Deep Sea Drilling Project Initial Reports Volume 62

31. SEDIMENTARY SEQUENCES AT DEEP SEA DRILLING PROJECT SITE 464:SILICIFICATION PROCESSES AND TRANSITION BETWEEN

SILICEOUS BIOGENIC OOZES AND BROWN CLAYS1

Anne-Marie Karpoff, Michel Hoffert, and Norbert Clauer, Centre de Sédimentologie etGéochimie de la Surface, (C.N.R.S.), Strasbourg, France

ABSTRACT

The relationships between mineralogical and geochemical data on the three successive sedimentary facies at DeepSea Drilling Project Site 464 are studied. The evolution of siliceous biogenic sediments is derived from the analyses ofone Fe-Ti smectite concretion, and of siliceous aggregates occurring in the pelagic "brown clays." Along the sedimen-tary section, the trace elements enriching the authigenic silicates and the Fe-Mn oxyhydroxides vary, depending on themarine environment. The proportion of clays and carbonates into the siliceous deposits controls the diagenetic evolu-tion of silica making up the quartz aggregates from the "brown clay" or the cristobalite cherts.

INTRODUCTION

Site 464 is on the northern flank of Hess Rise, north-west Pacific, in a water depth of 4637 meters (Fig. 1).According to shipboard lithological studies, the com-posite sequence overlying the basement comprises threemain units (Fig. 2). From bottom to top, the lowest unit(Unit 3, 219-m thick) contains cherts, limestones, andchalk of Albian to Cenomanian age. Unit 2 (53-mthick), of soft brown clays and pelagic oozes, is of EarlyCretaceous to late Miocene age, according to Doyle andRiedel (this volume). The youngest unit (Units 1A andIB, 33 m) consists of clays, siliceous biogenic muds, andoozes of late Miocene to Pleistocene age.

SEDIMENTOLOGY AND MINERALOGYMETHODS

The sediment lithology was defined on the basis of smear-slideobservations (Fig. 3 and Table 1).

The mineralogical composition of the sediments was determinedby X-ray-diffraction techniques. The X-ray-diffraction charts fromnon-oriented powders were obtained using bulk material or coarsefraction, under the following conditions: CuKα radiation, Ni filter, 98kV/18 mA, 0.1 to 1° slits, l<Vmin speed. The identification of clayminerals in the < 2-µm fractions was made on three types of orientedaggregates: untreated, ethylene-glycol-treated, and heated, accordingto the methods of the Institut de Géologie in Strasbourg (Mise aupoint collective, 1975).

Some clay fractions were also studied by micro-diffraction onisolated particles, by transmission electron microscope (TEM: PhillipsEM 300), using the method of Trauth et al. (1977).

RESULTSDescriptive data are given in Table 1. The composi-

tion of these lithologic units is as follows:

Unit IA: 3.5-19.0 meters (Samples 464-2-3, 1-3 cm;464-2-3, 50-52 cm; and 464-3-4, 145-146 cm)

This unit is made up of siliceous nannofossil ooze,which passes gradually to a muddy siliceous ooze and

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

clayey radiolarian ooze. The prevalent minerals arecalcite and amorphous silica or opal from pelagic bio-genic components such as coccoliths, discoasters, fora-minifers, radiolarians, and diatoms. Quartz, feldspars,and volcaniclastic fragments occur. Clays, as illite andsmectites, usually form small aggregates within andaround the siliceous organisms. Small amounts of zeo-lites and barite occur at the bottom of the sedimentarysequence, whereas the siliceous organisms are poorlypreserved and more dissolved. In these deposits, a light-yellowish-brown (2.5Y 6/4) concretion was found, theshape and surface of which are irregular (Fig. 4). Itsconcentric and colloform structure is similar to that ofmetalliferous nodules described by Sorem and Fewkes(1977). The mineralogical analyses establish that it ismade of smectites with some quartz. Very scarce man-ganiferous oxides as small patches between the suc-cessive clay layers are observable. Detailed geochemicaland microprobe studies were made on this clayey con-cretion.

Unit IB: 19.0-36.5 meters (Samples 464-3-5, 5-6 cm;464-5-3, 4-6 cm; and 464-5-3, 140-142 cm)

Unit IB comprises in the upper part a muddy siliceousooze, a gradual transition from Unit IA, and in thelower part siliceous clays. Similar to the sediments ofthe lower section of Unit IA, Unit IB muds are made ofamorphous silica (biogenic opal), quartz, trace amountsof plagiocase, barite, and phillipsite. The clay-mineralscontent increases and comprises illite, smectites, scarcekaolinite, and mixed-layer clays. Toward the bottom ofthis sequence, ferromanganese oxides and micronodulesbecome more abundant and volcanic glasses occur. Onthe X-ray charts from slides of untreated and orientedclay fractions, the main peak of smectites is at 13 to14A. The SEM study shows that the smectites arestrongly coupled to abundant siliceous-organism frag-ments, particularly diatoms. The smectite particles are"ringlet"-shaped and exhibit irregular outlines. Theyare related to authigenic clay minerals described by Hof-fert (1980) in Panama Basin.

