21. LITHOFACIES OF THE SHIKOKU AND PARACE VELA BASINS

Doris M. Curtis, Consultant, Robertson Research (U.S.) Inc., Houston, Texasand

Dorothy J. Echols, Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri;Consultant, Robertson Research (U.S.) Inc., Houston, Texas

ABSTRACT

The lithofacies of the Shikoku and Parece Vela Basins are typicalof marginal or back-arc basins of this type, consisting predominantlyof hemipelagic and pyroclastic sediments with minor pelagic compo-nents. Thickness relationships and distribution of principal facies ineach of five time-equivalent intervals (early Miocene through Pleisto-cene) at eight DSDP sites suggest the time-variable source areas forcoalescing clastic wedges within the basins and indicate the timing ofvolcanism in neighboring volcanic arcs.

Clastic sediments came principally from a northern and easternsource during the middle Miocene; from a northern and northeasternsource during the late Miocene; and from northern, northeastern,and northwestern sources during the Pliocene and early Pleistocene.The appearance of pelagic clay and calcareous biogenic sediments inthe upper Pleistocene at Site 297 probably marks the inception of theNankai Trough in mid-Pleistocene time, restricting the previouslyrapid accumulation of hemipelagic muds in the northern ShikokuBasin. Neogene sedimentation rates were generally lower in the Par-ece Vela Basin than in the Shikoku Basin.

Major volcanic activity during the middle Miocene was east ofthese basins, and volcanism was also prominent to the west andnorth. During the late Miocene, minor volcanism was restricted tothe eastern arc. Pliocene volcanism was active only to the northeast.During the late Pleistocene, there was active volcanism to the north.

INTRODUCTION

The object of this paper is to describe and interpretthe vertical and lateral distribution of lithofacies in twoadjoining back-arc or marginal basins, for the purposeof interpreting the sedimentary and tectonic history ofthese basins. To do this, we divided the sections pene-trated on four Deep Sea Drilling Project legs (Legs 6,31, 58, and 59) at eight sites (Sites 53, 54, 297, 442, 443,444, 449, and 450) into five readily identifiable time-equivalent intervals defined by paleontologic datumlevels, according to chronostratigraphic criteria used inthe reports on Sites 442, 443, and 444 (this volume). Thesequence penetrated in these two basins falls naturallyinto five time-stratigraphic subdivisions: early Miocene(22.5-16 m.y.), middle Miocene (16-10.5 m.y.), late Mi-ocene (10.5-5 m.y.), Pliocene (5-1.6 m.y.), and Pleisto-cene (1.6-0 m.y.). Figure 1 shows the location of theShikoku and Parece Vela Basins behind the Marianasarc, and the sites used in this study. Figure 2 shows theposition of each interval boundary by core number ateach site.

For each site, the lithologic details of each intervalare generalized into lithofacies (see White et al., this vol-ume), then plotted on ternary diagrams (Figure 3) to

show the relative dominance of each significant lithofaciescomponent for each interval at each site.

The lithofacies distribution has been plotted in threeways: (1) in a series of cumulative paleostratigraphiccross-sections, showing thickness and lithofacies atclose of each time interval (Figure 4); (2) in a series ofisopach maps with lithofacies symbols, showing lateralfacies distribution relative to source areas (Figure 5);and (3) in two composite time-stratigraphic sectionswhich show lithofacies distribution by time, withoutreference to relative thicknesses (Figure 6). The lithofac-ies are:

I Hemipelagic (clays and mudstones)II Resedimented (mudstones, sandstones, conglom-

erates)III Resedimented (carbonates)IV Pelagic (non-biogenic clays)V Pelagic biogenic

Va - calcareousVb - siliceous

VI Pyroclastic (volcanic ash, tuff, vitric volcani-clastics).

For this study, we have combined the facies units intothree principal groups, each of which contains one ormore genetically related facies, as follows:

701

D. M. CURTIS, D. J. ECHOLS

800 km

140c

Figure 1. Location of Shikoku Basin, Parece Vela Ba-sin, and DSDP sites.

