Basin evolution within the Kitakami Massif, northeast Japan: relationship between sedimentation, tectonics and volcanism in an incipient Neogene continental back-arc basin

Sedimentary Geology 133 (2000) 7–26 www.elsevier.nl/locate/sedgeo

Basin evolution within the Kitakami Massif, northeast Japan: relationship between sedimentation, tectonics and volcanism in an incipient Neogene continental back-arc basin K. Yagishita a,*, K. Komori b b

a Department of Geology, Iwate University, Morioka-City, Iwate 020-8550, Japan The Board of Education, Ichinohe Town-Hall, Ichinohe-Town, Iwate 028-5301, Japan

Received 16 December 1998; accepted 5 January 2000

Abstract The Ichinohe basin in the western margin of the Kitakami Massif, northeast Japan, consists of the Early to Middle Miocene Yotsuyaku and Keiseitoge formations and represents the incipient stage of Neogene back-arc sedimentation that extends toward the north and northwest. Megaclast breccias derived from Mesozoic basement rocks of the Kitakami Massif were transported from southern areas as debris flows. The debris-flow deposits of the Yotsuyaku Formation grade northwards along the basin axis into clast-supported, braided-stream deposits, where floodplains developed. In the center of the basin voluminous volcanic eruptives of the Keiseitoge Formation were derived from eastern and southeastern volcanic ranges and formed transversal distributaries. The deposition of both clastic and pyroclastic sediments suggests that volcanism and tectonic uplift took place simultaneously during basin evolution. Parallel-laminated, fine-grained sediments intercalated with lignite beds suggest lacustrine deposition, and large cross-beds of volcanic breccias indicate Gilbert-type progradation into the lakes. A number of small lakes in the center of the basin was probably formed as the result of damming tributary mouths by the main drainage from eastern and southeastern volcanoes or the reciprocal blocking by lahars and non-volcanic debris flows derived from the southern uplift. Damming episodes produced by thick lava flows were not recognized in this study. The evolution of the N–S trending, longitudinal Ichinohe Basin indicates that E–W extensional tectonism related to the opening of the Sea of Japan affected not only the Green Tuff Region, to the west of the studied basin, but also even the Mesozoic basement during the Miocene. Although the mingling of volcanic and basement-derived deposits indicates the complexities of sedimentary facies and provenances, our view of the stratigraphic architecture has relevance to the interpretation of the evolution of other extensional basin systems. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: back-arc basin; facies analyses; Lucustrine deposits; lahars; paleocurrent analyses; volcanism; tectonism

1. Introduction In the so-called Green Tuff Region, which was inti-

* Corresponding author. Tel.: ⫹ 81-196-21-6557; fax: ⫹ 81196-21-6557. E-mail address: [email protected] (K. Yagishita).

mately associated with the opening of the Sea of Japan, many Early Neogene basins have been studied in detail. However, the early phase of back-arc basin development within basement rocks, farther east of the Green Tuff Region, has been paid little attention to date. One such basin, the Ichinohe Basin (newly termed herein), is thought to be the incipient stage of the back-arc basin succession that formed on the

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Fig. 1. Geologic map along the Mabechi River Valley. Bs ˆ Mesozoic basement rocks (accreted subduction complexes) including some granitic rocks. Yt ˆ Yotsuyaku Formation, including Nisatai Dacite at the lowermost part of the formation (see Fig. 2). Kt ˆ Keiseitoge Formation, and both formations constitute the Ichinohe Basin (IB). Kd ˆ Kadonosawa Formation, Sm ˆ Suenomatsuyama Formation, and these two formations form the Kadonosawa Basin (KB). PsS ˆ post-sedimentation of the Shiratorigawa Group except Quaternary deposits (Qt). I ˆ Ichinohe Town, K ˆ Keiseitoge Pass. M.R. ˆ Mabechi River. GTR (inset) ˆ Green Tuff Region.

underlying Mesozoic basement of the Kitakami Massif, northeast Japan. The Ichinohe Basin, of Early and Middle Miocene age, is a small northward-opening, U-shaped basin with dimensions of 8 km in E–W, and 16 km in N– S directions. Development of the basin, however, eventually led to the evolution of some other Miocene–Pliocene basins toward the north and northwest. New roadcuts and fresh outcrops enable us to

expand significantly upon the early geologic mapping of the Neogene sequence by Chinzei (1958a,b, 1966) and to deal with the oldest sediments. Sedimentary processes in the basin are noteworthy in that silici-clastic (non-volcanic) and volcaniclastic sedimentation occurred simultaneously. The purpose of this paper is to describe the silici-clastic and volcaniclastic facies of the Ichinohe Basin and discuss how these sedimentary facies evolved in the context of

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Fig. 2. Stratigraphic column of the Shiratorigawa Group. Note that volcanogenic sediments are independently treated as the Keiseitoge Formation. IB ˆ deposits constituting Ichinohe Basin, and KB ˆ Kadonosawa Basin. Abbreviations of each formation, such as Kt and Sm, are the same as in Fig. 1.

