19. VOLCANICLASTIC CONSTITUENTS IN THE LEG 55 SEDIMENTS

Ivar Murdmaa, P. P. Shirshov Institute of Oceanology, U.S.S.R. Academy of Sciences, Moscowand

G. P. Avdeiko, Institute of Volcanology, Far East Science Center, U.S.S.R. Academy of Sciences,Petropavlovsk-Kamchatsky

This chapter was previously intended to trace volcan-ic episodes through the Neogene and Pleistocene geolog-ical history recorded in the sedimentary sections drilledon the Emperor seamounts. Drilling disturbance, poorcore recovery, and incomplete stratigraphic sections re-covered from the seamounts have frustrated that plan,however. Moreover, the Leg 55 sedimentologists foundin their smear-slide studies that transported island-arctephra is scarce in the sediments, if present at all. So wehave restricted our objective to description of the vol-caniclastic admixture in sediments, as determined bymineralogical and geochemical data. We studied geo-chemistry of bulk samples (see Murdmaa et al., thisvolume), coarse-fraction mineralogy, and additionalsmear slides. The results obtained, however, do not tellmuch more about the volcaniclastic matter than didshipboard core descriptions.

DISTRIBUTION AND COMPOSITION OFVOLCANICLASTIC MATTER

The sediment section from Site 430 on öjin Sea-mount comprises Paleocene and Eocene shallow-watercalcareous (clastic) ooze and sand, interbedded near thebase with 11.8 meters of volcaniclastic sediments, con-taining, according to shipboard smear-slide descrip-tions, 60 per cent volcanic glass. The upper portion ofthe section in Hole 430 contains mixed Eocene to Recentmicrofossils, ice-rafted and local rock fragments, andmanganese crusts and micronodules, and so represents a"non-depositional" environment on the guyot top. Ex-cept in the volcanic sand, volcanic glass occurs in smallamounts (5% or less, according to shipboard determina-tions).

We studied smear slides prepared from five samplesused in geochemical bulk analysis of the Site 430 sedi-ments (Murdmaa et al., this volume). The Al:Ti ratio inthe sediments is low (4.1 to 6.4), indicating, along withthe low SiO2:Al2O3 ratio (2.7 to 3.7), that the non-biogenic matter in the sediments is probably basaltic. Insmear slides, badly altered volcaniclastic particles arecommon; these are yellowish brown or orange peliticferruginous (smectite?) aggregates. But true alteredbasalt fragments also occur, as do zeolite crystals andrare pyroxenes. Alteration of basaltic clasts to soil isconfirmed by X-ray data (this volume) and by high K2Ocontent with respect to SiO2 and A12O3 contents.

A single coarse-fraction count (Table 1) shows a pre-dominance of ''unidentified" aggregate grains (mainly

altered basaltic glass) in the light fraction, whereas ironoxides and "black opaques" (Fe-Mn crusts) constitutemore than 80 per cent of the heavy fraction.

At Site 431, crushed manganese crusts, micronod-ules, zeolites, palagonite, and ice-rafted pebbles wererecovered. The mixture contains Quaternary microfos-sils, and indicates a long-period "non-depositional" en-vironment. Altered volcaniclastic constituents (phillip-site, palagonite) are undoubtedly older than the micro-fossils found in the cores.

Our four bulk analyses for Site 431 show low Al:Tiratios (1.3 to 8.2), indicating basaltic volcaniclastics.But the SiO2:Al2O3 ratio is rather high (4.0 to 4.9),perhaps because the samples contain terrigenous quartzand authigenic (?) chalcedony. In smear slides we found— along with ferromanganese crusts, phosphates, andcalcareous clasts — yellowish brown, completely de-composed basaltic glass and smectite matrix, probablya weathering product of volcaniclastic matter.

At Site 432 on the Nintoku Seamount, a thin cover ofQuaternary foraminiferal ooze is underlain by Eoceneshallow-water calcareous ooze, volcanic sand, sand-stone, and conglomerate (see core descriptions, thisvolume). A lateritic soil interbed, formed after basalticvolcaniclastic matter during subaerial weathering, oc-curs between two lava flows in Hole 432A. The volcani-clastic sand from Hole 432, according to shipboardsmear-slide and thin-section descriptions, is composedof volcaniclastic (basaltic, weathered basaltic) particles,mixed with carbonate clasts, Mn micronodules, fora-minifers, and quartz. In thin beds of volcanic sandstoneand conglomerate just over basalts in Hole 432A, thevolcanic particles, apparently epiclastic, are mixed withshallow-water benthic carbonate clasts, and are cement-ed by calcite.

A bulk analysis of calcareous-volcanic sand (seeMurdmaa et al., this volume, table 1, Sample 432-1-4,73-76 cm) shows characteristic evidence of alteredbasalts: low Al:Ti ratio (6.6), high K2O and Fe2O3 con-tents. In the smear slide we found abundant yellowishbrown aggregates and smectite (?) matrix of a similarcolor. The laterite soil sample is described in Murdmaaet al. (this volume).

