Deep Sea Drilling Project Initial Reports Volume 87 · TURBIDITE SEDIMENTATION, NANKAI TROUGH 1000...

12. TURBIDITE SEDIMENTATION IN THE NANKAI TROUGH AS INTERPRETED FROMMAGNETIC FABRIC, GRAIN SIZE, AND DETRITAL MODAL ANALYSES1

Asahiko Taira, Department of Geology, Kochi Universityand

Nobuaki Niitsuma, Institute of Geosciences, Shizuoka University2

ABSTRACT

At Site 582, DSDP Leg 87, turbidites ~ 560 m thick were recovered from the floor of the Nankai Trough. A turbiditebed is typically composed of three subdivisions: a lower graded sand unit, an upper massive silt unit, and an uppermostChondrites burrowed silt unit. The turbidites intercalate with bluish gray hemipelagic mud which apparently accumu-lated below the calcite compensation depth. In order to investigate the nature and provenance of the turbidites, we stud-ied the grain orientation, based on magnetic fabric measurements and thin-section grain counting, and grain size, usinga photo-extinction settling tube and detrital modal analysis.

The following results were obtained: (1) grain orientation analysis indicates that the turbidity current transport par-allels the trench axis, predominantly from the northeast; (2) Nankai Trough turbidites generally decrease in grain size tothe southwest; (3) turbidite sands include skeletal remains indicative of fresh-water and shallow-marine environments;and (4) turbidites contain abundant volcanic components, and their composition is analogous to the sediments of theFuji River-Suruga Bay area. Considering other evidence, such as physiography and geometry of trench fill, we concludethat the turbidites of Site 582 as well as Site 583 were derived predominantly from the mouth of Fuji River and weretransported through the Suruga Trough to the Nankai Trough, a distance of some 700 km. This turbidite transport sys-tem has tectonic implications: (1) the filling of the Nankai Trough is the direct consequence of the Izu collision in Plio-cene-Pleistocene times; (2) the accretion of trench fill at the trench inner slope observed in the Nankai Trough is con-trolled by collision tectonics; and (3) each event of turbidite deposition may be related to a Tokai mega-earthquake.

INTRODUCTION

The Nankai Trough is a unique trench for several rea-sons (Sugimura, 1978): (1) it is shallow with a maximumdepth less than 5000 m; (2) the Wadati-Benioff Zone isvague and short in length; (3) heat flow is high; and(4) the volcanic arc is poorly developed in southwesternJapan. The last three of these characteristics may be re-lated to the geologic youth of the subducting plate, whichconsists of the Oligocene-Miocene Shikoku Basin (Ka-gami, et al., 1983). The shallowness of the trench is di-rectly related to the thick sequence of sediments it con-tains, reaching a maximum thickness of 3000 m. The ac-tive accretion of sediments at the Nankai Trough, asdelineated from seismic reflection profiles (site chapter,Site 583, this volume), is probably controlled by thisthick sedimentary body, providing a valuable opportu-nity to study both the mechanism and the depositionalfades of trench sedimentation.

During Leg 31, Site 298 was drilled into the toe of theNankai accretionary prism. Unfortunately, core recov-ery was too poor to study the nature of trench sedimen-tation and no conclusive evidence was obtained regard-ing the transport and provenance of turbidite sands.

Kagami, H., Karig, D. E., Coulbourn, W. T., et al., Init. Repts. DSDP, 87: Washing-ton OJ S Govt. Printing Office).

•2 Addresses: (Taira, present address) Ocean Research Institute, University of Tokyo,Tokyo 164, Japan; (Niitsuma) Institute of Geosciences, Shizuoka University, Shizuoka 422,Japan.

During Leg 87, 11 holes were drilled off Shikoku bothin the undeformed and in the deformed trench sediments(Fig. 1). The recovered sediments consist of two units:an upper turbidite unit (Unit 1) and a lower hemipelag-ite unit (Unit 2). Unit 1, which is 500 m thick, is com-posed of interbedded sandstone and shale with turbiditesedimentary structures. The purpose of this study is toinvestigate the paleocurrent, detrital mode, and grain-size distribution of turbidite deposits in both Units 1and 2 in order to clarify the provenance, depositionalfacies, and mechanism of turbidite sedimentation in thistrench environment.

ANALYTICAL TECHNIQUES

Sampling

On board the Glomar Challenger, the working halves of cores weresampled using 2.2-cm3 plastic cases for magnetic analyses. Undisturbedparts of the core were sampled, but some disturbed parts were alsosampled for comparison. Routine 5-10-cm3 samples were also ob-tained for petrologic and grain-size analyses. In this study, about 500other samples from the area adjacent to the Nankai Trough were alsoexamined for grain size and provenance (see Fig. 1).

Magnetic Fabric Analysis

The magnetic susceptibility of most sediment is anisotropic be-cause of the preferred orientation of ferromagnetic minerals. Anisot-ropy of susceptibility in magnetite and titanomagnetite is related tograin shape; the maximum susceptibility is aligned in the direction ofthe longest axis, and the minimum susceptibility is oriented along theshortest axis. Therefore, sedimentary rocks usually contain a resultantsusceptibility anisotropy that may be approximated by a second-ordertensor; its main axes represented by Amax (maximum susceptibility),Kint (intermediate susceptibility), and Kvnm. (minimum susceptibility)(Rees, 1965; Taira and Lienert, 1979).

611

A. TAIRA, N. NIITSUMA

Sagami Trough

Izu Peninsula

Shiono-misaki Canyon/

Figure 1. Location map for DSDP sites and reference samples. Shaded area indicates the area from which more than 500samples were recovered. Dots locate piston core and grab samples.

Magnetic susceptibility was measured using a ring-core type flux-gate spinner magnetometer at Shizuoka University (Koyama and Niit-suma, 1983). The applied magnetic field was 25 µT. The shape of theanisotropy ellipsoid is represented by

Qn = - l o g :

(Kmax - Kint)(Kint - Kmin)

The ellipsoid is more oblate when qn is positive and more prolate whenqn is negative.

Settling Tube AnalysisPhoto-extinction settling tube analysis was used for grain-size anal-

ysis. The method has several advantages over other grain-size meth-ods, especially in its accuracy and precision. For DSDP samples, whichusually do not have substantial volume, this method is advantageousbecause it generally requires less than 2 g. The settling tube at KochiUniversity is 20 cm in diameter and 200 cm in settling distance. Theprinciple is essentially the same as described previously (Niitsuma,1971; Taira and Scholle, 1977). In this study, the analysis was limitedonly to the coarser sediment fraction, the lower limit being log V =-0.8 (V = settling velocity in cm/s, about 4.5 phi).

Petrographic Analysis

About 50 of the plastic cube samples used for magnetic fabric anal-ysis were impregnated in polyester resin, and thin sections were madefor petrographic and grain-orientation analysis. At the same time, thesieved coarse fraction (> 75 µm) was examined using thin sections anda binocular microscope. Immersion of the coarse fraction in water re-

veals various surface grain features such as color, surface morphology,and abrasion characteristics which were useful for provenance study.Grain point-counting was employed for petrographic analyses of thinsections of DSDP and other reference samples.

