Sedimentary Geologr', 45 (1985) 261-292 261 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THE ASHIGARA GROUP: A REGRESSIVE SUBMARINE FAN-FAN DELTA SEQUENCE IN A QUATERNARY COLLISION BOUNDARY, NORTH OF IZU PENINSULA, CENTRAL HONSHU, JAPAN

MAKOTO ITO

Institute of Geoscienee, Uniuersity of Tsukuba, 1-1-1 Tennodai, Sakuramura Niiharigun, Ibaraki 305 (Japan)

(Received November 5, 1984; revised and accepted March 14, 1985)

ABSTRACT

Ito, M., 1985. The Ashigara Group: A regressive submarine fan-fan delta sequence in a Quaternary collision boundary, north of lzu Peninsula, central Honshu, Japan. Sediment. Geol., 45: 261-292.

The coarse siliciclastic and volcaniclastic depositis of the Plio-Pleistocene Ashigara Group, as much as 4500 m thick, are distributed along a narrow belt which is assigned to the Quaternary collision boundary between the Izu Peninsula (lzu Block) and central Honshu.

Nine major depositional facies are discriminated in the Ashigara Group. Each characterizes the specific depositional environments of a regressive sequence from lower submarine fan/basin plain, through submarine volcano, mid-upper submarine fan/slope, to fan delta systems:

(1) The lower submarine fan/basin plain system is dominated by thin sheet turbidite sandstones and represents the initial deposition of the Ashigara Group under deep water. During this stage, subduction of the Philippine Sea plate beneath central Honshu gave rise to the uplift of the Tanzawa Mountains which formed a high source area.

(2) Doming intrusion of andesitic/dacitic magma, on the other hand, gave rise to trough and swell structures trending N W - S E on the landward slope. The submarine volcanism near the convergent margin was emplaced by the subducting Pacific plate beneath the Izu-Bonin arc. Volcaniclastics were debouched

from the submarine volcanoes on the swells to the side troughs. One of the troughs became a submarine canyon-like large recession along the present Sakawa River.

(3) The recessions around the swells were infilled with the gravelly mid-upper submarine fan deposits prograded from the NW and volcaniclastics from the submarine volcanoes. The trough and swell structures were eventually buried and levelled off by the muddy slope deposits prograded from the NW. (4) Later, progradation of a gravelly fan delta system onto the shoreline to offshore from the NE was emplaced by reactivation of the uplift of the Tanzawa Mountains and the shoaling together with northward tilting of the underlying deposits which resulted in extinction of the previous southeastward slope, as suturing continued.

INTRODUCTION

Plio-Pleistocene siliciclastic and volcaniclastic deposits of the Ashigara Group, as

much as 4 500 m thick, are distributed along a narrow belt between the Izu Peninsula and the Tanzawa Mountains in central Honshu, Japan (Fig. 1). The narrow belt

0037-0738/85/$03.30 © 1985 Elsevier Science Publishers B.V.

262

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Fig. 1. Geological sketch map around the Izu Peninsula (lzu Block) simplified from Kakimi et al. (1982). Depth contours of -200, -1000 and -2000 m are drawn. The study area is framed by the solid line. Open arrows in inset indicate convergent direction of the Pacific (PAC) and the Philippine Sea (PttS) plates to the Eurasia (EUR) plate adopted from Matsuda et al. (1978).

a lmost corresponds to the inland trace of the northeastern tip of the Philippine Sea plate from the shallow trenches of the Sagami and Suruga troughs (Fig. 1). The geological structures of the Cenozo ic and Mesozoic terranes and neotectonics

around the Izu Peninsula have been explained by coll is ion of the lzu-Bonin arc with central Honshu (e.g., Sugimura, 1972: Matsuda, 1976, 1977, 1978: Nakamura and

Shimazaki , 1981; Shimazaki et al., 1981). Recently much attention has been paid to the evolutionary processes of coarse

elastic deposits of mobi le zones, with reference to tectonic m o v e m e n t s (e.g., Crowell , 1974; Bluck, 1980; Steel and Gloppen, 1980; Miall, 1981: McLaughlin and Nilsen,

1982). Most of these studies, however, are dealing with non-marine to transitional

263

marine sequences in strike-slip basins. The Ashigara Group, on the other hand, represents marine to transitional marine coarse clastic deposits dumped near the Quaternary collision boundary between the Izu Peninsula (Izu Block) and central Honshu. Sedimentological and structural analysis of the Ashigara Group, therefore, bears particular interests for understanding the evolutionary processes of this kind of coarse clastic pile near the arc-arc collision boundary.

The focus of this paper is to interpret the depositional environments of the Ashigara Group and then discuss the evolution of the depositional systems near the young arc-arc collision boundary with reference to tectonism.

G E O L O G I C A L O U T L I N E

The Ashigara Group is distributed from Oyama in the eastern part of Shizuoka Prefecture to Matsuda in the western part of Kanagawa Prefecture. The outcrop belt is about 17 km in E-W direction and from 4 to 10 km wide. The group stretches in arcuate form, being convex northward, with a northward dip of 20-90 ° (Fig. 2). It is bounded on the north to the Miocene Tanzawa Group (Mikami, 1962; Sugiyama, 1976) by the Kannawa reverse fault (Kato, 1910; Matsushima and Imanaga, 1969) but is covered in the south by volcanics from the Hakone volcano on the Izu Peninsula. The Kannawa reverse fault has been later dislocated by dextral and sinistral strike-slip faults trending NW-SE and NE-SW, respectively (Sato, 1976; Hoshino and Hasegawa, 1977; Kano et al., 1978) and is then interpreted as a fault within the mechanical plate boundary between the Eurasia and Philippine Sea plates (Nakamura and Shimazaki, 1981; Shimazaki et al., 1981). The Ashigara Group has been further dislocated by several other active reverse and strike-slip faults and contains anticlinal and synclinal structures trending E-W with plunge of about 45°W in the southwestern part (Fig. 2).

