I. M. Varentsov, Geological Institute of the U.S.S.R ...

37. GEOCHEMICAL HISTORY OF POST-JURASSIC SEDIMENTATION IN THE CENTRALNORTHWESTERN PACIFIC, SOUTHERN HESS RISE, DEEP SEA DRILLING PROJECT SITE 4651

I. M. Varentsov, Geological Institute of the U.S.S.R. Academy of Sciences, Moscow, U.S.S.R.

ABSTRACT

Analysis of the distribution of major components and heavy metals in a section of post-Jurassic deposits ofsouthern Hess Rise (Deep Sea Drilling Project Site 465), the results of processing analytical data by factor analysis, in-terpretation of information on mineralogy and lithology of sediments, and evaluation of rates of component accumula-tion enable distinction of the main stages in the geochemical history: (1) Late Albian-early Cenomanian (early oceanic),in which the accumulation of mostly shallow-water carbonate sediments of turbiditic nature enriched in organic matterand volcanogenic materials was proceeding; the sediments are characterized by maximally high contents of Fe, Mn,SiO2, A12O3, and associated heavy metals, present in the form of basaltic volcaniclastic material and, to a lesser extent,hydrothermal and exhalation products and materials of their post-sedimentary transformation (smectite, hydromica,Mn-Fe-Mg-carbonates); extremely high rates of accumulation of components are representative of this given stage. (2)Late Cretaceous (middle Cenomanian-late Maastrichtian), in which the accumulating sediments were considerablyeroded along some hiatuses (middle Cenomanian-middle Turonian; late Coniacian; early Campanian); during theCenomanian-Coniacian, this region is assumed to have been in the subequatorial zone of high biological productivity,within the general northward movement of the Pacific Plate (Lancelot and Larson, 1975; Lancelot, 1978; van Andel,1974). Considering geochemical features, the sediments of this stage are similar to biogenic carbonate pelagic oozes ofthe open ocean. (3) Early Tertiary (early-late Paleocene), in which the beginning of the Tertiary is characterized by ac-cumulation of carbonate nannofossil oozes admixed with siliceous remains; sedimentation was proceeding in the north-ern oligotrophic zone of the Pacific Ocean. (4) Tertiary-Quaternary, in which foraminifer-nannofossil pelagic oozeswith geochemical features typical of these varieties were accumulated; during the Eocene-Pliocene biogenic carbonatesedimentation was repeatedly disturbed by hiatuses.

Thus, the established geochemical stages of post-Jurassic sedimentation of the region reflect, as a whole, the evolu-tion of post-Jurassic sedimentation in the central northwestern Pacific. It is noteworthy that although the timing ofevents of these stages (for the Cretaceous particularly) for certain regions (Mid-Pacific Mountains, Nauru Basin, etc.)are essentially different, the general tendency of geochemical evolution of the basins is common.

INTRODUCTION

Hess Rise is among the largest aseismic structures ofthe central northwestern Pacific. It is located muchhigher than the surrounding abyssal areas of the PacificPlate. Such disposition of the rise allows us to believethat a relatively complete section of biogenic sedimentscan be developed in this rather shallow-water region (seeSite 465 report, this volume).

This paper is a continuation of the series of works onthe geochemical history of post-Jurassic sedimentationin the central northwestern Pacific, based on sedimentspenetrated during DSDP Leg 62 (see papers by Varent-sov et al., on Sites 463 and 464, this volume).

As with the previous works, the objective of thisstudy is to elucidate the main features of the geochem-ical history of sedimentation in the southern region ofHess Rise, recorded in the chemical and mineral com-position of sediments, on the basis of study of thegeochemistry of the major components, heavy metals,trace elements, and data on mineralogy and lithology.

It should be emphasized that particular attention isfocused in this work on studying the local features ofthe geochemical history of sedimentation in the given

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

structural area, and their relationships with the generalevolution of geochemistry of post-Jurassic sedimenta-tion in the central northwestern Pacific.

MATERIALS AND METHODSThis work was based on the data of chemical and mineral composi-

tion of sediments penetrated by Holes 465 and 465A. The studies werecarried out at the Geological Institute of the U.S.S.R. Academy ofSciences, Moscow. The results of lithologic-mineralogical investiga-tions of sediments are presented in other chapters of this volume.

A detailed description of methodological features of the studieswas given in our paper on Site 463 (Varentsov et al., this volume).

It should be noted that the determination of chemical componentsof sediments was carried out at the Geological Institute of theU.S.S.R. Academy of Sciences: major components by methods ofbulk analysis, heavy metals by optical emission spectroscopy, with theuse of international standards (Zolotarev and Choporov, 1978; Kirk-patrick, 1979).

In order to exclude the diluting influence of carbonate and sili-ceous components (and terrigenous components being present), and toreduce the composition of sediments to a geochemically comparablebasis, the chemical analyses were recalculated to a clastic-free, silica-free, carbonate-free basis (Varentsov and Blazhchishin, 1976).

The analytical data were processed by a computer (EC-1022)(D. A. Kazimirov, P. K. Ryabushkin), following the program of fac-tor analysis (R- and Q-mode; Davis, 1973; Harman, 1967).

The determinations used in interpreting the data of chemicalanalysis of clay minerals in the < 0.001-mm fraction are given in ac-cordance with the data by Rateev et al. (this volume). In addition, allchemically analyzed samples were studied as thin sections under themicroscope, and by diffractometry methods, with necessary treatmentfor elucidation of their general composition (natural specimens).

819

I. M. VARENTSOV

PARAGENETIC ASSOCIATIONSOF COMPONENTS

Paragenetic associations of chemical componentswere identified through the results of processing ofanalytical data by the factor analysis method. Two typesof determinations were studied: (1) the data of thechemical analyses presented in weight percent (air-dry);(2) data recalculated to terrigenous-free, silica-free,carbonate-free. The established paragenetic groupingswere interpreted against the background of data on min-eralogy and lithology of sediments, and the possiblegeochemical conditions of their formation.

DATA OF CHEMICAL ANALYSIS(TABLES 1, 2, 3; FIG. 1)

the section (Fig. 1): nannofossil and foraminifer-nanno-fossil oozes of the Pleistocene/late Campanian.

Assemblage IIA (+): SiO2 (0.65)-Al2O3 (0.60)-Na2O(0.32)-K2O (0.74)-Fe (0.73)-Mn (0.41)-P (0.63)-Ni(0.40)-V (0.30)-Cu (0.17)-Ga (0.32)-Mo (0.28). This isan aluminosilicate phase represented mostly by mixed-layer minerals of the montmorillonite-illite type, closelyrelated to P and a set of heavy metals: Ni, V, Ga, Mo.

The predominant development of this associationwithin late Albian laminated limestones (e.g., Samples29-1, 81-82 cm; 37-2, 74-75 cm), appreciably enrichedin fine basaltic volcaniclastic material, can be inter-preted as an evidence that the given group of com-ponents is the products of alteration (hydromicatiza-tion, smectitization) of volcanogenic components.

Assemblage IIB ( - ) : CaO (-0.91)-CO2 (-0.88)-Assemblage IA( + ): SiO2 (0.39)-Al2O3 (0.59)-MgO Corg (-0.18)-Co (-0.52). This assemblage is repre-

(0.55)-Corg (0.75)-Fe (0.12)-P (0.49)-Cr (0.48)-Ni sented by calcium carbonate (main phase), Corg, andCo, which are slightly related to the main phase.

The assemblage is developed mostly in the lower partof Unit I (lower than Sample 9-4, 105-107 cm; depth120.55 m), and among the rocks of Unit II (laminatedlimestones) (Fig. 1).