PHYSIOGRAPHY AND BACKGROUNDGEOLOGY

Physiography

The Nankai Trough is a shallow trench and is the plateboundary between the Philippine Sea and Eurasianplates. As pointed out by Sugimura (1972) and Naka-mura and Shimazaki (1981), the plate boundary extendsnorthward to the Suruga Trough and then onshore atthe mouth of the Fuji River. It then continues to the Sa-gami Trough (Fig. 1).

The landward side of the trench displays a series ofwell-developed outer-ridge and forearc basins (Fig. 2).Two conspicuous canyons, the Tenryu and Siono-misakicanyons, cut into the outer ridge down to the troughfloor. Near the DSDP sites, the Ashizuri Canyon reach-es the southwestern part of Shikoku and may provideone avenue for sediment yield to the DSDP sites. Over-all, the submarine physiography suggests that most ofthe sediment in the Nankai Trough is provided along the

612

TURBIDITE SEDIMENTATION, NANKAI TROUGH

10002000

Suruga Trough

.Tenryu Canyon

2000

3000

2000

Figure 2. Submarine topography of the Nankai Trough and adjacent area (after Mogi, 1977).

axis of the trough, namely from northeast to southwest.The reasons are summarized below.

1. The Nankai-Suruga trough system is connected di-rectly to the highest mountain ranges in Japan includingthe Akaishi Mountains and Mt. Fuji.

2. The drainage from these mountains flows directlyinto the Suruga Trough without forming large coastalplains.

3. The trough becomes markedly deeper and widertoward the southwest.

4. Longitudinal channels follow the floor of thetrough (Mogi, 1977).

Background Geology

Subduction began at least as long ago as the Jurassictime and is responsible for the formation of the south-western Japanese "basement" geology (Taira et al., 1983).Several geologic terranes, referred to as "belts" in Japa-nese terminology, are subparallel to the Nankai Trough.These belts are conspicuous and are classified into aninner and an outer zone, which are separated by MedianTectonic Line (Fig. 3).

The outer zone geology, which is closely related tothe provenance of Nankai Trough elastics, is composedof three belts, the Sambagawa, Chichibu, and Shimantobelts. The Sambagawa Belt is made of high-pressure, met-amorphic rocks probably representing a part of a Jurassicsubduction complex. The Chichibu Belt is a heteroge-neous geologic terrane, including a Jurassic subductioncomplex and a strike-slip mobile zone with exotic tectonicblocks. The dominant lithology is sandstone, slate, chert,limestone, and metabasalt. The Shimanto Belt represents

a Cretaceous to Miocene subduction complex composedof sandstone, shale, chert, and metabasalt.

Neogene "post-Shimanto" sediments are limited to afew areas including the Miyazaki Group in Kyushu, theTanabe-Kumano Group in the Kii Peninsula, and thickclastic sedimentary rocks in the southern Fossa Magnaregion.

The distribution of volcanic rocks seems to be a keyto the evaluation of provenance. Tertiary-Quaternary vol-canic rocks occur only in the Kyushu and Fossa Magna-Izu regions. Felsic to intermediate-type explosive activ-ity represents the Kyushu volcanics, whereas mafic tointermediate volcanics are dominant in central Japan.

Neogene and Quaternary accretionary-complex andforearc-slope basin fill compose the offshore geology(Okuda, 1977; Taira, in press). Upper Pliocene sedi-ments are widespread in the upper to lower slopes of theNankai Trough region.

LITHOLOGY

Lithology and Sedimentary Structures, Unit 1

In the upper 300 m of Site 582 (Figs. 4 and 5), sedi-mentary structures are poorly preserved because of drill-ing deformation. The sequence below 300 m sub-bottomdepth shows various types of sedimentary structures char-acteristic of turbidite sedimentation. Examples of turbi-dite beds are shown in Figures 6 and 7.

The sequence comprises four divisions: Tl, T2, T3,and H. The Tl division is composed of massive to grad-ed sand. Sands are generally poorly sorted with mud con-tent exceeding 10%. Grain size ranges from very coarse

613


(Mainly Jurassicsubduction complex

£"| Neogene to Quaternaryfelsic volcanic rocksNeogene to Quaternary mafic tointermediate volcanic rocks

SSSiSil Chichibu Belt

Sambagawa Belt

Figure 3. Map showing the major features of background geology related to the provenance of Nankai Trough turbidites. Lines numbered from 1 to24 are topographic cross sections shown in Figure 23. Submarine contours are in meters.

to very fine grained sand but occasionally gravels werefound. Pumice, plant fragments (including stems of landplants), and abraded gravels of siltstone were found inthis division together with minor amounts of shells, echi-noderms, and foraminiferal tests. The T2 division is fin-er grained (very fine sand to silt) and usually structure-less, and parallel laminations are recognized occasion-ally in this part. In the Nankai Trough turbidites ingeneral, ripple laminations are rare. The microfossil con-tent increases uphole, especially calcareous nannoplank-ton which tend to concentrate at the uppermost part ofthis division, and consequently calcium carbonate con-tent also increases. Fresh-water diatoms also occur. Theseobservations indicate that Site 582 sands were emplacedfrom near-shore environments possibly very close to ariver mouth. The T3 division is composed of silt withabundant Chondrites burrows. Burrows are 1-2 mm indiameter and darker in color, being filled with slightlyfiner-grained sediments. The H division is composed of

blue greenish mud with fissility developed at the lowerpart of the Site 582 section. The mud is mottled by bur-rowing organisms. Smear-slide observations suggest thatmicrofossils are scarce in this division. Calcareous nan-noplankton are virtually absent, which is also indicatedby low calcium carbonate content. We attribute this con-trasting lithology between T3 and H division to the dis-solution of calcium carbonate. The H division is inferredas a "background" of slow accumulation of hemipelag-ic mud below the calcite compensation depth (CCD).

Shipboard measurements of carbon content within aturbidite bed reveal that total carbon content decreasesuphole whereas the organic carbon content stays essen-tially the same (Fig. 6). The trend of total carbon con-tent thus reflects the uphole decrease of carbonate con-tent as described above.

At Site 583, hydraulic piston coring revealed detailedsedimentary structures in a thinly bedded part of the up-per part of Unit 1. The turbidite beds observed are dif-

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ferent from the lower part of Unit 1 of Site 582. Asshown in Figure 8, the sequence is composed typicallyof three divisions: T2', T3', and H. The T2' division iscomposed of dark olive gray silt and very-fine-grainedsand with parallel lamination. The T3' unit shows mas-sive bedding of silty clay which contains microfossils.The H division is quite similar to the H division of theSite 582 sequence. The T2' division can be correlated tothe upper part of the T2 division, and the T3' divisionto the T3 division of the Site 582 sequence. Overall li-thology suggests that the upper 50 m of the Site 583 se-quence was deposited by turbidity currents of lower con-centration and lower flow-energy.