back-arc basin systematics. Several workers have recently reported some templates of basin evolution that included concurrent deposition of a large amount of volcaniclastics and non-volcaniclastics (e.g. Smith, 1987; Cole and Ridgway, 1993; Stollhofen and Stanistreet, 1994), and they detailed interactions between fluvial sedimentation and volcanism. Although the Ichinohe Basin provides an example of high-sediment input from volcanic-arc edifices (e.g. Dorsey and Burns, 1994), sedimentary facies and sequences are rather different from previous examples. Here we argue that the facies characteristics accord with modern fluvial–deltaic systems associated with the arc–trench volcanism (e.g. Kuenzi et al., 1979).

2. Geological setting The Ichinohe Basin is located just east of IchinoheTown, Iwate Prefecture (Fig. 1). The eastern, western and southern margins of the basin are bounded by the basement rocks of the northern Kitakami Massif. Paleomagnetic data suggest that prior to the Early Miocene the massif was part of the Eurasian Plate (Shikhote Alin) (Otofuji et al., 1985), and it consists of accreted subduction complexes of oceanic materials, such as chert, slate, basalt (including pillow

lava) and basic tuff (e.g. Saito and Hashimoto, 1982). During the Early and Middle Mesozoic the accreted massif was formed by the north- and northwest-ward movement of the subducting oceanic plate into the Eurasian Plate (Maruyama and Seno, 1986). The massif is also partly intruded by Early Cretaceous (ca. 110–120 Ma) granites (Kanisawa and Katada, 1988). The base of the sequence consists of the Early Miocene Yotsuyaku Formation, the lowermost formation of the Shiratorigawa Group (Fig. 2). The group is overlain conformably by the San-nohe Group (Miocene–Pliocene) (Chinzei, 1958a, 1966). The Ichinohe Basin is conveniently defined as the exposed extent of the Yotsuyaku and Keiseitoge formations (Fig. 1). The depocenter of the basin shifted northwards into a broader basin, the Kadonosawa Basin, which consists of deposits of the Kadonosawa and Sueno-matsuyama formations, mainly younger sediments of the Shiratorigawa Group (Chinzei, 1966). The oldest strata of the basin belong to the Koiwai Member, which is 30–40 m thick (Chinzei, 1958b). The Sugohata Member, up to 120 m thick (Chinzei, 1958b), overlies the Koiwai Member with locally erosional contact (Fig. 2). Volcaniclastic sediments of the Keiseitoge Formation are intercalated with the Sugohata Member of the Yotsuyaku Formation.

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K. Yagishita, K. Komori / Sedimentary Geology 133 (2000) 7–26

Chinzei (1958b) proposed that both the Koiwai and Sugohata members together with the Keiseitoge volcanogenic sediments should be treated as the Yotsuyaku Formation (his notations were A1 for Koiwai, A2 for Keiseitoge and A3 for Sugohata). Matsubara (1995) also assigned the sediments to four members of the formation. Clear-cut facies differences between the sediments of the Yotsuyaku Formation (the Koiwai and Sugohata members) and volcaniclastics of the Keiseitoge Formation, however, point to the stratigraphic relationships we depict in Fig. 2. The pyroclastic sediments together with volcanogenic debris-flow deposits should be distinguished from the mostly silici-clastic sediments of the Yotsuyaku Formation. In the recent geological map (1: 200,000) issued by the Geological Survey of Japan (Kamada et al., 1991), the volcanogenic materials are also recognized as the Keiseitoge Formation, and they are distinguished from essentially non-volcanic sediments of the Yotsuyaku Formation. Clastic materials in both the Koiwai and Sugohata members are derived from the basement rocks of the Kitakami Massif, together with minor amounts of intermediate volcanic rocks. Matsubara (1995) divided the Koiwai Member of Chinzei (1958b) into two further subdivisions: (1) the lower non-marine clastic sediments of the Matsukura Member; and (2) marine deposits of his Koiwai Member which corresponds to the upper half of the Koiwai Member of Chinzei (1958b). He also claimed that the Nisatai Dacite overlies the Matsukura Member, and cobbles and boulders of the dacite are contained in his Koiwai Member. Tagami et al. (1995) reported a result of their dating for the dacite, ranging approximately from 21 to 23 Ma. We did confirm the outcrop observed by Matsubara (1995), in which the dacite overlies the lignite-bearing, very fine-grained sandstone beds of the Koiwai Member. Thus the evolution of the basin can be traced back to the Early Miocene.

