Volcaniclastic sedimentation in a shallow-water marginal basin: the Early Miocene Koura Formation, SW Japan

Sedimentary Geology, 74 (1991) 309-321 Elsevier Science Publishers B.V., Amsterdam

309

Volcaniclastic sedimentation in a shallow-water marginal basin: the Early Miocene Koura Formation, SW Japan Kazuhiko Kano Geology Department, Geological Survey of Japan 1-3 Higashi 1-chome, Tsukuba, lbaraki 305, Japan Accepted for publication May 27, 1991

ABSTRACT Kano, K., 1991. Volcaniclastic sedimentation in a shallow-water marginal basin: the Early Miocene Koura Formation, SW Japan. In: R. Cas and C. Busby-Spera(Editors), Volcaniclastic Sedimentation. Sediment. Geol., 74: 309-321. The Early Miocene Koura Formation, SW Japan is composed mainlyof reworked volcaniclastics and andesite to rhyolite pyroclastics, and constitutes part of the volcaniczone of SW Japan of that time. Reworkedvolcaniclastic sequences are thick and mostly coarse-grained, and based on conventional facies concepts, are interpreted as shoreface to beach deposits, turbidites, and debris-flow deposits. Thick acid tufts intercalated in the volcaniclastic sequences individually show internal features characteristic of either subaerial or subaqueous ash-flowemplacement, or in some cases both. Andesite pyroclastic facies include surge and fall deposits. Facies relationships, together with the fossil record, indicate deposition in a shallow lacustrine environment. The deposition rate of the Koura Formation was probably high enough to rapidly fill the shallow lake, but sedimentation was balanced by rapid subsidence of the lake, similar to lakes present within the modern volcanic zones of Taupo, New Zealand and Hohi, Japan. The Koura Formation grades upward into marine deposits, and may represent the initial volcanism and volcaniclastic sedimentation during the early stages of back-arc or intra-arc rifting.

Introduction Facies and facies models are useful for better understanding of depositional environments and tectonic settings (e.g. Walker, 1984a), although facies models for volcaniclastic and volcanic successions still remain at a developmental stage (Cas and Wright, 1987). Models specific to volcaniclastic sedimentation in marginal basins have been presented by Karig and Moore (1975), Klein (1975), Cas and Jones (1979), Carey and Sigurdsson (1984), Busby-Spera (1984, 1988a, b) among others, and are mainly based on the back-arc rifting or spreading theory for the origin of marginal basins (e.g. Karig, 1971; Chase, 1978; Uyeda and Kanamori, 1979; Tamaki, 1985). There are, however, few models of volcaniclastic sedimentation in shallow-water marginal basins. The purpose of this paper is to describe and discuss the facies and associated volcaniclastic

sedimentation of the Early Miocene Koura Formation, Mihonoseki, Shimane Peninsula, SW Japan (Fig. 1) which is an example of volcaniclastic sedimentation in a shallow-water marginal basin. Though the regional geology and tectonic setting of the Koura Formation are not yet fully understood, some details of the geologic structure, lithology and depositional environment of the formation have been described by Yamauchi et al. (1980), Kano and Yoshida (1985), Kano and Nakano (1985, 1986) and Kano et al. (1989, 1991). Much of the Koura Formation is composed of freshwater to brackish lacustrine deposits yielding warm temperate-type plants and non-marine molluscs such as Corbicula and Fiviparus (e.g. Yamauchi et al., 1980; Kano and Yoshida, 1985; Kano and Nakano, 1985, 1986; Kano et al., 1989, 1991), and grades upward into the marine Josoji Formation (Kano and Yoshida, 1985; Nomura, 1986). The lower part of the Koura Formation is

0037-0738/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

~ll!

not exposed, but probably unconformably overlies pre-Miocene granitic rocks as suggested bv seismic refraction data and by the presence of gravels in the Koura Formation (Kano et al., 1989). The Koura Formation includes a large amount of rhyolite to andesite volcaniclastic rocks and is extensively distributed in an EW-oriented earliest Miocene volcanic zone (Fig. 1). The age is probably between 24-22 and 22-20 Ma (Kano et al., 1991), a time when mainland Japan (Honshu) began splitting from the Asian continent (Otofuji et al., 1985; Tosha and Hamano, 1988) and volcanism suddenly became active in the circumJapan Sea region (Tatsumi et al., 1989), The Koura Formation therefore may represent the initial volcanism and volcaniclastic sedimentation associated with the opening of the Japan Sea back-arc basin. Lithofacies The Koura Formation is weakly to moderately altered to form albite, chlorite, sericite, epidote, quartz and carbonate, but the original textures of constituent rocks are well preserved by pseudomorphs. The formation is exposed in places of the Shimane Peninsula to form cores of gentle anticlines. Though the exposure of the formation is poor due to thick vegetation, it can be best observed in the Mihonoseki district (Fig. 1), where 10 facies, Facies A to J, including volcaniclastic facies, are recognized. The distribution of those facies is somewhat complicated but can be traced by using 5 acid tuff t l - t 5 marker horizons (Figs. 1 and 8). Facies A