The stratigraphy of the Ashigara Group was first established by Hirabayashi (1898) and Kato (1910). Kuno (1951) divided the group into Lower, Middle and Upper zones. Imanaga (1978) divided the Ashigara Group into four stratigraphic units: the Hinata, Seto, Hata and Shiozawa formations in ascending order. In this study the stratigraphic units by Imanaga (1978) are adopted (Fig. 3). The Ashigara Group was mainly derived from volcaniclastic and metamorphic rocks of the Tanzawa Group and intrusive rocks in the Tanzawa Mountains (Fig. 3). Well-rounded pebbles of conglomerate, sandstone, shale and chert, however, were probably redeposited from the Middle Miocene conglomerates around the northwestern margin of the Tanzawa Mountains. The geological age of the Ashigara Group was estimated by Yokoyama (1921) and Otuka (1931, 1932) as Pliocene based on molluscan fossils. Kuno (1951) correlated the volcaniclastic rocks of his Lower zone with the Upper Miocene Hayakawa Tuff Breccias exposed at the base of the Hakone Volcano. Recently Matsushima (1982) and Ishikawa et al. (1983), respectively, examined molluscan shells and calcareous nannofossils from the group and esti-

264

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266

mated its geological age as Early to Middle Pleistocene. Hornblende-pyroxene andesite in the lower part of the Hata Formation, however, has been dated at 3.5 Ma (Y. Kobayashi, pers. commun., 1984). Calcareous nannofossils from the middle part of the Hinata Formation further indicate a Pliocene age (N. Oyama, pers. commun., 1984). The upper part of the Ashigara Group, therefore, probably was deposited during the Early to Middle Pleistocene except the lower part of the group which was probably deposited during the Pliocene.

DEPOSITIONAL FACIES

Nine major depositional facies are discriminated in the Ashigara Group (Fig. 2). Each facies is characterized by unique combinations of lithologies, sedimentary structures, textures, geometry and fossils.

F~cies 1

Facies I deposits predominate in the basal part of the Ashigara Group (Fig. 2). This facies is characterized by interbedded thin sandstones (beds, 3 22 cm, average. 7 cm thick) and thicker mudstones (beds, 1 469 cm, average, 30 cm thick) with some intercalations of pumice and scoria tuff (Fig. 4). Sandstones comprise about 15% of the deposits with mudstones forming most of the remaining 75-85%. The sandstones are medium- to very fine-grained with sharp or transitional basal contacts and fine upward into the overlying mudstones. Internally the sandstones are generally graded or massive and sometimes contain parallel a n d / o r ripple-cross laminae. Sandstone-mudstone couplets are interpreted as Bouma A-E, A-C-E, B-C-E, B-E and C-E sequences, respectively. The sandstone units are further characterized by rather high continuity and absence of lateral facies changes. No distinct vertical thickening- or thinning-upward sequencies can be recognized in this facies.

Facies 11

Facies II is characterized by two subfacies: (II-A) interbedded conglomerates and sandstones (Fig. 5A), and (II-B) mudstones with intercalations of lenticular con- glomerates and sandstones (Fig. 5B). The deposits of facies II abruptly overlie the muddy deposits of facies I and are well developed along the present Sakawa River (Fig. 2).

Subfacies II-A. lnterbedded conglomerates and sandstones comprise 57 90% and 10 27%, respectively, of the deposits with minor intercalations of mudstones (1 7%). Conglomerates (beds, 20-890 cm thick) are generally clast-supported and are composed of pebble- to boulder-sized fragments (Fig. 6A). Internally they are disorganized (16%), normally graded (35%), inversely (17%) or inverse to normally

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Fig. 5. A. lnterbedded conglomerates and sandstones of subfacies II-A along the Sakawa River. Beds younger to right. B. Mudstones intercalated with lenticular sandstones and conglomerates of subfacies II-B at Seto, along the Sakawa River. Beds younger to left.

(11%) graded with channeled bases. Graded stratified (7%) and cross-stratified (4%)

conglomerates, however, are sometimes intercalated. The clasts are well rounded and

some of them show well-developed imbrication with their long axis parallel to flow.

The clasts are composed predominantly of tuff and lapilli tuff, and andesite with

subordinate shale, diorite and basalt. The conglomerate units grade upward into the

overlying pebbly or coarse- to fine-grained sandstones. The sandstones (beds, 1 0 270 cm thick) are generally graded except for some

ungraded and inverse- to normally graded beds. Internally some contain parallel

a n d / o r ripple-cross laminae which are sometimes overlain by discontinuous mud-

sto~e drapes showing flaser-like bedding. The sandstone units are amalgamated to

make thicker deposits or are separated by thin sheets of massive or parallel-laminated

mudstones (Fig. 6A). Conglomerate-sandstone couplets generally show thinning- and fining-upward

sequences and are sometimes succeeded by thickening- and coarsening-upward

sequences (Figs. 5A and 6A).