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In contrast to the Yotsuyaku Formation, the Keiseitoge Formation is an assemblage of volcanogenic sedimentary rocks, ignimbrites, tuffs, tuffaceous silty sandstones and minor amounts of lava flows, probably derived from the present-day Keiseitoge Range. The lavas consist of augite–hypersthene–hornblende andesite, and the radiometric dating indicates the emplacement over the time interval from 15.9 Ma (Kimura, 1986) to 17.4 Ma (Ishizuka and Uto, 1995). The maximum thickness of the formation is more than 250 m in the central part of the basin, but it thins abruptly and pinches out west- and north-ward. Most outcrops of the Yotsuyaku and Keiseitoge formations show a monoclinic structure, gently dipping to the north or northwest. Some faults, particularly N–S trending faults, exist along the eastern margin of the basin (Fig. 1). The occurrence of littoral molluscan fauna in the Koiwai Member provides a marker horizon on each side of the normal faults and demonstrates a remarkable throw (more than 200 m) across the faults. Besides these, other normal faults exist in and around the basin, and they are thought to have been responsible for producing the Miocene Ichinohe Basin.

3. Facies description and interpretation Stratigraphic and lithofacies sections (Figs. 3a, b and 4) were constructed using a tape measure and altimeter for gently dipping or almost flat lying beds along high bluffs, and using a metric Jacob staff for monoclinic structures along some creeks. Facies codes used in this paper are as defined by Miall (1977) and Mathisen and Vondra (1983). Facies Gms. Common in the Sugohata Member of the southern part of the basin are angular to subangular boulder breccias of chert, slate, basalt, granite (up to 1.6 m long) and andesite enclosed in coarse- to

Fig. 3. (a) Detailed map of facies forming the Ichino Basin. Clastic sediments of the Yotsuyaku Formation (Yt) mainly consist of five sedimentary facies; Sh1 ˆ gray-colored, parallel-laminated fine- to medium-grained sandstone beds, Gms ˆ debris flow-deposits with basement rocks, Gm ˆ clast-supported fluvial conglomerates also with basement rocks, Sh2 ˆ medium-to fine-grained, brown colored sandstones intercalated with facies Gm, and Fl ˆ parallel laminated, very fine-grained sandstones and siltstones with lignite beds. Volcanogenic deposits of the Keiseitoge Formation (Kt) are divided into four sedimentary facies; Gvms ˆ debris flow deposits (lahar), Tms·tr ˆ pyroclastic flow deposit (facies Tms1 and Tms2) and pumiceous tuff beds (facies Tr), and facies Fl. Lv ˆ lava. Bs ˆ basement rocks. Psib ˆ post-Ichinohe Basin sedimentation (i.e. Kadonosawa and Suenomatsuyama formations). Qt ˆ Quaternary deposits, I ˆ Ichinohe Town. K ˆ Keiseitoge Pass. Letters a–i denote individual outcrops detailed in this paper, and arrows show the river flow directions. (b) Two cross-sections along lines A–A 0 –A 00 and B–B 0 , based on lithofacies logs in Fig. 4. Facies symbols are the same as in Fig. 3a.

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Fig. 4. Lithofacies logs in the Ichinohe Basin. Facies symbols are the same as in Fig. 3a.

medium-grained sands (Fig. 5A and B). The large clast (⬎20 mm diameter) content of the deposit is about 40% (i.e. the matrix is c. 60 %, using comparison chart for visual percentage estimation by Terry and Chilingar, 1955), and detrital frameworks of the

matrix mainly consist of slate, chert, metasedimentary and volcanic (both basalt and andesite) rock fragments (see Table 1). In this matrix-supported facies inverse and normal grading are present. Some petrified stumps and logs are also present.

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Fig. 5. (A) Inversely graded, debris-flow deposit (facies Gms), the Sugohata Member of the Yotsuyaku Formation; largest clasts are up to 1.6 m in diameter. The hammer is 0.3 m long. (B) Inversely graded, matrix-supported breccia bed (facies Gms). Angular clasts of the facies chiefly consist of slate, basalt and chert. The center of the outcrop shows imbrication of clasts (current is from right to left, location h in Fig. 3a, and location 13 in Fig. 12).

The boulder breccias of basement rocks supported by the coarse-sand matrix constitute debris-flow deposits derived from the uplifted high relief. As discussed below, even though clast composition

remains the same, paleocurrent analyses (see Section 4, Fig. 12) suggest that this matrix-supported facies grades northward (downward) to the clast-supported facies Gm. Progressive downward decrease in

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Table 1 Petrographic analyses for seived fractions from sandy matrix of three facies (facies Gms, Gm and Gvms) a Thin-section