Study of thin sections and correlation to the dis-cussed assemblage IB ( - ) enables us to consider groupIIB ( - ) mostly the product of recrystallization of the in-itial biogenic calcium carbonate in the course of its epi-genetic transformation. This conclusion agrees with thedata on determination of density (see Site 465 report,this volume): for Unit I below Core 10 (134-276 m), theaverage density is 1.60 ± 0.02 g/cm3; for Unit II(276-411.7 m) it is 2.22 g/cm3; whereas for the higherhorizons (0-135 m) the average density is 1.54 ± 0.04g/cm3. However, the process of recrystallization of bio-genic carbonate proceeds unevenly (the interval 120.55-134.0 m may be regarded as a zone of intermediate den-

yOTg

(0.60)-V (0.90)-Cu (0.81)-Mo (0.87). This assemblageis represented by aluminosilicate components in theform of basaltic volcaniclastics, considerably trans-formed into montmorillonite-illite minerals. The associ-ation is characterized by closely related organic carbon,P, and a number of heavy metals: Cr, Ni, V, Cu, Mo.

The distribution of this assemblage of components israther limited in the section (Fig. 1): late Albian-earlyCenomanian laminated limestones essentially enrichedin basaltic volcaniclastic material and dispersed sapro-pel-like organic matter.

Assemblage IB ( - ) : CaO (-0.23)-Na2O (-0.77)-CO2(-0.17). This assemblage is presented by calciumcarbonate in the form of unconsolidated biogenic re-mains: nannofossils, foraminifers, micrite. As a rule,this almost unaltered calcium carbonate is closely re-lated to Na taken up from sea water. Noteworthy is adistinct stratigraphic localization of this assemblage in

Table 1. Chemical composition of Cenozoic and Mesozoic sediments of the central northwestern Pacific, southern Hess Rise, DSDP Site 465 (%,wt, air-dry weight portion).

Sample(interval in cm)

465-2-1, 108-1103-2, 110-1124-2, 10-125-2, 70-72

465A-1-1, 104-106465-6-2, 98-100465A-3-1, 118-120465-10-5, 93-95465A-9A 105-107

10-1, 64-6811-1,26-3012-1, 70-7415-1, 120-12216-4, 80-8217-1, 140-14218-1, 136-13819-2, 146-14820-1, 94-9626-1, 57-5827-1, 76-7728-1, 81-8229-1, 81-8230-1,70-7132-1, 58-5933-1, 15-1634_1, 41-4236-2, 92-93

37-2, 74-75

38-1, 73-7439-1, 113-11440-2, 42-43

SiO2

5.750.440.261.452.520.570.722.610.06

—

0.910.420.300.32—

0.320.700.080.460.663.02

17.670.540.981.630.621.95

4.36

3.742.242.59

A12O3

1.570.010.050.160.100.050.100.160.05—

0.020.080.080.060.020.060.010.060.310.04

6.130.120.260.530.280.57

1.31

0.900.560.85

Fe2O3

0.740.150.060.060.200.190.200.200.070.350.200.200.200.520.200.130.130.130.130.370.353.580.220.130.220.130.26

0.33

0.330.260.55

CaO

49.4654.8454.9253.7852.5154.0752.8452.6453.8853.9652.7454.3954.2054.2154.1854.0653.8054.3653.8753.5848.6236.7054.0053.8852.7154.1950.76

46.39

48.4650.1551.35

(wt

MgO

0.420.410.420.220.07

0.09

—

0.340.170.520.260.350.340.440.260.181.230.350.180.350.180.44

0.35

0.440.440.71

. % , <

MnO

0.010.020.010.060.040.040.020.010.010.010.020.010.020.020.020.010.010.010.010.010.020.050.010.010.01

0.01

0.01

0.010.010.13

Component

lir-dry)

Na2O

1.281.081.08

(

.35

.45

.27).54.18.64.45.45.77.06.06.06.06.16

1.060.530.530.681.520.530.530.600.530.60

0.76

0.850.850.68

K2O

0.510.150.150.200.310.250.250.250.200.830.830.140.140.090.090.140.090.090.150.100.310.610.100.200 . 1 5

0.150.25

0.51

0.460.310.31

CO 2

38.9541.7543.45.41.3540.6042.3542.3040.5542.2042.3541.8042.8542.1540.2541.5542.9541.6041.8542.4541.8037.6027.1042.3541.9541.2542.5540.00

35.85

38.6539.4040.10

corg

—

——

—————

———

—0.880.831.930.030.710.571.150.651.12

-

2.420.54

P2O5

0.070.070.050.140.100.090.100.030.0040.0040.03

0.040.02

—

0.020.030.030.040.080.140.730.040.030.070.060.18

I 0.25 11(0.33)1

0.300.230.10

Fetot

0.520.100.040.040.140.130.140.140.050.240.140.140.140.360.140.090.090.090.090.260.242.500.150.090.150.090.18

0.23

0.230.180.38

Mn t o t

0.010.010.010.050.C30.030.020.010.010.010.020.010.020.020.020.010.010.010.010.010.020.040.010.010.01

0.01

0.01

0.010.010.10

Ptot

0.030.030.040.060.040.040.040.010.0020.0020.01

0.020.01

—

0.010.010.010.020.030.060.320.020.010.030.030.08

j 0.1111(0.14)1

0.130.100.04

Cr

<IO< 10< 10< 10<IO< 10< 10< 10<IO<IO< 10< 10< 10<IO<IO<IO<IO<IO

101115

<IO<IO< 10< 10< 10< 10

13

< 10< 10<IO

Ni

< 10< 10< 10< 10< 10< 10< 10< 10< 10< 10< 10< 10< 10<IO< 10<IO<IO<IO< 10

1112

<IO<IO<IO<IO<IO

20

49

3815

< 10

V

< 1 5< 15< 15< 15< 15< 15< 15< 15< 15< 15< 15< 15< 15< 15< 15< 15< 15< 15

4935724156596658

260

>500

280210155

(wt.

Cu

< 15< 15< 15< 15< 15< 15< 15< 1 5< 15< 15< 15< 15< 15< 15< 2 0< 2 0< 2 0< 2 0< 2 0< 2 0< 2 0< 2 0< 2 0< 2 0< 2 0< 2 0< 2 0

30

< 2 0< 2 0< 2 0

% 10"

Co

<IO< 10<IO<IO<IO<IO<IO< 10< 10< 10<IO<IO<IO<IO< 10< 10< 10<IO< 10<IO<IO<IO<IO<IO<IO<IO< 10

<IO

<IO<IO<IO

Pb

<IO< 10< 10< 10< 10< 10< 10< 10< 10< 10< 10<IO<IO<IO<IO< 10< 10< 10<IO< 10< 10< 10<IO<IO< 10<IO< 10

< 10

< 10<IO<IO

Ga

< 5< 5< 5< 5< 5< 5< 5< 5< 5< 5< 5< 5<5<5<5<5<5<5<5<5<5<5<5<5<5< 5<5

<5

<5<5<5

G«

<<<<<<<<<<<<<<<<<<<<<<<<<<<

<

<<<

Mo

<<<<<<<<<<<<<<<<<<

.55

.5

.5

.5

.5

.5

.5

.5

.5

.5

.5

.5

.5

.5

.5

.55

2.82.83.72.12.13.14.35.19.9

16.2

167.35.8

820

POST-JURASSIC SEDIMENTATION, SITE 465

Table 2. Results of factor analysis (R-mode) forchemical components (wt. °/o), Cenozoic andMesozoic sediments of the northwestern Pa-cific, DSDP Site 465.

No.