Facies A (Figs. 2 and 4A) is characterized by black to dark-brown or greenish dark-gray, massive to horizontally laminated mudstone, shale and siltstone. Parallel, varve-like laminations are not uncommon. Fossils and burrows are sparse and pyrite is common, suggesting reducing conditions at least within the sediments. Facies A is transitional to Facies B and C (Figs. 3 and 8), and probably comprises offshore deposits.

,

Kki~li

Facies B Facies B (Fig. 2) is characterized by thin- to thick-bedded volcaniclastic sandstones and conglomerates interbedded with mudstone, siltstonc and shale, and locally contains sparse plant remains and molluscs. Fine- to medium-grained sandstones interbedded with mudrocks arc thinbedded and massive to normally graded with sharp bases. Medium- to very coarse-grained sandstones are relatively thick-bedded, scour underlying beds and contain the S 1, S 2, S 3 and T t of high-density turbidite divisions of Lowe (1982). Some sandstone beds are entirely wavy or hummocky cross-stratified, and may be cross-stratified divisions of turbidites (Prave and Duke, 1990) or storm deposits (Walker, 1984b; Brenchley, 1989). It is, however, difficult to specify the deposit type of the sandstones because the orientations of wavy or hummocky stratification cannot be fully measured on the outcrops. Conglomerates are poorly sorted, thick-bedded, inversely to normally graded or massive, and sometimes transitional upward to high-density sandstone turbidites (S 1$2). These are features common to debris-flow deposits (Middleton and Hampton, 1973; Lowe, 1982). Facies B is transitional to Facies A, C and D (Figs. 3 and 8), probably forming sublacustrine fans, and may include storm deposits. Conglomerates are channel-fill deposits in the upper fan. Medium- to very coarse-grained sandstones form braided suprafan deposits, and fine- to mediumgrained sandstones are lower fan deposits, as suggested by repeated thinning-upward sequences of Facies B (Fig. 2). Cross-stratification and scour marks suggest that the flow directions of the sediments were from NNE to SSW or from SSW to NNE (Fig. 2). Facies C

Facies C shows the greatest diversity of any lacustrine facies (Figs. 3, 4B, 4C and 4D). This facies includes greenish-gray siltstone and very fine- to medium-grained volcaniclastic sandstone. Strata are parallel-laminated, wavy to crosslaminated, moderately- to well-sorted, burrowed

VOLCAN1CLASTIC

SEDIMENTATION

IN

A SHALLOW-WATER

MARGINAL

311

BASIN

O

o

e~ w

E

o

se.

o

t---

O

0

&

°~

t.a_ O

>.

t~ ffl ,

e'~

~ o ta.l ta..

O ¢.. °~

,

0-

_=

f ~1..

•

. ,..-.

SE

f

° i

.< 111

co

~

m ...a ID °

0

E

~-

°

d

~--=

~=

~°

o

312

--

Erosional contact 5' 'i'"

I~

'?.'i

Parallel stratificatio,~ Cross-stratification

concealed thickness unknown)

[]

I Mudrocks

1

I Sandstone Granule to pebblebearing sandstone Acid tuff

concealed thickness unknown)

N

f . ~

I

f . - -

concealed (thickness unknown)

I

-lOm

ms f.ss m.ss omitted (20-30 m thick)

C.SS

V.C.SS

I__

concealed (4 m thick) I

Fig. 2. Columnar section of Facies B (see Figs. 1 and 8 for loca~ty). The rose diagram in inset indicates flow directions inferred from the cross-stratifications and scour marks in Facies B.