Composition (%) Quartz

1.Gms-c-Gor 2.Gms-m-Gor 3.Gm-c-Sar 4.Gm-m-Sar 5.Gvms-c-Ane 6.Gvms-m-Ane

Feldspar

Lithic fragments

Miscellaneous

Qm

Qp1

Qp2

Pl

K–F

Ls

Lv

Lp

Lms

Ab

Ah

Apx

Apq

2 1 3 5 – –

5 1 4 1 – –

8 2 14 7 – –

6 11 3 5 23 30

tr 2 – 1 – –

23 42 37 46 – –

30 17 24 15 57 55

4 4 3 3 – –

11 6 7 9 – –

– 2 2 1 – –

6 4 1 3 4 4

4 7 1 2 14 9

tr 2 1 1 2 1

a Sediments from three facies matrix were sieved into coarse- and medium-grained sand size and impregnated by epoxi resin in film boxes, and thin-sections were made from these cylindrical boxes. 200 points were counted for each thin-section. Thin-section numbers of 1, 3 and 5 are coarse- grained fraction, whereas 2, 4 and 6 are medium-grained fraction. Sections 1 and 2 were made from facies Gms (sampling locality 12 in Fig. 12), 3 and 4 from facies Gm (sampling locality 3 in Fig. 12), and 5 and 6 from facies Gvms (sampling locality near k in Fig. 12), respectively. Qm, monocrystalline quartz; Qp1, polycrystalline quartz; Qp2, apahnitic polycrystalline quartz (i.e. chert); Pl, plagioclase; K–F, orthoclase; Ls, sedimentary rock (slate and shale) fragments; Lv, volcanic rock fragments; Lp, plutonic rock fragments; Lms, metasedimentary rock fragments; Ab, biotite; Ah, hornblende; Apx, pyroxene (mostly augite, others hypersthene); Apq, opaques.

maximum and mean clast sizes and angularity of facies Gms accompanies improved sorting and better-developed horizontal stratification, leading to deposition of facies Gm. Facies Gm. Limited to the Sugohata Member of northern part of the basin and the lower part of the Koiwai Member are locally imbricated, subrounded to rounded, pebble- to cobble-sized conglomerates of chert, slate, basalt and andesite, which show clastsupported and massive or crudely horizontal bedding (Fig. 6A). The sandy matrix occupies about 50% of the deposit, and the detritus of the matrix chiefly consists of chert, slate and volcanic (mostly andesite) rock fragments (Table 1). The average proportion of pebble- and cobble-sized andesite rock constituents relative to basement rock clasts of the Sugohata Member is less than 24%, whereas the Koiwai Member generally lacks andesites. Fining-upward or coarsening-upward grading is rare in both members. Laterally extended gravel beds of facies Gm with their external structure of flat bases and slightly convex-upward surfaces were probably formed by diffuse gravel sheets, some of which develop into longitudinal bars. The common occurrence of imbricated gravels showing the a-axis transverse alignment to the flow suggests the traction deposition. The rounded and subrounded andesite gravels of the Sugohata Member are probably from erosion of lavas

produced in an early stage of volcanisms during deposition of the Keiseitoge Formation, by which the lavas floored the Ichinohe Basin. Facies Sh1. Parallel-laminated, gray-colored finegrained sandstones or siltstones characterize the upper part of the Koiwai Member. Shallow marine molluscs, such as Crassostrea gigas, Vicarya yokoyamai Takeyama, Vicaryella sp. and Nipponomarcia nakamurai have been reported from the facies (Matsubara, 1995). The marine aspect of the facies in the Koiwai Member indicates that it was deposited under the low-energy conditions, likely in a calm embayment. Facies Sh2. Massive or parallel-laminated, coarseto medium-grained, brown-colored sandstone beds showing lateral continuity are intercalated with facies Gm in the Sugohata Member of the Yotsuyaku Formation. The facies is also discernible in the lower part of the Koiwai Member (Fig. 6D). Thickness is generally less than a half meter, and the bodies have flatness but erosively bounded upper surfaces. The facies represent fluvial deposits, and its external structure of the bodies particularly suggests longitudinal bar sediments. Facies Fl. Thinly parallel-laminated, very fine-grained sandstones or siltstones occasionally intercalated with lignite and tuff beds are very common in the Keiseitoge Formation (Fig. 6B; locality c in Fig. 3a). Many

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Fig. 6. (A) Imbricated, clast-supported conglomerate bed of facies Gm of the Sugohata Member, the Yotsuyaku Formation. The facies was probably formed after the matrix of facies Gms was winnowed away during transportation from southern source areas. The hammer is 0.3 m long, and current is from left to right (outcrop locality e in Fig. 3a, and locality 3 in Fig. 12). Note that debris from the top of the bluff partly cover the facies Gm. (B) Tuff beds (arrow) interbedded with thick very fine-grained sandstone beds (facies Fl), probably representing a lake deposit in the Keiseitoge Formation, outcrop locality c in Fig. 3a). (C) Very fine-grained sandstone beds of facies Fl are abruptly overlain by clast- supported conglomerates of facies Gm. The sandstone beds show lateral continuity (⬎200 m, locality f in Fig. 3a). (D) Imbricated, pebbly sandstone beds (facies Sh2) of the Koiwai Member, the Yotsuyaku Formation. The pencil is 0.1 m long. Current is from right to left (outcrop locality a in Fig. 3a).