Lithology and Sedimentary Structures, Unit 2

Greenish blue hemipelagic mudstone is the dominantlithology encountered within Unit 2 at Site 582. Micro-fossils are generally less abundant compared with theturbidites of Unit 1. However, local concentration of cal-careous fossils indicates that the hemipelagite was de-posited slightly above the CCD. Other features such assedimentary structures are essentially the same as the Hdivision of Unit 1. Planolites and Zoophycos burrowsare abundant with ash-layer intercalations. Organic mat-ter is concentrated in thin laminated layers. Unit 2 wasoriginally deposited within the Shikoku Basin and thenbrought to the present position by plate movement.

Bed-Thickness Distribution

The bed-thickness distribution of turbidite sequenceswas examined for the intervals of relatively good recov-

ery and preservation. At Site 582, the interval betweenCores 582B-22 to 582B-44 was selected and at Site 583,Cores 583A-1 to 583A-3 were selected. Figure 9 showshistograms for turbidite bed thickness and hemipelagite(division H) bed thickness distributions. The thickest tur-bidite bed observed is 105 cm, and the mean value is30.2 cm. The mean thickness of the hemipelagite inter-calations is 6.5 cm with maximum thickness of 20 cm.In Hole 583A, the maximum thickness of the turbiditebeds is 16 cm and the mean value is about 7 cm.

Examination of bed-thickness distribution using a log-probability plot indicates that both selected intervals atSites 582 and 583 show a log-normal distribution. Log-normality of turbidite bed thickness has been reportedpreviously (e.g., Niitsuma, 1974), and several examplesare shown for comparison (Fig. 10). Examples shownfor comparison are Pleistocene turbidites of the Koku-moto Formation in the Boso Peninsula (distal channel-levee association) and Eocene turbidites of the Shiman-to Belt in Shikoku (both proximal and distal channel-levee-overbank association). This comparison suggeststhat Site 582 turbidites are similar to distal channel-levee-overbank association of ancient examples in their bedthickness distribution. On the other hand, the upper50 m of Site 583 are more closely related to overbanksettings only.

Sedimentation Rate and Frequency of TurbiditeDeposition

The sediment accumulation rate at Site 582 was esti-mated based on paleontologic and magnetostratigraphic

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evidence (Fig. 11). The rate shows a remarkable varia-tion. Within the upper part of Unit 1 (0-90 m sub-bot-tom depth), sediment accumulated at a rate of 200 m/Ma, whereas in the lower part (90-560 m), it accumu-lated at a rate of 2000 m/Ma. This second value is com-parable to the Pleistocene sedimentation of alluvial plainsin Japan, an unusually high value for deep-sea sedimen-tation.

Assuming that turbidites were emplaced more or lessinstantaneously, we can infer that hemipelagite accumu-lation has taken up most of the time needed for sedi-ment accumulation. If we take the mean values of 30.2and 6.5 cm as representative thicknesses of turbidite andhemipelagite, respectively, each 6.5-cm-thick hemipela-gite accumulation in the lower part of Unit 1 representsabout 200 yr. In the lower part of Unit 1, therefore, anaverage of 30 cm of turbidite was emplaced every 200 yr.Given that the average recurrence time of great historicearthquakes off southwestern Japan is on the order of100 to 200 yr., a relationship between turbidite emplace-ment and major seismic events is likely.

Summary of Turbidite Facies

1. Sediment drilled at Sites 582 and 583 is divided in-to two units: turbidite facies (Unit 1) and hemipelagitefacies (Unit 2). Unit 1 can be further divided into an up-per finer-grained part (upper 50 m of 583) and a lowercoarser-grained part.

2. Turbidites of Unit 1 can be summarized in a gen-eralized facies model (Fig. 7).

3. The turbidite sequence at Site 582 shows a distalchannel-levee-type bed-thickness distribution, and theupper 50 m of Site 583 may represent overbank deposi-tional setting.

4. Turbidites were emplaced from nearshore rivermouth environments.

5. The emplacement of turbidites was every 200 yr.The mean thickness of turbidite layers is 30 cm in the in-terval between 90-560 m sub-bottom depth.

MAGNETIC FABRICS

In DSDP cores, the original orientation of core speci-mens is completely lost because of drilling-induced rota-tion; however, if the direction of remanent magnetiza-tion is used as a magnetic compass, the specimens canbe reoriented with reasonable accuracy. In this study,both remanent magnetization and anisotropy of mag-netic susceptibility were measured on the same sample.The direction of magnetic fabric is presented relative tothe magnetic north of the remanent magnetization (Ta-ble 1).

General Features of Magnetic Fabric

Many previous studies of the magnetic fabric on un-deformed sediments have demonstrated that sedimentshows a well-defined Kmin (minimum susceptibility) di-

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Figure 7. An example of a typical turbidite bed showing major observed features. Microfossil contents are shown on a relative scale inwhich the maximum content is ~ 5 % . Right-hand column is grain-size distribution in frequency and cumulative frequency (solidline) curves plotted on a probability scale. Grain size is shown in log V (V = settling velocity; cm/s) with phi scale as reference.

rection subperpendicular to the bedding plane (Rees, 1965;Graham, 1966; Taira and Scholle, 1979). The inclinationof Kmin direction usually lies within 90 to 60° relativeto the bedding plane, which is the angle of imbrication(Rees, 1965; Taira and Lienert, 1979).

The distribution of Kmax and Åinin directions onstereo-net are shown in Figures 12 and 13. The distribu-tion of ATmin directions of whole samples from Site 582(Fig. 12A) suggests that, although many are concen-trated around the pole of the bedding plane, a substan-tial number are at relatively low angles to the plane of

bedding. These low-angle fabrics are generally regardedas "deformed fabric" and the following reasons are pos-sible causes: (1) drilling-induced deformation; (2) synse-dimentary deformation, such as slumping and bioturba-tion; and (3) tectonic deformation.

At Site 582, the effect of tectonic deformation is min-imal because the beds are essentially flat-lying, and thereis no sign of deformation indicated by any of the seismicprofiles (Fig. 4).

The effect of both drilling deformation and biotur-bation can be calibrated by comparing deformed and

618


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HemipelagiteGreenish gray silty claybarren in microfossils except

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/ Parallel laminated dark olivegray silt and very fine sandwith microfossils.Upper parallel lamination

^d iv is ion of turbidite.

T3'

T2'

Figure 8. Turbidite sedimentary structures observed in Site 583 sam-ples.

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50Turbidite bed

thickness (cm)

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Figure 9. Bed-thickness distribution for the interval between Cores582B-22 and 582B-44. A. Turbidite bed thickness, total of 43 beds,mean thickness = 30.2 cm. B. Hemipelagite bed thickness, totalof 34 beds, mean thickness = 6.5 cm.

undeformed parts as shown in Table 2 and Figure 14. Asexpected, the Åinin directions within the deformed partare more diverse than those within the undeformed part,suggesting that drilling deformation and bioturbationhave altered the magnetic fabric. The effect of drillingdeformation is also clear from Figure 12C, which shows

the well-grouped ÅTmin directions below 400 m sub-bot-tom depth where the sediments are apparently less de-formed because of increased lithification. The Kmin di-rections cluster around the pole of the bedding planewith a dominant inclination of imbrication angle towardthe northeast.

