22. LITHOLOGIC-GENETIC CHARACTERISTICS OF SEDIMENTS IN A SECTIONAT SITE 463, DEEP SEA DRILLING PROJECT LEG 621

P. P. Timofeev, N. V. Renngarten, and V. V. Eremeev, Geological Institute of the U.S.S.R.Academy of Sciences, Moscow, U.S.S.R.

INTRODUCTION

Site 463 is situated in the northwestern part of theunderwater slope of the Mid-Pacific Mountains, at adepth of 2525 meters, the hole penetrated 820 meters ofsedimentary rocks and stopped before reaching the ba-saltic basement. The Mesozoic part of the sedimentarysection is roughly 765 meters thick; the upper 50 to 55meters are Cenozoic.

Deposits of the upper Barremian, Aptian, Albian,Cenomanian, and Turonian are most completely pre-sented here. During Barremian-Turonian time, thick(~ 565 m) sediments accumulated. These are overlain byrelatively thin (~80 m) deposits of the lower Turo-nian-Coniacian and upper Campanian. The Mesozoicpart of the section terminates with Maastrichtian beds,the total thickness being 130 meters. A hiatus begins atthe end of the upper Maastrichtian and ends at the be-ginning of the Eocene. Cenozoic deposits, approxi-mately 55-meters thick, are represented by lower Eo-cene, upper Oligocene, upper Miocene, lower and upperPliocene, and Pleistocene.

Sediments are dominantly biogenic, some purely car-bonate, some siliceous carbonate, and some essentiallysiliceous. The rocks composing the middle part of thelower Aptian deposits (Cores 69-74) are an interestingexception. These rocks are biogenic and pyroclastic.Quite different rocks, not biogenic, are found in thelowermost part of the section. These are breccias andsandstones, consisting of fragments of limestones, mol-lusk shells etc.

The shipboard party subdivided the section at Site463 into four lithologic units, and the uppermost unit(Unit I) was in turn subdivided into two sub-units.

The base of the section (Unit IV) contains rhyth-mically alternating clastic (shallow-water) and pelito-morphic (deep-water) limestones.

The overlying unit (Unit III) includes different rocks:limestones, marls, claystones, tuffs, and calcareous-sili-ceous rocks containing up to 4.3% organic carbon.

The second unit (Unit II) is composed of multi-colored limestones.

The lower sub-unit of Unit I (i.e., Sub-unit IB) con-sists of foraminifer-nannofossil and nannofossil sedi-ments of different degrees of lithification (limestone,chalk). The upper sub-unit (Sub-unit IA) is composed ofnannofossil oozes.

Initial Reports of the Deep Sea Drilling Project, Volume 62.

The shipboard subdivision of the section was madeduring visual examination of the cores, on the basis ofsuch features as color, degree of lithification, grain size,and sediment composition.

Laboratory treatment of the material (microscopicand other investigations of rocks) enabled us to specifyin detail the lithologic characteristics of the section,establish facies and mineralogical criteria for subdivi-sion of the section, and elucidate certain questions ofgenesis of the deposits and the history of accumulationof the section as a whole.

LITHOLOGIC AND FACIES ANALYSIS OF THESEDIMENTARY SECTION, SITE 463

Laboratory analysis of the core material from Site463 was intended to (1) reveal facies environments of ac-cumulation of primary sediments yielding Meso-Ceno-zoic rocks, (2) reproduce the geological history of for-mation of this section, reflected in particular in sedi-mentary structures and characteristic of post-sedimen-tary changes of rocks.

Extra attention was paid to one of the main principlesof genetic lithology that can be formulated as follows:the characteristics of each sedimentary rock are sub-divided into two groups: (1) all features inherited fromthe initial sediment and called primary or genetic; thisgroup of features reflects the facies conditions of ac-cumulation and the first stages of diagenesis of sedimen-tary material; (2) features acquired by a rock at variousstages of post-sedimentary changes in the process of for-mation and diagenesis of the pile; these features arecalled secondary, or superimposed; they can mask con-siderably the primary features of a sediment.

At Site 463, the primary features are (1) compositionof faunistic remains and relationships of biogenic formsof SiO2 and CaCO3, (2) the proportion of clastic (re-deposited) material, clay, and biogenic remains in thesediment, (3) admixture of ash particles, (4) presence ofsapropelic material, (5) character of primary laminationand degree of bioturbation, (6) diagenetic microlamina-tion in ooze. Primary (early-diagenetic) features includeauthigenic montmorillonite, zeolite, and siderite ap-pearing in decomposed volcanic glass under influence ofmineralized thermal waters in fresh sediment. Second-ary features of rocks are (1) recrystallization of organicremains, (2) leaching of biogenic silica in some cases andbiogenic carbonate in others, (3) transformation of or-ganic SiO2 into globular opal (in a stage of late diagene-sis), (4) transformation of amorphous SiO2 into a crys-talline variety optically determined as chalcedony and

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by X-ray examination as cristobalite, (5) replacement ofskeletal remains of radiolarians by calcite, and in fillingof voids in tests and sinuous fissures in hard rock withcalcite and barite. Traces of post-sedimentary changesof rocks gave an idea of geological factors controllingaccumulation of the sedimentary section. For example,leaching of biogenic silica from a rock followed by re-placement by calcite and filling of fissures with calcite(sometimes together with barite) suggests penetration ofalkaline solutions in an already lithified rock. Abundantpyroclastic material in rocks, with indications of dia-genetic reworking and participation of mineralized wa-ters, testifies to an outburst of volcanic activity in corre-sponding areas of the ocean, and to the proximity ofgeothermal fields.