313

VOLCANICLASTIC S E D I M E N T A T I O N IN A S H A L L O W - W A T E R M A R G I N A L BASIN

and sometimes slumped. Wave-generated sediments are indicated by wavy to cross-laminations (including hummocky cross-laminations) and oscillation ripple marks. Channels filled with sandstone turbidites are present, and molluscs and plant remains are not uncommon. This facies is transitional between Facies A and D or Facies B and D (Figs. 3 and 8) and probably consists of shoreface deposits. Facies D

Much of this facies consists of medium- to coarse-grained, parallel- to cross-stratified volcaniclastic sandstones (Figs. 3 and 5). Crossstratification commonly includes the low-angle plane-type, rippled trough-type, and sometimes wavy or hummocky type. Ripple laminations are

often transitional to wavy laminations and to plane parallel- or plane cross-laminations. Burrowing and molluscs are rare but plant remains are common. This facies is similar to Facies C, but it is coarser-grained than Facies C, and is often associated with Facies E (marsh deposits) (Fig. 5), suggesting that it was deposited in upper shoreface to backshore environments. Crest directions of ripples in Facies C and D indicate that waves acted normal to a WNW-ESE direction (Fig. 3), along which the shoreline probably developed. Facies E

Facies E (Fig. 5) is characterized by coal seams, light-gray tuffaceous siltstone, carbonaceous shale, very thin-bedded, very fine- to fine-grained,

[=;=~ Marine black shale r~]

Mudrocks Sandstone Conglomerate

B3

Coal Parallel stratification Wavy stratification

÷

@ f.

--'~Ripple lamination Planar cross-stratification Trough cross-stratification Foraminifera

D

-5m

Mollusca

ilci! i

F-~-~Wood root

. 0~o •0 :

r-~-~ Burrow

• (C).

""

F-~-~Wood trunk

"'

d 0:~:~ : .~5"~

®

Erosional contact

. o.'0. o . ., -0

)

Fig. 3. Columnar section of Facies A, B, C, D, E and F in the topmost part of the Koura Formation (see Figs. 1 and 8 for locality). The rose diagram in inset indicates flow directions and crest orientations of ripples in Facies C and D.

volcaniclastic sandstone. Wood roots and trunks are common. This facies is transitional to Facies D and F and is interpreted as marsh deposits. Facies F

Facies F is characterized by 3 to 15 m thick beds of moderately- to well-sorted, cross- to parallel-stratified, very coarse- to medium-grained volcaniclastic sandstone. Typically the beds form upward-fining sequences: coarser-grained sandstone near the base of the sequence shows trough cross-stratification and grades into finer-grained, parallel- to wavy-stratified or rippled sandstone (Fig. 5). This type of sequence, being associated with Facies E (Fig. 5), is common to fluvial deposits (Walker and Cant, 1984). Thick crossstratified sandstone beds are sometimes associated with Facies B (channel-filling debris-flow deposits, Fig. 3) and Facies D (Fig. 5), and may

have locally formed a wave-dominated delta iacies which was fed by a volcaniclast-dominated alluvial system, similar to those described from the Early Archacan Panorama Formation, Aus-tralia (DiMarco and Lowe, 1989). Facies G

Facies G is characterized by siltstone to coarse-grained sandstone turbidites (Fig. 6) composed of ash and subangular to subrounded glassy andesite fragments. Facies G encloses Facies H, and is intercalated with Facies B or C (Fig. 8). This facies is probably a turbidite facies reworked from Facies H. Facies H

Facies H is composed of repeated andesite lapilli t u f f - t u f f sequences. Individual sequences

Fig. 4. Offshore and shoreface deposits. A. Finely laminated offshore shale (Facies A). B. Parallel- and cross-stratified shoreface sandstone beds (Facies C). C. Wave ripple cross-laminated shoreface sandstone (Facies C). D. Hummocky cross-laminated shoreface sandstone (Facies C). Hammer is 30 cm long.

VOLCAN1CLASTIC SEDIMENTATION IN A SHALLOW-WATER M A R G I N A L BASIN

@

@ ® -Sm

,@

®

N o

t 4 tuff

ms

[ F ~ x ~ f.ss c.ss ,

.

cg

Fig. 5. Columnar section of Facies D, E and F above the t 4 tuff (see Figs. 1 and 8 for locality and Fig. 3 for legend).

consist of glassy andesite lapilli tuff which sometimes scours the underlying bed and typically grades upward into vaguely parallel-laminated fine- to coarse-grained tuff and then into parallel-laminated fine-grained tuff (Fig. 6). Accretionary lapilli up to 1 cm in diameter are sometimes present in the lapilli tuff beds, and glassy andesite clasts are blocky and moderately vesicular. Parallel laminations of fine- to coarsegrained tuff are outlined by parallel alignment of andesite clasts. These features of andesite pyroclastics are probably of surge and fall deposits (Fisher and Schmincke, 1984, pp. 231-264). Facies I and J