123456789

1011121314151617181920

Component

SiO2

A12O3

CaOMgONa2OK2O

co2CorgFeMnPCrNiVCuCoPbGaGeMo

Dispersion input (%)Total idispersion (%)

Factor Loadings(after rotation)

I

0.390.59

-0.230.55

-0.77-0.07-0.17

0.750.12

0.490.480.600.900.81

0.87

34.7534.75

II

0.650.60

-0.91

0.320.74

-0.88-0.18

0.730.410.63

0.400.300.170.52

0.32

0.28

17.0651.81

III

0.32

0.51

0.19

-0.51

0.38

-0.470.77

-0.34

9.6661.48

Table 3. Stratigraphic distribution of factor scores (R-mode) forchemical components (wt. %) in Cenozoic and Mesozoic sedi-ments of the northwestern Pacific, DSDP Site 465.

No.

123456789

10111213141516171819202122232425262728293031

Sample(interval in cm)

465-2-1, 108-1103-2, 110-1124-2, 10-125-2, 70-72

465A-1-1, 104-106465-6-2, 98-100465A-3-1, 118-120465-10-5, 93-95465A-9-4, 105-107

10-1, 64-6811-1, 26-3012-1, 70-7415-1, 120-12216-1, 80-8217-1, 140-14218-1, 136-13819-2, 146-14820-1, 94-9626-1, 57-5827-1, 76-7728-1, 81-8229-1, 81-8230-1,70-7132-1, 58-5933-1, 15-1634-1,41-4236-2, 92-9337-2, 74-7538-1,73-7439-1, 113-11440-2, 42-43

Stratigraphy

PleistoceneU. PaleoceneU. PaleoceneU. PaleoceneU. PaleoceneU. PaleoceneL. PaleoceneU. MaastrichtianL. MaastrichtianL. MaastrichtianL. MaastrichtianL. MaastrichtianU. CampanianU. CampanianU. CampanianU. CampanianU. CampanianU. CampanianU. CenomanianU. CenomanianU. AlbianU. AlbianU. AlbianU. AlbianU. AlbianU. AlbianU. AlbianU. AlbianU. AlbianU. AlbianU. Albian

Factor scores(after rotation)

I

-0.63-0.76-0.47-0.83-1.00-1.29-0.71-0.75-1.08-1.37-1.50-0.53-0.64-0.75-0.41-0.41-0.31

0.260.960.851.23

-0.470.990.871.021.191.671.921.361.380.72

II

0.97-0.27-0.86

0.010.430.420.310.10

-0.64-0.24

0.85-0.58-0.38-0.38-0.78-0.31-0.67-1.02-0.87-0.71

0.653.81

-1.12-0.88-0.55-1.00

0.111.881.120.140.45

III

-1.340.74

-0.20-1.68-1.23

0.910.830.031.231.321.330.78

-1.30-0.54-0.06

1.040.12

-0.90-0.23-0.10-0.12-0.95

0.29-0.04-1.25

0.530.192.060.87

-0.13-2.20

sities). The admixture of clay components and organicmatter inhibits the process of recrystallization, espe-cially in the lower parts of Unit II (see Fig. 1).

Assemblage IIIA (+): K2O (-0.19)-Ni (0.38)-Pb(0.77). This assemblage is composed of hydromica andK-feldspar components rich in nickel and lead.

It is noteworthy that the intervals of well-pronounceddevelopment of this assemblage are observed in thelower half of early Albian deposits (see Fig. 1) and inearly Maastrichtian sediments.

These deposits are characterized by noticeable enrich-ment in clay components developed after basic volcani-clastic material. In this case, in early Albian rocks themaximum content of basaltic volcaniclastic components(Assemblage I A; Fig. 1) corresponds to the maximum ofthe cluster concerned.

Assemblage IIIB ( - ) : A12O3 (-32)-MgO (-0.51)-Mn (-0.51)-Co (-0.47)-Ga (-0.34). The real phasecomposition of this assemblage is not clear. Study ofthin sections of rocks and sediments enables us to sup-pose that this assemblage is represented by manganesehydroxides associated with authigenic magnesian smec-tites and heavy metals (Co, Ga).

This assemblage is best pronounced near the contactwith basalts, the boundary between Units II and III,where considerable exhalation-hydrothermal influencetook place (Sample 465A-40-2,42-43 cm). At the otherlevels of the section, this assemblage is composed ofproducts of deep alteration of basaltic volcaniclasticmaterial (see Fig. 1).

DATA OF CHEMICAL ANALYSESRECALCULATED (TABLES 4-6; FIG. 2)

Assemblage IA (+): CaO (0.24)-Fe (0.30)-Mn (0.13).This cluster is represented by surplus amounts of lime,iron, and to a lesser extent manganese. The most ap-preciable values of factor loadings of this assemblagecorrespond to the similar parameters of the above-con-sidered Assemblage IIA (Fig. 1); this enables us to inter-pret such a cluster of components as evidence of thepresence of a specific calcium-iron smectite phase—aproduct of alteration of basaltic volcaniclastic material.Such an interpretation does not contradict the availabledata on mineralogy.

Assemblage IB (-): Na2O (- 0.41)-K2O (- 0.80)-Cr(-0.90)-Ni (-0.89)-V(-0.78)-Cu (-0.96)-Co(-0.89)-Pb (-0.89)-Ga (-0.94)-Ge (-0.80)-Mo( - 0.67). This group of components plays the role of adiluting agency in the general paragenetic associationwith the preceding Assemblage IA (+). If we assumethat both Assemblages IA (+) and IB ( - ) are repre-sented by mixed-layer minerals (montmorillonite/illite;see also assemblage IIA (+); Fig. 1), the given associ-ation of components (IB (+); Fig. 2) reflects the dis-tribution of illite packets proper and related heavymetals in the section. The predominant development ofassociations in upper Albian-lower Cenomanian rocksenriched with basaltic volcaniclastic material is indirectevidence of its epigenetic origin.

821

00

to FACTOR ASSEMBLAGES (FACTOR SCORES)

FACTOR LOADINGS

CLAY COMPONENTS

-_—_— Polymineral assemblage of illite, chlorite,— — — and a small admixture of montmorillonite.

× — × -- X - X

An assemblage represented by Fe(AD-montmorillonite, with a slight admixtureof illite, chlorite, and Fe-montmorillonitein some interbpds. Representative is thepresence of zeolite (heulandite) and chertysegregates composed of crystobalite—tridymite.

An assemblage consisting of Al-montmorillonite with a small admixture of illite,quartz is present too.

Figure 1. Stratigraphic distribution of factor scores of the main paragenetic assemblages of chemical components in the section of post-Jurassic deposits ofthe central northwestern Pacific, southern Hess Rise, DSDP Site 465 (data of chemical analysis recalculated to air-dry weight). Lithologic symbols are thoseused in DSDP Initial Reports.

POST-JURASSIC SEDIMENTATION, SITE 465

Table 4. Chemical composition of Cenozoic and Mesozoic sediments of the central northwestern Pacific, southern Hess Rise, DSDP Site 465. (wt.% recalculated to terrigenous-free, silica-free, carbonate-free).

