36. GEOCHEMICAL HISTORY OF POST-JURASSIC SEDIMENTATION IN THE CENTRALNORTHWESTERN PACIFIC, NORTHERN HESS RISE, DEEP SEA DRILLING PROJECT SITE 4641

I. M. Varentsov, B. A. Sakharov, M. A. Rateev, and D. Ya. Choporov, Geological Instituteof the U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.

ABSTRACT

Results describe the chemical composition of post-Jurassic sediments penetrated by Deep Sea Drilling Project Hole464 (northern Hess Rise), peculiar features in the distribution of chemical components through the section, and modesof their occurrence determined by factor analysis and from the data on mineralogy, lithology, and accumulation ratesof sediments and chemical components.

Three main stages in the geochemical history of sedimentation in the region are outlined: I. Late Mesozoic stage(Early Albian-Cenomanian). la. Early phase (Late Aptian-Early Albian). This early phase is characterized by ac-cumulation of dark-red and brown, calcareous silica-clay oozes, considerably enriched by volcanogenic (mostlyhyalopelite basalt volcaniclastic) material. High accumulation rates of sediments (22.5 mm 10~3yr-1) and chemicalcomponents are noted. Ib. Late phase (late Albian-Cenomanian). Accumulation of relatively light-colored, calcareous,essentially silicic sediments took place at that time. The amount of volcaniclastic material, iron, and other heavy metalsin these sediments is much less than in the early Albian deposits. Relatively high accumulation rates of sediments (17.9mm 10~3yr~') and of some of the components (SiO2, A12O3, etc.) were recorded. The obtained data are in good cor-relation with the model which implies that in the Albian the northern part of Hess Rise was in the equatorial zone, withhigh biological productivity, while the Pacific Plate moved northward. II. Late Cretaceous (Early Turonian?-MiddleMiocene). During that time, relatively local accumulation of fine volcaniclastic material (ash) took place; this materialwas transformed into clay (montmorillonite-hydromica) enriched by ferric hydroxide, and to a lesser degree by Mn-hydroxide and associated heavy metals. III. Late Cenozoic stage (upper Miocene-Pleistocene) was the time of ac-cumulation of clay-silica, mostly radiolarian oozes, in chemical composition and accumulation rates similar to thetypical clay-silica sediments of the modern open ocean at depths below the level of carbonate compensation. Theoutlined stages, phases of geochemical history of sedimentation in the region, represent the evolution of sediment ac-cumulation in the basin.

INTRODUCTION

Hess Rise is one of the largest submarine aseismicstructures of the northwestern Pacific. The Shatsky andMagellan Rises, the Ontong-Java and Manihiki sub-marine plateaus, and the Mid-Pacific Mountains alsobelong to this kind of structure. The data now availableallow us to surmise that during the greater part of post-Jurassic time these structures were normally above thecarbonate-compensation depth (CCD). Therefore, theymay contain a relatively complete section of sediments.

The objective of this work is to outline the majorfeatures of the geochemical history of sedimentation inthat region, as recorded in the chemical and mineralcomposition of sediments, on the basis of study of thechemistry of major components, heavy metals, traceelements, and data on mineralogy and lithology.

This article is one of the series of works on the geo-chemical history of sedimentation in the northwesternPacific bored on DSDP Leg 62 by D/V Glomar Chal-lenger (see Varentsov et al., this volume).

MATERIALS AND METHODS

The work is based on observational data on chemical and mineralcomposition, and lithologic features of deposits penetrated by Hole464, as analyzed at the Geological Institute, U.S.S.R. Academy ofSciences. Data on lithologic-mineralogical studies are presented in

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

other parts of this volume. The technique of research is given ingreater detail in the paper on geochemical history of sedimentation forHole 463 (Varentsov et al., this volume).

Determination of chemical components of sediments was made atthe Geological Institute: the main components by the method of bulkchemical analysis, and heavy metals by optical emission spectroscopy,with application of international reference standards (Zolotarev andChoporov, 1978).

The chemical analyses were recalculated on a terrigenous-free,silica-free, carbonate-free basis to exclude the diluting effect of bio-genic and clastic components, and to adjust the sediment to a certaingeochemically comparable basis (Varentsov and Blazhchishin, 1976).

Analytical data were processed by a computer (EC-1022) at theLaboratory of Mathematical Methods of Research, Geological Insti-tute (D. A. Kazimirov, P. K. Riabushkin), using the program of fac-tor analysis (R-, Q-mode; Davis, 1973; Harman, 1967).

It should be noted that the section of Hole 464 is represented by acomparatively limited number of core samples, especially the Meso-zoic deposits (early Cenomanian-Albian), mostly composed of car-bonate silicic rocks.