Facies I and J are typically represented by acid tufts, t 4 (a, b and c, Fig. 7) and t 5 (d, Fig. 7),

315

respectively. These tufts consist of a lower, poorly sorted mass of ash, pumice and lithic clasts (layer A) and an upper well-stratified layer of ash and pumice shards (layer B), which in turn may be overlain by an extremely fine-grained tuff (layer C, fall-out ash). Lithic clasts become smaller toward the top of layer A and are concentrated near the base, whereas pumice clasts become larger toward the top and form pumice swarms near the top. The upper part of layer A may be indistinctively parallel-stratified as represented by pumice alignment• Load marks are common on the sole and rip-up clasts occur in the basal part. Stratification of layer B is commonly represented by alternating pumice-rich and -poor layers• As documented by Kano (1990), the t 4 tuff (Facies I) commonly has gas-escape pipes, 5-60 cm in diameter and 1-10 m long, which are lined with coarse-grained clasts, are filled with finergrained clasts or both coarse- and fine-grained clasts, and extend nearly vertical toward the top from the basal part of the unit. Layer B exhibits parallel- to cross-stratification similar to those of dunes and U-shaped channels (Fisher, 1977). Layer B and the upper part of layer A also commonly contains armoured lapilli 5 mm to 8 cm in diameter. The t 5 tuff (Facies J) lacks gas-escape pipes, cross-stratification, and armoured lapilli in layer B. It is underlain and overlain by Facies B or C (Fig. 8), and shows features typical of subaqueous ash-flow tuffs (Fiske and Matsuda, 1964; Fisher, 1984; Yamada, 1984). Corbicula swarms are found in layer B, suggesting that this tuff was deposited in shallow water. In contrast with the t 5 tuff, the t 4 tuff is underlain by Facies C or B, and overlain by Facies D, C or B at locations a, b and c (Fig. 8). Most of the t 4 tuft resembles subaerial ash-flow tuff (e.g. Sparks et al., 1973; Fisher, 1979) but close to the base it resembles subaqueous ash-flow tuff. Kano (1990) suggested that the t 4 tuff advanced across a shallow-water body while pushing water aside and becoming wet. Lapilli were plastered with wet ash. Water incorporated into the ash flow was heated to steam and expanded the flow to form secondary surges, with the highly pressurized steam spouting to form pipes. The

@

® Fine-grained tuff

Bluish grey siltstone

Fine- to coarsegrained t u f f

01ive green fine-grained sandstone with parallel to wavy lamination

Lapilli tuff Im

[o

lm

Normally graded, pale green fine- to coarse-grained sandstone

I

0 Fig. 6. Typical columnar sections of Facies G and H.

long axes of U-shaped channels and channel-filling cross-stratification indicate that the t 4 tuff flowed from east to west (Kano, 1990)• To the west, the t4 tuff gradually loses gas-escape pipes,

armoured lapilli and the surge-like bedforms of layer B (Fig. 7), and therefore may grade into subaqueous ash-flow tuff of Facies J. A similar facies change is observed for the t 3 tuff, which

@

©

@

Layer • • ° ,% • _ .,u ~ , _

,..:•

.....

C

m

k " ,';" a &

--Pumice •

: "_"_,± ¢

Layer

A

,

"':

,o

b

t

~.'~

)

oa • , b: •

,

\

Q

t4

0

~

Crossstratification

~

Pumice

•

o • .e

~

, 4

• I

: IU,,J -~ ;il;"-'"

i

•

10 c m

~

o

,

0 II

o

e

-~

"

II " ii

~

i

~11" • I;

rmored

Gas-escape pipe

~.!1 • • h• • ,jr •

10 {~ ma x

arbonized wood lapilli

Q I,, •

~.(1: o

ip-up clast

t hic--k

' 1.0,/%

ithic

Q

i

.°~ ~ v°,.

clast ¢last

10 rn

I,.,

--

Parallel stratification

°

eQo

•

~

.I

---....

p

- t'--':

.

®

°.