The hydraulic piston cores of Site 583 are considera-bly deformed as far as their magnetic fabrics are con-cerned, although in appearance the bedding plane is fair-ly well preserved (Fig. 12D). We believe this is becausevery soft sediments recovered by the piston core were de-formed during coring, core-splitting, and sampling.

Paleocurrent AnalysisAs the direction of Kmax is approximately parallel to

the long axis orientation of grains and Kmin defines theimbrication, magnetic fabric is a useful indicator of pa-leocurrent direction (Rees, 1965; Taira and Scholle, 1979).For paleocurrent analysis, samples with a ATmin direc-tion of less than 60° inclination were discarded. Also,because the majority of samples taken from the intervalbetween 0 to 200 m sub-bottom depth are deformed,that interval was not taken into consideration.

Figure 15 shows the magnetic fabric at Site 582 below200 m sub-bottom depth. Sandy and muddy intervalshave contrasting magnetic susceptibility directions: ATmaxis concentrated around the southwest and the declina-tion of Kmin around the northeast for sandy intervals,whereas the muddy intervals have more scattered ATmaxdirections. Thus, paleocurrent direction indicated fromthe magnetic fabric runs predominantly from northeastto southwest, or parallel to the axis of the Nankai Trough.

The magnetic fabric of a single turbidite also sup-ports this conclusion. A 60-cm-thick turbidite from Sec-tion 582B-33-4 was sampled, and the magnetic fabricanalysis suggests that this bed was transported from north-east to southwest (Fig. 16).

In order to further investigate this conclusion, grainorientations in thin sections were counted according tostandard procedures. Results with vectors of magnitudeof less than 15% were not included in the data presenta-tion (Fig. 17). Magnetic fabric and grain counts corre-late well. The mean direction of grain-counting analysisfrom 16 thin sections shows that the paleocurrent direc-tion of turbidity currents is northeast to southwest.

Comparison between Sites 582 and 583Magnetic fabric data obtained from Site 583 suggests

that there is a slight difference in the distribution pat-tern of ATmin directions compared with the Site 582 da-ta. Kmin at Site 582 is concentrated around 30° declina-tion and 80° inclination, an indication of current-imbri-cated pattern (Fig. 12C). At Site 583, Kmin shows agirdle-type distribution around a N3OW-S3OE great cir-cle (Fig. 12B). The pattern of ATmax distribution, on theother hand, is similar for both (Fig. 13).

A possible explanation for the great circle distribu-tion of ATmin direction at Site 582 is induced tectonicstress. Graham (1966) presented a model for deforma-tion of original depositional fabric during the early stagesof sediment dehydration in Appalachian sedimentary

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Distalchannel- Ilevee- IoverbankIfacies ×

Proximalchannel-leveefacies

Cores 582B-22 to 582B-44Core 583A-1 to 582A-3Total of (×) + (•)Kokumoto Formation(Pliocene, Boso Peninsula)Naharigawa Formation(Eocene, Shimanto Belt, Shikoku)Naharigawa Formation(Eocene, Shimanto belt, Shikoku)Kurusuno Formation(Eocene, Shimanto Belt, Shikoku)

10 20 50 100 200 500

Bed thickness (cm)

Figure 10. Log-probability plots of turbidite-bed-thickness distributions for various examples.

Time (Ma)1.0

100 -

700 -

Figure 11. Sediment accumulation rates at Site 582.

rocks. The data obtained from Site 583 can be explainedby a similar model; namely, the Kmin direction was ro-tated slightly because of the northwest-southeast com-pressional stress without significantly changing the #maxdirection. Also, the value of qn is negative at Site 583,suggesting that the fabric is more prolate (Fig. 12A, B).Seismic profiles (Nasu et al., 1982) reveal an apparentshortening and thickening of sediments between the twosites. From these observations, it is suggested that therotation and reorientation of the short axis of sedimen-tary grains might have taken place between Sites 582 and583. If this inference is justified by further data fromother places, such reorientations may provide a meansof identifying tectonic stress in ancient accretionaryprisms. The interpretation of magnetic fabrics is sum-marized in Figure 18.

GRAIN-SIZE ANALYSIS

Photo-extinction settling tube analysis of 50 turbiditesamples from DSDP sites and 250 samples from the areaoffshore of Shikoku and Kyushu (Fig. 1) were carriedout.

Grain-Size Characteristics of Turbidites

Several examples of grain-size data from Site 582 arepresented in Figure 7, and additional examples are shownin Figure 19. The following characteristics are notewor-thy:

1. Grain sizes are generally poorly sorted, and distri-butions lack a notable modal peak.

2. Grading, which is indicated by a gradual decreaseof coarse-tail grain size (coarse-tail grading), is observed.

3. Two basic grain-size patterns occur in the Tl andT2 divisions of turbidite beds, respectively. A longer andwell-developed coarse tail characterizes the Tl-divisiontype, whereas a poorly developed coarse tail distinguish-es the T2-division type. Generally, there is a break be-tween the two; in other words, these types do not show agradual change.

The Nankai grain-size distributions differ from thePliocene Pico Formation, Ventura Basin of California,and from the Pleistocene Kokumoto Formation, BosoPeninsula of Japan (Fig. 19). The Site 582 and 583 tur-bidites are characterized by a lack of current-sorted dis-tribution. Taira and Scholle (1979) suggested that grad-ed division of a turbidite is emplaced by highly concen-trated non-Newtonian mass flow, whereas the part aboveis deposited by less concentrated Newtonian density (tur-bidity) current. The break in grain-size characteristicsbetween graded division and the above division is a con-sequence of this difference in flow characteristics. Thisbreak in grain-size characteristics is not as clear for theturbidites of Sites 582 and 583 as for turbidites from

620


Table 1. Examples of magnetic anisotropy, Hole 582B.