The shortage of core samples and their inadequate paleontologicaland biostratigraphical characteristics, and the few chemical analysesmake the obtained conclusions only tentative and speculative for thenorthern part of Hess Rise. Interpretation of this material can bemade only within the context of geological, lithologic, and geo-chemical, information on that region.

The stratigraphic subdivision of the "brown clay" unit was made,therefore, according to the results of study of ichthyolith remains(Doyle and Riedel, this volume). Moreover, the stratigraphic capacityof the lower part of the unit (Cores 9 and 10) dated by these authorson the whole as Late Cretaceous, is conditionally interpreted in thepaper as Turonian-Maastrichtian, from correlation with adjacentregions (Holes 171 and 310). It was inferred that the "brown clay"unit is composed of genetically similar deposits which accumulatedwithout notable hiatuses.

805

I. M. VARENTSOV, B. A. SAKHAROV, M. A. RATEEV, D. YA. CHOPOROV

PARAGENETIC ASSEMBLAGESOF COMPONENTS

The study of paragenetic assemblages of componentswas carried out using the analytical data processed byfactor analysis. Two types of determinations were in-vestigated: (1) actual data of chemical analyses, repre-sented in weight percent to air-dry weight, (2) recalcula-tion to a terrigenous-free, silica-free, carbonate-freebasis. In the first case, the determined groups of chemi-cal components, clusters, are definitely related to exist-ing mineral phases. In the second case, provided in par-ticular that the carbonate- and silica-dilution effect iseliminated, clearly outlined features of a number ofauthigenic mineral phases become more evident: hy-droxides, hydrothermal exhalates, products of post-sed-imentary transformations, etc.

Data of Chemical Analysis (Tables 1-3; Fig. 1)

Assemblage IA ( + ): SiO2(0.98)-Corg(0.21)-Ca(0.12).This assemblage is represented by biogenic silica of theopal-CT type, with which Co r g associates rather weakly.The assemblage is most conspicuous in the Cenomanianopal-clay sediment (Sample 11-1, 28-29 cm) with essen-tially altered hyalopelite material, relics of diatoms, pat-ches of chalcedony, quartz. Early Pliocene tuffogenicsediments (Sample 3-1, 60-64 cm; Fig. 1) are similar incomposition; the content of relics of diatoms in them isup to 40%, of spicules of siliceous sponges up to 20%,of radiolarians up to 10%.

Assemblage IB (-): A12O3( - 0.29)-Mg( - 0.97)-N2O(-0.52)-K 2O(-0.36)-Fe(0.86)-Mn(-0.70)-P(- 0.17)-Cr( - 0.34)-Ni( - 0.49)-V( - 0.74)-Cu( - 0.34)-Co(-0.25)-Pb(-0.56)-Mo(-0.48). This assemblage isrepresented by essentially altered hyalopelite basalticvolcaniclastic material, composed of tephra remains, Fe-and Mn-hydroxides, montmorillonite-illite compo-nents. This group can be considered as an example ofmultistage formation of an assemblage: (1) oxidation ofbasaltic hyalopelite volcaniclastics, development ofcrusts and patches of Fe- and Mn-hydroxides (Samples9-1, 30-32 cm; 5-5, 70-72 cm, etc.) after volcanogenicparticles promoting concentration of heavy metals; (2)transformation of volcaniclastic material into mixed-layer, fine-grained phases, i.e., montmorillonite-mica(Fig. 2) was followed by scavenging of Mg and K fromsolutions and from the bottom water.

Distinct localization of this group of components inthe section should be noted (Fig. 1). Its most obviousmanifestation is observed in sediments marking the be-ginning of a burst of explosive basalt volcanism, mainlyat the base of volcanogenic "brown clay" unit of theLate Cretaceous-middle Miocene (Doyle and Riedel,this volume); these clays are deposited on red-brownsilicates, to a lesser extent limestones of the Ceno-manian. To a relatively lesser extent, this assemblage isdeveloped in the lower layers of essentially volcanogenicsiliceous clays of the middle Miocene, and at the base ofargillaceous radiolarian oozes and argillaceous siliceousoozes of the late Pliocene-Pleistocene.

Assemblage IIA: SiO2(0.13)-Al2O3(0.38)-Na2O(0.78)-P(0.73)-Cr(0.32)-V(0.33)-Cu(0.72)-Mo(0.37).Comparison of the combination of components in thisgroup with observational data on mineral compositiongives grounds to assume that this assemblage is repre-sented by aluminosilicate phases of the zeolite type, withclosely associated phosphorus, copper, and a number ofheavy metals. The group is confined to volcanogenicsiliceous hyalopelite sediments in the top layers of theearly Cenomanian, and to the volcanogenic "brownclay" unit of the Late Cretaceous-middle Miocene.