As mentioned above, the Meso-Cenozoic sedimentsare divided into four parts (units), according to thegross features of rocks and their general composition.The study of primary features of initial sediments andsecondary transformations of rocks showed that such asubdivision is expedient; we suggest that the boundariesof some units be changed, and that the units themselvesbe characterized by different lithologic-genetic features.In addition sub-units were recognized in all units.

Thus, four successive series (units) of deposits wereoutlined in the Cenozoic-Mesozoic section at Site 463.Each of the units reflects the general environment ofsedimentation, which was rather stable within the re-gion. This environment and corresponding deposits arecalled a macrofacies. The regular change of macrofaciesthrough the section enables us to establish the majorstages of development of geological events within a re-gion. It is natural that in any part of a region local, less-stable conditions of sedimentation could exist. As aresult, the sediments acquired specific features in addi-tion to those peculiar to the given macrofacies. Thesemore-local conditions of accumulation of sedimentarymaterial are given the name facies. A macrofacies mayinclude several facies. If facies conditions originatedfrom a regular combination of sediments differing ingrain size and composition, each distinguished sedimentis regarded by us as a genetic type. A facies in turn canbe represented by several genetic types.

Figure 1 presents a scheme of subdivision of the sec-tion into units and sub-units, and brief characteristics(in historical succession) of sedimentation conditions ona regional scale {macrofacies).

The numbers of units and other subdivisions of thepile begin from the top and proceed downward, asadopted aboard the Glomar Challenger; description ofthe pile is carried out from bottom to top in the section,i.e., in chronological order.

Unit IV, Macrofacies 4 (Cores 75-92)*

Two sharply different types of rocks alternate in thisseries of deposits: (1) clastic carbonate rocks containingfragments of oolitic and organic-detrital limestones,

No samples were collected below Core :

etc., (2) biogenic, pelitomorphic limestones with skeletalremains of radiolarians.

All rocks of the macrofacies are characterized bystrong lithification and secondary carbonatization. Fis-sures and micropores filled with granular calcite (some-times with barite) are common. Radiolarian remains al-most everywhere do not preserve primary meshy micro-structure: they are represented now by globular opal—by globular opal with chalcedony, or by pure chalced-ony (cristobalite). In some cases, as result of secondaryprocesses, radiolarian skeletons were either entirelyleached out (and the rocks acquired micropore struc-ture), or metasomatically replaced by calcite (CaCO3content increased to 95%). One may think that the pro-cess of changing the primary opal skeleton of radiolari-ans proceeded by stages: at one stage of diagenesis theorganogenic SiO2 dissolved and reprecipitated as globu-lar opal; then, during epigenesis, the opal was recrystal-lized into cristobalite (chalcedony).

It is noteworthy that no foraminifer tests were ob-served in rocks of Unit IV.

Clastic Limestones or Calcareous Breccias(Cores 90, 88, 83-85, 79-81)

The clastic material of these rocks contains unsorted(clasts range from a millimeter to 2-3 cm) and com-monly angular fragments of organogenic-detrital andoolitic limestones (with granular, recrystallized cement);limestones with micro-lumpy structure; mollusk shells(some silicified); numerous single (broken and un-broken) oolites and their aggregates; a negligible admix-ture of silty and smaller particles of quartz, feldspar,chlorite, glauconite, and montmorillonite. There arealso remains of radiolarian skeletons formed of chal-cedony, and rare fragments of siliceous sponge spicules.Clastic material is cemented by pelitomorphic carbonatethat sometimes includes sinuous, thin veins of calcite. Itis noteworthy that almost every fragment of limestone,tests, oolites, etc. is coated by a thin, cryptocrystallinefilm that frequently grades into the cement of rocks andtherefore hardly can be recognized. This film likelyoriginated during transportation of the materials underconditions of dense, paste-like flow.

Pelitomorphic Biogenic Limestones(Cores 91-92, 89, 86-87, 82-83, 78, 75-78)

The ground mass of limestones is cryptocrystallineand strongly lithified; it contains rare semi-dissolvedcoccoliths—indicators of the biogenic nature of theoriginal sediments. The amounts of radiolarian remainsare variable; siliceous sponge spicules are common.Besides silica, there are appreciable amounts of claymatter (dominantly montmorillonite) in the carbonate-free part of these rocks; particles of quartz, feldspars,and chlorite are present. The thinnest clay interbeds(and short lenses) enriched with carbonaceous matter ofsapropelic origin are an exception. In some places,pyrite forms small authigenic accumulations and fre-quently develops after radiolarians. Radiolarian skele-tons formed by globular opal usually include mont-morillonitic particles within the globules.

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STRATIG-RAPHY LITHOLOGY UNIT (THIS PAPER)

MACROFACIES: CHARACTERISTICSOF SEDIMENTATION ENVIRONMENT

Pliocene

u. Miocene

Eocene

upper

Campanian

_33_

Pelitomorphic nannofossil, foraminifer-nanno-fossil and, nannofossil-foraminifer rocks. By degreeof lithification the Mesozoic part of the sectionis represented by "chalks," the Cenozoic by"oozes." Mesozoic deposits contain beds of chert(with relics of radiolarians and foraminifer tests,completely replaced by silica). Cherts are particu-larly abundant in upper Albian—Turonian rocks(Sub-unit I r J . Higher in the section, the pro-portion of cherty layers gradually decreases, andin Maastrichtian deposits (Sub-unit ! „ ) , it is rela-tively small. In Cenozoic oozes, no biogenic silicawas found. Mesozoic rocks are white, slightlypale-yellow, whereas Cenozoic rocks are predom-inantly pale-brown.