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Fig. 7. Sketch of bluff displaying debris-flow deposits (facies Gms) and alpha-type cross-stratification (facies Sp) of the Sugohata Member of the Yotsuyaku Formation. Dip of the foreset of facies Sp is toward north (0/28⬚, locality point s in Fig. 12). A ˆ debris-flow deposit bed (facies Gms), B ˆ parallel-laminated tuffaceous sandstone bed, C ˆ lignite bed, D ˆ tuffaceous sandstone bed with alpha-type cross-stratification (facies Sp), E ˆ parallel-laminated pebbly sandstone bed (Koiwai Member).

of the beds extend laterally more than a few tens of meters. The facies is also common in the Sugohata Member of the Yotsuyaku Formation, where the laterally extended (⬎200 m), very fine-grained sandstone beds are abruptly overlain by clast-supported conglomerate beds of facies Gm (Fig. 6C). Plant rootlets and leaf impressions, such as Cryptomercia sp., Sequoia sp., Metasequoia japonica and Liquidambar formosa Hance, are also discernible. Bedding surfaces locally display desiccation cracks, and these beds entomb a large number of spectacular petrified trees.

The facies was probably formed through the process of lake filling. Lacustrine very fine-grained sands and silts together with organic-rich materials are the typical sediments in lakes made by the blocking of tributary mouths. The tributaries are often dammed at their junction with the main trunk river that is characterized by rapid sedimentation of volcanic debris (e.g. Kuenzi et al., 1979). Facies Sp. Planar cross-bed sets, up to 5 m thick, with or without tuffaceous sands are present in the Sugohata Member (Fig. 7, and locality s in Fig. 12).

Fig. 8. Volcanogenic sediments of the Keiseitoge Formation. (A) Vesiculated pumices in tuffaceous sandstone of facies Tr overlain by matrixsupported breccias of facies Gvms. The contact is erosive (arrow, locality b in Fig. 3a). (B) Facies Gvms includes imbricated andesite breccias. Current is from left to right (locality d in Fig. 3a, and locality 7 in Fig. 12). (C) Debris-flow deposit (facies Gvms) showing hyperconcentrated flow deposit at the base of facies (arrow) (locality b in Fig. 3a). The bar is 1.5 m long.

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Fig. 9. Typical Gilbert-type planar cross-stratification (locality point k in Fig. 12). (A) Overview of the cross-bed set beyond the rice paddy. The dip of the foreset (F) is toward the west (262/29⬚), and T denotes the topset. (B) Detailed outcrop of the foreset with exclusively brecciated andesite clasts, showing partly inverse-grading. The hammer is 0.3 m long.

Foresets represent avalanche slopes, dipping up to 28⬚. This facies represents the alpha-type cross-stratification of Allen (1963). The thick planar cross-bed set may represent a Gilbert-type delta deposits in a local pond or a lake, which was once a tributary whose mouth became blocked by rapid and vertical agradation of another drainage system that probably originated from the Keiseitoge volcaniclastics. Overall the predominance of facies Gm and facies Sh2 generally suggests that the depositional environment for northern part of the Ichinohe Basin consisted

of a braided river system associated with numerous lakes or ponds. Facies Gvms. The occurrence of this facies is restricted to the Keiseitoge Formation. Angular blocks of andesite, of which size ranges from 0.1 to 0.8 m, are enclosed in very coarse- and coarse-grained sands and pumiceous sands, showing clay-poor matrix (Fig. 8A–C). This facies differs from facies Gms in that the breccias in the matrix are exclusively andesite, but they lack the Kitakami basement rocks such as chert, slate and basalt. The matrix, which occupies up to ca. 70% of the deposit, chiefly consists of detrital

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Fig. 10. Pyroclastic sediments of the Keiseitoge Formation. (A) Scoria-flow deposit of facies Tms1 with dense andesite clasts (white arrow) in the lower part and vesiculated andesite clasts (black arrow) in the upper part (outcrop locality g in Fig. 3a). (B) Gas segregation pipes of facies Tms2 (white arrow), displaying upward opening (outcrop locality i in Fig. 3a). Note the carbonized wood at the bottom (black arrow).

frameworks of plagioclase, pyroxene and volcanic (exclusively andestite) rock fragments (Table 1). Thick deposits of the facies locally overlie basal stratified units (Fig. 8C). These matrix-supported, exclusively andesite breccias without charred woods are volcanic debris flow deposits, i.e. cold lahars (e.g. Walton and Palmer, 1988; Rodolfo, 1989). Basal stratified units of the lahar were probably produced by hyperconcentrated flow generated by watery dilution of the debris-flow front (Smith and Lowe, 1991), and they typically grade upward into debris flows. Matrix of the facies is poor in mud content, and the fact differs from other debris-flow deposits (e.g. alluvial fanglomerates). The lahars in this study were probably caused by mobilization of fragmental pyroclastic and autoclastic debris

during or soon after eruption (i.e. with no development of soil layers due to vegetation). Smith and Lowe (1991) noted that any clays present were generally formed diagenetically by alteration of reactive volcaniclastic grains. Although a large number of silicified wood fragments exist in facies Gvms, the absence of clay matrix indicates that the diagenetic alteration has not yet taken place. Facies Gvp. Gigantic tabular cross-sets up to 9 m thick, with angular and sub-angular exclusively andesite cobbles and boulders, occur in the Keiseitoge Formation (Fig. 9A and B, and locality k in Fig. 12). Inverse-grading of the foreset can be discernible (Fig. 9B). Inverse-grading implies at least some inertial interaction of clasts, and while the debris was