Subfacies l l-B. Mudstones intercalated with lenticular conglomerates and sandstones

(3 60 m thick) rest on top of the thinning- and fining-upward sequences of the

interbedded conglomerates and sandstones of subfacles II-A (Fig. 6B). The mud- stones comprise about 82% of the deposits with the lenticular conglomerates 6% and

sandstones 12%. The lenticular conglomerates (beds, 10-60 cm thick) are mainly

composed of pebble-sized clasts. They are generally graded or graded stratified with channeled bases. The lenticular sandstones (beds, 1-60 cm thick) are coarse- to

fine-grained with some parallel a n d / o r ripple-cross laminae. They commonly pinch out into the mudstones in several tens of meters distance (Fig. 5B). Paleocurrent

directions of the lenticular conglomerates and sandstones are not so consistent. No distinctive vertical thickening- or thinning-upward sequences can be recognized (Fig.

6B). Some molluscan shells, such as Acila divaricata and Portlandia fischkei, are found

in the mudstones of facies II.

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Fig. 7. A. Channelized thick bedded sandstones of subfacies III-A following thinly interbedded sand- stones and mudstones of subfacies III-B at Yaga, along the Sakawa River. Beds younger to right. B. Pebbly mudstones intercalated in mudstones of subfacies III-B. Beds younger to right.

Facie.s" I l l

The conglomeratic deposits of facies 11 grade vertically and laterally into the facies Ili deposits (Fig. 2). Facies III is characterized by two subfacies: (Il l-A)

thickly amalgamated sandstones and (Ill-B) thinly interbedded sandstones and mudstones (Fig. 7A).

Subfacies III-A. The thickly amalgamated sandstones (beds, 4 360 cm thick) are very coarse- to fine-grained and are commonly graded containing parallel a n d / o r ripple-cross laminae which are generally overlain by mudstone drapes resulting in flaser-like bedding (Fig. 8A). The sandstones have flat or scoured basal surfaces but are sometimes sculptured by load casts and burrows. Some contain numerous mudstone intraclasts, rip-up clasts and diffused pebbles (Fig. 8A}.

Subfacies III-B. The sandstones thinly interbedded with mudstones are generally lenticular. They are medium- to very fine-grained and fine upward into the mud- stones with sharp, erosional or transitional contacts. These sandstones sometimes contain parallel- a n d / o r ripple-cross laminae and represent Bouma A, A-B, A-B-C or C divisions.

The mudstones interbedded with the sandstones commonly contain discontinuous sandstone streaks showing lenticular-like bedding. The mudstones are further char- acterized by many intercalations of chaotic deposits and structures, such as pebbly mudstones, slurry sandstones, slumps and slump scars (Fig. 8B). Pebbly mudstones (Fig. 7B) are generally lenticular and laterally pinch out into the mudstones. They have a sandy mudstone matrix and contain some shallow marine molluscan shell fragments. Some show coarse-tail grading and then fine upward into coarse- to medium-grained sandstones with Bouma A, A-C and C divisions. Minor clast-sup- ported conglomerates are also intercalated in the mudstones of subfacies III-B (Fig. 8B). The conglomerates are generally lenticular and ill-sorted containing many sandstone matrix and mudstone intraclasts.

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273

Thickening- and coarsening-upward sequences succeeded by thinning- and fining-upward sequences are recognized from the thinly interbedded sandstones and mudstones (subfacies III-B) to the thickly amalgamated sandstones (subfacies Ill-A; Fig. 8A)

Marine molluscan shells, such as Acila diuaricata, Portlandia lischkei and Limop- sis tokaiensis, are commonly found in the mudstones.

Facies I V

Facies IV is characterized by interbedded conglomerates, sandstones, mudstones and sandy mudstones (Fig. 9). The conglomerates comprise 63-87% of the deposits with sandstones comprising 4-20% and mudstones and sandy mudstones the re- maining 2-18%.

The conglomerates (beds, 15-320 cm thick) are generally clast-supported and are composed of pebble- to cobble-sized fragments. Multiple sets of cross- or horizon- tally stratified conglomerates (26%) together with graded or ungraded conglomerates (66%) with scoured basal surfaces represent facies IV (Fig. 9). Stratified con- glomerates generally show well-developed imbrication. Clasts are rounded and composed predominantly of tuff and lapilli tuff, and andesite with subordinate schist and diorite which abruptly increase from the facies IV deposits.

The coarse- to medium-grained sandstones rest on top of the conglomerate units or are interbedded with the mudstones or sandy mudstones. The sandstones are generally lenticular and graded containing some parallel or ripple-cross laminae.

Chaotic deposits and structures, such as pebbly mudstones, slurry sandstones, slumps, slump scars and irregularly arranged clastic dykes are intercalated in the mudstones and further characterize facies IV. Some marine molluscan shells are found in the mudstones of this facies.

Facies V

Facies V deposits overlie the deposits of facies III or IV. The lower part of facies V deposits is characterized by interbedded medium- to fine-grained sandstones, sandy mudstones and pebble-sized conglomerates (Fig. 10A). Sandstones (beds, 5-120 cm thick) have sharp/f la t or transitional basal contacts and some show convex upward geometry. The sandstone units internally contain parallel laminae overlain by ripple-cross laminae. The ripple-cross laminated thin sandstones, how- ever, are predominant in the lower part o f the sequence (Fig. 10A). The sandy mudstones sometimes contain trough-cross or ripple-cross laminations. Articulated shallow marine molluscan shells, such as Phacosomajaponicum, Meretrix lusoria and Mya arenaria oonogai are found in the sandy mudstones. Well-preserved molluscan shells are also found in the channel-shaped pebbly mudstones and sandstones pinching out into the sandy mudstones. Thickening- and coarsening-upward se- quences are recognized in the lower part of the facies V deposits (Fig. 10A).