Deep-sea zone (2500-3000 m). Only biogenicsediments with a minimum admixture (5—10%)of dispersed clay material were accumulated. Inthe upper Albian—Turonian, permanent accumula-tion of nannoplankton remains and foraminifertests took place, with periodic influx, sometimesrather abundant (Sub-unit I Q ) , of biogenic silica(predominantly skeletons of radiolarians, rarelysponge spicules). The Coniacian—Maastrichtiandoes not show particularly high productivity ofsiliceous organisms. Mostly carbonate-biogenicsediments, with rare interlayers of biogenic silica.The Cenozoic (Facies 1) is characterized by com-mon hiatuses and very slow sedimentation. Thincarbonate oozes were deposited. No accumulationof silica took place.

I . I

Pelitomorphic limestones with foraminifers,frequently with shell detritus and interlayers richin radiolarians. Their color is diversified: darkermottles, lenses, wavy-discontinuous bands againstwhite and pale yellow-pink background. Suchcolor results from uneven pigmentation of lime-stones by dispersed Fe-hydroxides.

Relatively deep-sea zone. Only fine fragmentsof shell detritus and Fe-bearing colloid could reachthe zone from the shelf. Remains of nannoplank-ton and foraminifer tests composed the bulk ofsediments, radiolarian skeletons being sometimesnumerous. Rate of accumulation was low; theoxidizing film was retained on the surface of sedi-ments for a long time; and oozes were colored byFe-hydroxides with pale-yellow-pink color.

67 I I

I I

IIILimestones with radiolarians, tuff, tuffogenic

montmorillinitic siltstones, calcareous radiolariteswith organic carbon (up to 7%). Diagenesis led toformation of montmorillonite, zeolite, siderite,calcite. The rocks are dark grayish-green.

Ill

IIIIII ç

III

3 Relatively deep-sea zone. It accumulatedremains of nannoplankton, biogenic SiO~, andvitroclastics. Sometimes organic-mineral colloidswere brought into the sediments. Ash falls increas-ed the sedimentation rate; fresh ash activatedmineral formation during diagenesis.

"8 =

IV

Intercalation of pelitomorphic limestones (withradiolarians) and calcareous breccia (composed offragments of oolitic and other limestones, mollusksshells, etc.). The rocks are strongly lithified, withintersecting veinlets of calcite and barite. Manyskeletons of radiolarians are completely leachedand replaced by calcite. Limestones are white orpale-yellow, the breccia grayish-pale-yellow andpale-grayish-green.

IV

IV

Relatively deep-sea zone, but adjacent to themargin of an island shelf. Because of its position,mud flows bearing fragments of oolitic and otherlimestones, fragments of mollusk shells, etc.were periodically delivered into the area of purelybiogenic, radiolarian-nannofossil sediments fromthe shelf.

IV

Foraminifernannofossil ooze

Foraminifernannofossil chalk

Limestone

Limestone withradiolarians

Radiolarite withorganic carbon

Figure 1. Lithologic units and macrofacies at Site 463 (according to the results of this study).

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Clastic limestones form approximately four indepen-dent horizons within Unit IV, whereas pelitomorphiclimestones form six. Breccias of all three horizons aresimilar, their origin likely being related to one genetictype of sediments. Pelitomorphic limestones, on thecontrary, are diverse; this fact shows pecularities of in-itial sediments and differences in the processes of theirsubsequent transformations.

There is a certain regularity in the distribution ofclastic and biogenic limestones and lithogenetic types ofpelitomorphic limestones through the section. It allowsus to divide Unit IV into at least three sub-units. Eachsub-unit includes deposits of one facies zone withcharacteristic features of sediments (genetic types).

Figure 1 gives a detailed scheme of alternation ofrocks differing lithologically (and genetically) and com-posing Unit IV, facies distinguished by us (sub-units),and lithologic-genetic types of deposits. Unfortunately,we had very poor core recovery in this interval (12%).Therefore, the boundaries between heterogeneous bedswere drawn conditionally.

Sub-unit IVC, facies 4c (Cores 86-92) is character-ized by predominance (-70%) of pelitomorphic vari-eties of carbonate rocks over clastic ones (-30%). Pe-litomorphic limestones are frequently composed of car-bonate material (90-92%); skeletal remains of radiolari-ans are replaced by calcite; thin veins of calcite and ba-rite are frequently observed as well. Clay matter is innegligible amounts. Near brecciated horizons, the rockscontain fine shell detritus and single fragments of lime-stones.

Sub-unit IVB, facies 4b (Cores 79-85) differs fromthe underlying one by a reverse ratio of thickness of pe-litomorphic (-30%) and clastic limestones (-70%).The main difference consists in the character of pelito-morphic rocks; they originated from nannofossil oozesenriched in clay and radiolarians. The clay matter formssmall lenses and thin interbeds; some clay interbeds arecolored brown by colloidal sapropelic organic matter.The rocks show pinnate, wave-like, thin discontinuoushorizontal lamination.

Sub-unit IVA, facies 4a (Cores 75-78) is composedsolely of pelitomorphic limestones, which are ratherhomogeneous. The carbonate-free part of the rocks,amounting to not more than 10 to 15%, is representedby clay and amorphous SiO2. The groundmass of lime-stones is represented by cryptocrystalline carbonate anddisplays microporosity at the expense of completelyleached, tiny radiolarian skeletons. Only at the verybase of the sub-unit (Core 77, Section 1), radiolarian re-mains are replaced by calcite; there are veins filled withcalcite.