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Fig. 11. Interbedded pyroclastic sediments of the Keiseitoge Formation with debris-flow deposits of the Sugohata Member of the Yotsuyaku Formation. The debris-flow deposits including megaclast breccias were derived from the southern uplift of Mesozoic basement rocks. X denotes the outcrop shown in Fig. 5B and Y in Fig. 10A.

avalanching into a relatively deep lake or pond, dispersive pressure might have produced the grading (e.g. Walton and Palmer, 1988). The large-scale Gilbert-type cross-stratification suggests that the lake may have been active for a relatively long-time (probably more than a few decades) while producing such a delta reworking (see, Vessell and Davies, 1981). Facies Tr. They are found in the Keiseitoge Formation, and the rocks are vesiculated pumices in reworked tuffs and tuffaceous sandstones exhibiting parallel lamination or small-scale trough crossstratification. Such sedimentary structures indicate

that sediments of facies Tr were reworked by water. Facies Tms. Two types of pyroclastic flow deposits are discernible; One consists of inverse-graded, dense, non-vesicular cognate clasts, and vesiculated pumices concentrated at the top of the facies (scoria-flow deposit of Cas and Wright, 1987; facies Tms1 in this paper, Fig. 10A); the other is pumice-flow deposits with conspicuous gas segregation pipes (facies Tms2, Fig. 10B). Although there is no welded example in facies Tms1, monolithologic clast composition and charred

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Fig. 12. Paleocurrent directions deduced from dips of foresets and from pole plots of imbrication plane of conglomerates of the Yotsuyaku and Keiseitoge formations. The imbrication of 50 pebble and cobble clasts in each outcrop of facies Gm, Gms and Gvms were selected along the bedding plane. All the data obtained in the field were corrected for structural dip, and all plots are shown by lower-hemisphere projection. Dip direction of large-scale foresets is toward the west (point k) in the Keiseitoge Formation and toward the north (point s) in the Sugohata Member, respectively. Broken line indicates the basin outline.

woody remains may be the field criteria for distinguishing such pyroclastic flow deposits from other sedimentary deposits, such as debris flow deposits (Cas and Wright, 1987, p. 111). Facies Tms2 is limited to the western margin of the Ichinohe Basin (point h in Fig. 3a). Both facies belong to the Keiseitoge Formation, but

facies Tms1 is interbedded with facies Gms of the Sugohata Member, the Yotsuyaku Formation (Fig. 11). The segregation pipes of facies Tms2 represent gas escape structures from settling hot pyroclastic flow deposits that are confined to topographic lows (Colella and Hiscott, 1997). However, the relative scarcity of

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the Ichinohe Basin. In this study two methods for paleocurrent analyses were adopted: (1) imbrication of clasts which can be measured readily where the clasts have weathered out from the matrix; and (2) azimuths of admittedly scarce planar cross-sets (facies Sp and Gvp). 4.1. Imbrication

Fig. 13. Lithology of gravels comprizing the Yotsuyaku and Keiseitoge formations. Numbers in centers of individual circular histograms denote the same localities as shown in Fig. 12 (A: Koiwai Member, B: Sugohata Member, C: Keiseitoge Formation, and L: Legend, sd ˆ basement sedimentary rocks except chert, vr ˆ intermediate volcanic rocks (Miocene), ch ˆ chert, igb ˆ basic tuffs and basalt (pillow lavas) derived from the Mesozoic basement, msd ˆ metasedimentary rocks (mainly hornfels) of the basement.

outcrops of the facies does not enable us to determine the paleogeography and the regional extent of the facies.

Imbrication of gravels is a reliable indicator for analyzing paleoflow patterns (e.g. Yagishita, 1992, 1997). Fifteen localities of facies Gm, Gms and Gvms were chosen in both the Yotsuyaku and Keiseitoge formations. In each outcrop 50 clasts that showed well-developed imbrication of the a–b plane (i.e. the plane including both the a- and b-axes) were selected in the bed. Paleocurrents for facies Gm of fluvial conglomerates in the Koiwai Member generally reflect infilling from the basin margins (Fig. 12). The measurements of facies Gms and Gm in the Sugohata Member record sediment dispersal away from highs to the south of the basin, into a longitudinally northward-flowing drainage system along the basin axis (Fig. 12). Lithologic diversity between the clasts of facies Gms and those of facies Gm is not discernible in the Sugohata Member (Fig. 13). This means that winnowing of matrix produced a proximal to distal facies change, i.e. from boulder breccias of debris flows on alluvial fans to predominantly pebble-sized conglomerates of braided streams (Fig. 12). Because of the presence of thick volcanogenic sediments of the Keiseitoge Formation in the center of the basin, however, the transition from facies Gms to facies Gm cannot be observed. Paleoflow directions deduced from the imbrication of andesite clasts of facies Gvms in the Keiseitoge Formation (Fig. 12) show that most of these sediments constitute transversal tributaries, principally derived from the present-day Keiseitoge Range. However, there seems to be no remarkable facies change of the volcanic breccias in the flow direction; i.e. from megaclast-rich (clast-supported) to interclast matrixrich (matrix-supported), as shown by Palmer et al. (1991). 4.2. Planar cross-sets