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Fig. 11. A. Channelized cross- and horizontally stratified conglomerates interbedded with sandy mud-

stones of facies V, about 2 km east of Oyama Town. Beds younger to left. Tetrapod at left side middle is

about 1 m in height. B. Spreading dendritic colony of Crassostrea gigas in sandy mudstone interbeds of

facies V, about 2.5 km northeast of Oyama Town.

The middle part of the facies V deposits is characterized by interbedded sandy mudstones, coarse- to medium-grained sandstones and pebble- to cobble-sized conglomerates (Figs. 10B and llA). The sandy mudstones comprise from 17 to 38% of the deposits, sandstones 11-18% and conglomerates 44-68%. The sandy mud- stone units generally contain ripple-cross laminae but they are disturbed by strong burrowing in the upper part of each unit. Ostrea banks and other shallow marine molluscan shells such as Meretrix lusoria and Macorna incongrua are commonly found in the sandy mudstones (Fig. llB). The sandstone units contain parallel and /or trough-cross laminae. Some units have sharp/flat basal surfaces and convex upward geometry. The others, however, are channel-shaped and laterally pinch out into the sandy mudstones. The conglomerate units are clast-supported and chan- neled into the underlying deposits. Internally ungraded or graded beds (57%) are predominant with subordinate horizontally (20%)- and cross-stratified (23%) beds. Ungraded or horizontally stratified cobble-sized conglomerates, overlain by parallel- or trough-cross laminated sandstones, predominantly occupy the lowest part of the fining-upward sequences (Fig. 10B). Clasts are rounded and consist mainly of tuff and lapilli tuff, diorite and schist.

The upper part of the facies V deposits is characterized by interbedded pebble- to boulder-sized, poorly sorted conglomerates and coarse-grained sandstones with intercalations of sandy mudstones (Fig. 12). The conglomerates comprise about 87% of the deposits with sandstones 6% and sandy mudstones 7%. The conglomerates are generally clast-supported and are channeled into the underlying conglomerates or sandstones. They are sometimes overlain by parallel- or trough-cross laminated sandstones on the scoured surfaces. The conglomerate units commonly show coars- ening-upward sequences from cross-stratified, through horizontally stratified, to disorganized bedding (Fig. 12).

276

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Fig. 12. Representative section of the upper part of the facies V deposits, about 1 km east of Oyama

Town. Legend as in Fig. 4. Note multiple sets of coarsening-upward sequences from cross stratified.

through horizontally stratified, to disorganized bedding of the conglomerate units.

Facies VI

Facies VI deposits represent the uppermost part of the Ashigara Group (Fig. 2). They are dominated by poorly sorted, clast-supported conglomerates with clast size ranging from pebble to boulder (180 cm maximum diameter). Clasts are composed predominantly of diorite and schist fragments• The maximum clast size increases abruptly from the boundary between the facies V and VI deposits. Sedimentary structures are noticeably lacking in the upper part of the facies Vl deposits but

277

Fig. 13. Horizontally stratified cobble- to boulder-sized conglomerates of facies VI at Shiozawa. Beds

younger to right.

cross- or horizontally stratified conglomerates are predominant in the lower part (Fig. 13). Much plant debris and drift wood is intercalated in facies VI. Large-scale (60-100 m thick) coarsening-upward sequences are recognized in this facies.

Fig. 14. Massive matrix-supported conglomerates of facies VII, about 1 km north of Matsuda Town. Beds younger to right. Note well-rounded clasts floated in lapilli tuff matrix.

278

Facies 1/71

Facies VII deposits are massive, poorly sorted conglomerates with clast size from pebble to cobble. Clasts are rounded and consist predominantly of diorite, tuff and lapilli tuff. The clasts are widely spaced in tuff or lapilli tuff matrix and show a matrix-supported appearance (Fig. 14). Internally sedimentary structures are noticeably lacking in this generally massive facies. Facies VII deposits are graded from the deposits of facies IX but they grade laterally into the deposits of facies I1 and IIl (Fig, 2).

Facies VI I I

Facies VIII is intercalated in facies 1I and III. It is well developed in the southwestern part of the Ashigara Group within an anticlinical structure showing a semicircular dome about 1.5-2.0 km in diameter (Fig. 2). Facies VIII deposits are mainly composed of volcaniclastic rocks, such as volcanic breccia, tuff, lapilli tuff, and meso- and mega-breccias of Lipman (1976) associated with intrusive rocks of sill, sheet and dyke of andesite and dacite. Most of these volcanic rocks are pyroxene-andesite, pyroxene-hornblende andesite and pyroxene-hornblende dacite (Kuno, 1951). The intrusive and extrusive rocks occur together and intertongue with the conglomerates and mudstones of the surrounding deposits of facies 11 and 1II (Fig. 2). Many slumps are recognized in some of the volcaniclastic rocks.

Sills and sheets branch out from the dykes with irregularly waving walls. They are partly brecciated to grade into the volcanic breccias or contain parallel-laminated flow structures a n d / o r blocky structures. Some andesite sills and volcanic breccias are further traceable near a quartz-diorite plug in Mt. Yaguradake (Fig. 2). Nearly concordant contacts between the quartz-diorite and the surrounding deposits of facies llI are observable. The quartz-diorite has a chilled marginal zone of hyper- sthene-hornblende andesite (Kuno, 1951).