This series of deposits (Unit IV), characterized byalternation of lithologically and genetically differentrocks, is in the focus of attention of researchers. Yet,there is no unanimous opinion on general environmentof accumulation of initial sediments composing thisseries. The materials obtained from one site only, andpoor core recovery, do not allow us to speak with con-fidence of the origin of the section as a whole. Never-theless, certain assumptions are possible.

We believe that biogenic planktonic nannofossil andradiolarian oozes accumulated in a relatively deep-waterzone. But that zone adjoined a less-deep-water zone, azone of the peripheral part of island relief. Therefore,dislocation of boundaries of these zones took placeperiodically (likely because of tectonic factors). As a re-sult, biogenic carbonate-siliceous sediments alternatedwith limy-clastic, brecciated ones. When judged by com-position of clastic material of breccias, it follows thatthe parent rocks were represented by oolitic, lumpy, andskeletal clastic limestones of shallow-water genesis (withremains of echinoderms, mollusks, etc.). By the time ofredeposition these limestones had already been heavilylithified, partly recrystallized, and partly silicified. Theshallow-water limestones appear to be older than Barre-mian.

Shallow-water sediments in the central parts of theoceans are known to be due to the existence of islands ofvolcanic or tectonic origin (intense rise of separateblocks). Clastic material of Barremian breccias was sup-plied into the sedimentation area by means of densepaste-like flows. These flows were likely distributedover the periphery of the shelf.

It is noteworthy that the deposits grouped by us intoUnit IV underwent some post-sedimentary transfor-mations. They were strongly lithified; nannofossil re-mains were recrystallized into a homogeneous pelito-morphic mass, and most radiolarian skeletons were re-placed by either reprecipitated globular opal or cristo-balite. Some changes in rocks could have proceededbecause of hydrothermal processes. For instance, com-plete removal of SiO2 from rocks and replacement ofthe latter by carbonate should have taken place underinfluence of concentrated alkaline solutions.

Unit III, Macrofacies 3(Core 67, Section 3 to Core 74, Section 3)

This series of deposits, of lesser thickness (-45 m)than the underlying Unit IV is represented by lithologic-ally different, but genetically related rocks: radiolarianlimestones, tuffaceous siliceous-montmorillonitic clay-stones, calcareous radiolarites enriched with sapropelicmaterial, etc. General conditions of sediment accumula-tion of macrofacies 3 (Unit III) were considerably dif-fered from those of macrofacies 4 (Unit IV), in the fol-lowing: (1) almost no fragments of limestones were sup-plied to the zone of sediment accumulation; biogenicsedimentation was persistent; (2) volcanic ash (some-times abundant) was constantly supplied to sediments.As result of abundant ash falls, specific post-sedimen-tary (diagenetic) processes were proceeding in sedi-ments: authigenic montmorillonite, zeolite, and sideriteformed.

The regular change of lithologically different rocksthrough the section enabled us to subdivide Unit III intofour sub-units that correspond to four facies types ofinitial sediments.

Sub-unit HID, facies 3d (Core 71, Section 4 to Core74, Section 7) has the greatest thickness (~ 30-35 m) andconsists of rocks similar to pelitomorphic limestones ofUnit IV. The main components of Sub-unit HID are

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biogenic pelitomorphic limestones rich in radiolarian re-mains, with fragments of small siliceous sponge spic-ules, and thin interbeds and micro-lenses of clay matter.Radiolarian remains are formed in most cases of chal-cedony or chalcedony together with globular opal; ra-diolarian skeletons with primary opal organogenicstructure are rare; in some interbeds, radiolarian re-mains are replaced by calcite. The rocks can be broken(across the bedding) by thin fissures filled with calcite orchalcedony.

Near the base of Sub-unit HID, there occurs an in-terbed (Core 74, Section 1, 15-16 cm) of pure tuff:pyroclastic glass with a slight admixture of plagioclasesand hyalobasalts, with single skeletons of radiolarians;the carbonate content of the tuff is low, roughly 10 to15%.

In Core 73, Section 3, a bed of calcareous breccia wasrecorded. It is similar to clastic limestones of Unit IV.The breccia consists of fragments of oolitic, skeletal-clastic and micro-lumpy limestones, fragments of largetests, echinoderm remains, etc. Fragments of effusiverocks of andesitic composition sometimes can be ob-served (phenocrysts of entirely calcitized plagioclasesare submerged in the pilotaxitic groundmass). Cementin the breccia is pelitomorphic and granular calcite.

In Core 71, Section 4, 41-56 cm, single fragments ofskeletal-detrital limestones are observed in pelitomor-phic limestones with radiolarians. Many skeletons of ra-diolarians are carbonatized here.

Pale-green authigenic montmorillonite was recog-nized in all rocks of Sub-unit HID; it usually fills thecentral part of a radiolarian skeleton (the outer coatingremains opal).