4. Paleocurrent analyses Paleocurrent directions vary considerably within

At a few localities the maximum dip orientations of facies Sp coincides with the paleoflow direction

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mouth is blocked by rapid agradation by another drainage that carries voluminous volcanic sediments such that the river consequently becomes a flanking lake, has been detailed in modern fluvial-deltaic systems accompanied with arc–trench (forearc) volcanism in western Guatemala (Kuenzi et al., 1979). Similarly, small lakes formed by pre-1980 lahar deposits were also reported from Mount St. Helens (Crandell, 1987, p. 80, 81; Yamaguchi and Hoblitt, 1995). Damming episodes produced by thick lava flows have been described from some basin analyses (e.g. Waresback and Turbeville, 1990; Turbeville, 1991; Stollhofen and Stanistreet, 1994). However, there is no field evidence of extensive, thick lava flows enough to block the streams in this study. As stated, the lakes were probably formed by blocking of rivers by repeated sedimentation of volcaniclastics. There is another possibility, however, that the dammed lakes were produced by an abrupt debris avalanche descended down the flank of the Keiseitoge volcano. The case of a gigantic collapse of Bandai Volcano, which resulted in formation of three dammed lakes is well known (Sekiya and Kikuchi, 1890). Owing to no field evidence of huge volcanic boulder to form hummocks, however, there might not have been such a large-scale, abrupt debris avalanche in this study. Fig. 14. Paleogeographic diagrams illustrating evolution of the central part of the Ichinohe Basin. (1) Early stage when the shallow sea invaded from the north (sedimentation of the Koiwai Member). (2) Middle stage when the rapid uplift of the southern province provided voluminous basement rocks toward the north (deposition of the Sugohata Member). (3) Late stage when volcanogenic sediments were supplied from the eastern volcanoes, and the mutual blocking of clastic- and volcaniclastic-sediment distributaries produced a number of ponds and lakes.

deduced from the gravel imbrication of facies Gm., i.e. similarly oriented towards the north. As stated, this Gilbert-type cross-bed set was the northwardly progradational product in the lake that came into the existence from blockage by northwestward-running debris flows of the Keiseitoge volcaniclastics. Dip orientation of the large cross-bed set of facies Gvp (Fig. 9A and B) is toward the west. Cobble and boulder size clasts exclusively consist of andesites, and the facies was also produced by progradation of lahars into another lake. The process, in which a river

5. Lithology of gravels Gravel lithology of the Koiwai Member depends entirely upon the local petrology of basement source rocks (Fig. 13). One of the lithologic features of the gravels is that they generally contain very few andesite clasts. During deposition of the member, volcanism in the southeastern range was probably inactive. Imbrication in the Sugohata Member shows that most gravelly deposits were transported from southern elevated sources towards the north. Petrographic diversity between breccias in the southern area and conglomerates in the northern area is not great (Fig. 13). Detritus in the matrix of facies Gms and Gm do not show the remarkable difference in composition (Table 1). Andesitic gravels of the member might be produced by the erosion of the Miocene Keiseitoge lavas that had already floored the basin, and the

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gravels were mixed with the Mesozoic basement rocks and transported from the south.

6. Basin evolution and tectonism Many workers have evaluated the effects of volcanism on modern and ancient fluvial systems (e.g. Kuenzi et al., 1979; Vessell and Davies, 1981; Smith, 1987). In the Ichinohe Basin, two representative drainage systems, (i.e. volcaniclastics and nonvolcaniclastics), constitute the deposits. Distribution of different sedimentary environments, paleocurrent data and variation of maximum clast sizes allow construction of a depositional model for basin-filling. Very coarse sedimentary breccias derived from the Mesozoic basement rocks were transported from the southern uplift as debris flows (Fig. 12), and their deposits grade northward into clast-supported braided-stream sediments, where extensive floodplains developed. The voluminous amount of volcanic debris was transported from the southeastern range, and they interfingered with non-volcanic matrixsupported debris-flow deposits from the south (Fig. 11). This implies that deposition of eruptive materials took place concurrently with the uplift during basin evolution (Fig. 14). Chinzei (1966) suggested that there was no high relief in the hinterland at the initial stage of the basin evolution, for he did not observe large clasts in sediments. It is clear now that large clasts do occur in debris-flow deposits above the Kitakami basement rocks, and the uplifted basement highs must have existed. Uplifted horsts or steeply inclined basin walls were present, and such features together with the existence of some faults in and around the basin (Figs. 2, 3a and b) can argue for an extensional tectonism, as stated below. It is well known that numerous N–S trending basins formed areas farther to the west of the Kitakami Massif, i.e. in the so-called Green Tuff Region, during the Early to Middle Miocene. Evolution of such backarc Miocene basins was produced under an extensional stress regime related to the opening of the Sea of Japan (Yamaji, 1989; Sato and Amano, 1991; Sato, 1994). Felsic to intermediate volcanism was also remarkable during this time (Tatsumi et al., 1989; Ohki et al., 1993). The sporadically distributed N–S