Oc •

i"

t5

I° l/

Q

0

0

cast

11, 0

lOem

t4

Q

lOom

['~

corbicula

Mean diameter of ~max I0 largest pumice and lithic clasts

t4

Q Fig. 7. Typical columnar sections of Facies I, t 4 tuff and Facies J, t 5 tuff, compiled after Kano and Nakano (1985) and Kano (1990) (see Figs. 1 and 8 for locality).

VOLCANICLAST1C SEDIMENTATION

IN A S H A L L O W - W A T E R

317

MARGINAL BASIN

it"

West

@ I

® I

@ I

@ I .9.3

@ I

East

@ I

"-

-'9

o

ts

t4 t3 ©

8

O

LKm

~

- 6(~) : Column no.

(see F i g .

I for

1oca1~ty)

Fig. 8. An EW-oriented schematic cross-section of the Koura Formation showing the distribution of Facies A to J.

flowed from east to west as indicated by crossstratification (Kano and Nakano, 1985). The flow directions of ash-flow tufts t l , t 2 and t 5 are not known. Discussion The following model of the Koura Formation, Mihonoseki district (Fig. 9) is based on an interpretation of the distribution and paleocurrent directions of Facies A to J (Figs. 2, 3 and 8). The Koura Formation was deposited in a shallow lake of an EW-trending depression over 15 km wide and over 70 km long. The shorelines were in a WNW-ESE direction, and surrounding the lake there were marshes, alluvial systems and volcanoes possibly of a composite type. Coarse-grained volcaniclastic sediments were deposited in shoreface to backshore environments (Fig. 8). As observed for tectonically formed large lakes (e.g. Link and Osborn, 1978), but not for small lakes

such as crater lakes (Smith, 1986), sublacustrine fan deposits, which are also composed mainly of volcaniclastics, developed on and off the volcaniclastic shore deposits (Fig. 8). These volcaniclastic deposits make up more than 60 vol.% of the Koura Formation, derived mainly from rhyolite and andesite pyroclastics. They were transported into the lake through alluvial systems from the deposits of surrounding volcanoes, possibly in a manner similar to the cases of the Deshutes Formation (Smith, 1987) and the Puye Formation, New Mexico (Waresback and Turbeville, 1990) where volcaniclast-dominated alluvial systems developed with volcanogenic-alluvial fans (Waresback and Turbeville, 1990), alluvial plains and fluvial channels. Ash flows intermittently came down from adjacent volcanoes, and were emplaced in and around the lake. Ash falls accompanying these ash flows may have been common but were probably extensively reworked. Andesite hydroclastic eruptions

Coarse-grained volcaniclastic deposits pl(~graded repeatedly in association with emplacement of ash-flow tufts and andesite surge and fall deposits (Fig. 8). ]'his implies that voluminous volcaniclastics flowed into the lake, mainly during and immediately after episodic explosive eruptions, and volcaniclastic sediment supply waned as a result of volcanic quiescence. According to its age interval (possibly 2 m.y.: Kano et al., 1991), thickness (thicker than 700-800 m: Kano and Yoshida, 1985; Kano and Nakano, It,~85) and number of ash flows (at least 5), the deposition rate of the Koura Formation was larger than 3 - 4 x 10 4 m / y and major pyroclastic eruptions occurred possibly at a mean interval shorter than 4 × 105 yr. Voluminous volcaniclastic supply repeatedly occurred and must have been large enough to fill the shallow lake in a short time. However, the lake was not filled, probably due to

repeatedly occurred on the lakeshorc, in the backshore marshy area or in the shallow lake, resulting in thick accumulation of surge and fall deposits, with andesite volcaniclastic turbidites being deposited around the deposits. Waterchilled andesite lavas less commonly occur in the Koura Formation in the area adjacent to the Mihonoseki district (Kano and Yoshida, 1985), and provide evidence for sublacustrine andesite volcanism. The t 5 tuff, on which in situ swarms of Corbicula occur, is about 15 m thick, suggesting that the lake was somewhat deeper than 15 m where turbidity currents occurred. Thick beds with wave-generated sedimentary structures, which are not common in shallow lacustrine environments, may possibly have been deposited when Tsunamis were induced by inflow of pyroclastic or volcaniclastic debris into the shallow lake or by large earthquakes.

\

A1luvlal plain (not deposits preserved)

~Upper shorefaceto . deposits "":.__~_~.. backshore sits

~~ " " L -

•

Shorefacedeposits

-)ll,

Fluvial., deposits

: Marshdeposits

....-

%

L . , L ,L

(~

/

P

J explosion

Alluvia] fan (not deposits preserved)

\ Sublacustrine and channel deposits Offshore deposits X

®

®

/

:.