Meso- and mega-breccias are well developed in the central part of this facies. They are mainly composed of a few centimeters to a few tens of meters of andesite breccias together with other breccias composed of mudstone, conglomerate, interbedded sandstone and mudstone which all have the same lithologies as the rocks surrounding the breccias (Fig. 15). The meso- and mega-breccias are poorly sorted and have some matrix of decomposed andesite grains and mudstone chips. They are then intruded by pyroxene-hornblende andesite dykes and are partly affected by hydrothermal alterations with pyrite. The size of the breccias decreases from the base to the top, but tends to increase near the surrounding rock walls. The dip and strike of the facies III deposits, surrounding the facies VIII deposits, are widely inconsistent between nearby outcrops in the southwestern side of the meso- and mega-breccias with some fragmental materials. The facies 111 deposits are

279

Fig. 15. Meso-breccias of facies VIII, about 5 km southwest of Yamakita Town.

further intruded by pumice dykes and are intercalated with some pumice tuffs and volcanogenic sandstones.

Facies IX

Facies IX is also characterized by volcaniclastic rocks of volcanic breccia, tuff breccia, lapilli tuff and tuff. Sedimentary structures are noticeably lacking in generally massive volcanic breccias and tuff breccias but parallel stratifications can be recognized in tufts or lapilli tuffs. Resedimented volcanogenic sandstones are interbedded with mudstones containing Bouma A a n d / o r B divisions. Slurried beds of tuff and lapilli tuff are commonly intercalated in the mudstones. Relative abundance of tuff breccias and volcanic breccias of this facies gradually increase from the west to the east of the Ashigara Group. Facies IX deposits then intertongue with the deposits of facies I, II and III in the central part of the AshigaraGroup (Fig. 2).

DEPOSITIONAL ENVIRONMENTS

The nine major depositional facies of the Ashigara Group can be assigned to specific depositional environments from lower submarine f a n / b a s i n plain, through mid-upper submarine fan/slope-submarine volcano, to fan delta systems. Each depositional environment is characterized by unique combinations of physical and biological processes.

280

Lower submarine fan-basin plain

The thin sheet-like turbidite sandstones and the interbedded thick mudstones of facies I are interpreted as lower submarine fan or basin plain deposits. These deposits are characterized by some diagnostic features of distal turbidite facies including thin bedding, well-developed mudstone layers and parallel-sided regular bedding (Walker, 1967). Facies I is further characterized by absence of lateral facies changes and bed continuity of the sandstone units. Parallel- a n d / o r ripple-cross laminated thin sandstones also characterize lower submarine fan or basin plain deposits (Walker, 1967; Walker and Mutti, 1973; Mutti and Ricci Lucchi, 1978). The lack of distinctive vertical thickening- or thinning-upward sequences further suggests basin plain environment (e.g., Mutti and Ricci Lucchi, 1978). The benthic for- maminiferal assemblage from the mudstones of facies I indicates a water depth of 1000 2000 m (Ishikawa et al., 1983) and supports the above interpretation.

Mid-upper submarine fan

The depositional environments of coarse-grained deposits commonly associated with typical flysch sequences are interpreted as slope, base of slope or submarine fans (e.g., Walker~ 1978, 1979). Interbedded conglomerates~ sandstones and mud- stones of facies II are interpreted as channelized mid-upper submarine fan deposits. Mid- and upper submarine fan channels, however, are not readily distinguishable in facies II. The facies I1 deposits generally lack lobe deposits (e.g., Walker. 1978) but abruptly follow the lower submarine fan/bas in plain deposits. Facies II may alternatively be interpreted as conglomeratic submarine channel deposits in base of slope.

The interbedded conglomerates and sandstones of subfacies I I-A were deposited out of high-concentrated sediment gravity flows (Middleton and Hampton, 1976). The conglomerate units of facies It show many characters of submarine resedi- mented conglomerates (e.g., Walker, 1975, 1978: Nemec et al., 1980; Hein, 1982). Disorganized, inversely- or inverse-, to normally graded conglomerates were accu- mulated due to highly concentrated turbulent flows on steep slopes (Davies and Walker, 1974). Normally graded conglomerates were successively deposited out of suspension as concentration decreased. These conglomerates can represent deposits in thalweg channels and grade upward into the sandstones. The sandstones were deposited out of suspension from turbidity currents of lower concentration than those that deposited the thalweg conglomerates. Amalgamation and mudstone drapes in the sandstone units probably represent fluctuating concentrations and velocities of the turbidity currents.

The couplets of the conglomerates and the overlying sandstones of subfacies II-A generally show thinning- and fining-upward sequences (10-15 m thick) and can be interpreted as submarine fan braided channel and marginal terrace deposits (Eriks-

281

son, 1982; Hein and Walker, 1982). Thickening- and coarsening-upward sequences succeeding thinning- and fining-upward sequences in subfacies II-A probably repre- sent lateral shifting of the main channel and the associated terrace systems (Hein and Walker, 1982). Cross- and/or horizontally stratified, traction-produced bedding in the conglomerate units of subfacies II-A could be the result of layer-by-layer deposition of gravels from subaqueous turbulent flows. They could be deposited on submarine fan braided channel bars as documented by Winn and Dott (1979) and Hein and Walker (1982).

The mudstones intercalated with lenticular sandstones and conglomerates of subfacies II-B probably represent overbank deposits emplaced by sediment-laden overflows from the main channels. The intercalated coarse sandstones and con- glomerates with channeled bases can be interpreted as crevasse-splay deposits. Paleocurrent dispersion and no distinct vertical thickening- or thinning-upward sequences also characterize the overbank deposits. Molluscan shells found in the mudstones of subfacies II-B indicate the bathyneritic or bathyal zone. The benthic foraminiferal assemblage from the mudstones of facies III indicates a water depth of 200-600 m (Ishikawa et al., 1983). These paleontological data support the above interpretation.