Sub-unit IIIC, facies 3 Core 71 to Core 71, Section 3)has a very small thickness (-4.5-5 m), but is character-ized by great diversity and pecularity of rocks. These aremostly radiolarian-montmorillonitic (zeolitized in in-terbeds) and montmorillonitic (with authigenic siderite)claystones. All these rocks resulted from biogenic,mostly radiolarian oozes saturated with fine ashy mate-rial. The latter has been diagenetically altered; authi-genic montmorillonite, zeolite, siderite, and calcite wereformed. Fine fragments of volcanic glass can be seenunder a microscrope (in immersion specimens) in finefractions of these rocks. Worth attention is the charac-ter of siliceous and carbonate material of rocks of Sub-unit IIIC. Besides relics of skeletal silica (radiolarian,less frequently siliceous sponges) one can often see areprecipitated globular opal that develops irrespectiveof radiolarian skeletons forming lenses. These appear asdiffuse mottles of a homogeneous isotropic mass. Mostradiolarian remains were replaced by pale-green mont-morillonite or an aggregate of colorless zeolite crystals(clinoptilolite). It should be emphasized that in all rocksof Sub-unit IIIC no skeletons replaced by globular opalor chalcedony (cristobalite) were observed. This showsthat the processes of metasomatic replacement of radio-larian remains by montmorillonite and zeolite tookplace in the earliest stages of diagenesis.

Biogenic carbonate in the rocks is almost absent. Itmay be that a relatively high rate of ash accumulationprevented deposition of considerable amounts of such

fine and light material as remains of nannoplankton. Itis noteworthy that in places where the montmorilloniticgroundmass of claystones contains coccoliths the latterare well preserved and do not bear distinct traces of dis-solution. This again indicates that montmorillonitiza-tion of the whole sediment took place at some earlystage of diagenesis. In some interbeds (e.g., Core 70,Section 7), the groundmass consists of reprecipitatedcalcite enclosing remains of radiolarians (with biogenicopal skeleton or entirely replaced zeolite), relics of vol-canic glass fragments, and authigenic siderite. Equantand irregularly shaped granular aggregates (sometimeswith obscure spherulitic structure) of authigenic, iron-containing carbonate (siderite?) are present in mont-morillonitic claystones as well.

Sub-unit IIIB, facies 3b (Core 70, Section 6 to Core69, Section 3), (up to 9-m thick) contains specific rocksthat require extra attention. These are chiefly calcareousand siliceous, with a relatively high content of organiccarbon (up to 4.5-7.0%). Timofeev and Bogolyubovacarried out a detailed study of these rocks and organicmatter and described the results in a separate paper (thisvolume).

Some siliceous rocks are almost carbonate-free (5-7%CaCO3), some appreciably calcareous (up to 35%CaCO3). Silica in the form of chalcedony (cristobalite)and globular opal replaces skeletal remains of radiola-rians and siliceous sponge spicules. Remains of radiola-rians prevail in almost all rocks. Therefore, radiolaritesare the main components of Sub-unit IIIB. In some in-terbeds, however (Core 70, Section 3) sponge spiculesare of importance, so the rocks can be called radio-larian-spicularite. In addition to silica that is spatiallyrelated to remains of micro-organisms, the repreci-pitated amorphous SiO2 forms irregular lens-shapedareas; sometimes it makes up a homogeneous ground-mass of rocks.

Organic carbon is represented by brown colloidalmatter and tiny, rounded, opaque bodies which, as sug-gested by L. I. Bogolyubova, are remains of micro-algae(cysts of dinoflagellates).

The rocks are characterized by thin, interrupted,wave-like, lens-shaped micro-lamination due to the dis-tribution of organic colloid and small short lenses ofpale-green montmorillonite which is elongated in thesame direction. In all lenses, montmorillonite is meta-colloidal: it is recrystallized into a wavy and radially ex-tinguishing, brightly polarizing mass. These lenses showno scaly terrigenous structure of clay material. Themontmorillonite in lenses appears to be authigenic; itprecipitated from colloidal solution and underwent re-crystallization in the process of diagenesis. The lenticu-lar and wave-like distribution of organic matter andauthigenic montmorillonite appears to be a result of se-parate coagulation of a complex organic-mineral hy-drogel that saturated siliceous ooze sediments. Whileprecipitating out of the colloidal solution, hydrosols be-came isolated; this imparted a corresponding micro-lamination to the diagenetically transformed sediment.

In Sub-unit IIIB there occurs an interbed (Core 70,Section 3) of carbonatized tuffaceous siliceous rock. Itis pale-greenish-yellow and contains small fragments of

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acidic volcanic glass, authigenic aggregates of zeolite,and relics of radiolarian skeletons; it consists mostly ofreprecipitated carbonate (CaCO3 -8 .9%; grains dif-fuse).

In the overlying section, the rocks gradually becomemore carbonate-rich, the organic-carbon content de-creasing.

Sub-unit III A, facies 3a (Core 67, Section 3 to Core69, Section 2) is roughly 9-meters thick. It is composedmostly of greenish-gray, pelitomorphic nannofossil lime-stones, with interbeds of calcareous radiolarites (CaCO3— 35%). Radiolarian remains in the latter are formed byglobular opal together with chalcedony. Relics of theformer admixture of ashy particles are visible. Pyro-clastic fragments of hornblende and pyroxene grains arerarely recognized. In Core 69, Section 3, 15-17 cm, ra-diolarian skeletons in the sediment are carbonatized. Insome rocks of Sub-unit IIIA equant aggregates of authi-genic, strongly oxidized siderite are dispersed in thepelitomorphic mass.