Assemblage IIB ( - ) : CaO( - 0.86)-CO2(0.87)-Corg(-0.62)-Ge(-0.62). This assemblage is represented bybiogenic calcium carbonate, with which Co r g is closelybound. Being one of the group, Ge is obviously associ-ated with Co r g and forms metallo-organic compounds.The assemblage occurs in deposits of the early Plio-cene-Pleistocene, with carbonate nannofossil micrite re-mains (up to 20%).

Assemblage IIIA ( + ): Al2O3(0.80)-MgO(0.16)-K2O(0.65)-Fe(0.35)-Mn(0.67)-P(0.53)-Cr(0.80)-Ni(0.82)-V(0.49)-Cu(0.30)-Co(0.93)-Pb(0.57)-Ga(0.90)-Ge(0.47)-Mo(0.72). Results of mineralogical studies andthe composition of this assemblage provide evidence tobelieve that it is represented by fine hyalopelite basalticmaterial, essentially altered to illite components. Thistransformation may have been accompanied by the ap-pearance of Fe- and Mn-hydroxide patches, encrusta-tions, and aluminophosphate and iron phosphate com-pounds in close association with heavy metals such asCr, Ni, Co, Ga, and Mo.

Assemblage IIIB (-): CO2(-0.46)-SiO2(-0.09)-C( - 0.01)-CaO(?). The nature of phases in this assem-blage is not quite clear from the study of its compo-

Table 1. Chemical composition of Mesozoic-Cenozoic deposits, central northwestern Pacific, northern Hess Rise, Site 464.

Sample(interval in cm)

464-2-1, 115-1193-1,60-644-1, 130-1345-5, 70-726-2, 95-976-3, 94-967-3, 120-1228-2, 13-159-1, 30-3211-1, 28-29

SiO 2

40.5351.5143.7344.8546.5346.7546.3646.9926.5982.68

A12O3

4.673.964.48

12.5312.7714.6512.9812.937.%3.68

Fe 2 O 3

6.745.255.88

10.1513.687.98

11.3312.2828.77

2.14

CaO

16.308.30

11.813.743.013.971.401.632.630.82

MgO

3.222.623.123.813.122.943.622.875.481.76

MnO

0.170.210.072.270.841.770.280.402.690.01

Na2O

4.004.534.213.462.513.183.462.892.991.05

Components

(wt.%)

K2O

2.052.012.403.293.723.294.586.102.440.96

co211.305.107.75

—-―

__

—

C

0.150.01___

__

—

P2O5

0.110.070.131.480.721.400.240.380.990,55

Fe

4.713.674.117.109.575.587.928.59

20.121.50

Mn

0.130.160.051.760.651.370.220.312.080.01

P

0.050.030.060.650.310.610.100.170.430.24

Cr

18202258586260704014

Ni

4811265

500178500

9875

21515

V

434972

140170130135142400

28

(wt.% ×

Cu

11510862

202190160135130245152

Co

13201887448424322710

10 ~ 4 )

Pb

16161967365911146410

Ga

5779

1198855

Ge

< l1.31.211.11.41

< l< l< l

Mo

1.62.81.5

8030

1003.9

8980

1.5

806

POST-JURASSIC SEDIMENTATION, SITE 464

Table 2. Results of factor analysis (R-mode) forchemical components (wt.%), Cenozoic andMesozoic sediments of central northwesternPacific, northern Hess Rise, Site 464.

No.

123456789

1011121314151617181920

Factor Loading (after

Component

Siθ2A12O3CaOMgONa2θK 2OCO2CFeMnPCrNiVCuCoPbGaGeMo

Input in dispersion

(%)1

Total dispersion (%)

I

0.98-0.29

-0.97-0.52-0.36

0.010.21

-0.86-0.70-0.17-0.34-0.49-0.74-0.34-0.25-0.56

0.12

-0.48

51.13

-51.13

rotation)

II

0.130.38

-0.860.030.78

-0.87-0.62

0.730.32

-0.060.330.72

-0.620.37

19.96

71.09

III

-0.090.80

0.16

0.65-0.46-0.01

0.350.670.530.800.820.490.300.930.570.900.470.72

11.77

82.87

Table 3. Stratigraphic distribution of factor scores (R-mode)for chemical components (wt.%) in Cenozoic and Mesozoicsediments of central northwestern Pacific, northern HessRise, Site 464.

No.