Core-Section(interval in cm)

41-5, 59-6141-5, 76-7841-5, 97-9942-1, 40-4242-1, 53-5542-2, 81-8342-2, 118-12042-4, 5-742-4, 63-6442-5, 32-3544-2, 40-4244-2, 53-5550-1, 41-4350-2, 54-5650-3, 73-7550-7, 15-1751-1, 58-6051-1, 62-6352-1, 101-10352-1, 94-9652-3, 60-6252-3, 79-8152-3, 4-654-2, 58-60

Maximumsusceptibility

Kmax(×10~3)

16.442.646.99.96

11.612.84.86

36.047.629.89.54

20.513.632.3

1.4516.93.50

11.865.99.81

54.1108.061.315.5

D

104224302116109193213284241310208149180229281156282

8961

143281

71122268

I

- 2- 1 8- 1 6- 1 1- 1- 4- 2 2- 1 0- 1 8- 3- 6- 6- 1 0- 7- 1- 7- 1 1- 4- 3- 3- 1- 2

- 3

Intermediatesusceptibility

A•int(×10~3)

15.841.945.89.92

11.412.54.63

35.047.129.5

9.1719.812.429.40.91

16.73.06

11.265.8

8.5352.8

10660.413.8

D

19431720923

19910311718914541975889

1391916316

181331234

11340

32177

/

- 8- 8- 8- 9- 6- 5- 1 5- 2 9- 1 6- 2- 7 4- 8- 4- 3- 1 6- 2 5- 2 2- 3 4- 3- 1 2- 9- 7- 4~ l l

Minimumsusceptibility

Kmva(×10~3)

15.241.244.79.58

10.711.74.52

34.545.228.79.12

18.211.925.5

0.66215.72.85

10.962.36.55

50.5101.054.012.1

D

26996

25511

3173543118

170299272337

2416

25916835320340

183175254

11

/

- 8 2- 7 0- 7 2- 7 5- 8 4- 8 4- 6 3- 5 9- 6 5- 8 6- 1 5- 8 0- 7 9- 8 2- 7 4- 6 4- 6 5- 5 6- 8 6- 7 8- 8 1- 8 3- 8 5- 7 9

-0 .04-0.13

0.023.322.231.48

-1 .02-0 .96

2.011.44

-2 .921.44

-1 .350.49

-1 .131.89

-1 .10-0 .63

5.480.630.841.322.82

-0 .06

Note: D = declination (in degrees, 360° clockwise; 0° = magnetic north); / = inclination (in degrees, negativeupward; 0° = horizontal).

other areas. This observation, together with a lack ofwell-developed parallel and ripple lamination is a uniquecharacteristic of the Nankai Trough turbidites. The rea-son is uncertain—perhaps the grain size of sedimentsabove the Tl division is usually too fine-grained to formeither flat beds or current ripples. The turbidites wereemplaced as muddy, highly concentrated non-Newtoni-an flow with less-developed diluted secondary flow.

Grain-Size Distribution in the Surrounding Area

It is not within the scope of this report to describe allavailable data; however, the following facts pertain tothe provenance and transport direction of turbidites.

Available data on the grain size of sediments offshoresouthwestern Japan (mainly from the shaded area inFig. 1) can be classified into three basic types: (1) near-shore to shelf, (2) slope, and (3) base of slope or trough(Fig. 20). Type 1 shows a well-developed peak suggestiveof sorting processes. Type 2 is a composite type with bi-modal or multimodal peaks and occasionally large mudcontent. Type 3 is characterized by a lack of modal peaksand by a large mud content.

Sediments offshore of Shikoku are generally finegrained; coarse elastics are limited to the nearshore envi-ronments. The Ashizuri Canyon sediments also have ba-sically Type 2 patterns, and there is no sign of emplace-ment of a large quantity of nearshore sands along thecanyon axis.

A phenomenal exception is recognized at the Surugatrough area where a large volume of nearshore sands isemplaced. Type 1 grain-size distribution occurs in theSuruga Trough at depths exceeding 3000 m.

Paleocurrent analysis of Nankai Trough turbidites sug-gests, as mentioned above, transport from northeast tosouthwest along the axis of the Nankai Trough, and thecomparison of the few available samples shows a nota-ble grain-size change along strike (Fig. 21). In pistoncore KH-72-2-21, located south of the Kii Peninsula,graded beds are coarser grained and apparently have abetter sorted distribution than at Site 582. Sample KT-78-19-24, which is in the Suruga Trough, contains well-sorted medium sand. Other piston core locations, in theSuruga Trough include much coarser-grained sands andgravels (Sato, 1962; Ohtsuka, 1980). This lateral changein grain-size and sorting characteristics along the Nankai-Suruga Trough supports the northeast to southwest flowpattern deduced from the grain-orientation analysis.

DETRITAL MODAL ANALYSIS

The above discussion indicates that the turbidites atSites 582 and 583 were transported from northeast tosouthwest along the axis of the Nankai Trough. In orderto track down the exact provenance, it was necessary tocarry out a petrographic analysis.

Compositional Characteristics of the Turbidite Sands

The following features characterize the compositionof the turbidite sands.

1. As mentioned previously, sands at Sites 582 and583 include abundant land-derived materials such as plantdebris and fresh-water diatoms. On the other hand, ma-rine organisms are less abundant, especially in the sandypart. The available reference samples from offshore ofsouthwestern Japan indicate that most of the marine sands

621


W -

qn = -0.08

•r . . .. . ••: ffiVy

•

Km in

W -

Kmin

••' f *!'

• *** "T•

••••••••

Kmin

Figure 12. ATmin directions of Site 582 and 583 samples. A. Site 582, 66-442 m sub-bottom depth. B. Site 583, 66-442 m sub-bottom depth. C. Site582 below 400 m sub-bottom depth. D. Site 583 HPC cores. Mean values of qn are shown in A and B.

are rich in skeletal debris of marine organisms such asmolluscan shells and foraminiferal and echinoderm frag-ments. These components suggest that most of the sandswere directly derived from river mouths, not from mucholder shelf deposits.

2. The most abundant detrital components are blackslate, sandstone, and chert. These sedimentary rock frag-ments are probably derived from the outer zone of geo-logic belts, such as the Shimanto and Chichibu belts.

3. Especially noteworthy is the presence of a substan-tial volcanic component, including volcanic rock frag-ments, pumice, and volcanic glass, plagioclase, pyrox-ene, and olivine. Examination under both polarizing andbinocular microscopes reveals both basic and intermedi-ate volcanic rock fragments; some of these are red incolor. These volcanic grains are critical for provenancedetermination, because the distribution of volcanic rocksis limited to two areas in the outer zone of southwestern

622

Site582


Site583

W -«-• , - E W -

Km ax

Figure 13. Kmax direction for Sites 582 and 583.

Kmax

Upper hemisphere

Table 2. Calibration for deformation of magnetic fabric, Hole 583D.

Core-Section(interval in cm)

22-1, 77-7922-2, 123-12522-3, 47-4922-3, 110-11222-4, 67-69a

22-4, 70-72a

22-5, 61-63b

22-5, 68-7022-6, 49-51

Maximumsusceptibility

Am ax(×IO~3)

23.915.319.16.075.382.587.30

17.225.0

D

14313419824415215477

23269

/

- 3- 4 7- 1 8- 2 0- 3- 5 9- 4 1- 1 1- 5

Intermediatesusceptibility

Kint(x lO~ 3 )

23.714.718.85.695.052.256.74

16.323.3

D

522299

33560

282309137161

/

- 1 7- 1 9- 2 2- 3- 3 6- 1 9- 3 5- 2 4- 2 0

Minimumsusceptibility

Amin(×10~ 3 )

22.014.617.25.084.742.126.04

15.021.3

D

24227732274

24621

195347326

/

- 7 3- 3 7- 6 0- 6 9- 5 3- 2 2- 3 0- 6 4- 6 9

Qn

3.18-1.99

2.000.69

-0.06-1.32

0.320.660.30

Note: D = declination (in degrees, 360° clockwise; 0° = magnetic north); / = inclination (in degrees, negativeupward; 0° = horizontal).

a Drilling-induced deformation.Bioturbation.