Unit II, Macrofacies 2 (Cores 52-67, Section 2)This unit is about 130-meters thick, and consists of

rocks which sharply differ from the rocks of the lowerunits (III and IV). These are pelitomorphic limestoneswith foraminifer tests and, commonly, fine shell detri-tus (usually undefinable; ostracode valves are rarelyrecognized in Cores 61, 62); single fragments of largemollusk shells are observed as an exception (Cores 61,Section 1; 62, Section 1; 65, Section 2; 66, Section 2).The limestones are slightly clayey (10-15% of particles<O.Ol mm), biogenic silica being not so representativeas for instance for rocks of Unit III. According to Dean(this volume), the content of free SiO2 in rocks of UnitII is, on the average, close to 27%, whereas in rocks ofUnit III it amounts to about 64%. Beds enriched withbiogenic silica (remains of radiolarians) are present inCores 53, 55, 58, and 66, where the SiO2 content canreach 54%. In places where skeletons of radiolariansprovide large accumulations, they are composed of fine-grained, crystalline silica. If radiolarian remains werescattered in ooze, many skeletons either entirely leachedaway, or were completely replaced by calcite.

The groundmass of limestones is pelitomorphic,almost entirely recrystallized, with semi-dissolved coc-coliths hardly visible.

The limestones of Unit II are distinguished by theircolor and structural pecularities. While describing themoist core material on board Glomar Challenger, theserocks were called multi-colored limestones. In a drystate, they are of pale colors: greenish, pale-yellow,pinkish pale-yellow. The "texture" of limestones visiblewith the naked eye is very peculiar. In a section acrossthe bedding of the limestone, there appear micro-pinnate, lenticular, discontinuous-horizontal, and otherlaminations. However, a thorough study raises doubtswhether this reflects the primary lamination of the pri-mary sediments, i.e., the lamination functionally relatedto dynamics of bottom waters. Any primary laminationis known to be expressed or emphasized by something.It can be due to differences in grain size or materialcomposition of beds, various angles of inclinations of

oblique series of laminae, etc. The character of lamina-tion sometimes emphasizes bedding planes of ter-rigenous micaceous and coaly particles, etc. In the givencase, the bedding in so-called "laminated" rocks doesnot prove primary and related to the character of migra-tion of water masses during sediment accumulation. Itmay be said to be false bedding that appeared in diagen-esis of sediments and owes its origin to correspondingdistribution of meta-colloidal Fe-hydroxide in sedi-ments. It should be emphasized that the common pale-yellow and pale-pink color of rocks is related to slightbut even pigmentation of carbonate material with finelydispersed Fe-hydroxides.

Unit II is composed mainly of rather homogenousrocks, and is not subdivided as easily as underlyingunits. Nevertheless, a certain succession of beds is rec-ognized in this series, too; this enables us to distinguishfive sub-units.

Sub-unit HE, facies 2e (Cores 64-67, Section 1) in-cludes pelitomorphic limestones enriched with radiola-rian remains (filled with finely crystalline silica) in inter-beds. The limestones contain small, scattered fora-minifer tests, and oval and rounded micropores fromleached radiolarians. Aggregates of authigenic siderite(strongly oxidized) are present.

Sub-unit IID, facies 2d (Core 60, Section 2 to Core63) includes pelitomorphic limestones with small andrelatively large foraminifer tests. There are smallamounts of shell detritus enriching separate interbeds(Core 36, Section 1, 62-63 cm). Tests of small foramini-fers in the lower part of the sub-unit may be filled withcalcite, whereas in the upper part they are usuallyhollow.

Sub-unit IIC, facies 2c (Cores 59-60, Section 1)unites a rather peculiar group of rocks. These are pelito-morphic limestones with small foraminifers, rare re-mains of radiolarians, and a small amount of shell detri-tus. Almost all remains of radiolarians are replaced bycalcite, and some are entirely leached. Appreciable py-ritization is characteristic. Pyrite develops after discretetests of foraminifers, and forms aggregates stretchingalong the lamination of rocks. In some places, smallclay lenses and hardly recognizable patches of colloidalorganic carbon are noticed. At the base of this sub-unit(Core 60, Section 1, 75-78 cm), there occurs a pelito-morphic limestone devoid of faunal remains, and rich inaggregates of oxidized authigenic siderite.

Sub-unit IIB, facies 2b (Cores 56-58) consists of pel-itomorphic limestones with small foraminifer tests (withhollow chambers), and (very characteristic) greatamounts of shell detritus containing fragments of ostra-code valves. The arrangement of shell detritus empha-sizes the horizontal lamination of rocks.

Sub-unit IIA, facies 2a (Cores 52-55) consists ofpelitomorphic limestones with small foraminifers (withhollow chambers) and interbeds (Cores 53, 55) enrichedwith radiolarians. The latter are represented by micro-and cryptocrystalline silica; their content in limestonescan reach 50%.

Accumulation of sediments of Unit II proceeded in arather deep-water oceanic environment, where mostlyforaminifer and nannofossil material was supplied, ad-

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mixed periodically with shell detritus (fragments of os-tracode valves, etc., and sometimes fragments of mol-lusk tests). The latter were brought directly from thearea of the island shelf. The presence in some rocks ofleached or calcite-replaced skeletons of radiolarians,and also clearly authigenic siderite, suggests to us thatduring formation of the deposits ash was present, andthe subsequent penetration of mineralized thermal solu-tions could have taken place, although not systematic-ally.

Unit I, Microfacies 1 (Cores 1-51)

This unit has a thickness of about 450 meters, and iscomposed of rocks genetically associated with deep-water biogenic sediments. This series of deposits coversa large time interval from late Albian to Pleistocene,inclusively.