The massive poorly sorted conglomerates of facies VII show a matrix-supported appearance and may represent submarine debris flow deposits on the slope to the slope base. Abundant volcanogenic matrix in the conglomerates suggests proximity of submarine volcanoes.

Submarine slope

Muddy facies associated with many chaotic deposits of facies III are interpreted as submarine slope deposits. The coarse chaotic deposits of pebbly mudstones (Crowell, 1978) and slurry sandstones (Hiscott and Middleton, 1979) represent submarine debris flow deposits. Slumps and slump scars (e.g., Laird, 1968; Piper et al., 1976; Clari and Ghibaudo, 1979) were induced by sea-floor failure and char- acterize shelf edge to upper slope environments.

Sandstones and pebbly sandstones of subfacies III-A and lenticular con- glomerates of subfacies III-B represent deposition from high-density turbidity cur- rents in submarine channels incised into the slope. These deposits are characterized by amalgamation and fining-upward sequences which were emplaced by repetitive waning of the turbidity currents. Multiple sets of parallel- or ripple-cross laminae and mudstone drapes in the sandstone units were deposited out of suspension of lower-density turbidity currents with highly fluctuating velocities.

Thin-bedded turbidites associated with the submarine channel deposits of sub- facies III-A can be interpreted as interchannel deposits (e.g., Mutti and Ricci Lucchi, 1978) on the slope. They were deposits from lower-density turbidity currents overflowed from the channels. Progradation and abandonment of the channel and

282

interchannel systems on the slope can be inferred from thickening- and coarsening- upward sequences of the thinly interbedded sandstones and mudstones (subfacies Ill-B) succeeded by thinning- and fining-upward sequences of the thickly amalga- mated sandstones (subfacies Ill-A: Fig. 8A)(e.g., Walker and Mutti, 1973; Walker, 1978, 1979). Channel-shaped, laterally truncated conglomerates and sandstones are alternatively interpreted as deposits in depressions on the slope. Molluscan shells found in the mudstones of subfacies III-B indicate an upper hathyal zone (Matsushima, 1982). Benthic foraminiferal assemblages in the mudstones of sub- facies III-B further suggest a water depth of 100 300 m (Ishikawa et al., 1983). These paleontological data support the above interpretation.

Fan delta prodelta-distal delta ,fi'ont

lnterbedded conglomerates, sandstones, mudstones and sandy mudstones con- taining shallow marine molluscan shells of facies IV, V and V1 respectively represent specific depositional environments of a fan delta system (e.g., Holmes and Hohnes, 1978: Wescott and Ethridge, 1980) prograded out into shoreline to offshore environ- ments from the adjacent highlands of the Tanzawa Mountains.

Facies IV deposits overlie the submarine slope deposits of facies I11 and can be interpreted as prodelta to distal delta front deposits. Multiple sets of cross- or horizontally stratified conglomerates characterize distal coarse-grained braided al- luvium deposited by migration of sinuous-crested dunes during flood stage or transverse bars under conditions of reduced sediments and water discharge (Hein and Walker, 1977; Miall, 1977, 1978; Rust, 1979). The cross-stratified conglomerates above the erosional bases can be alternatively interpreted as gravelly infills on channel scours. Graded and ungraded conglomerates, on the other hand, were probably emplaced by sediment gravity flows during high discharge of submarine rivers which scarred the fan delta prodelta.

Thin-bedded sandstones and mudstones or sandy mudstones can be interpreted as interchannel deposits. The sandstone units generally show characteristic features of turbidite deposits and were probably emplaced by sediment-laden overflows along the prodelta. Chaotic deposits and structures, such as pebbly mudstones, slurry sandstones, slumps and slump scars, were emplaced by gravity slide which is an important process in distal delta front environments (Wescott and Ethridge, 1980). Irregularly arranged clastic dykes may further indicate instability of the prodelta-distal delta front environments.

Proximal delta front

The relatively thin sandstones interbedded with fossiliferous sandy mudstones of the lower part of the facies V deposits can be interpreted as storm-generated sublittoral sheet sandstones (e.g., Johnson, 1978) deposited in lower shoreface to

283

offshore environments. Ripple-cross laminae succeeding parallel laminae generally show characteristic wave-built structures (e.g., Raaf et al., 1977; Reineck and Singh, 1980) and suggest lower flow regime, waning current condition or post-storm reworking. Pebbly mudstones and sandstones containing well-preserved molluscan shells are interpreted as debris flow deposits emplaced by sea-floor failure during storm (Masuda et al., 1981). The thickening- and coarsening-upward sequences recognized in the lower part of the facies V deposits indicate progradation of the fan delta system onto lower shoreface to offshore region. Molluscan shells found in the sandy mudstones of the lower part of the facies V deposits mainly consist of shallow embayment elements (Matsushima, 1982) and support the above interpretation.

The interbedded conglomerates, sandstones and sandy mudstones of the middle part of the facies V deposits generally show fining-upward sequences and represent gravelly braided channel and interchannel systems intruded into shoreline to off- shore environments. Multiple sets of ungraded or horizontally stratified con- glomerates are generally predominant in the lower part of each fining-upward sequence and are interpreted as gravelly longitudinal or diagonal bars.(Smith, 1974; Miall, 1977, 1978; Rust, 1979). Thin sheet sandstones interbedded with the con- glomerate units probably represent bar edge sand deposited at the lateral margin of the gravel bars during waning stage (Rust, 1972). Cross-stratified conglomerates, on the other hand, grade upward into parallel-laminated sandstones and represent smaller channel scour deposits. Crevasse-splay deposits can be represented by channel-shaped, lenticular pebbles and coarse-grained sandstones laterally pinching out into the sandy mudstones. Both medium- to fine-grained sandstones showing convex upward geometry and ripple-cross laminated sandy mudstones probably represent sheet flow deposits with high concentration of suspended mud. Strong bioturbation of the upper part of the ripple-cross-laminated sandy mudstone units indicates post-flood reworking. No remarkable reworking and modification by wave or other marine processes can be recognized. Deposition of this facies, therefore, could be dominated by fluvial induced processes rather than marine processes. Brackish embayment molluscan shells from the sandy mudstones support the above interpretation.