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A.-M. KARPOFF, M. HOFFERT, N. CLAUER

3462

170° 180° 170° 160°

Figure 1. Location of Site 464 and nearest previous sites. Isobaths 4500 and 3500 meters.

Unit II: 36.5-89.0 meters (Samples 464-7-5,80-82 cm; and 464-10-4, 70-72 cm)

Unit II belongs to the "red clays" or pelagic "brownclay" facies. The sediment is made up of 95% clays, 5°7oFe-Mn oxyhydroxides, with trace amounts of radiolari-ans, coccoliths, volcaniclastic particles, and glasses.Microcline, quartz, and phillipsite were also identifiedby X-ray-diffraction analysis on non-oriented powdersof bulk sediment. The two available and analyzed sam-ples are located at the upper part of the unit (first sam-ple), and at the bottom, near the limit with underlyingUnit 3 (second sample). Similar in bulk mineralogicalcomposition, the two samples differ from each other bytheir age (Plio-Miocene and Cretaceous, respectively),and also by the composition and nature of coarse(<63-µm) and clay (<2-µm) fractions.

The SEM and TEM investigations were made on thebulk sediment, on the coarse and clay fractions.

Upper Deposits of Unit II

The upper part of the sequence contains phillipsite,which occurs on sponge spicules and appears to be re-lated to this particular restricted environment (Plate 1,

Figs. 1 and 2). In the clayey matrix of the sediment,manganiferous micronodules are observed, growinginto cavities or pores, probably from residual dissolvedsiliceous organisms (Plate 1, Figs. 3 and 4). Smectitesprevail in the clayofraction; on X-ray charts, they have amain peak at 15Å. Very scarce palygorskite is associ-ated. In TEM photomicrographs, the smectite particlesshow very diffuse and fibrous outlines, as laths (Plate 1,Figs. 5 and 6), and differ from those of overlying UnitII. They are similar to those formed from dissolvedradiolarian silica, described by Hoffert (1980).

Lower Part of Unit II

The coarse fraction of the Cretaceous brown claycomprises numerous white slabs, microconcretions, ag-gregates, and particles. These fragments are made ofquartz with low contents of cristobalite (opal-CT). SEMobservations show that these microconcretions are silici-fied coccoliths (Plate 2, Fig. 1) or radiolarian, withsome zeolite crystals among the siliceous "lepispheres"(Plate 2, Figs. 2, 3, 4). The zeolite occurrence was de-fined by spectrochemical micro-diffraction (Tracor sys-tem). The clay fraction of Cretaceous brown sedimentscontains prevalent smectites (15 Å), scarce palygor-

760

SEDIMENTARY SEQUENCES

SITE 464

Figure 2. Lithologic core description of sedimentary units of Site 464.

UNIT

IA

IB

62-464

2-3,1-3

2-3,50-52 -

3-4,145-6 -

3-5,5-6

5-3,4-6

5-3,140-2

7-5,80-82

20 40 60 80

1 i<•y.<yyy×mf—

r —

10-4,70-72-

Figure 3. Composition of soft sedimentary Units I and II at Site 464,established by smear-slide data. 1. Calcareous organisms (nan-nofossils and foraminifers). 2. Siliceous organisms (radiolarians,diatoms, and sponge spicules). 3. Clays. 4. Micronodules andFe-Mn oxides. 5. Volcanic glass and volcaniclastic minerals.

skite, and also traces of detrital clays, as illite and chlo-rite. The SEM investigations show the occurrence ofpalygorskite laths with the bigger smectite particles andscarce small siliceous globules (Plate 2, Figs. 5 and 6).

Unit III: 89.0-307.5 meters (Samples 464-13,CC;464-26-1, 82-84 cm; and 464-26-1, 88-90 cm)

The thick Unit III comprises brown-red cherts,, sili-cified chalk, and limestones, of late Albian to Ceno-manian. It overlies the basalts of Unit IV. Cherts aremade of prevalent cristobalite (opal-CT), associatedwith quartz and sometimes calcite. Traces of radiola-rians are visible in thin sections.

In summary, the mineralogical variations of the sedi-mentary sequence at Site 464 illustrate a gradual tran-sition from essentially siliceous biogenic oozes to thebrown clay facies, which lies on cherts. Successive anddifferent mineral assemblages correspond to each unitof the sequence, with a progressive change of the pro-portion of the calcareous or siliceous biogenic phases,and of the detrital or authigenic minerals (Fig. 3).

From top to bottom in Unit I, the calcareous biogenicphases decrease when the siliceous organisms increase.In the lower part of this lithologic unit, siliceousorganisms are poorly preserved and dissolved, and thecontent of biogenic phases decreases, with a correlativeincrease in clay content. The pelagic biogenic oozesrange downward to clayey sediments. On top (Unit IA),the clay fraction essentially is composed of detritalminerals, and the scarce authigenic minerals comprisebarite, related to the dissolution of organisms, andzeolite. Clayey and quartz nodules occur in the softdeposits. At the base (IB), clays are prevalent and areessentially authigenic "ringlet"-shaped smectites; me-talliferous oxyhydroxides appear.

The pelagic clays of Unit II are characterized by theabundance of authigenic smectites, with fibrous out-lines, and by the occurrence of palygorskite. Siliceousmicroconcretions, as quartz and opal-CT, occur in thelower part of the soft sediments overlying the cherts.