Japan; one is in Kyushu and the other is in central Ja-pan.

4. Other components include micas, hornblende, andsome metamorphic minerals such as alkaline amphiboles.The first two are possibly derived from a granitic ter-rane. These minerals are also diagnostic of provenance.

Provenance of the Turbidite Sands

Point counting was used to investigate and comparethe detrital components of the sands at Sites 582 and583 and surrounding areas (Table 3). A special combina-tion of end-member components was devised to discrim-inate the outer zone sediments of southwestern Japan:

1. Component OZ (outer zone background) includessandstone, shale, chert, and metamorphic rock fragments,which are derived from "basement" geologic terranes ofthe outer zone of southwestern Japan, such as the Shi-manto, Chichibu, and Sambagawa belts.

2. Component AV (acidic volcanic) includes plagio-clase, pyroxene, glass, and pumice, which are derivedmostly from eruptive acidic to intermediate pyroclasticflows.

3. Component VR (volcanic rock fragments) includesvolcanic rock fragments, in particular, red, brown, andblack fragments, which are mostly intermediate to basicin composition.

623


Hole 583D, Core 22

Kmax Kmin

Uπdeformedpart * x

Deformedpart ° Δ

(B = bioturbation)

Figure 14. Calibration for drilling and burrowing disturbance on mag-netic fabrics.

The AV component is closely related to the Quater-nary volcanic rocks of Kyushu such as the Aso and Airavolcanics. The VR component is related to the volcanicrocks of central Japan and to the Tertiary volcanic rocksin Kyushu.

A ternary plot of OZ-AV-VR components togetherwith a conventional Q(quartz)-F(feldspar)-R(rock frag-ment) plot is shown in Figure 22. Generally, sands of theouter zone of southwestern Japan are classified as lithicwackes or lithic greywackes (Fig. 22A). Sands from theKyushu area are more feldspathic than others.

The OZ-AV-VR ternary plot clearly discriminates thesands of the outer zone of southwestern Japan into fourcompositional provinces (Fig. 22B). Sands from Kyushuare rich in the AV component, sands from Shikoku arevirtually devoid of volcanic components, and sands fromTokai are slightly more feldspathic than those from Shi-koku. Sands at DSDP sites, other areas of the NankaiTrough, the Suruga Trough, and the Fuji River area arealmost identical.

The composition of Site 298 sands (Harrold andMoore, 1975) is essentially the same as the sands at Sites582 and 583 (Fig. 22).

The detrital components, grain-size distributions, andmagnetic fabric together with physiographic observationssuggest strongly that the sands of Sites 582 and 583 werederived from the mouth of the Fuji River trough, theSuruga Trough, and along the axis of the Nankai Trough,involving a transport distance of 700 km.

SEDIMENTARY FACIES AND SEDIMENTATIONMODEL

Sediment Budget

The sedimentation rate based on magnetobiostratig-raphy indicates an enormous sediment yield rate in the

interval from 0.6 to 0.4 Ma. Naturally, a fundamentalquestion occurs regarding sediment budget; could theFuji River drainage basin alone supply the sediments de-posited in the Nankai Trough?

The present-day Nankai Trough area includes rough-ly 10,000 km2 of turbidite sedimentation area with a meanthickness of about 500 m. The total volume of turbi-dites is thus 5000 km3. The Fuji drainage basin is about5000 km2. Present-day erosional rates of central Japan,including the Ohi and Tenryu drainage basins, has beencalibrated previously, based on artificial reservoir sedi-mentation rates (e.g., Kaizuka, 1977). The value is 5 to6 mm per unit area per yr.

To provide 5000 km3 of sediment to the NankaiTrough within 0.2 Ma requires 5 mm per unit area peryr., a unit rate almost identical to the present-day ero-sional rate. Of course, not all sediments eroded fromthe Fuji drainage basin were deposited as turbidites inthe Nankai Trough. Considering additional sediment sup-ply to the Suruga Trough by the Ohi, Abe, and Kano riversand possible longshore transport from the Omaezaki area(sands of the Tenryu River), we may reasonably con-clude that the magnitude of a drainage basin like that ofthe Fuji River is capable of producing enough sedimentto account for the volume of the entire Nankai Troughturbidite sequence.

One problem concerning this evaluation is the role ofMt. Fuji. The maximum age of this gigantic volcano isabout 0.08 Ma, which postdates the period of maximumsediment supply. The configuration of the paleo-Fujidrainage system before the existence of Mt. Fuji is un-known and remains a subject for future study.

A longitudinal cross section from the Fuji River tothe Nankai Trough reveals that the proximal part of thissediment transport system has a high gradient (Fig. 23).From the summit of Mt. Kitadake, the second highestpeak (highest nonvolcanic mountain) in Japan (3192 m)to the southern margin of the Suruga Trough, the eleva-tion difference of 7000 m occurs within 160 km of lat-eral distance (in fact, the elevation difference probablyexceeds 10,000 m if the top of the ocean crust is taken asa datum). This gradient is one of the largest land-con-nected slopes on the Earth and is comparable to the gra-dient of the Himalayas, which ranges from the foot ofthe lesser Himalayas to 8000-m peaks in about 150 to170 km lateral distance. Obviously this gradient providesthe potential energy for the yield of coarse elastics intothe deep-sea environments.

Trench Sedimentary Fades

Inspection of bathymetric maps and of longitudinaland transverse cross sections in available seismic profilesindicates some aspects of trench-fill processes. The lon-gitudinal cross section of the Suruga and Nankai troughsshows that it is composed of a step-like topography (Fig.23, also Kato et al., 1983). The first slope is developedat the mouth of the Fuji River, which makes up the mostconspicuous slope. This slope apparently makes up theFuji River fan-delta system. The many gullies developedon this slope are indicative of active gravity emplace-ment (Ohtsuka, 1980).

624


Holes 582, 582A, 582B

5 0 -

100-

150-

2 0 0 -

2 5 0 -

3 0 0 -

* 3 5 0 -

•? 4 0 0 -

4 5 0 -

5 0 0 -

5 5 0 -

6 0 0 -

6 5 0 -

7 0 0 -

750 •

• Km ax

XKmin

Facies

[ . •. l Sandy turbidites

[H-rEH-rEI:] Muddy turbidites

I I Hemipelagites

Upper hemisphere

T.D. = 249.4 m (Sampled interval corresponding to stereo-net)

Figure 15. Magnetic susceptibility directions of Site 582 samples. A-E represent the stratigraphic interval for measured samples. Arrow indicates theinterpreted paleocurrent direction.

625


582B-33-4

H Hemipelagiteh60

Chondrites

Massive silt

-80

100

"Graded sand

• KmaxxKmin

582B-2O-2, 95 -97 cm

Upper hemisphere• • - 1 - 1 2 0

• Sampled horizon

Figure 16. Magnetic susceptibility directions of a turbidite bed at Site582. Arrow indicates the interpreted paleocurrent direction.