12345

6

789

10

Sample(interval in cm)

2-1, 115-1193-1, 60-644-1, 130-1345-5, 70-726-2, 95-96

6-3, 94-96

7-3, 120-1228-2, 13-159-1, 30-3211-1, 28-29

Age

U. Plio.L. Plio.L. Plio.Mio.U. Eoc.L. Mio.U. Eoc.L. Mio.Paleoc.Paleoc.U. Cret.Cenom.

Factor Score(after rotation)

I

-0.600.630.06

-0.420.09

0.39

-0.080.14

-2.212.00

II

-0.82-1.86-1.38

0.430.43

-0.16

0.380.510.941.52

III

-1.46-0.02-0.56

1.030.95

1.75

0.050.36

-0.71-1.39

nents, but correlation of the intervals of its most defin-itely manifested development with the actual mineralcomposition of sediments, as observed in thin sectionsunder microscrope and revealed by X-ray diffracto-grams, implies that it is represented by calcium carbon-ate, which occurs as nannofossil micritic remains. Inthis aspect, the group is similar to assemblage IIB (-)discussed above. As indicated, the latter is representedmostly by calcium carbonate in the form of foraminiferand nannofossil micritic components.

Data on Chemical Analysis Recalculated(Tables 4-6; Fig. 3)

Assemblage IA ( + ): Fe(0.84)-Mn(0.72)-P(0.31)-Cr(0.64)-Ni(0.39)-V(0.79)-Co(0.15)-Ga(0.29)-Mo(0.79).This assemblage is represented mostly by oxyhydroxidecompounds of iron and manganese, and associatedheavy metals. The ferrous phosphate phases are appar-ently of special importance. The group is distinctly con-fined to the volcanogenic "brown clay" unit (Unit II) ofthe Late Cretaceous-middle Miocene. Elimination ofthe diluting effect of silicate components allows therevelation of the geochemical significance of Fe- andMn-hydroxides in basaltic volcaniclastics, and as sepa-rate patches (see assemblages IIA (+), IIIA (+); Fig. 1;Tables 1-3). The combination of components of thisassemblage testifies, therefore, that the geochemicalrole of collectors, concentrating heavy metals, belongsmostly to Fe- and Mn-hydroxides.

Assemblage IB ( - ) : CaO(-0.12)-MgO(-0.85)-Na2O(-0.83)-Cu(-0.36). The combination of compo-nents in this group, peculiar features of its occurrence inthe section (Fig. 3), and the data on mineral composi-tion allow us to state that the group is represented by asmectite phase. This phase is not connected with vol-caniclastics, which are a rather essential component ofthe intervals in the section of the lower Pliocene/Pleis-tocene and lower Cenomanian (Fig. 3).

Assemblage IIA (+): CaO(0.35)-Mg(0.17)-K2O(0.59)-P(0.65)-Cr(0.72)-V(0.39)-Cu(0.67)-Co(0.43)-Ga(0.90)-Ge(0.58)-Mo(0.15). This assemblage is repre-sented by illite phases, which occur separately and asmixed-layer components of the montmorillonite-micatype, developed in fine volcaniclastic, mostly hyalopel-ite material of basaltic composition. Phosphorus (alum-ophosphate compounds) and heavy metals, predomi-nantly Cr, Ga, Cu, Ge, etc., are closely associated withthe illite minerals proper. The group is confined to sedi-ments in the top part of the lower Cenomanian and tothe volcaniclastic "brown clay" (Unit II), probablyLate Cretaceous-middle Miocene (Fig. 2).

Assemblage IIB ( - ) : Na2O(-0.39)-Fe(-0.12)-Mn( - 0.27). This assemblage is represented by hydrox-ides of manganese and to a lesser extent iron, and by as-sociated sodium. The assemblage contains no heavymetals, and it is located at the base of the volcaniclastic"brown clay" and siliceous, essentially volcanogenicsediments (Units IA, IB; early Pliocene-Pleistocene);these facts allow to state that this group is representedby late-diagenetic, epigenetic patches of Mn-hydrox-ides, with a slight iron admixture, which formed as theresult of volcaniclastic alteration. Similar patches aredistinctly observed, in fairly limited amounts, in thinsection under the microscope (not more than 5%).

Assemblage IIIA ( + ): Na2O(0.12)-K2O(0.24)-Fe(0.31). This assemblage is represented by hydroxides ofiron and K- and Na-bound alkalis. The genetic nature ofthis group and its occurrence in the section are similar tothose of assemblage IIB (-) , described above. In this

807

00o00

LITH0L0GY FACTOR ASSEMBLAGES (SCORES)

FACTOR ASSEMBLAGESCLAY COMPONENTS

Polymineral assemblage, mainly illite,mixed-layer montmorillonite—illite, andchlorite, with an admixture of quartz,tridymite, and cristobalite

Dominantly mixed-layer montmorillon-ite—illite, with a slight admixture ofillite .chlorite, and zeolite

1.0

0.8

0.6

0.4

0.2

0

0.2

0.4

0.6

0.8

1.0

1

o u

Figure 1. Stratigraphic distribution of factor scores for the main paragenetic assemblages of chemical components in the post-Jurassic sedimentary section of the central northwesternPacific, northern Hess Rise, DSDP Site 464 (chemical analyses recalculated to air-dry weight). Lithologic symbols are those used in the DSDP Initial Reports.