Terrigenous (I, II, III)Pelagic (IV, Va, and Vb)Pyroclastic (VI)

These make up the three major components in the ter-nary diagrams (Figure 3). The diagrams are subdividedto show relative proportions of each major component,and the symbols used in Figure 3 are repeated to showthe facies distributions plotted in Figures 4, 5, and 6.

EARLY MIOCENE (22.5-16 m.y.)

Sedimentary facies deposited during this time intervalare difficult to define because of poor core recovery,poor age determinations, and very short sections. Ingeneral, sediments of this age tend to occur as thin inter-beds between basalt layers. Thus, they may be baked,brecciated, or otherwise altered. They appear to be ab-sent at Site 450, and may also be absent at Sites 297,443, and 444, where age determinations are uncertain.Where they can be identified, however, the dominantlithofacies are pelagic. In the Shikoku Basin, this is theonly time interval in which true pelagic sediments weredeposited. In the Parece Vela Basin, where sedimenta-tion rates are generally lower, pelagic sediments aremore common. The facies include pelagic non-biogenicclays, pelagic biogenic calcareous oozes, and pyroclas-tics. The non-biogenic pelagic facies is dominant at Sites442, 444, and 449. The pelagic calcareous facies is pres-ent as calcareous ooze at Site 449, and as limestone atSites 442, 443, 54, and 53. The limestone is brecciated.Pyroclastic deposits are conspicuous only at Site 53, butare also present at Sites 297, 442, and 449.

No attempt was made to map the thickness distribu-tion of this unit, because of the fragmentary recoveryand dubious age identifications. Sediments assigned tothis interval, however, appear to be thickest (51 m ormore) at Site 449, and possibly at Site 53.

-5.3os 3

T3 .2

S. 2,

s ^-IS U. •§

2 o '

co co i co i

' T t co co oa oa

- T

3 I

1

Iε8X0

41

to

I

8

I3

O

1

•S s;

702

LITHOFACIES

PYROCLASTIC

Pleistocene

Note: Lithofacies inferredat sites 297, 443, 444, 450

53442, 449

443, 444 (Va)54, 450 (Va),

TERRIGENOUSHemipelagicResedimented

PELAGICNon-biogenicBiogenic calcareousBiogenic siliceous

442 .297 L297 (lower)

Figure 3. Ternary diagrams of major lithofacies components (three major components and seven combinations ofthese) for five intervals.

703

D. M. CURTIS, D. J. ECHOLS

Shikoku Basin

A 297 442 443

*r** ifi•> k \ \ \ \ l *VV++?

* Lower Miocene not identified atthis site; lithofacies inferred.

444 m

;

-100

449

+ + + +

Parece Vela Basin

54 450 53 16

442 443 54 450 53

-100 + + + +

-200

-300

443 54 OMa

- 2 0 0

- 4 0 0

- 5 0 0

- 6 0 0

- 7 0 0

(1) 22.5-16 Ma(2) 16-10.5 Ma(3) 10.5-5 Ma(4) 5-1.6 Ma(5) 1.6-0 Ma

Figure 4. Cumulative paleostratigraphic cross-sections showing thicknessand lithofacies at the eight sites for: (A) early Miocene; (B) early andmiddle Miocene; (C) early, middle, and late Miocene; (D) early, mid-dle, and late Miocene, and Pliocene; (E) early, middle, and lateMiocene, Pliocene, and Pleistocene; and (F) thickness of total sequenceat each site referred to present sea-floor topography. Lithofacies sym-bols explained in Figure 3.

704

LITHOFACIES

Shikoku Basin442 443

Parece Vela Basin54 450 5 3 10.5 Ma

•×m

442

-100

H200

443 5 3 1.6 Ma

442

-200

- 3 0 0

-500

- 6 0 0

54 450 534300 meters below present sea level

-4500

-4600

-4700

-•« = sea floor

- 4 8 0 0 '

-4900

-5000

-5100

-5200

Figure 4. (Continued).