Sample(interval in cm)

465-2-1, 108-1103-2, 110-1124-2, 10-125-2, 70-72

465A-1-1, 104-106465-6-2, 98-100465A-3-1, 118-120465-10-5, 93-95465 A-9A 105-107

10-1,64-6811-1,26-3012-1,70-7415-1, 120-12216-4, 80-8217-1, 140-14218-1, 136-13819-2, 146-14820-1,94-9626-1,57-5827-1, 76-7728-1,81-8229-1, 81-8230-1, 70-7132-1, 58-5933-1, 15-1634-1,41-4236-2, 92-9337-2, 74-7538-1,73-7439-1, 113-11440-2, 42-43

CaO

47.695—

36.21927.1324.682

—36.5324.299

——

21.55263.17639.992

31.19539.001

20.09233.01122.9580.179

28.6813.666——

24.728——

4.614

MgO

13.18211.9978.6456.9752.149——

2.834_—__

15.3743.553

16.994

14.01212.85932.04817.3195.241

11.72730.13911.83326.66818.14214.65510.819

23.73631.886

Na2θ

65.31631.66577.66146.37152.66270.67165.43746.17782.84357.45463.17684.86940.04923.04234.74387.58046.55640.81653.69035.71938.73426.86746.95340.59252.78562.94957.22437.35961.90351.94233.286

K 2 O

15.7754.371

10.3026.146

10.78013.54328.8128.9769.761

32.88836.05714.2935.9921.7882.854

10.9373.5733.166

11.1336.359

11.747—

7.42312.8526.965

13.51517.04817.48025.63414.6079.955

Fe t o t

4.2142.875_

0.4494.4806.800

4.4482.1249.272

5.8527.6304.495

3.5333.089

17.1166.024

31.61811.6263.6055.4980.365

0.480

L82612.236

M n t o t

_

0.292—

1.5541.0821.661

0.3740.496

—

0.9150.4320.653

0.4000.378

0.6701.0240.4100.8590.7050.733

0.320

5.029

p t o t

1.2970.8792.8672.0511.4422.2184.8350.3700.0910.0790.432

0.9150.214

0.8120.4000.3781.9772.0163.4346.3931.7440.7052.5663.5317.7765.5969.6416.1701.918

Cr

0.0110.0290.0700.0310.0350.0540.1160.0350.0490.0400.0430.1060.0450.0210.0320.0800.0400.0370.0830.0730.066

0.0800.0700.0640.1100.0700.0320.0380.0440.031

Ni

0.0110.0290.0710.0310.0350.0550.1160.0350.0500.0400.0430.1060.0450.0210.0320.0800.0400.0370.0940.0730.048

0.0800.0700.0640.1100.1690.2290.2600.0760.076

V

0.0270.0440.0810.0480.0520.0820.1750.0550.0740.0590.0650.1610.0670.0320.0490.1210.0600.0560.4890.2350.4040.0020.4920.4470.5770.6822.5522.6172.0971.2970.773

Cu

0.0540.0440.1070.0340.0530.0830.1790.0570.0750.0590.0650.1640.0680.0320.0650.1640.0800.0760.1980.1350.1080.0040.1760.1490.1650.2310.1790.1390.1300.1130.093

Co

0.0430.0290.0720.0340.0360.0550.1210.0390.0500.0400.0430.1100.0460.0220.0330.0820.0400.0380.1010.0670.0540.0090.0890.0760.0820.1880.0900.0480.0690.0570.047

Pb

0.0430.0290.0720.0340.0360.0550.1210.0390.0500.0400.0430.1100.0460.0220.0330.0820.0400.0380.1010.0670.0540.0090.0890.0760.0820.1180.0900.0480.0690.0570.057

Ga

0.0160.0150.0350.0170.0170.0270.0590.0190.0250.0200.0220.0540.0230.0110.0160.0410.0200.0190.0470.0330.024

—0.0430.0360.0370.0560.0400.0160.0310.0250.021

Ge

0.0040.0030.0070.0030.0040.0060.0120.0040.0050.0040.0040.0110.0050.0020.0030.0080.0040.0040.0100.0070.005

—0.0090.0780.0080.0120.0090.0050.0070.0060.005

Mo

0.0070.0040.0110.0050.0050.0080.0180.0060.0080.0060.0070.0170.0070.0030.0050.0120.0060.0060.0290.0190.0220.0020.0190.0240.0380.0620.0980.0860.1220.0450.030

Table 5. Results of factor analysis (R-mode) ofchemical components (wt. °7o, recalculated toclastic-free, carbonate-free, silica-free), Ceno-zoic and Mesozoic sediments of the northwest-ern Pacific, DSDP Site 465.

Table 6. Stratigraphic distribution of factor scores (R-mode) forchemical components (wt. °?o, recalculated to clastic-free, carbon-ate-free, silica-free), in Cenozoic and Mesozoic sediments, centralnorthwestern Pacific, DSDP Site 465.

No.

1

2

3

4

5

6

7

8

9

10

11

12

1314

15

16

Components

CaOMgONa2θK 2 OFeMnPCrNiV

CuCoP b

GaGe

M o

Dispersion input (%)Total dispersion (%)

Factor Loadings(after rotation)

I

0.24

-0.41-0.80

0.300.13

-0.94-0.89-0.78-0.96-0.89-0.89-0.94-0.80-0.67

58.2558.25

II

0.910.62

-0.73

0.800.92

-0.04

-0.05

-0.36-0.36

-0.20

15.5673.81

III

-0.160.49

-0.140.82

-0.16

0.53

0.63

9.8283.63

No.

123456789

10111213141516171819202122232425262728293031

Sample(interval in cm)

465-2-1, 108-1183-2, 110-1124-2, 10-125-2, 70-72

465A-1-1, 104-106465-6-2, 98-100465A-3-1, 118-120465-10-5, 93-95465A-9-4, 105-107

10-1, 64-6811-12-15-16-̂17-18-

, 26-30,70-74, 120-122

>, 80-82, 140-142, 136-138

19-2, 146-14820-1, 94-9626-1, 57-5827-1, 76-7728-1, 81-8229-1, 81-8230-1, 70-7132-1, 58-5933-1, 15-1634-1, 4i_4236-2, 92-9337-2, 74-7538-1, 73-7439-1, 113-11440-2, 42-43

Stratigraphy

PleistoceneUpper PaleoceneUpper PaleoceneUpper PaleoceneUpper PaleoceneUpper PaleoceneLower PaleoceneUpper MaestrichtianLower MaestrichtianLower MaastrichtianLower MaastrichtianLower MaastrichtianUpper CampanianUpper CampanianUpper CampanianUpper CampanianUpper CampanianUpper CampanianLower CenomanianLower CenomanianUpper AlbianUpper AlbianUpper AlbianUpper AlbianUpper AlbianUpper AlbianUpper AlbianUpper AlbianUpper AlbianUpper AlbianUpper Albian

Factor scores(after rotation)

I

1.080.68

-0.020.600.33

-0.23-0.76

0.24-0.12

0.470.57

-0.91-0.02

1.110.23

-0.270.170.24

-0.65-0.69-0.43

4.36-0.96-1.32-0.85-1.02-0.69-0.49-0.31-0.10-0.21

II

-1.000.78

-1.320.520.580.15

-1.460.63

-0.11-0.99-1.69-1.41

0.790.951.06

-1.700.710.77

-0.861.181.02

-0.070.831.280.82

-0.53-0.93

0.81-1.52-0.50

1.21

III

0.18-0.22

0.26-0.23-0.52-0.99-0.12-0.67-1.68-1.27-0.75-2.12-0.37-0.47-1.74-0.63-0.49-0.43

0.910.290.451.320.330.080.611.031.821.621.401.550.85

823

OOFACTOR ASSEMBLAGES (FACTOR SCORES)

C L A Y C O M P O N E N T S

= _ — _ — Polymineral assemblage of i l l i te , ch lor i te ,= = = and a small ad mix ture of moπtmoπl lon i te .

× — X —

An assemblage represented by Fe(AI ) •montmorillonite, with a slight admixtureof ill ite, chlorite, and Fe-montmorillonitein some interbeds. Representative is thepresence of zeolite (heulandite) and chertysegregates composed of crystobalite— tridymite.

An assemblage consisting of Al-montmoriilonite with a small admixture of illite,quartz is present too.

1.0

0.8

0.6

0.4

0.2

0

0.20.40.60.81.0

FACTOR LOADINGS

II III

fcβ

Figure 2. Stratigraphic distribution of factor scores of the main paragenetic associations of chemical components (recalculated to terrigenous-free, carbonate-free, silica-free) in the section of post-Jurassic deposits of the central northwestern Pacific southern Hess Rise, DSDP Site 465. Symbols as in Figure 1.