POST-JURASSIC SEDIMENTATION, SITE 464

SAMPLE 7-5, 45-48 cm

3.23

BSAMPLE 10-3, 72-75 cm

12,0

1) NATURALORIENTED

2.572.52

2) GLYCERINATED

3.34

3.21 10.0

3) HEATEDAT 550 °C

Figure 2. X-ray diffractograms of natural samples of volcanogenic "brown clays" (radiation: CuKα), Site 464. 1.Natural, oriented sample. 2. Oriented after saturation with glycerine. 3. After heating to 550°C. Sample 7-5,45-48 cmis represented by disordered, rather finely dispersed mixed-layer montmorillonite-mica, with variable contents of ex-panding and non-expanding layers; trace quantities of hydromica are present; admixture: basic plagioclase oflabradorite-bytownite type, zeolite of phillipsite type, quartz. Sample 10-3, 72-75 cm is composed of rather finely dis-persed, mixed-layer, disordered montmorillonite-mica; admixture: hydromica, zeolite of clinoptilolite type, quartz.

Table 4. Chemical composition of Mesozoic-Cenozoic sediments, central northwestern Pacific, northern Hess Rise, Site 464.

Sample(interval in cm)

464-2-1, 115-1193-1,60-644-1, 130-1345-5, 70-726-2, 95-976-3, 94-967-3, 120-1228-2, 13-159-1, 30-3211-1, 28-29

CaO

8.1498.8948.705

15.63313.10818.8596.5587.2077.250

12.886

MgO

20.69417.74020.16915.92513.58713.96616.95712.69015.10627.657

Na2θ

29.08035.47930.87714.46210.93115.10716.20812.7798.242

16.500

K2O

10.75011.90313.69013.75216.20015.62921.45426.972

6.72615.086

Fe

30.08524.46525.83129.67741.67626.50837.10037.98255.46123.571

Mn

0.8751.1980.2707.3572.8316.5081.0311.3715.7340.157

Component(wt.

I. M. VARENTSOV, B. A. SAKHAROV, M. A. RATEEV, D. YA. CHOPOROV

Table 5. Results of factor analysis (R-mode) forchemical components (recalculated wt.%) ofCenozoic-Mesozoic sediments, central north-western Pacific, northern Hess Rise, Site 464.

No.

123456789

10111213141516

Input(%)

Factor Loading (after rotation)

Component

CaOMgONa2θK2OFeMnPCrNiVCuCoPbGaGeMo

in dispersion

Total dispersion (%)

I

-0.12-0.85-0.83

0.840.720.310.640.390.79

-0.360.150.080.29

0.79

43.77

43.77

II

0.350.17

-0.390.59

-0.12-0.27

0.650.72

0.390.670.430.060.900.580.15

21.57

65.34

III

-0.91

0.120.240.31

-0.56-0.54-0.19-0.84

-0.79-0.92-0.24

-0.50

14.61

79.95

Table 6. Stratigraphic distribution of factor scores (R-mode) forchemical components (recalculated wt.%) in Cenozoic-Mesozoicsediments, central northwestern Pacific, northern Hess Rise, Site464.

No.

123456789

10

Sample(interval in cm)

2-1, 115-1193-1, 60-644-1, 130-1345-5, 70-726-2, 95-976-3, 94-967-3, 120-1228-2, 13-159-1, 30-3211-1, 28-29

Age

U. Plio.L. Plio.L. Plio.Mio.U. Eoc.-L. Mio.U. Eoc.-L. Mio.Paleoc.Paleoc.L. Cret.Cenom.

Factor Scores(after rotation)

I

-1.11-1.08-0.93

0.400.630.330.511.191.51

-1.45

II

-1.10-0.98-0.98-0.08

0.800.130.550.55

-1.032.13

III

0.67-0.20

0.41-1.58-0.31-1.83-1.39

1.200.200.05

Among the altered volcaniclastic components of thevolcanogenic unit (II) of brown clay, two groups can bedistinguished: (1) the main, dominating group, IA ( + ),represented by hydroxides of iron and to a lesser extentmanganese, by ferrous phosphates and associated heavymetals; it occurs mainly in the lower half of the unit(Late Cretaceous-Paleocene; Fig. 3); (2) a relativelysubordinate group, IIIB (-) , which contains Mn-hy-droxides, phosphates, and associated heavy metals, andappears in the upper half of the unit (late Eocene/late-middle Miocene; Fig. 3). The separation obviousthroughout the section between groups of componentsassociated with Fe- and Mn-hydroxides suggests varyingcontributions from volcanic exhalates and hydrother-mal solutions, along with the dominating volcaniclasticmaterial at the initial and final phases of sedimentaryaccumulation in this unit. Iron compounds were preva-lent in initial phases; manganese compounds dominatedat final stages.