In the Albian-Turonian, accumulation of nannofos-sil-foraminifer and foraminifer-nannofossil oozes wasproceeding, this carbonate material frequently beingassociated with radiolarian remains (less frequentlysponge spicules: Core 33,CC). During later Cretaceoustime (Coniacian-Maastrichtian), the participation ofbiogenic silica in formation of oozes was gradually de-creasing. Cenozoic sediment accumulation was alreadyessentially biogenic carbonate.

During drilling, the siliceous interbeds produced brec-cias, and variously sized fragments of chert were pressedinto rather soft Cretaceous rocks.

Contrary to all underlying older rocks consolidatedto limestones, the rocks of Unit I are less lithified,especially those of Cenozoic age. Therefore, all Meso-zoic rocks (beginning with the upper Albian) from thissection were called "chalk," whereas Cenozoic oneswere called simply "ooze." All rocks of Unit I containforaminifer tests that did not undergo any appreciablerecrystallization; their chambers are hollow everywhere,and not incrusted with secondary calcite. In cases whereoozes were rich in biogenic silica, many tests of fora-minifers in post-sedimentary processes were entirelyreplaced by chalcedony (all their contours remaining).Post-depositional changes in the sediment mainly af-fected the siliceous oozes; as a result, no primary opalskeletons of radiolarians are preserved. Transforma-tions of amorphous biogenic siliceous material resultedin formation of hard (with chonchoidal fracture), com-pact siliceous masses of crystalline silica with inclusionsof reprecipitated globular opal.

Carbonate rocks, poor in or devoid of siliceousmaterial, have high CaCO3 content (-95%). The in-soluble residue usually consists of clay matter; some-times it contains a negligible admixture of silty andsmaller particles of quartz, feldspars, and micas. Inrocks enriched with silica, the amount of carbonate canfall below 40% (Core 36, Section 1, 1-3 cm), and in theinsoluble residue, besides clay material, there can benumerous radiolarians and nuclei of silicified tests offoraminifers.

Already on board the ship, the fresh core materialrevealed differences in the character of Cenozoic andMesozoic rocks, and this enabled us to distinguish Sub-

unit IA and Sub-unit IB in the section concerned.Laboratory studies of the core material from theMesozoic part of Unit I allowed us to single out threesub-units and, consequently to subdivide Unit I intofour sub-units.

Sub-unit ID, facies Id (Cores 31-51) consists of an al-ternation of chert and soft sediment. The core recoveryin this part of the section was consequently very poor.Nannofossil-foraminifer and foraminifer-nannofossil,slightly cemented rocks (chalks) alternate with hardcherts. The latter are diverse: some consist of a homoge-neous, recrystallized mass of silica with rare, scarcelyvisible relics of radiolarians and foraminifers, others ofcrystalline silica and irregular pieces of opal. In car-bonate rocks of this sub-unit, there are usually scatteredskeletons of radiolarians and single, entirely silicifiedtests of foraminifers.

Sub-unit IC facies Ic (Cores 16-30) is composed offoraminifer-nannofossil chalks, with thinner beds ofcherts, and rare radiolarians and silicified foraminifersin a carbonate groundmass. The total carbonate contenthere is higher than in Subunit ID, and core recovery bet-ter. Remains of nannoplankton commonly prevail overthose of foraminifers.

Sub-unit IB, facies Ib (Cores 8-15) unites foramini-fer-nannofossil and nannofossil-foraminifer chalks, al-ready with very rare interbeds of cherts and almostwithout inclusions of radiolarian skeletons. The totalcarbonate content is constantly high; the insolubleresidue usually consists of clay material; single particlesof quartz, feldspar, and mica are rare.

Sub-unit I A, facies la (Cores 1-7) includes Cenozoicdeposits separated from Mesozoic deposits by a majorhiatus (upper Maastrichtian beds are directly overlainby lower Eocene). The total thickness of the Cenozoicseries is not great (~55 m). All the sediments in a drystate are pale-brown, and practically devoid of biogenicsilica. The total carbonate content of the oozes is high(up to 99%). The insoluble residue consists of clay par-ticles; silt-sized particles of quartz, feldspar, and micaare sporadic; there is coal dust and very typical authi-genic fibrous chalcedony (ellipsoidal and roundedgrains with a black cross under crossed nicols).

The diversity of rocks of this sub-unit consists in dif-ferent ratios of nannoplankton and foraminifer re-mains, and in the degree of sorting of foraminifer tests.When judged by the available samples, the followingregularity can be outlined: Eocene-Oligocene sediments(Cores 5-7) abound in nannofossil-foraminifer oozeswith small, approximately equally sized tests of fora-minifers; in Miocene deposits (Core 4), pelitomorphicnannofossil oozes with rare large tests of foraminifers;the groundmass of Pliocene-Pleistocene oozes consistsof nannoplankton remains, with unevenly distributedsmall and large tests of foraminifers.

The sediments in Core 1 accumulated in a deep-waterzone of the ocean (-2500-3000 m), without any influ-ence on the process of sedimentation of the island shelf.Almost no ashy pyroclastic material was supplied andno hydrothermal processes were recorded throughoutthe Campanian-Pleistocene. Worth attention is the fact

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that Cenozoic deposits are characterized here by in-significant thickness and frequent hiatuses. The periodof formation of this sedimentary series was likely char-acterized by extra activity of tectonic movements thatdid not promote the burial of sediments and theirlithification, or by increased flow of bottom waters.

CONCLUSION

These are brief conclusions obtained as a result ofour lithologic-facies investigations of the Ceno-Meso-zoic section of Site 463.