GEOCHEMISTRYMETHODS

The geochemical data refer to the same set of studied samples.Major- and trace-element analyses were performed following themethod described by Besnus and Lucas (1968) and Besnus andRouault (1973), using arc spectrometry and an ARL quantometer.The method consists of melting the sample in a mixture of lithiumtetraborate and introducing the melt into a glycolated solvent. Traceelements were determined using graphite disks, as described by Besnusand Lucas (1968). Na and K were determined by emission spec-trometry. Relative precision is ±2% for major elements, and ±20%for trace elements.

ResultsBulk-chemistry data are presented in Tables 2 and 3

for major and trace elements. The prevalent-componentvariations along the sequence are shown in Figure 5.

The variation of the major elements and of the traceelements supplement the mineralogical data and definethe evolution of the sedimentary components. Selectivemicroprobe investigations detailed the chemical rela-tionships between some mineral phases.

761


Table 1. Results of smear-slide investigations and X-ray-diffraction analyses of sedimentary deposits from Site 464.

Sample(interval in cm)

464-2-3, 1-3

464-2-3, 50-52

464-2-3, 145-146

464-3-5, 5-6

464-5-3, 4-6

464-5-3, 140-142

464-7-5, 80-82

464-10A 70-72

13.CC

LithologicUnit

Unit IA

Unit IB

I Inir TTwlllL 11

Unit III

Sediments

Radiolarian-nannofossilooze

Clayey radiolarian-nannofossil ooze

Radiolarian ooze withclays and authigenicphases

Siliceous clay withauthigenic andvolcaniclasticparticles

Siliceous clay withauthigenic andvolcaniclasticcomponents

Siliceous clay withauthigeniccomponents

Brown clay (pelagicclay)

Brown clay, pelagicclays with coarsewhite slabs

Siliceous limestoneand cherts

Smear-Slide Components

Coccoliths, discoasters, and fragmentedforaminifers; radiolarians and diatoms,scarce silicoflagellates, clays as reddishaggregates in or around the siliceousorganisms, quartz

Diatoms and radiolarians, coccoliths anddiscoasters, clays in organisms, scarcequartz and volcaniclastic particles,feldspars

Radiolarians and diatoms, poorly pre-served, clays and globules of Fe-oxyhydroxides, coccoliths andunspecified carbonates, quartz, scarcevolcaniclastic particles and pyroxenes

Radiolarians, poorly preserved, well-preserved diatoms, fragmentedcoccoliths and unspecified carbonates,quartz, volcaniclastic particles

Radiolarians, fragmented, clays andreddish aggregates of oxyhydroxides,quartz (some coarse grains), scarceblack micronodules, volcaniclasticparticles

As above: clays and brownish globules ofof oxyhydr oxides, poorly preservedsiliceous organisms (radiolarians anddiatoms), volcanic particles and glass,coarse grains of quartz

Brownish globules, very abundant,sometimes in residual organisms,clays, micronodules, radiolarians,scarce fragments of sponge spicules,few coccoliths, volcaniclastic particles

Clays and diffused oxyhydroxideshomogeneous on the slide, splinteredfragments of glass or siliceousorganisms, radiolarians and spongespicules, scarce fragments of cocco-liths and unspecified carbonates,quartz

Mineralogy ofBulk Sediment

Calcite, quartz, clays(illite and smectite)feldspars

Calcite, quartz,amorphous silica,clays (illite andsmectite) feldspars

Quartz and amor-phous silica, clays(illite and smectite),calcite, phillips-ite(?), barite,feldspars

Quartz and amor-phous silica, clays,plagioclases, barite,phillipsite (traces)

Clays, quartz andamorphous silica,phillipsite, barite,plagioclases, Mn-Fe oxides (traces),volcanic glass

Quartz, clays, amor-phous silica andvolcanic glass,phillipsite, barite,Mn-Fe oxides

Clays, microline,calcite, quartz,phillipsite, Mn-Feoxides (traces)

Quartz, clays, silica ascristobalite, Fe-hydroxide(goethite), calcite(traces), ecolite(philipsite),volcanic glass,feldspars (traces);

Coarse fraction: whiteslabs are made ofquartz and scarcecristobalite

White zones ofsiliceous limestoneare made ofcristobalite, calcite,and quartz

Mineralogy ofClay Fraction

Illite, smectite(14Å),kaolinite,interstratifiedminerals

Illite smectites(13-14Å),interstratifiedminerals,kaolinite

Illite smectites(13-14Å),chlorite,interstratifiedminerals

Smectites (15Å),palygorskite(scarce)

Smectite (15Å),illite, chlorite(traces),palygorskite(traces)

Major-Element Variations:Signature of the Main Mineral Phases

Bulk Sediments from Units I and II

The variation of major elements reflects the mineral -ogical composition, its fluctuations and evolution. InUnit I, occurrence of calcareous nannofossils is shownby Ca contents, that of siliceous organisms by corre-spondingly higher SiO2 contents and Si/Al ratios. Thegradational and simultaneous increases of Al, Fe, Tiand Mg contents correspond to those of clay phases insediments of Unit I (Figs. 5 and 6); the presence of

authigenic minerals such as barite, zeolite and oxyhy-droxides is indicated by the Ba, K, and Mn contents.