705

D. M. CURTIS, D. J. ECHOLS

ANorthwest

Ma 297

Shikoku BasinSoutheast

444

Figure 5. Lateral facies distribution and thickness rela-tive to source area for each interval: (A) 16-10.5 Ma;(B) /0.5-5 Ma; (C) 5-1.6 Ma; (D) 1.6-0 Ma; (E) toto/sediment thickness. Data points correspond to sitesshown in Figure 1. Arrows indicate sediment influx.Thickness in meters. Lithofacies symbols explainedin Figure 3.

1 2 -

14 -

mm

S ^ I 1BWest

Ma

Parece Vela Basin

54

East

2 -

4 -

6 -

8 -

1 0 -

1 2 -

1 4 -

16

mI-tHK

Figure 6. Lithofacies distribution by time: (A) ShikokuBasin; (B) Pαra:? Vela Basin. Lithofacies symbols ex-plained in Figure 3.

Since very little volcanic material was deposited atSite 449, and the 51 meters of section is largely pelagic,it is obvious that the Palau-Kyushu Ridge was sub-merged (not supplying sediments) at this time, and hadlittle or no active volcanism. If the time designation iscorrect for Site 53, the West Mariana Ridge could havebeen an early-Miocene source for volcaniclastics in theeastern Parece Vela Basin. However, Site 450 (whichoffsets Site 53) has no sediments definitely identified asearly Miocene. The Japanese Islands, to the north, mayhave already begun to supply volcaniclastics at Site 297.The age of the ash at Site 297 is in doubt, however. Asmall quantity of volcaniclastic material identified atSite 442 could have come either from the north or fromthe Palau-Kyushu Ridge on the west.

MIDDLE MIOCENE (16-10.5 m.y.)

The dominant lithofacies during this time interval ispyroclastic in the eastern Parece Vela Basin (Sites 53,54, 450), pelagic (both siliceous and calcareous biogen-ic) at Site 449 in the western Parece Vela Basin, andhemipelagic (mudstones) plus resedimented sandstonesand pyroclastics in the Shikoku Basin.

A calcareous biogenic facies is present at Sites 449and 444, and in the upper and lower parts of the intervalat Site 443. There also are minor amounts of calcareous

706

LITHOFACIES

biogenic facies at Site 450, and at the base of the intervalat Site 54. The siliceous biogenic facies is present at Sites449 and 444, and (a very minor occurrence) in the lowerpart of the interval at Site 442. Pelagic non-biogenicclays are present in the upper part of the sequence at Site450, and are a very minor component at the bottom ofthe interval at Site 444.

Resedimented mudstone occurs, along with hemipel-agic mudstone, in the lower half of the interval at Sites443 and 444. Pyroclastics constitute almost all the inter-val at Sites 450, 53, and 54, and they are very prominentbut decrease upward at Site 297. The pyroclastics arealso significant at Site 443, especially in the lower half.They are present at Site 444, and prominent in the lowerhalf of the interval at Site 442.

The thickest sediments (up to 292 m) deposited dur-ing this time interval are at Sites 450, 54, and 53, in theeastern Parece Vela Basin, where they virtually form apyroclastic wedge of sediments. In the Shikoku Basin,the section at Site 443 is thickest (141 m ?), followed bythat at 297, then 444. The thinnest section is at Site 442(48 m).

The facies and thickness distributions for this time in-terval suggest the following interpretation: The Japan-ese Islands to the north supplied hemipelagic and pyro-clastic sediments at Site 297, where a clastic wedgebegan to develop at this time. The volcanic materialsand mudstones at Site 442 could have come from thenorth as well, but it is also possible that their source wasa slightly elevated and volcanically active Palau-KyushuRidge during the early middle Miocene. The Iwo JimaRidge was volcanically active, especially during the firsthalf of the middle Miocene, and was also an emergentsource area for the resedimented and hemipelagicmudstones. A clastic wedge began to develop in theeastern Shikoku Basin during this time.

In the eastern Parece Vela Basin, the West MarianaRidge was a very active volcanic source, becomingslightly less active toward the latest middle Miocene.The slight amount of volcanics at Site 449, along withthe pelagic sediments, indicates that the southern part ofthe Palau-Kyushu Ridge probably was still submergedand was not supplying sediments to the western PareceVela Basin.

