほぼ 平行に走る重信川断層(新称)の意義を明らかにした。断層の両側の和泉層群を層序学的 に対比することはできない。摺曲構造も断層両側で不連続である。そこで,和泉層群泥岩 の孔隙率を測定して,この断層の規模を推定した。和泉層群泥岩の孔隙率は0.5-9.7 の範囲の値を示し,日本の新第三系油田地域泥岩の孔隙率一埋没深度ダイヤグラムに基づ くと,和泉層群の最大埋没深度は断層の北側で約5.5km,南側で約3.6kmと見積られる。 これは,和泉層群中の酸性ガラス質凝灰岩の沸石埋没続成帯が断層の両側で異なり,かつ 最大埋没深度の差が1 -2kmと推定されることと矛盾するものではない。 この結果に基づいて和泉堆積盆の解析を行った。後期白亜紀には,和泉堆積盆は三波川 結晶片岩の海底のたかまりを越えて南方に広がっていた。そのたかまりは白亜紀の終りに は消滅した。重信川断層が古第三紀に活動し,断層北側の和泉層群や領家花繭岩が隆起し 侵食され,その砕屑物が南-運ばれ,久万層群明神層が堆積した。.

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