trending basins in the Green Tuff Region, in which Lower Miocene basin-fills are often bounded by basement rocks (mostly granites) with normal faults, characterize the incipient stage of the rifting (Yamaji, 1989). The east–west extensional stress regime during the Early to Middle Miocene time can be confirmed from other lines of evidence, particularly from directional analyses of dyke swarms (e.g. Uyeda, 1982) and, in the Ichinohe Basin, by a longitudinal basin-fill. The N–S trending Ichinohe Basin therefore might have formed as a rifting basin within the Kitakami Massif. This means that extensional tectonism during the Early to Middle Miocene affected even the Mesozoic basement east of the Green Tuff Region. Thus the rifting within the basement and the opening of the Sea of Japan were probably caused under a similar tectonism. However, paleomagnetic data from the Nisatai Dacite, which is intercalated with sediments of the Koiwai Member, the lower part of the Yotsuyaku Formation (Fig. 2), show a westerly deflection in declination (see Fig. 5 of Otofuji et al., 1985). Recently Hoshi and Matsubara (1998) have also reported such additional data. These data suggest that the Kitakami Massif was still a part of the Eurasian continent at the very beginning stage of the basin evolution. The extensional tectonism to produce the longitudinal basin-fill, therefore, might have effectively worked during deposition of the Sugohata Member of the upper part of the Yotsuyaku Formation and its concurrent sedimentation of the Keiseitoge volcaniclastics. Namely, the basin evolved while the Kitakami Massif was being separated from the Eurasian continent.

7. Conclusions 1. The Ichinohe Basin of Early and Middle Miocene age is located within Mesozoic basement rocks of the Kitakami Massif. The basin developed farther east to the Green-Tuff Region and is thought to be the early phase of a back-arc basin. 2. The basin consists of sediments of the Yotsuyaku and Keiseitoge formations, in which the former is characterized by basement-derived rocks and the latter by volcanogenic sediments. 3. A systematic facies change is observed in the

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Sugohata Member of the Yotsuyaku Formation; matrix-supported, debris-flow deposits in southern areas grading into clast-supported, fluvial conglomerates in northern areas. 4. Voluminous volcanogenic sediments of the Keiseitoge Formation derived from the east or southeast were deposited by transversal distributaries. The sediments show lacustrine sedimentary facies and large-scale Gilbert-type cross-sets dipping into the lakes, where damming of tributary mouths by the main trunk river or mutual blockages by lahars and the non-volcanic drainage system from southern areas took place. 5. The N–S trending, longitudinal basin-fill of the Ichinohe Basin was formed as a rifting basin within the Mesozoic basement rocks. The rifting of the basin and the opening of the Sea of Japan were probably caused under a similar extensional tectonism. 6. Although the mingling of volcanogenic and basement-derived deposits indicates the complexities of sedimentary facies and paleocurrent patterns, our view of the stratigraphic architecture has relevance to the interpretation of early stage of back-arc basin evolution in continental or mature island-arc settings.

Acknowledgements We wish to thank Prof. Emeritus K. Chinzei (Kyoto University) for his review of the first version of the article. His critical comments greatly improved the manuscript. The second version of the manuscript significantly benefited from a careful review and editing suggestions by Prof. B.R. Pratt (University of Saskatchewan, Canada). Thanks are also extended to Prof. K. Amano (Ibaraki Univeristy) and Drs J. Itoh and K. Uto (Geological Survey of Japan), T. Matsubara (Hyogo Prefectural Museum of Human and Natural History), M. Watanabe (Tohyoh University) and N. Tsuchiya (Iwate University) for their discussions on Neogene volcanism and the basin evolution. Mr H. Kawamorita (Ichinohe Town-Hall) gave his support in the field. Finally, the manuscript was significantly improved by many critical but helpful comments from two reviewers, Profs. R. Cas (Monash University, Austra-

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lia) and K. Ridgway (Perdue University, USA). Prof. A.D. Miall (University of Toronto and Co-Editor of this journal) gave his helpful advice and encouragement to complete the manuscript.

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