LATE MIOCENE (10.5-5 m.y.)The dominant lithofacies in the Shikoku Basin for

this time interval is hemipelagic. In the Parece VelaBasin, the dominant facies is a combination of hemi-pelagic clay and pelagic non-biogenic clay.

The calcareous biogenic facies is present in the upperpart of the interval at Sites 443 and 444. The siliceousbiogenic facies is present at Site 444 in the lower part ofthe interval, and in very minor amounts along with thecalcareous facies in the lower part at 443. A siliceousbiogenic facies is present also at Site 53, and, with thecalcareous facies, at 450.

Site 297 has prominent resedimented sandstones.Pyroclastics are prominent at Sites 443 and 444, and inthe lower half of the interval at Sites 450 and 53.

The sediments are thickest at Site 297 (157 m), andthinnest at Site 442. The sequence at Sites 443 and 444 is

thick (98 and 95 m) compared with the thin sequences at442 and in the Parece Vela Basin.

Thickness distribution and facies distribution lead tothe interpretation that the principal sources of sedimentduring the interval were to the north and east of theShikoku Basin. Clastic wedges continued to accumulateat Site 297, 443, and 444. The Japanese Islands wereemergent, but there is no indication of active volcanismin that area. The Iwo Jima Ridge was emergent andprobably had minor volcanism, especially during theearly part of the late Miocene. The northern part of thePalau-Kyushu Ridge could have been supplying sedi-ments at a very low rate at Site 442, or these thinsediments could have come from the Japanese Islands tothe north.

In the Parece Vela Basin, almost no sediments werebeing supplied on the west at Site 449 from the probablysubmerged Palau-Kyushu Ridge. On the east, minorvolcanism in the West Mariana Ridge was supplyingpyroclastics at a relatively low rate, decreasing fromearlier to later late Miocene.

PLIOCENE (5-1.6 m.y.)The lithofacies of this interval is almost entirely

hemipelagic, with prominent resedimented sandstonesin the northern Shikoku Basin at Site 297 in the lowerhalf of the interval. Minor pyroclastics are present in theeastern part of both basins (Sites 443, 444, 450, and 53).Although no ash is present at Site 442, Chamley (thisvolume) has observed abundant clays of volcanic originat this site for this time interval.

Pelagic non-biogenic sediments are present at PareceVela Basin sites. Biogenic facies are conspicuously ab-sent. Sites 443 and 444 have very minor occurrences ofcalcareous biogenic components.

The thickest sedimentary sequence in this interval isat Site 297 (275 m). A thickness of 95 meters at Site 443,eastern Shikoku Basin, and 75 meters at 442, westernShikoku Basin, indicate relatively rapid sedimentationat these sites. Very thin sequences were deposited in theParece Vela Basin, and at Site 444.

Facies and thickness suggest an emergent source tothe north in the Japanese Islands, from which a clasticwedge continued to be deposited in the northern Shiko-ku Basin. The sequence at Site 442, including hemipel-agic mudstones and volcanic clays, could have comefrom the Palau-Kyushu Ridge, or from the north andfrom a more distant source for the volcanic clays. Awestern Shikoku Basin clastic wedge began no earlierthan Pliocene time. An emergent Iwo Jima Ridge on theeast, with minor volcanism, supplied sediments in theeastern Shikoku Basin. The southern part of the Palau—Kyushu Ridge and the West Mariana Ridge were prob-ably submerged, because very little clastic sediment wasbeing supplied to the Parece Vela Basin.

PLEISTOCENE (1.6-0 m.y.)The dominant Pleistocene facies in the Shikoku Basin

is hemipelagic mudstone. In the Parece Vela Basin,almost no sediments were deposited (recovered?), ex-cept for very thin deposits of pelagic non-biogenic clays.