POST-JURASSIC SEDIMENTATION, SITE 465

Assemblage IIA (+): CaO (0.91)-MgO (0.62)-Fe(0.80)-Mn (0.92). This assemblage is represented byprofoundly altered basaltic volcaniclastic material andrelated segregates, crusts, and coatings of Mn- and Fe-hydroxides. The assemblage is developed mostly in up-per Albian-lower Cenomanian and upper Campaniansediments.

Assemblage IIB ( - ) : Na2O (0.73)-Co (-36)-Pb(-0.36). This assemblage of components is negativelycorrelated with Assemblage IIA ( + ). One can assumethat Assemblage IIB ( - ) is represented by volcanogeniccomponents relatively rich in Na2O, and that Co and Pare rather moderately associated with these compo-nents. Elimination of the diluting effect of carbonateand siliceous components enables us to establish arather even distribution of these two assemblages in thesection.

Assemblage IIIA (+): MgO (0.49)-P (0.82)-V(0.53)-Mo (0.63). This assemblage is represented by aset of excessive (against the accepted norm) compo-nents; their distribution is clearly confined to upper Al-bian-lower Cenomanian rocks (Fig. 2). The similar dis-tribution in the section, as mentioned above, is repre-sentative of assemblage IA (+) (Fig. 1). Thus, the iden-tity of mineral composition of these two assemblages isevident: basaltic volcaniclastic material transformed in-to montmorillonite-illite minerals and associated heavymetals.

Assemblage IIIB ( - ) : CaO ( - 0.16)-Mn ( - 0.14)-Cr(-0.16). The slight significance of factor loadingsenables us to assess the composition of the assemblagerather tentatively. Worthy of attention is the clear local-ization of this assemblage in the section: Unit I (upperCampanian-Pleistocene). A similar stratigraphic dis-tribution is characteristic of the above-considered as-semblage IB ( - ) (Fig. 1), represented by biogenic car-bonates (nannofossil oozes and foraminifer-nannofossiloozes).

It should be emphasized that Assemblage IIIB ( - ) iscomposed of a set of components that are excessiverelative to normative carbonate molecules. The identityof distribution of both assemblages and peculiarities ofthe composition of components allows us to think thatAssemblage IIIB ( - ) may be represented by remnantphases after recrystallization of biogenic carbonates,diagenetic Mn-hydroxides in particular. The reality ofsuch post-sedimentary recrystallization of biogenic car-bonate is evidenced by the results of study of the sedi-ments under a microscope, and the features of distribu-tion of paragenetic groups (IB ( - ) , IIB ( - ) ; Fig. 1) inthe section.

AVERAGE CONTENTS OF COMPONENTS ANDRATES OF THEIR ACCUMULATION

(TABLES 1, 4, 7; FIGS. 3-5)

Distribution of Average ContentsAnalysis of the specific features of distribution of

average contents of the major components of sediments

and rocks in the main geochronological subdivisions ofthe section (Tables 1, 4; Figs. 3, 4), with allowance forthe above-mentioned forms of their occurrences (seeFigs. 1,2), enables us to outline the following stages andphases in the geochemical history of sedimentation:

Late Albian-early Cenomanian (early oceanic). Thisphase is characterized by relatively high concentrationsof SiO2, A12O3, Fe, Mn, P, and associated heavy metals,and high contents of organic carbon (Late Albian,0.0-2.42, avg. 0.83); early Cenomanian, 0.83-0.88, avg.0.86). Two phases can be defined in the stage con-cerned: (1) a late Albian phase in which the sedimentsare characterized by relatively high SiO2, A12O3, Fe,Mn, and P contents, and associated heavy metals; en-richment in these components is related to the consider-able amounts of volcaniclastic components of basalticcomposition, and to hydrothermal and exhalative prod-ucts; (2) an early Cenomanian phase, in which the litho-logic features and chemistry of major components(CaCO3, Corg, etc.) in the sediments are similar to thelate Albian; nevertheless, an appreciable decrease inconcentrations of the components of volcanogenicorigin (SiO2, A12O3, Fe, Mn, P) and heavy metals isobserved.

Late Cretaceous (middle Cenomanian-late Maas-trichtian). The chemical composition of sediments ofthe initial phases of this stage (middle Cenomanian-Santonian) remains obscure because of the hiatuses andpoor core recovery, and the predominance of cherts inthis part of the section. However, fragmentary data ofthe shipboard descriptions (see Site 465 report, this vol-ume) enable us to believe that the sediments of this stageare formationally homogeneous.

The contents of major components and associatedheavy metals in sediments of the later period of thisstage (late Campanian-late Maastrichtian) do not ex-ceed the values characteristic of pelagic foraminifer-nannofossil oozes of the open ocean. Noteworthy is asomewhat higher content of normative molecules ofMnCO3, FeCO3, and MgCO3 in early Campanian-earlyMaastrichtian sediments, due to epigenetic alteration ofan admixture of mafic volcaniclastic material.

Early Tertiary (early-late Paleocene). Essential pre-dominance of CaCO3 in the form of foraminifer-nannofossil oozes considerably masks the features ofauthigenic components recognized while analyzing therecalculated data; for instance, P and K2O concentra-tions (early Paleocene; Fig. 4); noteworthy are high con-centrations of the normative molecules MnCO3 andFeCO3 (early Paleocene) and MgCO3 (late Paleocene)(Fig. 3).

Tertiary-Quaternary. A sharp decrease of thicknessin this stage and their redeposited character (Eocene-Pliocene sediments) resulted in very restricted chemical-analysis data. The information of Pleistocene sediments(see Tables 1, 4, 7; Figs. 3, 4) shows that they are repre-sented by foraminifer-nannofossil oozes with an admix-ture of basaltic volcaniclastic material. Such volcano-genic components resulted in relatively high contents of

825

I. M. VARENTSOV

Table 7. Average contents and mean rates of accumulation of chemical components for the main geochronological subdivisions of post-Jurassicsediments, central northwestern Pacific, southern Hess Rise, DSDP Site 465.

Unit

I

II

III

Lithology

Nannofossil,foraminifer-nannofossiloozeLaminatedlimestoneTrachyte

Cores

1-11, 1A-25A

26A-40A

40A-46A

Sub-bottomDepth(m)

0-276

276.0-411.7

411.7-476.0

Thickness(m)

276.0

135.7

64.3

Stage(substage)

PleistoceneU. PaleoceneL. PaleoceneU. Maastricht.L. Maastricht.U. CampanianU. TuronianL. Cenoman.U. Albian

Cores

0 to 2-1, 130 cm2-6, 60 cm to 6.CC7-1 to 8,CC9-1 to 8A.CC9-lAto 1212-1 to 20.CC

25.CC to 27.CC28-1.4O.CC

Sub-bottomDepth

(m)

0.0-2.39.1-48.5

48.5-61.561.5-106.0

106.0-144.5144.5-229.0

267.0-295.5295.5-419.5

Thickness(m)

2.339.413.043.538.584.5

28.5123.5

Density(g/cm*)

1.541.541.541.541.541.60

2.222.22

WaterContent

(%)

40.0(?)35.035.0(?)35.035.134.8

5.6012.7

Duration(m.y.)

1.86.55.02.52.54.0

2.54.0

A12O3 and Fe, and in post-sedimentary dissolution inappreciable amounts (see Fig. 3) of normative moleculesof FeCO3 and MnCO3.

Rates of Component Accumulation (Table 7; Fig. 5)Methodological aspects of calculation of average

linear rates of sedimentation, accumulation of compo-nents, selection of chronostratigraphic scales, and dura-tion of the main chronostratigraphic intervals are givenin our work on the geochemical history of sedimenta-tion at Site 463 (Varentsov et al., this volume).