AVERAGE CONTENTS AND ACCUMULATIONRATES OF COMPONENTS (Table 7; Figs. 4-6)

Distribution of Average Contents

The basic elements of calculation of average contentswere discussed in our paper on site 463 (Varentsov et al.,this volume). It should be noted that in order to revealgeneral geochemical tendencies in the history of sedi-mentation the average contents were calculated fordefinite lithologic types of the main geochronologicalsubdivisions.

Analysis of distribution of average contents of com-ponents (Table 7; Figs. 4, 5) in the studied section allowsto distinguish three major geochemical stages of sedi-mentation: (1) late Mesozoic (late Aptian-Cenomanian);(2) Late Cretaceous (Turonian?)/middle Miocene (timeof accumulation of the volcanogenic "brown clay"unit); (3) late Cenozoic (late Miocene-Pleistocene).

Late Mesozoic (late Aptian-Early Cenomanian)

Lithologic and geochemical data on the sedimentsconstituting this interval of the section are rather frag-mentary and tentative. According to the shipboard files(see Site 464 report, this volume), the sediments form aunit of red-brown cherts (III). However, considering theextreme core shortage, waste of debris of chalk, marl,lithified clays, and speed of drilling, the authors of thesite report (Thiede, Valuer, et al., this volume) believethat chalk and marl dominate in the composition of theunit, whereas chert makes not more than 10%. There-fore, the data on average contents and accumulationrates given below can be analyzed only provided an al-lowance is made for the noted restrictions.

On the whole, the late Mesozoic is characterized byaccumulation of maximally high contents of silica andphosphorus in this section of Mesozoic and Cenozoicdeposits (Table 7; Fig. 4).

Recalculation of data (Fig. 5) distinctly reveals ex-cessive amounts of K2O and MgO, which might in-directly indicate the presence of illite-smectite com-ponents formed after volcaniclastic material.

Late Cretaceous (Turonian?)/Middle Miocene(?)(accumulation period of the "brown clay"

volcanogenic unit)

In this volume, the age interval of this stage is deter-mined by ichthyoliths (Doyle and Riedel, this volume).

Volcanogenic nature of these deposits and their im-pregnation by Fe- and Mn-hydroxides are distinctlyrecorded in concentrations of iron, manganese, phos-phorus, and associated heavy metals, which are unusu-ally high for normal sediments (Tables 1, 7; Fig. 4). Theoccurrences of these components are described above.The data on recalculation (Table 4; Fig. 5) also give evi-dence of high contents of Fe, Mn, and heavy metals, ascompared to sediments in the other intervals of the sec-tion.

The following features should be emphasized: (1) thelower, basal horizons of the unit (Turonian-Maastrich-tian?) are distinguished by maximal contents of Fe(20.12%) and Mn (2.08%), at relatively low valuesof aluminosilicate components (SiO2, 26.59%; A12O3,

810

LITHOLOGY FACTOR ASSEMBLAGES (SCORES)

Middle-lateioceng

Early MioceneLate Eocene

CLAY COMPONENTSPolymineral assemblage, mainly illite,mixed-layer montmorillonite—illite, andchlorite, with an admixture of quartz,tridymite, and cristobalite

Dominantly mixed-layer montmorillon-ite—illite, with a slight admixture ofillite, chlorite, and zeolite

FACTOR ASSEMBLAGES

II

o o

III

" O

1.0

0.8

0.6

0.4

0.2

0

0.2

0.4

0.6

0.8

1.0

Figure 3. Stratigraphic distribution of factor scores for the main paragenetic assemblages of chemical components in the post Jurassic section of sediments in the central northwesternPacific, northern Hess Rise, DSDP Site 464 (chemical analyses recalculated). Symbols as in Figure 1.

O

tin

c>in

nmö

m

I

I. M. VARENTSOV, B. A. SAKHAROV, M. A. RATEEV, D. YA. CHOPOROV

Table 7. Average contents and mean linear accumulation rates of chemical components for major geochronological subdivisions of the sectionof post-Jurassic sediments, central northwestern Pacific, northern Hess Rise, Site 464.

Unit

IA

IB

II

III

Lithology

Clayey radiolarian oozes,clayey silicic oozes

Siliceous clays

Brown volcanogenic clays

Red-brown silica, chalk,limestone

Cores

2-3

3-5

5-11

11-34

Sub-bottomDepth

(m)

3.5-18.8

18.8-36.1

36.1-89.0

89.0-307.6

Thickness(m)

15.3

17.3

52.9

218.6

Stratigraphy

L. Pleist.U. Plio.L. Plio.

U.-M. Mioc.

M. Mioc.L. Mioc.-U. Eoc.L. Eoc.-Paleoc.U. Cret.U. Maastr.(?)-Turonian

POST-JURASSIC SEDIMENTATION, SITE 464

Table

Age(m.y.)