707

D. M. CURTIS, D. J. ECHOLS

Calcareous biogenic sediments are prominent in themiddle Pleistocene at Site 297 and in the early Pleisto-cene at Sites 443 and 444. The siliceous biogenic facies iscommon in late-Pleistocene sediments at Sites 297 and442. Minor volcanism produced ash deposits at Sites 443and 444, lesser deposits at 442, and only in the latePleistocene at 297.

The thickest Pleistocene sequences are at Sites 297(162.5 m), 442 (123.5 m), and 443 (121 m). The thick-ness and facies distribution indicate that a northernsource continued to supply clastic sediments at Site 297until mid-Pleistocene time. The appearance of theplanktonic calcareous biogenic facies and clay at thissite probably marks the inception of the Nankai Troughin mid-Pleistocene time, restricting the previously rapidaccumulation of hemipelagic muds in the northern Shi-koku Basin. Development of the northern clastic wedgeceased at this time. The Iwo Jima Ridge on the east andthe northern Palau-Kyushu Ridge on the west wereemergent during this time and continued to supply hem-ipelagic mudstones to the western and eastern clasticwedges, but volcanism was not prominent on eitherridge. The West Mariana Ridge and the southern Pa-lau-Kyushu Ridge were low or submerged.

SUMMARY AND CONCLUSIONS

Generalized dominant facies sequences in each basinare as follows:Northern Shikoku BasinLate Pleistocene

Western Parece Vela Basin

Early Pleistocene andPliocenePliocene andlate Miocene

Late Miocene andmiddle MioceneMiddle Miocene andearly Miocene

Eastern Shikoku BasinPleistocene andPlioceneLate Miocene

Middle Miocene

Early Miocene

Western Shikoku BasinPleistocene, Pliocene,and late Miocene

Middle Miocene

Early Miocene

Pelagic biogenic (calcareousand siliceous); hemipelagic;pyroclasticHemipelagic mudstone

Hemipelagic mudstone andresedimented sandstone

Hemipelagic mudstones

Pyroclastics

Hemipelagic mudstones withminor pyroclasticsHemipelagic mudstones andpyroclasticsResedimented sandstones, pe-lagic biogenic sediments, andpyroclasticsPelagic clays

Hemipelagic mudstones; pos-sibly volcaniclastic clays in thePlioceneHemipelagic mudstones andpyroclasticsPelagic clays and pyroclastics

Pleistocene, Pliocene,and late MioceneMiddle Miocene

Early Miocene

Pelagic clay and hemipelagicclayPelagic biogenic sediments andpyroclasticsPelagic clays with minor pyro-clastics

Eastern Parece Vela BasinPleistocenePlioceneLate MioceneMiddle Miocene andearly Miocene

Pelagic claysPelagic claysPelagic clays and pyroclasticsPyroclastics

A summary of the geologic history of sedimentsource areas, and the accompanying sedimentation ratesin the basins, follows:Japanese Islands (northern source) and northern Shiko-ku BasinPleistocene Source emergent; inception of Nan-

kai Trough in mid-Pleistocene; de-velopment of northern clastic wedgeceased in mid-Pleistocene time; vol-canism in the late Pleistocene; sed-imentation rate 101.6 m/m.y.

Pliocene Source emergent; no volcanism; north-ern clastic wedge; sedimentation rate81 m/m.y.

Late Miocene Source emergent; no volcanism; north-ern clastic wedge; sedimentation rate28.5 m/m.y.

Middle Miocene Source emergent; active volcanism;northern clastic wedge began; sedi-mentation rate 18.2 m/m.y.

Early Miocene Active volcanism; insufficient evi-dence for other conclusions

Northern Palau-western ShikokuPleistocene

Pliocene

Late Miocene

Middle Mioceneand earlyMiocene

-Kyushu Ridge (western source) andBasin

Source emergent; little volcanism;western clastic wedge; sedimenta-tion rate 77.2 m/m.y.Source emergent; no volcanism evi-dent, except clays of volcanic origin;western clastic wedge began; sedi-mentation rate 25.2 m/m.y.Source very low, emergent; no vol-canism; sedimentation rate 3.5 m/-m.y.Source emergent, low; active vol-canism; sedimentation rate 8.7 m/m.y.