Analysis of the recalculated data on average linearrates of sedimentation and accumulation of componentsand the values of their average contents and forms ofoccurrence in the sediment permitted definition ofstages of geochemical history of sedimentation of theregion. On the whole, these stages and phases corre-spond to the intervals established in the analysis ofdistribution of average contents of the components. It isimportant to emphasize that hiatuses in the lower partof the section (middle Cenomanian-middle Turonian;late Coniacian; early Campanian; Fig. 5) result in thefinal analysis in decrease of already-minimal values ofrates of sediment and component accumulation. Thus,interpretation of these data can be carried out onlywithin the context of information on geochemistry,lithology, mineralogy, and sedimentation conditions.

Late Albian—early Cenomanian (early oceanic). Thisstage is characterized by maximum accumulation ratesof sediments and components. Noteworthy are ratherhigh rates of accumulation of components supplied inthe form of volcaniclastic material of basaltic composi-tion, and products of hydrothermal exhalations: Fe,Mn, A12O3, SiO2, P, and heavy metals (Bender et al.,1971; Boström, 1973; Mac Arthur and Elder field, 1977).Sediments of turbiditic nature accumulated under con-ditions of a relatively shallow-water stagnant basin(deficiency of oxygen in bottom water).

For the late Albian phase of this stage, the rates ofsedimentation and rates of accumulation of componentsrepresentative of the initial (proto-oceanic) stages ofdevelopment of a basin have been established (Tiercelinand Faure, 1978); in particular, they are close to param-eters for the late Barremian-early Aptian phase in the

western part of the Mid-Pacific Mountains, Site 463(Varentsov et al., this volume).

For the early Cenomanian phase of the stage, thevalues of rates of sedimentation and accumulation ofcomponents are appreciably lower than those for thelate Albian (Table 7; Fig. 5). This may be related to aconsiderable extent to the presence of a lower middleCenomanian-middle Turonian hiatus, and to a lesserextent to a lower rate of sedimentation with the deepen-ing of the basin.

Late Cretaceous (middle Cenomanian-late Maes-trichtian). The given geochronological intervals arepresented by deposits of drastically decreased thick-nesses, as a result of hiatuses (Fig. 5). This is confirmedby the fact that the calculated average rates of sedimen-tation and accumulation of components do not corre-spond with the rates that might have been expected inthe absence of hiatuses during the northward movementof the Pacific Plate at that time. According to the modi-fications of the horizontal movement of the PacificPlate, the southern region of Hess Rise should havecrossed the equatorial zone of high biological produc-tivity either during the Cenomanian-Coniacian (Lance-lot and Larson, 1975; Lancelot, 1978), or near that in-terval of time (van Andel, 1974). The available datashow that during the entire post-Jurassic the position ofthe region under study was higher than the CCD.

It is known that the northern and southern bound-aries of the recent near-equatorial zone of high bio-logical productivity of the Pacific Ocean are approx-imately within 10°N and 10°S. It is assumed that duringthe early Maastrichtian the southern region of Hess Risehad not yet moved beyond the northern boundary of theequatorial zone of high biological productivity. Thecrossing of the boundary likely took place in the lateMaastrichtian, as evidenced by the continued highvalues of rates of sedimentation and accumulation ofcomponents (Table 7; Fig. 5).

Along with this interpretation, the possibility shouldbe taken into consideration that essential increasing ofthickness (i.e., rates of accumulation) of the earlyMaastrichtian sediments may have been caused by localcontribution and redeposition of sedimentary materialfrom tectonic uplift of nearby structural blocks (details

826

POST-JURASSIC SEDIMENTATION, SITE 465

Table 7. (Continued).

(mm 10 ~3

1.286.12.6

17.415.421.2

11.430.9

Rate of sedimentation

ur~1) (mg cm"2 10~3 yr~1)

146726309

207015202645

24706467

(%)

5.751.050.722.610.350.29

0.563.58

Av

SiO2

Accum.Rate

8.47.62.2

54.05.37.7

13.9231.5

erage Contents of Components (wt. <7o) and Rate of Accumulation(mg cm~2 10~3 yr~1)

A12O3

(%)

1.570.070.100.160.040.05

0.171.12

Ace.Rate

2.30.50.33.30.61.3

4.272.4

CaCO3

W

88.2795.1494.3092.2395.8794.70

95.6188.07

Accum.Rate

128.9690.7291.4

1910.01457.02550.0

2390.05695.5

W

0.520.090.140.140.140.15

0.170.40

Fe

Ace.Rate

0.80.70.42.92.14.0

4.225.9

(%)

0.010.030.020.010.010.01

0.010.02

Mn

Ace.Rate

0.010.200.060.210.150.26

0.251.30

P

Ace.(%)

0.030.040.040.010.0030.01

0.020.08

Rate

0.040.290.120.210.050.26

0.505.17

on the structural position of the site are given in the Site465 report, this volume).

Early Tertiary (early-late Paleocene). The beginningof the Cenozoic is characterized by a considerable de-crease in rates of foraminifer-nannofossil-ooze sedi-mentation in the northern oligotrophic zone of the Pa-cific Ocean. For the major components and associatedheavy metals (Table 7; Fig. 5), the accumulation ratesdo not exceed the values characteristic of carbonatepelagic oozes (Arrhenius, 1963, 1967; Bezrukov andRomankevich, 1970; Bogdanov and Chekhovskikh,1970; Lisitsin, 1974, 1978; MacArthur and Elderfield,1977). Worthy of attention, however, are relativelyhigher values of Mn and P accumulation for the latePaleocene (mg cm-2 10 ~3 yr~1): Mn, 0.20; P, 0.29.This can be due to the presence of appreciable amounts(up to 10-15%) of an admixture of hyalopelitic, siltymaterial of basaltic composition, essentially trans-formed into smectite-illite, considerably diluted withcarbonate: CaCO3 to 95.14% (see Figs. 1, 2).

Late Tertiary-Quaternary. A considerable part ofthis period, as mentioned above, was rather poorly re-covered. Thicknesses of Eocene-Pliocene deposits aresharply decreased, because of numerous hiatuses ac-companied by partial redeposition and washing of sedi-ments (see Site 465 report, this volume).

Relatively limited information on late Pliocene andPleistocene sediments (Table 7; Fig. 5) enables us tothink that foraminifer-nannofossil oozes with typicalrates of component accumulation were deposited at thattime (Table 7; Fig. 5).

GEOCHEMICAL HISTORYOF SEDIMENTATION

The geochemical history of post-Jurassic sedimenta-tion of southern Hess Rise can be subdivided into stagesand phases that correspond considerably to geochrono-logical intervals established by interpretation of averagecontents and rates of component accumulation, studiedwithin the general context of data on geochemistry,mineralogy, and lithology of sediments.

Late Albian-early Cenomanian (early oceanic). Rep-resenting this time are thinly-laminated, olive-gray lime-stones, with subordinate interbeds of gray limestones

accumulated in the southern region of Hess Rise (seeSite 465 report, this volume). These deposits are charac-terized by appreciable amounts of sapropel-like organicmatter, and a significant admixture of basaltic volcani-clastic material.

Geochemical features of these sediments were de-scribed above (Figs. 1-4). They are characterized by thehighest contents of SiO2, A12O3, Fe, Mn, P, andassociated heavy metals, which were contributed withbasaltic volcaniclastic material, and to a lesser extentwith hydrothermal-exhalative activity, especially in theearly intervals (the first half of the late Albian).

Accumulation of sediments containing fragments ofshallow-water mollusks, reef-building fossils, and frag-ments of volcanic edifices and rocks (basalts, trachy-basalts, etc.) took plate in the initial phase (late Albian),in a relatively shallow-water, apparently depression-type basin, with stagnant bottom waters, characterizedby notable hydrodynamic activity. The sediments havedistinct features of turbidites. The basin was boundedby island volcanoes, reef banks, etc.