Two more steps (Profiles 3 and 5 in Fig. 23) are recog-nized toward the entrance of the Nankai Trough. Seis-mic profiles (Kato et al., 1982) reveal a difference inshape of the longitudinal channel from a sloped bottomto a flat bottom. In the sloped bottom of the trough, anarrow and relatively deep channel is recognized, where-as in the flat trough bottom, a wide and shallow channelis identified (Figs. 2 and 23).

Some of the available seismic profiles reveal an as-pect of interaction between the development of trenchsedimentary facies and tectonic deformation. The trenchtopography can be divided into four categories; outerslope, trench channel floor, inner swell, and toe of ac-cretionary prism (Fig. 24). The last two topographic fea-tures coincide with the protothrust zone and the lowerportion of the imbricated thrust zone of the NankaiTrough (Kagami et al., 1983).

A possible sedimentation pattern and facies modelbased on this trench topography classification empha-size two important aspects of trench sedimentary facies(Fig. 24). First, the trench turbidites are funneled througha longitudinal channel, thus forming a lenticular andtroughlike sand body in transverse cross section. Sec-ond, as the trench-floor elevation gradually increases to-ward the toe of the accretionary prism, there is a widezone of "overbank" deposition where diluted turbiditycurrents and occasional gigantic flows are accommodat-ed. This facies pattern creates a fining-upward trench-filldepositional cycle similar to the one pointed out in theMiddle American Trench (Moore et al., 1982). The fa-cies starts from the trench central channel facies to leveefacies, then to the finer-grained thin-bedded overbankturbidite and to slope hemipelagite. A similar fining-up-ward cycle is also recognized from the Cretaceous Shi-manto subduction complex of Shikoku (Taira, in press).

Tectonics and Trench Sedimentation

The tectonism of central Japan has been controlledby two collisional systems; the Izu-Honshu collision (Ni-itsuma and Akiba, in press) and that of northeastern Ja-pan against southwestern Japan at the Fossa Magna (Na-

582B-3O-3, 43-45 cm

N

582B-31-1, 126-1 28 cm

N

582B-39-6, 104-106 cm

Figure 17. Grain orientation measured in thin sections, Site 582. Lowerright shows a rose diagram for the mean directions of preferredorientation direction from 16 thin sections.

Current direction

Slopemud

facies

Hemipelagitefacies

Figure 18. Illustration for paths of sediment dispersal and possiblecause of sediment deformation.

626


582B-41-5

T 2

H

T2

T1

~ - – -50 E

CD

ül

9-|-

6 -

_3 -

-

2.0

9-

6 -

3 -

-o-

2.0

9

-

6-

3-

_D-

2.0

-4--2•

1.0

1.0

-1.0

Log

-0-1-2-Phi

3 CD

O Jr

r99.9

yf

I1

jj/

-99-90. 7 0= 50

30"10-1-0.1

0.0 -1.0

r99.9- 9 9- 9 0.70=50

30"10-1L0.1

0.0 -1.0

I r 99.9

/,,-Y

-•" J

-99-90

70E50

30"10-1n 1

0.0 -1.0K

—3-4 5

V \ GT4-A y

\ \ GT4-B

\ \ GT4-D

\ GT4-F

\ GT4-J

\ GT4-L

/

A/ •

^ ^

^―-

Jj

V

2 1.5 1 0.5 0 -0.5 - 1Log V

- 4 - 2 0 1 2 3 4 5Phi

0 1 2 3 4 5Phi

-100

Figure 19. Grain-size (settling velocity) distributions for selected turbidite examples. A. Site 582. B. Pico Formation, Ventura Basin, California (afterTaira and Scholle, 1979). C. Kokumoto Formation, Boso Peninsula, Japan (Niitsuma, 1974). See Figure 7 for explanation of frequency curves.

kamura, 1983). As the continuation of the Suruga Troughplate boundary seems to extend onto the coast of Shi-zuoka Prefecture, an event that created the sedimentpathway to the Nankai Trough, we suggest that the Izu-Honshu collision triggered the enormous yield of clasticsediment into the Nankai Trough. As a consequence ofthis collision, which began ~ 2 Ma (Niitsuma and Aki-ba, in press), the trench extends on shore between twoland masses, and the deformation on land created theJapan Alps. A steep drainage systemr formed along thestrike-slip faults, provided coarse clastic sediment, cre-ating a major fan-delta system at the river mouth. Occa-sional gigantic earthquakes destroyed the slope of thefan-delta and caused turbidity currents which flowedalong the axis of the trough. Because this sediment yieldwas too rapid to be subducted, an accretionary prismformed at the base of inner trench slope.

SUMMARY

1. Turbidites encountered at Sites 582 and 583 have acharacteristic sequence (Fig. 7).

2. Turbidites were emplaced from northeast to south-west as indicated by magnetic fabric analysis.

3. Settling tube analysis suggests that turbidity cur-rents were composed of muddy, viscous flow with minoramounts of diluted secondary flow.

4. Petrographic analysis reveals that sands in DSDPsamples were provided mainly from the Fuji River drain-age system.

5. Physiographic considerations and grain-size dataalso support the above conclusion (Item 4) on prove-nance.

6. A trench turbidite-facies model involves a fining-upward cycle (Fig. 24).

7. Rapid yield of trench turbidites was initiated byIzu-Honshu collision tectonics.

8. Rapid trench sedimentation was responsible for thecreation of an accretionary prism.

ACKNOWLEDGMENTS

The authors thank Drs. Hideo Kagami and Dan Karig for theirhelpful discussions during the cruise. Reference samples used in thisstudy were provided by the Geological Survey of Japan, Hydrographi-cal Department of the Japanese Maritime Safety Agency, Ocean Re-search Institute of the University of Tokyo and Tokai University. A. T.thanks M. Imajo, M. Miyake, and Y. Okamura for their help in analy-sis of samples and preparation of the manuscript. N. N. thanks Mr.M. Koyama for his help in measuring magnetic anisotropy. Drs. G.deV. Klein and J. Casey Moore reviewed the manuscript.

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627


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Okuda, Y., (Ed.), 1977. Geological Map off Outer Zone of SouthwestJapan. Marine Geology Map Series, No. 8, Geol. Surv. Jpn., To-kyo.

Rees, A. I., 1965. The use of anisotropy of magnetic susceptibility inthe estimation of sedimentary fabric. Sedimentology, 4:257-271.

Sato, T., 1962. Sand and gravel bed cored from the bottom of the Sur-uga Bay. J. Geol. Soc. Jpn., 68:609-617. (In Japanese)

Sugimura, A., 1972. Plate boundaries around Japan. Kagaku, 42:170-181. (In Japanese)

, 1978. Iwanami-koza Chikyukagaku (Iwanami Earth ScienceSeries), 10: Tokyo (Iwanami Shoten). (In Japanese)

Taira, A., in press. Sedimentary evolution of Shikoku subduction zone:Shimanto Belt and Nankai Trough. In Nasu, N., Kobayashi, K.,Uyeda, S., Kushiro, I., and Kagami, H. (Eds.), Formation of Ac-tive Margins, 1983 Oji Seminar Proceedings: Tokyo (Terra Publ.Co.).