Iwo Jima Ridge (eastern source) and eastern ShikokuBasinPleistocene Source emergent; very minor vol-

canism; eastern clastic wedge; sedi-mentation rate 33 to 75.6 m/m.y.

708

LITHOFACIES

Pliocene

Late Miocene

Middle Miocene

Early Miocene

Source emergent; minor volcanism;eastern clastic wedge; sedimentationrate 8.5 to 27.9 m/m.y.Source emergent; minor volcanism;eastern clastic wedge; sedimentationrate 17.8 m/m.y.Source emergent; active volcanism;eastern clastic wedge began; sedi-mentation rate 17.3 to 25.6 m/m.y.No ridge

West Mariana Ridge (eastern source) and eastern PareceVela BasinPleistocene

Pliocene

Late Miocene

Middle Miocene

Early Miocene

Source submergent; no volcanism;sedimentation rate <3.5 m/m.y.Source submergent; minor active vol-canism; sedimentation rate 3.5(?)m/m.y.Source submergent; active volcan-ism; sedimentation rate 4.2 to 7.8m/m.y.Source submergent; active volcan-ism; sedimentation rate 21.4 to 53.1m/m.y.Source submergent; active volcan-ism; no thickness data

Southern Palau-Kyushu Ridge (western source) andwestern Parece Vela BasinPleistocene Source submergent; no volcanism;

sedimentation rate 0 to 2.5 m/m.y.Pliocene Source submergent; no volcanism;

sedimentation rate 3.2 m/m.y.Late Miocene Source submergent; no volcanism;

sedimentation rate 5.5 m/m.y.Middle Miocene Source submergent; minor volcan-

ism; sedimentation rate 2.5 m/m.y.Early Miocene Source submergent; minor volcan-

ism; no thickness dataEarly-Miocene active volcanism took place in the

area of the Japanese Islands, northern Palau-KyushuRidge, and West Mariana Ridge. Middle-Miocene active

volcanism continued in the area of the Japanese Islands,northern Palau-Kyushu Ridge, and West MarianaRidge, and began on the Iwo Jima Ridge. Late-Mioceneminor volcanism took place only on the West MarianaRidge. Pliocene volcanism was minor. Pleistocenevolcanism was generally minor on Iwo Jima Ridge andin the Japanese Islands during the late Pleistocene. Weassume that the relative geographic positions of thesefeatures remained the same, whichever spreading modelis used.

During the early Miocene, prominent sources ofsediments did not exist. Middle-Miocene sources pro-ducing high sedimentation rates were the JapaneseIslands and the Iwo Jima Ridge. During the lateMiocene, the Japanese Islands and Iwo Jima Ridge werethe ony prominent sources of sediments. Pliocene sedi-ment sources were the Japanese Islands, Iwo JimaRidge, and the northern part of Palau-Kyushu Ridge.During the Pleistocene, prominent sources were theJapanese Islands (early Pleistocene only), northernPalau-Kyushu Ridge, and Iwo Jima Ridge.

Sedimentation rates (m/m.y.) are tabulated below:

PleistocenePliocene

Late MioceneMiddle Miocene

Early Miocene

N. Shikoku W.

101.6

81

28.518.2

Shikoku

77.2

25.2

3.58.7

E. Shikoku

33-75.6

8.5-27.9

17.8

17.2-25.6

(insufficient data to permit calculation

E. Parece

<3.5

3.54.2-7.8

21.4-53.1

of rates)

W. Parece

0-2.53.2

5.5

2.5

The facies are typical for marginal or back-arc basinsof this type, consisting predominantly of hemipelagicand volcaniclastic sediments with very minor pelagiccomponents. Thickness relationships and depositionalsequences for each time interval suggest the time-variable development of coalescing clastic wedges atseveral loci within the basins, and indicate the timing ofvolcanism in neighboring volcanic arcs.

ACKNOWLEDGMENTS

This manuscript was reviewed by John Warme, ColoradoSchool of Mines, and Lee Russell, Shell Development Com-pany.

709