Multiple sets o f coarsening-upward sequences from cross-stratified, through horizontally stratified, to disorganized bedding of the pebble- to boulder-sized conglomerates of the upper part of the facies V deposits indicate further proximal part of the delta front environments. The three divisions of a coarsening-upward sequence can be respectively interpreted as platform, organized supraplatform and disorganized supraplatform of gravelly medial or longitudinal bars. This superposi- tion of the three divisions reflects migration of bar head and bar tail gravels over bar platform (Steel and Thompson, 1983). Superposition of the sequence suggests lateral migration and abandonment of the gravelly bars. Irregular bounding surfaces between the conglomerate units and the overlying sandstone units indicate falling stage modification of the gravelly bars and deposition of bar edge sand.

284

Proximal fan delta plain

Coarse gravel and rapid increase of clast size in proximal reach are the important characters of alluvial fans (Heward, 1978; Rust, 1979). Horizontally stratified clast-supported cobble- to boulder-sized conglomerates of the lower part of the facies VI deposits are the predominant facies in the proximal reaches (Miall, 1977, 1978; Rust, 1979). Disorganized poorly sorted cobble- to boulder-sized con- glomerates of facies VI were emplaced by mass transport mechanism during high discharge of fluvial channels on the head of the fan delta from the adjacent highlands. No matrix-supported conglomerates or breccias of debris flow origin can be observed. This suggests that this proximal fan delta plain was deposited under rather humid climatic conditions (e.g., Schumm, 1977; Rust, 1979; Wescott and Ethridge, 1980). Large-scale coarsening-upward sequences (60-100 m thick) prob- ably represent progradation of the proximal fan delta plain in response to relative uplift of the adjacent mountains. Facies VI deposits stratigraphically overlie the deposits of facies V and probably represent subaerial environments. The presence of much plant debris and drift wood supports this interpretation.

Submarine volcano

The intrusive and extrusive rocks of facies VIII, developing a semicircular depositional center and a dome structure (Fig. 2), can be interpreted in terms of doming intrusion of andesitic or dacitic magma and its related submarine volcanism on the slope or submarine fan environments. The intertonguing relations between the deposits of facies VIII and those of facies II a n d / o r III indicate that the submarine volcanism was contemporaneous with the deposition of facies II and IIl. Some blocky structures and flow structures of the intrusive rocks suggests that their intrusions were caused by injection of andesitic or dacitic magma into weak, low-density, water-laden sediments within shallow portions of the sea floor. The irregularly waving walls of some dykes and the brecciated sills or sheets, commonly branching out from the dykes, are consistent with this interpretation (e.g., McBirney, 1963; Einsele, 1982). The anticlinal structure of the facies II and III deposits, corresponding to the depositional center of facies VIII, can then be interpreted as a syndepositional warping structure uplifted by the doming intrusion of the andesitic and dacitic magma into the unconsolidated soft sediments of facies II and III. Minor hydrothermal deposits of pyrite are observable within the deposits of facies III and VIII. Similar dome structures were recently reported by Lonsdale and Lawver (1980) from the Guaymas Basin and Lonsdale (1983) from the East Pacific Rise, where they are accompanied by normal faulting.

Meso- and mega-breccias mainly distributed in the central part of the dome structure are interpreted as subaqueous caldera collapse breccias. These are subaque- ous equivalents of caldera collapse breccias described by Lipman (1976) in the

285

western San Juan Mountains. Wide inconsistency of dip and strike of the surround- ing deposits of the meso- and mega-breccias also indicates accumulation of a chaotic assemblage of large blocks by the caldera formation. The volcaniclastic rocks thickly deposited on the northern flank of the dome intertongue with both the meso- and mega-breccias and the surrounding deposits of facies III (Fig. 2). These field relations suggest that the submarine caldera was caused by extrusion of the volcaniclastics during the deposition of facies III. The asymmetrical deposition of the volcaniclastic rocks suggests that the extrusions of the volcaniclastics were caused by slumping or faulting of the overlying sediments induced by asymmetrical intrusion of the andesitic and dacitic magma. Some volcanogenic sandstones interbe- dded with the mudstones of facies III can be interpreted as the deposits from the suspended materials arising from the caldera collapse episode.

Post-caldera volcanism is indicated by pumice dykes of facies III deposits, andesite dykes intruded into the meso- and mega-breccias and hydrothermal alter- ations of the breccias. The intimate occurrence of the quartz-diorite, andesite sills and volcaniclastic rocks of facies VIII (Fig. 2) suggests that the quartz-diorite indicates a slowly cooled facies of a magma closely related to those which gave rise to doming intrusion and some extrusive submarine volcanism.

The volcaniclastic rocks of facies IX are widely distributed in the eastern part of the Ashigara Group but show proximal to distal facies changes from east to west. Facies IX deposits can also be interpreted as the deposits caused by submarine volcanic activity which took place in the far eastern area of the Ashigara Group.