1) The formation of the sediments took place farbeyond the continental shelf. It was an area in the oceaninaccessible to allochthonous products of erosion fromcontinental land. Purely local autochthonous materialwas deposited and redeposited in that zone. Mostly bio-genic oozes were formed from remains of radiolarians,siliceous sponges, and nannoplankton in some places,foraminifers and nannoplankton in others. There wereepisodic conditions favorable for accumulation of cal-careous breccias, the clastic material was supplied by lo-cal source areas of oceanic origin. Outbursts of volcanicactivity in the ocean were reflected in the accumulationof great amounts of pyroclastic material; hydrothermalprocesses become more intense, and the composition ofbiogenic remains changed (participation of siliceousorganisms increased), etc.

2) Four different environments of sedimentation(four macrofacies) are observed. Each macrofacies wascharacterized first of all by deep waters and remotenessfrom the area of underwater (or surface) oceanic rises ofthe island type (Mid-Pacific Mountains). The Mesozoicpart of the section clearly shows a certain succession inchange in time of deposits of all four macrofacies. Thissuccession enables us to outline four main historicalstages respectively in development of the sediments, andto recognize the general tendency in the evolution ofsedimentation conditions in the given area of the oceanfor the period from late Barremian to early Maastrich-tian.

3) The earliest historical stage (macrofacies 4) of ac-cumulation of the above-mentioned deposits covers theperiod from late upper Barremian to the beginning ofthe early Aptian. The accumulation area under studywas at that time near the island shelf, with a very mobileboundary. Therefore, relatively deep-water biogenicnannofossil-radiolarian sediments frequently alternatedwith clastic sediments (breccias). Clastic material wasbrought in by mudflows from the area of denudation ofislands. These contained skeletal-detrital, oolitic, andother limestones (surely of shallow-water origin), andandesite-basalt rocks. The next historical stage (macro-facies 3) was very short (it occupies only the middle partof the lower Aptian), but very peculiar. Activation ofvolcanic activity in corresponding regions of the waterarea is responsible for abundant ash falls in the area ofsedimentation. Highly siliceous and vitroclastic sedi-ments appeared, which in the early stages of diagenesiswere transformed into montmorillonitic clays. In this

period, the flows rich in fragments of limestones, largetests, etc. were not formed, indicating an increased dis-tance to shallow carbonate banks. Only clay and or-ganic carbon (sapropelic) were supplied from the shelfzone. We suggest that separate coagulation of hydrogelsoccurred in the ooze. This resulted in a fine, lenticular-discontinuous microstructure of carbonaceous clayey-siliceous sediments. The second stage of sedimentation(macrofacies 2) was proceeding from the end of theearly Aptian to the end of the middle Albian. The gen-eral environment of sedimentation in the ocean becamequite different. Ash falls ceased, and hydrothermalprocesses synchronous with sediment accumulationwere no longer manifest. Radiolarians and sponge spic-ules were not preserved; remains of nannoplankton andforaminifers became predominant in the sediments.Finely crushed, repeatedly redeposited shell detritus,and ostracode valves supplied by turbidity flows con-taminated foraminifer-nannofossil oozes. The sedimen-tation rate decreased promoting the conversion of all re-active iron into a hydroxide. The sediments were cycliclycolored pale-pink, pale-green, or pale-brown. The lasthistorical stage of Mesozoic sediment accumulation(macrofacies lb) began after the late Albian and endedin the early Maastrichtian. It was the period when sedi-ment accumulation was proceeding in the deepest-wateroceanic environment (2300-3000 m). The process of ma-terial accumulation was proceeding very slowly, withlarge hiatuses, resulting in formation of mostly carbon-ate-biogenic (foraminifer-nannofossil) oozes. Duringpre-Coniacian time, appreciable amounts of organo-genic silica were periodically supplied to carbonateoozes; this might have resulted in the appearance (dur-ing post-sedimentary transformations of sedimentarymaterial) of hard cherty interbeds within slightly lithi-fied chalks. Later, the amount of biogenic SiO2 in sedi-ments gradually decreased, and was very low in theMaastrichtian.

Cenozoic sediment accumulation was proceedinghere approximately under the same facies conditions asin the Turonian-Maastrichtian, i.e., at large depths andvery slowly, with considerable hiatuses, with almost noaccumulation of biogenic silica.

4) From the base upward, the rocks are progressivelyless lithified. Deposits from the upper Barremian tomiddle Albian are represented by hard (if not soaked inwater) limestones (depth of occurrence —450-820 m).Beginning with the upper Albian and ending in thelower Maastrichtian (depth of occurrence ~ 55-450 m),there is a gradual transition from hard limestones to lessand less consolidated rocks (chalk-like and even ooze-like). Cenozoic deposits crowning the sedimentary pileand resting at a depth of 55 meters are scarcely lithifiedand oozy. Noteworthy is the fact that deposits groupedin Units IV and II (depth 585-820 m) are especiallystrongly lithified. Once can observe that the increasingoverburden down-section intensified pressure and hy-drothermal processes. The influence of these processeson already-lithified rocks resulted in filling of fissureswith calcite and barite, leaching of SiO2 from remains

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of radiolarians, and formation of pseudomorphs of cal- accumulation area and increasing remoteness from shal-cite after radiolarian skeletons. low-water banks. During the late Barremian to early

5) Accumulation of Ceno-Mesozoic sediments took Aptian, sediment accumulation and diagenesis wasplace against a background of gradual deepening of the strongly affected by regional volcanism.

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