In the upper (pelagic) section at Site 433 — lithologicUnits 1 and 2 (upper Miocene to upper Pliocene) — vol-caniclastic constituents are virtually absent, accordingto shipboard smear-slide descriptions (this volume). In asmear slide from the surface core of Hole 433, we found

503

I. MURDMAA, G. AVDEIKO

TABLE 1Coarse-Fraction (0.1 to 0.05 mm) Mineralogy

Sample,Interval (cm)

430-2-1,52-60432-1-2, 33-37433A-1-1, 10-15433A-1-2, 72-74433A-2-1, 30-35433A-4-1, 80-81433A-4-3 64-fifi~ J JA ^ J , v " \J\)

433A-5-2, 60-65433A-9-4, 45-47433A-10-3, 70-74433A-13-1, 30-36433A-19-1, 32-37433B-2-1, 19-23

paqu

esB

lack

o

384

172

1615

_-9114

Pyr

ite

____—+

17-1940

333

Fe

ox

id4845

561

840

724537388652

Gar

net

1_1+11

_+___-

nde

Hor

nble

21014

2141

1121

+1

Heavy Fraction

'ro

xen

eO

rth

op

)

11

12+16

6

+2

11411

rox

ene

Cli

nopy

32432

93113

5-12

372

gro

up

Ep

ido

te

127242

+-11_1

Zir

con

+

_1_]_

_

__

1_1

Ap

atit

e

+

11_1+

_-1__

-

Sph

ene

__+__1

+-+_

+

Bar

ite

__+_8_2

-+__-

ifie

dU

nide

n 1

61323_13_2

538

1211

15

Qu

artz

11+8+1

+-6+2-

rN <

1.

54F

elds

pa:

+1+2_1

+-21--

ise

N >

1.

54P

lagi

ocl;

1-18_-

+-6_--

Light Fraction

Zeo

lite

_-

-

-

_--+26-

S3- 1

Vol

e, g

l

_

11

222036

23

-12+7

bo

nat

eO

rg.

car

2591773512141 0j . \J

84100

579755

100

a.oaó

o_-11

593886ou

3---3

-

ifie

dU

nide

nl

738

1723

81022

-17

1--

a few colorless and light brown glass shards, along withhornblende, biotite, plagioclase, epidote, and severalother heavy minerals. We also found glass (very scarce)in smear slides from Sections 433A-3-1 and 433A-5-1.Altered basaltic (?) glass occurs in Sample 4-2, 50-55cm.

The coarse (0.1 to 0.05 mm) light fraction separatedfrom the diatom-nannofossil ooze of Cores 433A-1 to433A-5 (Table 1) contains 12 to 36 per cent acidicvolcanic glass with a low refraction index (less than1.54). The glass predominates over other light clasticmineral grains, which are feldspar, quartz, and ag-gregates (including green chlorite or glauconite aggre-gates). In the heavy fraction, pyroxenes (perhaps vol-canic) predominate; clinopyroxene (augite) is approxi-mately twice as abundant as orthopyroxene (hypers-thene). Most of other heavy minerals — hornblende,garnet, zircon, epidote, apatite, sphene — are probablyterrigenous.

We have not noted any chemical evidence of basalticvolcaniclastic matter or its alteration products in the up-per Pliocene portion of the pelagic sequence (Murdmaaet al., this volume). The Al:Ti ratio is high (24 to 34),and indicates acidic rather than basaltic volcaniclastics.In Cores 4 and 5, this ratio decreases somewhat (16 to18), and we assume that a minor admixture of basalticproducts may be present. Smear-slide data confirm this.

Sample 433A-6-7, 44-46 cm contains lower Miocenesiliceous-calcareous ooze, transitional from a pelagic toa shallow-water ooze, and composed of nannofossils,spicules, radiolarians, palagonite, altered basalticgrains, and scarce heavy minerals (clinopyroxene, bio-tite). Its bulk chemical composition (Murdmaa et al.,this volume) shows a low Al:Ti ratio (9), indicating ba-saltic volcaniclastics. The basaltic particles, however,are epiclastic, sub-rounded, and reworked during trans-portation in high-energy bottom waters. Depositiontook place probably on an upper slope.

In the Paleocene to Eocene shallow-water sediments(Unit 5), volcanic clasts occur occasionally in tracequantities, as noted by the shipboard scientists. We alsofound scarce altered basaltic volcanic glass (palagon-ite?) in most smear slides of the sequence. A relativelylow Al:Ti ratio (10 to 16) provides additional evidenceof basaltic matter. It is most abundant in the calcareousinterbed between two lava flows (Section 433C-3-3),where the Al:Ti ratio is lowest (5.9). In Sample433A-19-1, 32-37 cm, abundant zeolites (phillipsite)occur in the light fraction (Table 1).

CONCLUSIONSIn the Paleogene shallow-water sediments, which

were deposited on the seamounts simultaneously withbasaltic volcanism or just after its cessation, the volcani-clastic admixture is entirely of basaltic composition, andwas probably derived from local sources. Most of thebasaltic matter in these sediments is epiclastic, previous-ly weathered, and partly rounded in an active wave envi-ronment.

Fine-grained acidic tephra, derived from island arcvolcanoes, was found only in upper Miocene to Pliocenepelagic sediments, but even there only in small quan-tities. Ash layers, which are widespread in the North Pa-cific abyssal floor hemipelagic sediments (Hays andNinkovich, 1970; Repechka, 1974) and in Bering Seasediments (Hein et al., 1978) were not found on the sea-mounts.

ACKNOWLEDGMENTS

This paper was reviewed by Dr. G. N. Baturin and Prof. A.P. Lisitsin, both of the Institute of Oceanology, Academy ofSciences of U.S.S.R., Moscow, U.S.S.R.

REFERENCES

Hays, J. D. and Ninkovich, D., 1970. North Pacific deep-seaash chronology and age of present Aleutian underthrusting.

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VOLCANICLASTIC CONSTITUENTS IN SEDIMENTS

In Geological Investigation of the North Pacific: Geol. Soc. Repechka, M. A., 1974. Ash layers in bottom sediments in theAm. Mem. 126, p. 263-289. transitional zone between Asian continent and Pacific. In

Hein, J. R., Scholl, D. W., and Miller, J., 1978. Episodes of The Problems of Geology and Geophysics of the MarginalAleutian ridge explosive volcanism, Science, v. 199, p. 137- Seas of NW Pacific: Vladivostok, p. 26-41 (in Russian).141.

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