\ Ash-flowQdeposit/

Andesi volcaniclastic turbidites

@

@ r

Andesitesurge and fall deposits

r~/~ -

.... . . . . . . . :.,....• ...-.%

/

. . " -"" " . ' ' ' . . . .

"'- " • "• " ' "

Fig. 9. F a c i e s o r g a n i z a t i o n a n d i n f e r r e d d e p o s i t i o n a l p a l a e o e n v i r o n m e n t s o f t h e K o u r a F o r m a t i o n , M i h o n o s e k i d i s t r i c t (see text f o r facies abbreviations).

VOLCANICLASTIC SEDIMENTATION IN A SHALLOW-WATERMARGINALBASIN

subsidence at a rate nearly equal to the deposition rate. Similar environments are present in the Taupo Volcanic Zone, New Zealand (Cole, 1979; Cole and Lewis, 1981; Stern, 1985) and the Hohi Volcanic Zone, Japan (Kamata, 1989). Both the Taupo and Hohi volcanic zones are back-arc or intra-arc rifts still now subsiding and which are coupled with active andesite to rhyolite volcanism. According to the data of Wilson et al. (1984) for the Taupo Volcanic Zone and Kamata (1989) for the Hohi Volcanic Zone, the volcanic zones have subsided at a rate of 10-4-10 -3 m/y. Thick accumulations of lavas and volcaniclastics are present, with volcaniclastic sedimentation occurring in shallow-water to subaerial environments (Martin, 1961; Tamanyu, 1985). In these volcanic zones, major ash flows have been repeatedly generated at an interval of 1-2 × 105 yr (Wilson et al., 1984; Kamata, 1989). The facies model presented here and the deposition rate, together with the geologic setting suggest that the Koura Formation was deposited in an EW-trending depression where rapid subsidence and active volcanism occurred, probably similar to the Taupo and Hohi volcanic zones. In situations where volcanism and volcaniclastic sedimentation are coupled with a rapid sealevel rise or more enhanced subsidence, the style of volcanism and volcaniclastic sedimentation may change to those typical of marine environments (Fisher, 1984; Carey and Sigurdsson, 1984; Cas and Wright, 1987, pp. 435-440). In the Shimane Peninsula, this can be observed in the marine Josoji Formation that overlies the Koura Formation: acid volcanism was still active and subsidence continued resulting in accumulation of thick subaqueous lavas, subaqueous ash-flow tufts and volcaniclastic sediment-gravity flow deposits (Kano and Yoshida, 1985; Kano and Nakano, 1985, 1986; Kano et al., 1989, 1991).

Conclusions (1) The Early Miocene Koura Formation in the Mihonoseki district, Shimane Peninsula, is a shallow lacustrine formation composed mainly of

319

volcaniclastic shoreface to beach and sublacustrine fan deposits, subaerial to subaqueous acid ash-flow tufts, and andesite surge and fall deposits. (2) The lake of the Koura Formation occupied an EW-trending depression within the volcanic zone of that time. It subsided rapidly in association with active volcanism, but remained shallow due to rapid inflow of voluminous volcaniclastics, subaerial to subaqueous ash-flow tuffs and hydroclastic eruption products. (3) The Koura Formation grades upward into marine deposits, and may represent the early stages of rifting in this volcanic zone.

Acknowledgements I wish to thank R.A.F. Cas, B.N. Turbeville, R.M. Easton, F. Masuda and G.J. Orton for their critical reading of this manuscript, and F. Yoshida, K. Yanagisawa, S. Nakano, K. Takeuchi and T. Yamamoto for their help in the field survey and instructive discussions.