1.0

{2.0

10.0

4.525.04.5

11.5

27.0

8.53.0

7. (Continued).

SedimentationRates

(mean linear)

(mm10~3yr )

7.5

10.5

0.45

1.110.181.111.65

0.70

17.8822.50

(mg.çm 2

lO~V~1)

367.5

499.8

21.42

84.8013.7584.80

126.06

53.48

3540.244455.0

40.53

47.62

44.8546.14

46.68

26.59

82.68

SiO2Ace. Rate

148.9

238.0

38.036.34

58.84

14.22

2927.07

4.67

4.22

12.5313.71

12.96

7.96

3.68

Average content (wt.%) and Accumulation Rates of Components(mg c m - 2 1 0 - 3 yr~ !)

A12O3Ace. Rate

17.16

21.1

10.621.89

16.34

4.26

130.28

CaCθ3

(

I. M. VARENTSOV, B. A. SAKHAROV, M. A. RATEEV, D. YA. CHOPOROV

Figure 4. Distribution of average contents (wt. %, air-dry) of SiO2, A12O3, Fe, Mn, P, and the normative molecule CaCO3 in post-Jurassic sediments of the central northwestern Pacific, northern Hess Rise, DSDP Site 464.

cial handling of actual values of accumulation rates ofcomponents (Table 7, Fig. 6). Nevertheless, assumingon the basis of mineralogy and lithology of these sedi-ments a reasonable variation range of the chemical com-position of late Aptian-early Albian and middle Al-bian-Cenomanian deposits, it is possible to note rela-tively high accumulation rates of SiO2, A12O3, and othercomponents. These data can be interpreted as evidencethat the northern part of Hess Rise during the Albianwas in the equatorial zone of high biological productiv-ity, with the general northward movement of the PacificPlate. Somewhat reduced values of these rates, as com-pared to the known values of that zone, and largeamounts of siliceous material, imply that sediment ac-cumulation took place considerably below the lysocline,and sometimes below the CCD.

Early phases of this stage are extremely interesting.Limited amounts of material allow to assume only that

during the late Aptian-Early Albian a significant role inthe total sedimentation balance belonged to accumula-tion of volcaniclastic material, i.e., products of explo-sive volcanism from the surrounding islands, whereas atthe very base of the section the same role belonged tometal-bearing sediments (Site 464 report, this volume).

Late Cretaceous (Turonian-Maastrichtian?)/Middle Miocene (accumulation time ofvolcaniclastic "brown clay" unit)

As emphasized earlier, the data on ichythyoliths(Doyle and Riedel, this volume) allows us to believe thatthe volcanogenic "brown clay" unit (53-m thick) coversthe period from Late Cretaceous (Turonian?) to middle(Late?) Miocene. The bottom part (Cores 8-10; LateCretaceous-Paleocene) contains particles of brown ba-salt glass of sand-silt size (up to 20-40%), hyalopelitematerial essentially altered to clayey matter, patches of

814

POST-JURASSIC SEDIMENTATION, SITE 464

+ + + ++ + 27.66+ ++ + + +

Figure 5. Distribution of average content (wt. °/o, recalc.) of Fe, Mn, P, K2O, and MgO in the section of post-Jurassic sediments of the central northwestern Pacific, northern Hess Rise, DSDP Site 464.

hematite (up to 20%), zeolite (up to 10-20%), basic pla-gioclases (labradorite-bytownite; up to 5-10%). Thebasal part (Turonian-Maastrichtian?) of the unit sam-ple 9-4, 106-109 cm) contains gravel debris of recrystal-lized silicified limestones. It is important to note that thegroundmass (up to 70-90%) of these sediments is com-posed of hyalopelite and silty basaltic material essen-tially transformed into clayey matter. Analysis of X-raydiffractograms of these sediments (Fig. 2) shows thatthe main components are represented by a combinationof disordered, finely dispersed mixed-layer phases, i.e.,montmorillonite-mica, with a variable content of ex-panding (montmorillonite) and non-expanding (mica)layers; quartz, heulandite, and plagioclases of labrador-ite-bytownite type are present as admixtures. In the up-per half (middle Miocene-Paleocene) of the unit (Sam-ple 5-7,CC), the general mineral composition of sedi-

ments, on the whole, remains the same. It should benoted, however, that in separate intervals occur appreci-able amounts of colorless glass of acid to intermediatecomposition (up to 10-20%; for example in the Paleo-cene, Sample 7-6, 43-46 cm), patches of nannofossilmicrite, opal-quartz material, and relative reduction inthe amount of Fe-hydroxides (by as much as 10%). Ac-cording to X-ray diffractograms, the type of zeolite ischanged; phillipsite dominates.

Mineral compositions have good correlation withgeochemical results, in particular with the distributionof paragenetic groups of chemical components, ana-lyzed above (Figs. 1,2).