From top to bottom, the deposits of Unit II are dif-ferentiated by several chemical criteria (Figs. 5 and 6).In the upper part, the proportionally higher Ti, Fe, andK contents are related to the occurrence of volcaniclasticparticles and more-abundant zeolites. These variationsare also related to the clay fraction of the sediment,which could contain more Fe and Ti than that of overly-ing Unit I. In the lower part, the occurrence of quartzaggregates and concretions, metalliferous oxyhydrox-ides, and micronodules induce, respectively, the increas-

762


Figure 4. Photograph of a section of a clayey siliceous concretionfrom the deposits of Unit IA. Sample 464-2-3, 45-47 cm (× 1.5).

ing Si, Mn, and Fe contents, relative to Al and Ti (inclays).

Clayey Nodule from Unit 1

Structural FormulaThe clayey concretion found in the Unit I deposits

was also analyzed. The structural formula of the smec-tite forming this nodule is established using Millofscalculation (1949). The 9.4% silica excess (assumingthat no tetrahedral Al substitution exists in the clay) cor-responds to the authigenic quartz micro-crystals inter-layered between the successive clay laminae. The in-fluence of volcaniclastic particles and feldspar frag-ments from the core (or nucleus) of the nodule, was con-sidered negligible in the calculated composition of thesmectite. The formula is as follows:

Inter layered cations: (Ca0 0gNa0.049K0.19)

The excess of octahedral cations appears to be due tothe presence of Mg in the interlayer position. This for-mula is characteristic of the iron-rich beidellites andnontronites.

Microscopic InvestigationsMicroscopic and spectrochemical analyses were per-

formed on the polished section of the concretion, byX-ray-dispersive energy spectroscopy (SEM Cameca 07and Tracor system). Maps of the distribution of theprevalent elements (Si, Al, Ca, Fe, Ti, and K) selected ina zone of the sample are given in Plate 3. These datashow the chemical composition of the concretion, andparticularly of iron smectite. The successive clayeylaminae are characterized by variable Fe/Ti ratios (Plate3, areas a and b). However, the prevalent smectite isrich in iron and titanium (a). The microprobe analysesconfirm also the occurrence of (1) several zones ofquartz micro-crystals, essentially located between thenodule core and the first clay lamina (d), (2) small spotsof Fe-Ti oxides in the clayey matrix of the nodule core(e), (3) fragments of Ca feldspars in the nodule core (/),and (4) small amounts of a potassium-rich silicate, aszeolite (c), and small concentrations of Ca as residualnannofossils (g), interlayered with the successive claydeposits.

The outermost clay deposits contains small patchesof manganiferous oxides which do not concentrate anyiron, but are potassium- and copper-rich.

Therefore, the clays forming the indurated nodulehave a different chemical composition than those of thesurrounding soft sediments. The clay minerals of thesiliceous ooze (Unit I A) are a mixture of detrital clays,as illite, scarce kaolinite and chlorite, and authigenicsmectites which are formed on and as replacement of si-liceous organisms. The chemical composition of thisclay assemblage from the soft sediments appears homo-geneous and constant through the sequence, and is char-acterized by a strong correlation of Al, Fe, and Ti con-

Table 2. Bulk chemical analyses of sediments from Site 464 (wt.<Vo).


464-2-3, l - 3 a

2-3, 45-472-3, 50-523 ^ , 145-1463-5, 5-65-3, 4-65-3, 140-1427-5, 80-8210A 70-7210-4, 70-72b

13.CC,26-1, 82-8426-1, 88-90

SiO2

41.060.048.360.064.155.752.250.571.492.570.898.095.6

A12O3

6.94.78.49.69.8

12.514.012.65.80.50.70.2c

O.2C

MgO

2.134.362.542.842.893.323.765.102.610.260.410.02c

0.02c

CaO

18.81.0

12.74.21.01.11.71.34.51.8

14.20.51.5

Fe 2 O 3

3.215.03.84.34.45.46.1

10.64.20.20.50.40.4

M n 3 O 4

0.2630.4290.2180.1050.1210.9881.690.2001.020.0870.0320.0160.01c

TiO2

0.310.550.380.410.440.540.591.470.250.020.050.02c

0.02c

BaO

0.480.020.550.830.841.041.160.010.040.01c

0.010.01c

0.01c

Na2O

2.823.213.704.694.714.985.294.292.190.320.340.110.10

K2O

1.241.921.832.572.782.913.003.961.730.200.310.070.06

Loiat 1000°C

21.656.24

17.5611.108.399.43

10.048.305.162.31

12.961.412.14

Total

98.8097.4399.98

100.6099.4797.9199.5398.3398.9098.20

100.31100.75100.06

Si/Al

5.2311.235.065.505.763.923.285.53

10.83162.8089.00

431.20420.64

Mn/Fe

0.080.030.060.020.030.190.280.020.250.450.060.040.02

CaCθ3

33.61.8

22.77.51.81.93.02.38.03.2

25.30.92.7

a Contains 0.24% NiO and 0.52% CuO." White slabs from the > 63-µm fraction.c Value below detection limit; total iron was calculated as F e 2 θ 3 , and total manganese as Mn3θ4; CaCθ3 was calculated using the total CaO.