Accumulation of sediments in such a rapidly sub-siding basin was characterized by rather high rates of ac-cumulation of major components and associated metals(Fig. 5; Table 7). Such high rates of accumulation aredue to three factors: (1) high rates of sedimentation inthe shallow-water, progressively subsiding basin; (2) anactive volcanic influence that proved responsible forrelatively high contents of some major components(e.g., Fe, Mn, A12O3, SiO2) and associated heavymetals, and supply of considerable amounts of basalticvolcaniclastic material, and lesser amounts of hydro-thermal exhalation products; (3) intensive biologicalproductivity of the planktonic zone of the basin as aresult of the position of the region near the equator atthat time, and the contribution of nutrient mineral com-ponents from volcanic sources. High biological produc-tivity of this shallow basin resulted in accumulation ofconsiderable amounts of sapropelic organic matter incarbonate sediments.

Thus, the initial (early oceanic) stage of developmentof the southern region of Hess Rise is characterized bywell-pronounced geochemical features. A similar geo-chemical stage can also be observed in other regions (for

827

00to0O

AGE

1.0 2.0 3.0 4.0

AI 2 O 3

0.2 0.4 0.6 0.8

CaCO3

2.0 4.0 6.0 8.0

Fe

0.1 0.2 0.3 0.4

COMPONENTS (wt %)

Mn

0.01 0.03

Corg

0.2 0.4 0.6 0.8

MnCO

0.005 0.020

Fe2CO3

0.1

MgCO3

0.1 0.4 0.8 1.2

Pleistocene 0.52 0.0.. X × X X

< X X X × >JX X X X X i 0.0

L. Pliocene © © © © © © © © ©E. Pliocene © © © © © © © © © ©L. Miocene

M.Miocene

E. Miocene

L.Oligocene •Φ• -ay -©- -©- -<z> <z>E. Oligocene

•( Lacuna)

L. Eocene

M. Eocene

E. Eocene

L. Paleocene

E. Paleocene

L. Maastrichtian

0.0

0.0 × × 0 . 0 3 2 × Λ 0.0

0.0 0.0 0.0 0.0

E. Maastrichtian 0.003 0.0

L. Campanian

E. Campanian (Lacuna)

L. Santonian

E. Santonian © © © © © © © © ©L. Coniacian

E. Coniacian

L. Turonian

M. Turonian (Lacuna)

E. Turonian

L. Cenomanian

M. Cenomanian

Figure 3. Distribution of average contents (wt. °7o, air-dry): SiO2, A12O3, Fe, Mn, P, and normative molecules CaCO3, MnCO 3, FeCO3, MgCO3 in a section of post-Jurassic deposits ofthe central northwestern Pacific, southern Hess Rise, DSDP Site 465.

POST-JURASSIC SEDIMENTATION, SITE 465

AGE

Pleistocene

L. Pliocene

E. Pliocene

L. Miocene

1. Miocene

E. Miocene

L.Oligocene

E. Oligocene

L. Eocene

M. Eocene

E. Eocene

L. Paleocene

E. Paleocene

L. Maastrichtian

E. Maastrichtian

L. Campanian

E. Campanian

L. Santonian

E. Santonian

L. Coniacian

E. Coniacian

L. Turanian

1. Turonian

E. Turonian

L. Cenomanian

COMPONENTS (wt.%)

Fe

2.0 4.0 6.0 8.0

©©

©

0.0

Φ•

Mn

2.0 4.0 6.0 8.0

0.0

©©

©

0.0

-©-

1.0 2.0 3.0 4.0

©©

(Lacuna)

\\W\V1

(Lacuna)

©–

(Lacuna)

MgO

4.0 8.0 12.016.0

©©

©

0.0

0.0

©-

K 2 0

5.0 10.015.0 20.0. . . . . . . . I

©©

©

• © •

Figure 4. Distribution of average contents (wt. °/o, recalculated to terrigenous-free, silica-free,carbonate-free) of Fe, Mn, P, MgO, and K2O in the section of post-Jurassic deposits of the centralnorthwestern Pacific, southern Hess Rise, DSDP Site 465.

instance, the western Mid-Pacific Mountains, Site 463,Shatsky Rise, Nauru Basin, etc.) of the Pacific Ocean,Atlantic, and Indian Oceans (Schlanger and Jenkyns,1976; Arthur and Schlanger, 1979) covering variousgeochronological intervals of the Cretaceous.

Late Cretaceous (middle Cenomanian-late Maas-trichtian). The sedimentological significance of an mid-dle Cenomanian-middle Turonian hiatus remains ob-scure. According to some modifications of the modeldescribing the northward movement of the Pacific Plate(Lancelot and Larson, 1975; Lancelot, 1978; van Andel,1974), the southern region of Hess Rise during theCenomanian-Coniacian was in the equatorial zone ofhigh biological productivity. Such a location of the re-gion (if the accepted model proves true, as for manyareas of the northwestern Pacific) could have resulted inaccumulation of considerable amounts of carbonatesediments. It should be emphasized that during thewhole Meso-Cenozoic history of sedimentation thesouthern part of Hess Rise was situated above the CCD.

It is believed that the development of this hiatus inthe region is related to activation of Late Cretaceous

analogues of equatorial currents and their northwesternbranches (Luyendyk et al., 1972).

During the Santonian, the accumulation of nanno-fossil and foraminifer-nannofossil oozes with an admix-ture of siliceous components took place. Relatively de-creased thicknesses of these sediments and low rates oftheir accumulation enable us to regard them as residualafter the early Campanian hiatus.

An early Campanian hiatus is the highest of the LateCretaceous hiatuses of similar nature (Fig. 5).

The final phase of Late Cretaceous sedimentation,during which nannofossil and foraminifer-nannofossiloozes accumulated was the late Campanian-late Maas-trichtian. These sediments are characterized by contentsof major components and heavy metals characteristic ofcarbonate pelagic varieties. Worthy of attention arerather high rates of sedimentation and accumulation ofmajor components during the late Maastrichtian (Fig. 5;Table 7). As mentioned above, these data evidence thatlate Maastrichtian sediments were not affected by ero-sion, and accordingly they adequately reflect the sedi-mentation rates of that time. At the same time we can

829

I. M. VARENTSOV

COMPONENTS (mg-cm•2.10'3.yr•

(mm-cm .

10"3.yr"1)5 10 15 20

Figure 5. Distribution of average rates of sedimentation and accumulation of components in a section of post-Jurassic deposits of the centralnorth-western Pacific, southern Hess Rise, DSDP Site 465.

assume that during the late Maastrichtian the southernregion of Hess Rise had not yet passed across the north-ern boundary of the sub-equatorial zone of high bio-logical productivity. Only in the latest Maastrichtianwas the region displaced into the northern ologotrophiczone of the Pacific Ocean. However, the relatively in-creased thickness (rate of sedimentation) of the Maas-trichtian deposits may also have been caused to a certainextent by the local supply and redeposition of sedimentsfrom blocks upraised at this time.

Early Tertiary (early-late Paleocene). The beginningof the Tertiary was characterized by accumulation ofnannofossil oozes, with a small admixture of radiola-rians and siliceous remains transformed into chertnodules. The composition of major components andheavy metals of these deposits differs slightly from thatof typical biogenic carbonate pelagic oozes of the openocean. Worthy of attention are somewhat higher ratesof Mn and P accumulation (Table 7; Fig. 5), the natureof which can be due to the appreciable amounts of finevolcaniclastic material of basaltic composition. Theavailable geochemical data, as well as data on mineral-ogy and lithology of these sediments, show that in theregion the transition from Mesozoic to Cenozoic was

not marked by well-pronounced changes of geochemicalparameters of sedimentation.

Tertiary-Quaternary. During the Eocene-Pliocene,the sedimentation phases appeared to alternate withnumerous hiatuses. This finally resulted in accumula-tion of residual redeposited materials, with sharply de-creased thickness (Site 465 report, this volume).