DEPOSITIONAL HISTORY

The Ashigara Group represents a regressive sequence embracing a coarsening-up- ward fan deltaic succession succeeding a fining-upward submarine volcano-mid-up- per submarine fan /s lope succession on lower submarine fan /bas in plain deposits (Fig. 3). The four major depositional systems are further characterized by unique paleocurrent patterns as shown in Fig. 16. A schematic stratigraphic profile of the Ashigara Group is illustrated in Fig. 17. The Ashigara Group is a coarse clastic pile derived mainly from the northern Tanzawa Mountains onto the adjacent narrow shelf, through the submarine slope-submarine fan, to the basin plain during Pliocene to Middle Pleistocene. The evolution of the Ashigara Group near the young arc-arc collision boundary can then be explained by a depositional model given in Fig. 18.

The Ashigara Group began with the deposition of thin sheet turbidite sands and volcaniclastics on a lower submarine fan /bas in plain (Fig. 17). Turbidity currents, which bypassed the northwestern slope to the southeast, turned southwestward on the lower submarine fan/bas in plain. The volcaniclastics were debouched south- westward from the submarine volcano situated to the east along the basin axis. During this stage, subduction of the Philippine Sea plate beneath central Honshu gave rise to the uplift of the Tanzawa Mountains which formed a high source area.

286

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Trough and swell structures trending NW-SE, on the other hand, were initially accompanied by doming intrusions of andesitic and dacitic magma in the swells on the landward slope (Fig. 18). One of the troughs resulted in a submarine canyon-like large recession along the present Sakawa River, which could receive a large amount of coarse clastics from the adjacent Tanzawa Mountains. These horst and graben-like structures and the doming intrusions suggest that the extensional stress field was dominating on the landward slope during their construction. The calc-alkaline magmatism near the convergent margin was probably raised by subduction of the Pacific Plate beneath the Izu-Bonin arc. The submarine canyon and the smaller troughs were then filled with gravelly submarine fan deposits prograded from the NW and volcaniclastics debouched from the submarine volcanoes on the swells.

All these were eventually overlain by muddy slope deposits. The progradation of the muddy slope deposits from the NW buried and levelled off the trough and swell structures and then accomplished an overall fining-upward sequence of submarine volcano-mid-upper submarine fan /s lope systems (Fig. 17). Progradation of the muddy slope deposits suggests the end of active submarine volcanism and bypassing

288

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or ephemeral decline of successive shedding of coarse clastics from the Tanzawa Mountains.

Restoration of more supply of coarse clastics from the Tanzawa Mountains is indicated by the progradation of a gravelly fan delta system onto the shoreline to offshore from the NE (Fig. 16), which established an overall coarsening-upward sequence (Fig. 17). The abrupt increase of schist and diorite clasts from the fan delta system (Fig. 3) suggests the uplift of the Tanzawa Mountains and reach of base level of erosion into the deeper rock facies of the mountains. The uplift of the source area is further represented by coarsening-upward cycles of the proximal fan delta plain deposits. The southwestward progradation of the fan delta system and the shoaling would require the extinction of the previous southeastward slope and initiation of northward tilting of the underlying deposits together with the uplift of the Tanzawa Mountains as the result of suturing. Later, as the suturing continued, the coarse clastic pile of the Ashigara Group was uplifted and deformed, giving rise to the generally northward tilt of the present structures by the subsequent tectonism following the collision of the Izu Peninsula (Izu Block) with the southern flank of the Ashigara Group since Middle Pleistocene.

289

CONCLUSIONS

The Ashigara Group represents a regressive sequence stacked near the Quaternary collision boundary between the Izu Peninsula (Izu Block) and central Honshu during Pliocene to Middle Pleistocene. The regressive sequence was introduced by pro- gradation from lower submarine f a n / b a s i n plain, through submarine volcano-mid-upper submarine fan/slope, to fan delta systems. Evolution of the four major depositional systems responds to the uplift of the Tanzawa Mountains and syndepositional deformation induced by the suturing of the Izu-Bonin arc with central Honshu together with submarine volcanism on the landward slope emplaced by the subducting Pacific plate beneath the Izu-Bonin arc.

Each depositional environment of the Ashigara Group can be found in the Sagami trough, Okinoyama Bank Chain and narrow shelf of the present Sagami-nada Sea (Fig. 1). The Sagami trough is a broad and shallow trench where the northeast- ern tip of the Philippine Sea plate has obliquely subducted northwestward (e.g., Ando, 1974). The Okinoyama Bank Chain is a cluster of fault blocks on the landward slope bounded by normal and /o r strike-slip faults (Kimura, 1976). It can be interpreted as an extensional structure in the convergent margin. Several sub- marine canyons divide the banks along the fault lines. Submarine fans, on the other hand, are developed at the mouth of the canyons (Mogi, 1977). Another peculiar feature of the Okinoyama Bank Chain is that there are several small volcanic cones (ca. 1-2 km in diameter) on the top of the Banks (Kimura, 1976). Along the northwestern coast of the Sagami-nada Sea, gravelly coastal and fluvial sediments are accumulated today.

ACKNOWLEDGEMENTS

I would like to thank Drs. Hiroshi Noda, Tadashi Sato and Fujio Masuda of the University of Tsukuba for their many helpful discussions and guidance. Special thanks are extended to Dr. Harold G. Reading of the University of Oxford and Dr. Andrew D. Miall of the University of Toronto for their critical reading of an early version of the manuscript and instructive comments. I am grateful to Drs. Tokihiko Matsuda and Kazuaki Nakamura of the University of Tokyo and Dr. Yoji Kobayashi of the University of Tsukuba for their constructive discussions. Thanks are also due to Dr. Keith A.W. Crook of the Australian National University and an anonymous reviewer for their valuable suggestions for improvement of the paper.

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