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Table 3. Bulk chemical analyses of sediments from Site 464 (pPm)


464-2-3, 1-32-3, 45-472-3, 50-523 A 145-1463-5, 5-65-3, 4-65-3, 140-1427-5, 80-8210-4, 70-7210-4, 70-72a

13.CC,26-1, 82-8426-1, 88-90

Sr

103214975855540647454792

43489

2893134

V

5583399499

1371441295212102921

Ni

126_ c13915516235143014619033

81011

Co

25_ c

365245

1021583054123

139

Cr

51336378768487

26126

5 b

820

B

10756

118136138131149131109101236579

Zn

10224410910893

10213081

15624

3123

Ga

1418192931403722172263

Cu

44c

699490

236289113250

24102740

Pb

259706d

289321345289452112161

2 b

127

a White slabs from >63-µm fraction.b Value below detection limit,c Value over the upper detection limit." Extrapolated value.

tents (Fig. 6), whereas the authigenic smectites of thenodule are richer in iron and titanium, with correspond-ingly less aluminum. The trace-element analyses alsobring out the difference between the two types of clayassemblages.

Variations of Trace-Element Contents:Characterization of the Authigenic Components

The relationships between the trace elements andassociated major elements indicates the differences and

evolution of the biogenic components, the detrital orprimary minerals, and the authigenic phases.

Strontium and Barium Contents

In the upper radiolarian-nannofossil ooze (IA), theSr-bearing biogenic carbonates, and Ba-bearing sili-ceous organisms supply the trace elements which arefound thereafter with the authigenic minerals, such asclays and zeolites for Sr and barite for Ba. The occur-rence of these elements in biogenic components of pe-lagic sediments is often cited (El Wakeel and Riley,1961; Brongersma-Sanders, 1967; Turekian, 1968; Chowand Goldberg, 1970); their release from dissolved bio-genic phases and precipitation with secondary mineralshas been suggested by Hoffert et al. (1978) and Karpoff(1980). The occurrence of barite and its formationunder volcanic influences and (or) in sediments rich inorganic material is cited by Goldberg et al. (1969), Bos-tröm et al. (1973), Cronan (1974), and Dean and Schrei-ber (1978). In Site 464 pelagic oozes, the occurrence ofbarite, and small amounts of zeolites, appears to be aresult of a diagenetic evolution under a slight volcanicinfluence.

The main geochemical characteristic of the brownclay (Unit II) consists in the very low barium content, inthe bulk soft sediment and the siliceous slabs or concre-tions. The cherts from Unit III have the same peculiarity

LITHOLOGICUNITS

Fe2O3 Mn3O4 TiO2 BaO K2O0 10% 0 1% 0 1% 0 1% 0 2%

2-3,1-3 cm - .

2-3,45-47oconcretion \

2-3,50-52/

3-4,145-146-

3 5,5-6

5-3, 4-6

5-3,140-142-

Sr Ni0 500ppm 0 200

7-5,80-82 -

10-4,70-720white fragments10-4,70-72 f

, A Å

A A

. A i

A A

13,CC

26-1,82-84 -

26-1,88-90 J

Figure 5. Variations of prevalent elements (oxides in percent, and traces elements in ppm) along the sedimentary column at Site 464. Thickness ofdeposits not to scale. Arrows indicate high contents (see Tables 2 and 3).

764


15

as 10

* 11.Top

-H-NODULE

0.5 1.0TiO2(%)

1.5

15

10

* 11.Top

*NODULE

10Fe2°3 (

15

Figure 6. Relationships between A12O3, TiO2, and Fe2O3 contents ofsoft sediments of Site 464 (Units IA, IB, and II), and the clayey-siliceous concretion from Unit IA. 1. Regression: %A12O3 = 24.2%TiO2 - 0.53; correlation coefficient Al-Ti = 0.996. 2. Regres-sion: %A12O3 = 2.47 %Fe2O3 - 1.03; correlation coefficientAl-Fe = 0.999.

as the coarse siliceous slabs of Unit II: they do not con-tain any barium. In the soft deposits, the silica releasedfrom the dissolved siliceous organisms precipitates andforms quartz-rich concretions, or transforms by epi-genesis of calcareous nannofossils (Plate 2, Fig. 1). Thissecondary silica does not concentrate barium, which isnot retained as barite as it is the case in the clayeysiliceous ooze (Unit I). The cherts appear to result fromthe same diagenetic process which is indicated by the Sr-isotope investigations.

Nickel, Copper and Other Trace Elements

In the clayey siliceous deposits (Unit IB), the well-known trace-element assemblage related to clay min-erals (B, Ga, Pb, and V) cannot easily be discriminatedfrom that of metalliferous oxyhydroxides (Ni, Co, Cu,and Pb), because of the simultaneously increasingamounts of both phases. Thereby, the relationships oftrace elements with detrital clays, authigenic smectite,and oxyhydroxides from the soft sediments were notestablished. However, bulk and microprobe analyses onthe clayey nodule (Unit IA) show that the authigenicFe-Ti beidellites are poor in boron and gallium,preferentially concentrate high contents of Ni, and con-tain Co, Pb, and Zn. The scarce Mn-oxides betweenclay laminae are rich in copper only.