Taira, A., and Lienert, B. R., 1979. The comparative reliability ofmagnetic, photometric and microscopic methods of determiningthe orientation of sedimentary grains. J. Sediment. Petrol., 49:759-772.

Taira, A., and Scholle, P. A., 1977. Design and calibration of photo-extinction settling tube for grain size analysis. J. Sediment. Petrol.,47:1347-1360.

, 1979. Deposition of resedimented sandstone beds in the Pi-co Formation, Ventura Basin, California. Geol. Soc. Am. Bull., 90:952-962.

Taira, A., Saito, Y., and Hashimoto, M., 1983. The role of obliquesubduction and strike-slip tectonics in the evolution of Japan. InHilde, T. W. C , and Uyeda, S. (Eds.), Geodynamics of the West-ern Pacific-Indonesian Region: Washington (Am. Geophys. Un-ion), Geodynamic Series 10:303-316.

Date of Initial Receipt: 3 April 1984Date of Acceptance: 6 May 1985

628


Type 1:

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Figure 20. Selected examples of settling-velocity distributions for southwestern Japan offshore samples.

629


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630


Table 3. Tabulated point-count data (in °7o) for sands of Site 582 and reference areas of southwestern Japan.

Sample

1. 582A-1-12. 582A-1-3, 49-513. 582B-2-1, 49-514. 582B-2-1, 75-775. 582B-5-1, 52-546. 582B-11-1, 136-1387. 582B-15-1, 59-618. 582B-18-2, 10-129. 582B-18-2, 10-12

10. 582B-3O-3, 43-4511. 582B-30-3, 43-4512. 582B-33-4, 104-10613. 582B-39-4, 104-10614. 582B-42-4, 63-6415. KH72-2-21, a-616. KH72-2-21, a-717. KH78-19-218. KH78-19-2119. KH78-19-2420. Fuji River 121. Fuji River 222. BG 20223. BG 21524. BG 24325. BG 27326. HG 33027. HG 36728. HG 37429. Miyazaki Airport30. Hitotsuba Beach31. Hitotsuse River32. TS 1833. TS 3134. TS 6335. MR 14536. JM 2137. JM 2238. JM 2539. JM 32

Quartz

19.717.711.816.523.423.920.216.913.819.115.736.523.831.239.326.414.822.914.923.917.921.222.117.213.010.614.018.24.29.9

16.017.834.817.930.128.139.232.137.7

K-feldspar

3.52.32.01.81.83.72.00.00.02.20.01.03.80.01.20.71.61.30.02.20.73.53.53.20.00.00.00.02.82.01.21.10.00.01.28.35.84.60.9

Plagioclase

5.110.05.9

11.910.810.114.18.59.2

11.211.85.88.6

10.44.85.73.13.85.04.36.0

41.230.223.715.612.926.331.829.618.822.2

5.64.37.10.07.49.27.38.5

Sandstone

18.79.2

24.76.4

12.613.86.18.5

13.814.610.814.411.47.81.2

10.717.23.8

11.92.2

11.24.74.75.47.8

20.09.68.0

23.913.922.223.315.930.415.713.29.28.3

10.4

Shale

21.220.828.718.320.718.317.214.116.526.918.619.220.029.932.117.930.555.735.630.422.4

3.53.55.47.87.12.65.75.65.0

16.017.829.025.012.011.615.022.017.0

Chert

8.65.45.97.35.42.82.04.27.35.6

10.85.86.73.94.8

15.05.51.34.01.15.23.51.23.22.67.10.90.04.24.06.28.94.30.0

15.77.44.2

10.110.4

VRF

21.716.914.930.314.411.920.239.423.913.515.74.8

16.26.53.68.6

13.35.1

18.817.418.77.1

27.916.110.415.37.0

13.622.524.8

7.40.00.00.01.21.70.81.80.9

Glass

0.01.50.00.01.81.85.12.80.01.11.01.01.92.64.84.30.01.30.00.00.00.00.03.25.23.5

22.08.00.00.00.01.10.00.01.20.00.00.00.0

MRF

1.02.32.01.82.70.90.01.41.82.21.91.91.90.00.02.12.32.54.00.01.50.00.00.00.02.40.90.00.00.00.00.07.21.8

18.14.15.01.84.7

Mica

0.03.80.01.80.95.55.10.05.51.15.95.81.95.22.43.64.71.31.07.60.70.00.00.00.01.20.00.00.00.00.00.00.03.60.0

12.48.3

10.17.5

Pyroxene

0.05.40.02.83.63.73.04.25.52.22.91.03.80.02.41.43.11.32.04.3

11.92.42.3

10.810.42.47.98.05.6

18.88.61.10.00.00.04.10.00.01.9

Others

0.54.63.00.91.83.75.10.02.80.04.92.93.82.63.63.63.90.03.06.53.7

12.94.7

11.827.317.67.06.81.43.00.0

23.34.38.94.81.71.71.80.0

Note: VRF = volcanic rock fragments; MRF = metamorphic rock fragments. DSDP samples (Samples 1-14) are expressed as hole-core-section, interval incm. Samples 1-14 are from Site 582, 15-16 from the Nankai Trough, 17-19 from the Suruga Trough, 22-31 from Kyushu, 32-35 from Shikoku, and 36-39 from Tokai.

a Duplicate sample.

Quartz

OZ(sandstone + shale + chertmetamorphic rock fragments)

* DSDP Site 582

» Nankai Trough

* Suruga Trough

• Fuji River

* Tokai

o Tosa Bay

x Kyushuoffshore and rivers

+ Site 298 mean(Harrold andMoore, 1975)

Feldspar Rock VRfragments (volcanic

rock fragments)

AV(plagioclase

+ pyroxene + glass+ pumice)

Figure 22. Ternary plots for compositions of sands of southwestern Japan offshore and river samples.

631


100 Mt. Kitadake'-3000-2000-1000- 0 <r>

|

Depth (km)

Figure 23. Longitudinal and transverse topographic cross sections for the Suruga-Nankai Trough. See Figure 3 for location of cross sections.

10

km

Toe of prism Inner swell Trench Outerslope

Hemipelagiclayer

Plioceneturbidite

Ocean crust

Trench channel-levee faciesSlope Inner swell

hemipelagite overbank-leveefacies

Outerslopeoverbank-leveeturbidite facies

Ocean crust

Pelagic tohemipelagicsediments

Figure 24. A. Representative seismic cross section at the Nankai Troughtrench axis (after Aoki et al., 1982). B. Trench sedimentary faciesmodel.

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Deep Sea Drilling Project Initial Reports Volume 87 · TURBIDITE SEDIMENTATION, NANKAI TROUGH 1000...

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