These data allow us to believe that the "brown clay"unit is a deep-water (below CCD) accumulation of rela-tively fine-grained hyalopelite volcaniclastic materialsof basaltic composition, which are products of explosive

815

I. M. VARENTSOV, B. A. SAKHAROV, M. A. RATEEV, D. YA. CHOPOROV

COMPONENTS (mg • cm"2- 10"3• yr"1)

L./M. Mio.

POST-JURASSIC SEDIMENTATION, SITE 464

Late Cenozoic Gate Miocene-Pleistocene)

During this period, pelagic clayey-siliceous sedimentsaccumulated, forming Sub-units LA (clayey radiolarianoozes, clayey siliceous oozes) and IB (siliceous clays).These sediments (Unit IB) are deposited on volcano-genie "brown clay" (Site 464 report, this volume).

The major biogenic components of both sub-unitscontain in variable proportions remains of radiolarians,diatoms, sponge spicules, and to a lesser extent nanno-fossil micrite. The groundmass is represented by finehyalopelite material altered to clay matter. A charac-teristic admixture is represented by sand-silt particles ofbrown basalt (up to 5-15%), to a lesser extent by color-less acid glass. A slight admixture of micronodules ofFe- and Mn-hydroxides is also noted.

The following tendency is observed in this interval:on the one hand, a relative reduction of volcaniclasticcomponents upward in the section, and, on the otherhand, an increase in the content of siliceous (and to alesser extent carbonate) biogenic remains.

According to X-ray structural analysis, the greaterpart of the clay (Cores 2-5) is represented by two majorcomponents present in approximately equal propor-tions: (1) illite and a mixed-layer phase of mica-mont-morillonite with a few (

I. M. VARENTSOV, B. A. SAKHAROV, M. A. RATEEV, D. YA. CHOPOROV

Thus, in the geochemical history of post-Jurassicsedimentation of the northern part of Hess Ress, themost essential phases of development can be outlined,with different degrees of clarity, which reflect thegeneral evolution of the basin.

ACKNOWLEDGMENTS

The authors express their gratitude to the colleagues at theGeological Institute, U.S.S.R. Academy of Sciences, P. K. Ryabush-kin, D. A. Kazimirov, N. I. Kartoshkina, and N. Yu. Vlasova, for ef-ficient assistance in processing analytical data by computer, recalcula-tion of analyses, and technical help; to V. A. Drits and T. G. Eleseevafor X-ray structural analysis of samples; to P. P. Timofeeva and V. I.Koporulin for the provided materials and helpful discussions of work;to V. S. Zelinsky, N. K. Mirskaya, and K. A. Scheponnikova forassistance in graphic presentation of materials; and to L. L. Morozov-skaya for translation of the papers into English.

The authors greatly appreciate the reviewing of the papers andhelpful comments by N. G. Brodskaya and A. G. Kossowskaya.

REFERENCES

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Bender, M. L., Brocker, W., Gornitz, V., et al., 1971. Geochemistryof three cores from the East Pacific Rise. Earth Planet. Sci. Lett.,12:425-433.

Bezrukov, P. L., and Romankevich, E. A., 1970. Rate of sedimenta-tion in Pacific ocean. In Bezrukov, P. L. (Ed.), Pacific Ocean:Sedimentation in Pacific Ocean (Vol. 2): Moscow (Nauka), 288-300.

Bogdanov, Yu. A., and Chekhovskikh, E. M., 1979. Rates of sedi-mentation and absolute masses. In Smirnov, V. I. (Ed.), Metal-liferous Sediments of South-Eastern Pacific Ocean: Moscow(Nauka), pp. 280.

Boström, K., 1973. The origin and fate of ferromanganoan activeridge sediments. Stockholm Contri. Geol, 27:149-243.

Davis, J. C , 1973. Statistics and Data Analysis in Geology: New York(Wiley).

Harman, H. H., 1967. Modern factor analysis: Chicago (Univ. ofChicago Press).

Lisitzin, A. P., 1974. Sedimentation in Oceans: Moscow (Nauka).

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MacArthur, J. M., and Elderfield, H., 1977. Metal accumulationrates in sediments from Mid-Indian Oceanic Ridge and MarieCeleste Fracture Zone. Nature, 266:437-439.

Varentsov, I. M., and Blazhchishin, A. I., 1976. Ferromanganesenodules. Geology of Baltic Sea: Vilnus (Mokslas), pp. 307-348.

Zolotarev, B. P., and Choporov, D. Ya., 1978. Petrochemistry of ba-salts D/V. Glomar Challenger, Leg 45, Holes 395, 395A, 396. InMelson, W. G., Rabinowitz, P. D., et al., Init. Repts. DSDP, 45:Washington (U.S. Govt. Printing Office), 479-492.

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