In the second sedimentary unit, trace elements aredistributed among clay minerals (Ga, Pb, V), volcani-clastic particles (V, Cr), micronodules, and ferroman-ganese oxides (Ni, Cu). Those oxides from the lowerpart of the section seem to contain less copper thanthose of the overlying unit.

Competition between primary minerals, authigenicsilicates, and metalliferous oxides to trap the trace ele-ments is confirmed. Particularly, the nickel is trapped inauthigenic Fe-smectite when the manganese oxides arescarce, whereas the copper is located in both phases. Avolcanic influence probably promoted the formation ofsuch Fe-Ti- and Ni-rich smectites, and their concretion-ary-nodule structure.

Isotope Analyses: Indications of the DiageneticEvolution of the Siliceous Horizons

METHODSRb- and Sr-isotope analyses were carried out the siliceous slabs

from the bottom of the brown clay, the cherts, and associatedlimestones (Table 3). The Sr concentrations and isotopic compositionon the one hand, and the Rb concentration on the other hand, weremeasured on mass spectrometers with 30- and 15-cm radii respec-tively, and corresponding 60 and 90° deviation angles. The blanks ofthe complete chemical procedure lie at about 4 ng/g Rb and 20 ng/g Srfor a 1-g theoretical sample dissolution. The individual error of each87Sr/86Sr ratio is given at two standard deviations from the mean. TheRb/Sr ratios are estimated as less than ±1%. The results are correctedwith the usual constants: 85Rb/87Rb = 2.591, and 86Sr/88Sr = 0.1194;the 87Sr/86Sr ratios are also adjusted to 0.71014 for the NBS 987standard. Further analytical and instrumental conditions are de-scribed by Clauer (1976).

Results

Isotopic ages are calculated assuming that the initialstrontium trapped in the siliceous and calcareous mate-rial had the same isotopic composition as the sea-waterSr contemporaneous with the sediments: 0.70743 ±0.00042 for Albian samples (Dasch and Biscaye, 1971),and 0.70759 ±0.00027 for Late Cretaceous samples(Peterman et al., 1970). For the Cenomanian, no valuesare available; therefore, we used the Albian value. Theisotopic ages are systematically older than biostrati-graphic data (Table 4). This discrepancy can be ex-plained by an alteration of minerals, as well as by ex-changes with the interstitial environment, which could

765


Table 4. Rb/Sr isotopic data on siliceous, calcareous, and bulk material from the"red clays" and cherts from Site 464.

Sample

464-10^, 70-72 cm

464-13.CC, bulk chert464-13.CC, calcite464-13.CC, silica (chert)464-13.CC, silica (other chert)

464-26-1, 82-84 cm, chert464-26-1, 88-90 cm, bulk464-26-1, 88-90 cm, calcite464-26-1, 88-90 cm, silica

BiostratigraphicAge

Late Cretaceous:80±20 m.y.

Cenomanian:98 ±2 m.y.

Albian:100±6 m.y.

Rb

4.22

9.37

12.74.82

0.720.80

1.32

Sr

25.7

248796

4.536.49

6.9919.7

5111.40

8 7 R b / 8 6 S r

0.476

0.109

8.1002.152

0.2960.118

2.732

8 7 Sr/ 8 6 Sr*

0.70903 ±21

0.70756 ±10)0.70769 ±27 \0.73489±29>0.71505 ±18

0.70977 ±220.70779 ±21 )O.7O765±ll }0.71188±65 J

IsotopicAge

215 ±90 m.y.

240±10m.y.

250 ±30 m.y.

555 ±90 m.y.

115 ±20 m.y.

The 8 7Sr/86sr ratios were corrected for a 8 6 S r / 8 8 S r ratio at 0.1194 and adjusted to 0.71014 for the NBS 987standard. The individual error of each run (2σA/N) is given in 10 ~ ^, Ages are calculated with the new decay con-stant λ 8 7 Rb = 1.42 × 10" n × β ~ l .

be enriched in 87Sr, as was already observed on the caseof interstitial waters from "red clays" (Clauer et al.,1975). The initial Sr amounts of siliceous material arevery low, (1.4-7 ng/g), smaller than those of presentday sea water. A slight "contamination" from the sur-rounding environment therefore can explain the dis-crepancies in the ages. Thus, the siliceous compoundsevolved by diagenesis in an open system since the dis-solution of the biogenic silica, until chert and aggregateformation by recrystallization. On the other hand, thecalcite associated with cherts shows 87Sr/86Sr ratiosclose those of the contemporaneous of deposits seawater, respectively + 22 × 10 ~5 for the Albian sampleand +10 × 10-5 for the Cenomanian sample (Table 4).The calcite certainly has also evolved in an open systemduring the chert formation, but "contamination" isnegligible because of the high initial Sr contents (>500

These results confirm those from a previous study onsiliceous rocks from central Pacific deposits (Karpoff,1977). They also emphasize the difficulties encounteredwhen dating siliceous material, considered by Bruecknerand Snyder (1979), whereas the 87Sr/86Sr ratios of thelittle-transformed calcareous residual inclusions incherts can be used for dating these formations.