During the late Pliocene and Pleistocene, the ac-cumulation of foraminifer-nannofossil oozes with aslight admixture of siliceous remains and basaltic hyalo-pelitic-silty (up to 10-15%) material (Pleistocene) tookplace. The contents of major components and heavymetals in the sediments under study and rates of theiraccumulation differ slightly from those typical of bio-genic carbonate pelagic oozes of the open ocean.

CONCLUSIONS

Study of specific features of distribution of majorcomponents and heavy metals in the section of Meso-zoic and Cenozoic deposits of southern Hess Rise(DSDP, Site 465), analysis of forms of their occurrenceon the basis of processing of analytical data by thefactor-analysis method, interpretation of results of min-eralogic-lithologic studies, and evaluation of distribu-

830

POST-JURASSIC SEDIMENTATION, SITE 465

tion of average contents of components and rates oftheir accumulation for the major geochronological sub-divisions enable us to outline the main stages in thegeochemical history of sedimentation of the region.

Late Albian-Early Cenomanian (early oceanic)

Accumulation of shallow-water carbonate sedimentsof turbiditic origin took place. The sediments were en-riched with sapropelic organic matter, basaltic volcani-clastic material, and to a lesser extent with hydrother-mal and exhalation products. Volcanogenic componentsresulted in relatively high concentrations of Fe, Mn,SiO2, A12O3, and associated heavy metals, presentmostly in the form of basaltic volcaniclastics and ma-terial of their post-sedimentary transformations: smec-tite-illite, Mn-Fe-Mg-carbonates. The rates of sedimen-tation and accumulation of components were maximalfor the post-Jurassic history of the region.

Late Cretaceous (middle Cenomanian-late Maastrichtian)

This stage is characterized by sedimentation, theresults of which were considerably destroyed by the ma-jor hiatuses (middle Cenomanian-middle Turonian; lateConiacian-early Campanian). According to a model ofthe northward movement of the Pacific Plate (Lancelotand Larson, 1975; Lancelot, 1978; van Andel, 1974),the region under study could have been situated duringthe Cenomanian-Coniacian in the zone of high biologi-cal productivity. Only relatively high rates of sedimenta-tion and accumulation of components for the late Maas-trichtian enable an approximate evaluation of the finalphase of sediment formation near the northern bound-ary of this near-Equator zone. However, the local con-tribution and redeposition of sedimentary materialsfrom upraised blocks at this time may somewhat in-crease the values of the sedimentation rate of the Maas-trichtian.

On the whole, the sediments of this stage are similarto biogenic carbonate pelagic oozes of the open ocean,when judged by the known characteristics.

Early Tertiary (early-late Paleocene)

The beginning of the Tertiary was characterized byaccumulation of carbonate nannofossil oozes, admixedwith siliceous remains. The available geochemical datado not speak of somewhat abrupt changes of geochem-ical parameters of sedimentation at the Maastrichtian-Paleocene boundary.

Tertiary-Quaternary

In the Eocene-Pliocene, biogenic carbonate sedimen-tation is interrupted many times by hiatuses. This re-sulted in accumulation of redeposited residual sedi-ments of much-reduced thickness. In the late Plioceneand Pleistocene, deposition of foraminifer-nannofossilpelagic sediments with normal geochemical characteris-tics was proceeding.

Thus, in the geochemical history of post-Jurassicsedimentation of southern Hess Rise, four stages areestablished. They reflect the evolution of sedimentation

from the environment of a relatively shallow-waterbasin (late Albian-early Cenomanian) to the open ocean(Late Cretaceous-Pleistocene). In our previous workson geochemistry of sedimentation in the region of thewestern Mid-Pacific Mountains (Site 463) and northernHess Rise (Site 464), geochemically similar stages ofpost-Jurassic sedimentation were identified. However,their geochronological duration (for the Cretaceous pe-riod particularly) differed significantly, a well-pro-nounced evolutionary tendency being common.

ACKNOWLEDGMENTSIt is a pleasant duty to express sincere gratitude to my colleagues at

the Geological Institute of the U.S.S.R. Academy of Sciences: P. K.Ryabushkin, D. A. Kazimirov, N. I. Kartoshkina, N. Yu. Vlasova,for appreciable help in processing the analytical data in a computer,recalculation of analyses, and preparation of the materials; V. A.Drits, B. A. Sakharov, T. T. Eliseeva, for X-ray structural analysis;D. Ya. Choporov, for careful control of analytical data; P. P.Timofeev and V. I. Koporulin, for the provided materials and discus-sion of the results in the course of the study; V. S. Zelinsky and K. A.Shchepoinnikova, for assistance in graphic presentation of the mate-rial; G. N. Surovtseva and G. V. Kozlovskaya, for translation of thepaper.

Critical review of the paper and comments made by N. G. Brod-skaya and A. G. Kossowskaya were useful and provided improve-ment.

REFERENCES

Arrhenius, G., 1963. Pelagic sediments. In Hill, M. N. (Ed.), The Sea(Vol. 3): New York (Interscience), 655-727.

, 1967. Deep Sea sedimentation: a critical review of U.S.works. Trans. Am. Geophys. Union, 48:604-631.

Arthur, M. A., and Schlanger, S. O., 1979. Cretaceous Ocean anoxicevents as caused factors in development of reef-reservoired giantoil fields. Bull. Am. Assoc. Petrol. Geol., 63:870-885.

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), p. 280.

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

Harman, H. H., 1967. Modern Factor Analysis: Chicago (Univ. ofChicago Press).

Kirkpatrick, J., 1979. Interlaboratory comparison of Leg 46 stand-ards. In Dmitriev, L., Heirtzler, J., et al., Init. Repts. DSDP, 46:Washington (U.S. Govt. Printing Office), 293-297.

Lancelot, Y., 1978. Relations entre evolution sédimentaire et tecton-ique de la Plaque Pacifique depuis le Crétacé inférieur. Mém. Soc.Geol. France, 57(134):40.

Lancelot, Y., and Larson, R. L., 1975. Sedimentary and tectonic evo-lution of the north-western Pacific. In Larson, R. L., Moberly, R.,et al., Init. Repts. DSDP, 32: Washington (U.S. Govt. PrintingOffice), 925-939.

Lisitzin, A. P., 1974. Sedimentation in Oceans: Moscow (Nauka)., 1978. Processes of Oceanic Sedimentation: Lithology and

Geochemistry: Moscow (Nauka).Luyendyk, B. P., Forsyth, D., and Phillips, J. O., 1972. Experimental

approach to the paleocirculation of the oceanic surface water.Geol. Soc. Am. Bull., 83:2649-2664.

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

Schlanger, S. O., and Jenkyns, H. C , 1976. Cretaceous oceanic an-oxic events: causes and consequences. Geol. Mineral. Jahrb.,55:179-184.

831

I. M. VARENTSOV

Tiercelin, J. J., and Faure, H., 1978. Rates of sedimentation and ver- Varentsov, I. M., and Blazhchishin, A. I., 1976. Ferromanganesetical subsidence in neorifts and paleorifts. Tectonics and Geo- nodules. Geology of Baltic Sea: Vilnius (Mokslas), pp. 307-348.physics of Continental Rifts, (Vol. 2): Moscow (Dordrecht),41-47. Zolotarev, B. P., and Choporov, D. Ya., 1978. Petrochemistry of ba-

van Andel, Tj. H., 1974. Cenozoic migration of the Pacific Plate, salts D/V Glomar Challenger, Leg 45, Holes 395, 395A, 396. Innorthward shift of the axis of deposition and paleobathymetry of Melson, W. G., Rabinowitz, P. D., et al., Init. Repts. DSDP, 45:the Central Equatorial Pacific. Geology, 2:507-510. Washington (U.S. Govt. Printing Office), 479-492.

832