References Brenchley, P.J., 1989. Storm sedimentation. Geology Today, July-August, pp. 133-137. Busby-Spera, C.J., 1984. The lower continental margin and marine intra-arc sedimentation at Mineral King, California. In: J.K. Crouch and S.B. Backman (Editors), Tectonics and sedimentation along the California Margin. Pacific Section, Soc. Econ. Paleontol. Mineral., pp. 135-156. Busby-Spera, C.J., 1988a. Evolution of a Middle Jurassic back-arc basin, Cedros Island, Baja California: evidence from a marine volcanic apron. Geol. Soc. Am. Bull., 100: 218-233. Busby-Spera, C.J., 1988b. Speculative tectonic model for the early Mesozoic arc of the southwest Cordilleran United States. Geology, 16: 1121-1125. Carey, S.N. and Sigurdsson, H., 1984. A model of volcanogenic sedimentation in marginal basins. In: B.P. Kokelaar and M.F. Howells (Editors), Marginal Basin Geology. Geol. Soc. London, pp. 37-58. Cas, R.A.F. and Jones, J.G., 1979. Paleozoic interarc basin in eastern Australia and a modern New Zealand analogue. N.Z.J. Geol. Geophys., 22: 71-85. Cas, R.A.F. and Wright, J.V., 1987. Volcanic Successions. Allen Unwin Ltd., London, 528 pp. Chase, C.G., 1978. Extension behind island arcs and motions relative to hot spots. J. Geophys. Res., 83: 5385-5387.

Cole, J.W., 1979. Structure, petrology, and genesis of Cenozoic volcanism, Taupo Volcanic Zone, New Zealand---a review. N.Z.J. Geol. Geophys., 22: 631-657. Cole. J.W. and Lewis, K.B., 1981. Evolution of the TaupoHikurangi subduction system. Tectonophysics. 72: 1-21. DiMarco, M.J. and Lowe, D.R., 1989. Shallow-water volcaniclastic deposition in the Early Archean Panorama Formation, Warrawoona Group, eastern Pilbara Block. Western Australia. Sediment. Geol., 64: 43-63. Fisher, R.V., 1977. Erosion by volcanic base-surge density currents: U-shaped channels. Geol. Soc. Am. Bull., 88: t 287-1297. Fisher, R.V., 1979. Models of pyroclastic surges and pyroclastic flows. J. Volcanol. Geotherm. Res.. 6:305-318 Fisher, R.V., 1984. Submarine volcaniclastic rocks. In: B.P. Kokelaar and M.F. Howells (Editors), Marginal Basin Geology, Geol. Soc. London, pp. 5-27. Fisher, R.V. and Schmincke, H.-U., 1984. Pyroclastic Rocks. Springer-Verlag, Berlin, 472 pp. Fiske, R.S. and Matsuda, T., 1964. Submarine equivalents of ash flows in the Tokiwa Formation, Japan. Am. J. Sci., 262: 76-106. Kamata, H., 1989. Volcanic and structural history of the Hohi volcanic zone, central Kyushu, Japan. Bull. Volcanol., 5l: 315-332. Kano, K., 1990. An ash-flow tuff emplaced in shallow water, Early Miocene Koura Formation, southwest Japan. J. Volcanol. Geotherm. Res., 40: 1-9. Kano, K. and Nakano, S., 1985. Geology of the Mihonoseki district (with geological sheet map at 1:50,000). Geol. Surv. Japan, 28 pp. (in Japanese with English abstract). Kano, K. and Nakano, S., 1986. Geology of the Etomo district (with geological sheet map at 1 : 50,000). Geol. Surv. Japan, 30 pp. (in Japanese with English abstract). Kano, K. and Yoshida, F., 1985. Geology of the Sakaiminato district (with geological sheet map at 1:50,000). Geol. Surv. Japan, 57 pp. (in Japanese with English abstract). Kano, K., Takeuchi, K., Oshima, K. and Bunno, M., 1989. Geology of the Taisha district (with geological sheet map at 1:50,000). Geol. Surv. Japan, 58 pp. (in Japanese with English abstract). Kano, K., Takeuchi, K. and Matsuura, H., 1991. Geology of the Imaichi district (with geological sheet map at 1:50,000). Geol. Surv. Japan, 80 pp. (in Japanese with English abstract). Karig, D.E., 1971. Origin and development of marginal basins in the western Pacific. J. Geophys. Res., 75: 239-255. Karig, D.E. and Moore, G.F., 1975. Tectonically controlled sedimentation in marginal basins. Earth Planet. Sci. Lett., 26: 233-238. Klein, G., 1975. Sedimentary tectonics in Southwest marginal basins based on leg 30 Deep Sea Drilling cores from the South Fiji, Hebrides, and Coral Sea Basin. Geol. Soc. Am. Bull., 86: 1012-1018.