DISCUSSION: INFLUENCE OF THESEDIMENTATION ENVIRONMENT ON

SILICATES AND SILICA AUTHIGENESIS

Authigenic ClaysAnalytical data indicate that several types of clay

authigenesis are represented in the sedimentary column.Nevertheless, results were not sufficient to point to thecauses of the segregation and differentiation of authi-genic smectites forming concretions in a siliceous ooze,which also contains another clay-mineral assemblage.However, the ferroan and titaniferous nature of thenodule smectites is similar to that of authigenic smec-tites formed in a submarine environment with strongvolcanic influences, particularly where nontronites andFe-Mn crusts are found, Sites 430 and 431 (Fig. 1) (Kar-poff et al., 1980). Therefore, a local volcanic influenceseems to have acted upon Pliocene clayey-siliceousoozes, and helped the formation of Fe-Ti-smectitesdistinct from those resulting from the dissolution of

organisms. This volcanic influence was certainly veryslight; it favored zeolite and barite formation, and didnot end with Fe-Mn-oxide precipitation.

In the brown clay, smectites are essentially authigenicand result from the dissolution of siliceous organisms.Glasses and volcaniclastic particles are the only traces ofa volcanogenic contribution, which helps however theformation of Fe-Mn oxides and micronodules, andsmall amounts of zeolites. Palygorskite-type particlesform the clay fraction, related to the hypersilicic natureof the deposits and the presence of quartz aggregates.Authigenic palygorskite in siliceous sediments is fre-quently encountered in alkaline environments (Isphod-ing, 1973; Donnely and Merrill, 1977). These clays havealso been ascribed to weathering of volcanic material ortransformation of smectites (Hathaway and Sachs,1965; Bonatti and Joenssu, 1968; Lancelot, 1973; Cou-ture, 1977). Small amounts of palygorskite encounteredin brown oozes from Site 464 result from the hypersilicicenvironment. This authigenesis follows those which arerelated to the dissolution of organisms.

Silica AuthigenesisCristobalite from bentonites, shales, and cherts is

often ascribed to authigenesis related to dissolution ofbiogenic opal, volcanic glass, and sometimes quartz(Peterson and von der Borch, 1965; Wise et al., 1972;Wise and Weaver, 1974; Klasik, 1975; Wilding et al.,1977). Cristobalite is sometimes described as the precur-sor of quartz in the SiO2 evolutionary maturation se-quence which starts with biogenic opal or amorphoussilica (Ernst and Calvert, 1969; Mitzutani, 1970). Thepossibility of a mixed volcanogenic-biogenic origin ofthe cherts has been suspected (Gibson and Towe, 1971;Castellarini and Sartori, 1978). Kastner et al. (1977)established the following diagenetic succession: siliceousooze (opal-A) —• porcellanite (opal-CT) — chert (chal-cedony, cryptocrystalline quartz). These authors re-viewed the common hypotheses for silica diagenesis.According to Wiley (1978), the ratio between clays andbiogenic-silica amounts affects silica dissolution. Simi-larly, von Rad (1979), after discussing authigenic anddiagenetic processes of SiO2 evolution through time,suggests that opal-CT formation is faster in carbonatethan clayey sediments, and that the clayey environmentslows the transformation of opal-CT into quartz.

766


Greenwood (1973), Lancelot (1973), and Kastner et al.,(1977) show that cherts are cristobalite in clayey hori-zons, whereas quartz is most common in carbonate sedi-ments. The possibility of quartz precipitation during anearly stage of porcellanite diagenesis or epigenesis oforganisms is also suggested by von Rad (1979).

In Site 464 brown clay, authigenic quartz with smallamounts of cristobalite is common, whereas cherts areessentially cristobalite. Evidence supporting the diage-netic formation of Albian-Cenomanian cherts agreeswith studies by Kastner et al. (1977), Riech and von Rad(1979), and von Rad (1979).

On the other hand, the occurrence of authigenicquartz in deep-sea sediments, particularly in siliceousooze and manganese-crust-bearing volcanic sedimentarydeposits, has already been described (Karpoff, 1977;Hoffert et al., 1978). Therefore, authigenic microcrys-talline quartz in the clayey nodule from Unit IA appearsto have formed along with the concretion of iron smec-tite in siliceous pelagic deposits under a slight volcanicinfluence; this is seen to be a stage of silica evolution inthis marine environment.

However, in brown clay from Site 464 quartz-concre-tion crystallization and epigenesis are due to diageneticprocesses similar to those of chert formation. It resultsfrom dissolution of biogenic silica and is strongly af-fected by clay- and oxyhydroxide-rich environments.Quartz-concretion authigenesis cannot be consideredthe result of restricted and local phenomena because oftheir abundance. They are an essential stage of brown orred clay evolution through time. Thus, differences be-tween the two siliceous phases, concretions and cherts,from Site 464—such as their age, the deposition depths,and the carbonate, clay, and Fe-Mn-oxide contents—have controlled the diagenetic evolution of silica. Theimportance of these factors in chert formation hasalready been mentioned by Lancelot (1973), Keene(1975), and Kastner et al. (1977).

CONCLUSIONS

The lithologic sequence at Site 464 appears to be aresult of interaction of three connected evolutionaryfactors:

1) The biogenic components, as calcareous nanno-fossils and siliceous organisms, form the upper deposits,and their dissolution and a slight volcanic influence pro-mote the authigenesis of clays, zeolites, and barite.

2) The clay assemblages, as detrital clays and authi-genic smectites, vary with the sequence. From the disso-lution of siliceous organisms, the smectites are distinctringlet-shaped particles, or laths, in the upper siliceousooze and clay and in the brown clays, respectively.Under a volcanic influence, authigenic Fe-smectites areformed which are chemically distinct from those of thesoft host pelagic sediments. Authigenic palygorskite oc-curs on brown clay characterized by a hypersiliceous en-vironment.

3) The silica cycle and evolution induce the authi-genic formation of microcrystalline quartz associatedwith clayey nodules in the siliceous ooze. In the brownclay, the quartz-rich concretions and aggregates appear

to be derived from the same diagenetic process as thechert, but probably influenced by a previous calcareousenvironment.

These factors have conditioned the mineralogical andchemical characteristics of the brown clay. Therefore,the lower part of this unit, of Cretaceous age (Doyle andRiedel, this volume), and the Paleocene upper part, ap-pear to be similar to the "red clay" facies, formed re-spectively from calcareous biogenic ooze and from sili-ceous biogenic ooze, as described by Hoffert (1980).

The intermittent volcanic influence during depositionof the Site 464 sediments was more marked during thePaleocene-Eocene and Miocene-Pliocene, and is in ac-cordance with that suggested by the studies of sedimentsfrom Sites 310 and 433.

Nevertheless, the marine environment—location,depth of deposition, geologic history, and hiatuses—in-duces varying lithologic sequences in these sites, corre-spondingly more calcareous pelagic deposits at HessRise (Site 310), and more volcanogenic sediments on theEmperor Seamounts (Sites 430, 433), compared to thepelagic sediments at Site 464.

ACKNOWLEDGMENTS

The analytical work was performed with the technical support ofthe Centre de Sédimentologie et Géochimie de la Surface of theC.N.R.S. and of the Institut de Géologie, Strasbourg, especially D.Trauth, I. Zimmerlin, B. Kiefel, R. A. Wendling, and R. Winkler.The authors are grateful to José Honnorez for helpful criticism, andto Yves Lancelot and Michel Steinberg for suggestions.

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•–

Plate 1. SEM photomicrographs of the bulk sediment (Figs. 1 to 4)and TEM photomicrographs of the clay fraction (Figs. 5 and 6) ofthe brown clay from the top of Unit II, Sample 464-7-5, 80-82 cm.

Figure 1. General view of the sediment with a sponge spicule; scale bar50 µm.

Figure 2. High magnification of Figure 1, showing the inner cavity ofsponge spicule with authigenic crystals of zeolite; scale bar 5 µm.

Figure 3. Beginning formation of a micronodule in the brown clay;scale bar 50 µm.

Figure 4. High magnification of Figure 3; crystallization of manga-nese oxide mixed with clays; scale bar 10 µm.

Figure 5. Clay fraction with big particle of smectite fringed by finefibers; scale bar 2 µm.

Figure 6. General view of the clay fraction of authigenic smectites(particles and fibers); scale bar 2 µm.

769


Plate 2. SEM photomicrographs of the white slabs of the coarse frac-tion (Figs. 1 to 4) and TEM photomicrographs of the clay fraction(Figs. 5 and 6) of the brown clay from the base of Unit II, Sample464-10-4, 70-72 cm.

Figure 1. White fragment composed of silicified coccoliths; scale bar 5µtn.

Figure 2. White slabs composed of siliceous "lepispheres" and coatedcrystals of zeolite (Z); scale bar 20 µm.

Figure 3. Detail of a siliceous slab with residual radiolarian; scale bar10 µm.

Figure 4. Detail of the siliceous "lepispheres" with successive laminaeof silica; scale bar 20 µm.

Figure 5. Clay fraction containing smectite particles and palygorskitelaths; very small globules of silica occur (―); scale bar 2 µm.

Figure 6. General view of the clay fraction; the smectites prevail;scale bar 4 µm.

770


üH11111pi

§1Si

Plate 3. SEM chemical studies on polished section of the clayey-siliceous concretion from the sedimentary deposits of Unit IA,Sample 464-2-3, 45-47 cm.

Figure 1. General view of the polished section. Contact between thenodule core and a feldspar crystal and the successive layers of claysforming the cortex; the square shows the location of the analyzedarea.

Figure 2. SEM microphotograph of the analyzed area, with theequivalent maps of Si, Al, Ca, Fe, Ti, and K contents.

Figure 3. Schematic drawing of the various components, a. PrevalentFe-Ti-rich clays, b. Fe-clay layer without Ti. c. InterlayeredK-silicates as zeolite, d. Layer of fine crystals of quartz, e. Fe-Tioxides, f. Feldspar crystal, g. Micropatches of carbonate ascalcareous microfossils.

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Deep Sea Drilling Project Initial Reports Volume 62