Link, M.t|. and Osborn, R.H., 1978. Lacustrine facies in tl~e Plioeene Ridge Basin Group: Ridge Basin,( alifornia, h~: A. Matter and M.E. Tucker (Editors), Modern and A n eient Lake Sediments, Spec. Publ. Int. Assoc. Scdimentol., 2: 169..-187. Lowe, D.R., 1982. Sediment gravity flows. Depositional models with special reference to the deposits of high-densily turbidity currents, J. Sediment. Petrol., 52: 279-297. Martin, R.C., 1961. Stratigraphy and structural outline of the Taupo Volcanic Zone. N.Z.J. Geol. Geophys., 4:449-478. Middleton, G.V. and Hampton, M.A., 1973. Sediment gravity flows: mechanics of flow and deposition. In: G.V. Middleton and A.H. Bouma (Editors), Turbidites and Deep-water Sedimentation, Short Course, Pacific Section Soc. Econ. Paleontol. Mineral., May 1973, pp. 1-38. Nomura, R., 1986. Geology of the central part in the Shimane Peninsula, Part I. Stratigraphy. J. Geol. Soc. Jpn., 92: 405-420 (in Japanese with English abstract). Otofuji, Y., Matsuda, T. and Nohda, S., 1985. Paleomagnetic evidence for the Miocene counter-clockwise rotation of Northeast Japan--rifting process of the Japan Arc. Earth Planet. Sci. Lett., 75: 265-277. Prave, A.R. and Duke, W.L., 19911. Small-scale hummocky cross-stratification in turbidites: a form of antidune stratification? Sedimentology, 37:531-539. Smith, G.A., 1987. The influence of explosive volcanism on fluvial sedimentation: the Deschutes Formation (Neogene) in central Oregon. J. Sediment. Petrol., 57: 613-629. Smith, R.M.H., 1986. Sedimentation and palaeoenvironments of Late Cretaceous crater-lake deposits in Bushmanland, South Africa. Sedimentology, 33: 369-386. Sparks, R.S.J., Self, S. and Walker, G.P.L., 1973. Products of ignimbrite eruption. Geology, t: 115 118. Stern, T.A., 1985. A back-arc basin formed within continental lithosphere: the central volcanic region of New Zealand. Tectonophysics, 112: 385-409. Tamaki, K., 1985. Two modes of back-arc spreading. Geology, 13: 475-478. Tamanyu, S., 1985. Stratigraphy and geologic structures of the Hohi geothermal area, based mainly on the bore hole data. Rep. Geol. Surv. Jpn., 264:115-142 (in Japanese with English abstract). Tatsumi, Y., Otofuji, Y., Matsuda, T. and Nohda, S., 1989. Opening of the Sea of Japan back-arc basin by asthenospheric injection. Tectonophysics, 166: 317-329. Tosha, T. and Hamano, Y., 1988. Paleomagnetism Tertiary rocks front the Oga Peninsula and the rotation of Northeast Japan. Tectonics, 7: 653-662. Uyeda, S. and Kanamori, H., 1979. Back-arc opening and the mode of subduction. J. Geophys. Res., 84: 1049-1062. Walker, R.G., 1984a. General introduction: facies, facies sequences and facies models. In: R.G. Walker (Editor), Facies Models. Geosci. Can. Reprint Ser., 1 : 1 - 9 (2nd ed.).

VOLCANICLASTIC SEDIMENTATION IN A SHALLOW-WATER M A R G I N A L BASIN

Walker, R.G., 1984b. Shelf and shallow marine sands. In: R.G. Walker (Editor), Facies Models. Geosci. Can. Reprint Ser., 1:71-89 (2nd ed.). Walker, R.G. and Cant, D.J., 1984. Sandy fluidal systems. In: R.G. Walker (Editor), Facies Models. Geosci. Can. Reprint Ser., 1:71-89 (2nd ed.). Waresback, D.B. and Turbeville, B.N., 1990. Evolution of a Plio-Pleistocene volcanogenic-alluvial fan: the Puye Formation, Jemez Mountains, New Mexico. Geol. Soc. Am. Bull., 102: 298-314. Wilson, C.J.N., Rogan, A.M. and Smith, I.E.M., 1984. Caldera

321

Volcanoes of the Taupo volcanic zone, New Zealand. J. Geophys. Res., 89, B10: 8463-8484. Yamada, E., 1984. Subaqueous pyroclastic flows: their development and their deposits. In: B.P. Kokelaar and M F. Howells (Editors), Marginal Basin Geology. Geol. Soc. London, pp. 29-35. Yamauchi, S., Mitsunashi, T. and Yamamoto Y., 1980. Miocene of the Shimane Peninsula. A field guide book presented at 87th annual meeting of the Geological Society of Japan, 39 pp. (in Japanese).