carol j. pudsey, department of geology, university of leicester1982/05/27 · carol j. pudsey,...
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12. X-RAY MINERALOGY OF MIOCENE AND OLDER SEDIMENTS FROM DEEP SEADRILLING PROJECT LEG 78A1
Carol J. Pudsey, Department of Geology, University of Leicester2
ABSTRACT
One hundred and fifty-three samples of Recent to Late Cretaceous sediments from Sites 541, 542, and 543 were ana-lyzed by X-ray diffraction. The main constituents are quartz, feldspar, clinoptilolite, opal-CT, calcite, and clay minerals(kaolinite, illite, smectite, and palygorskite). Minor components (generally less than 10% of the total) include amphi-bole, dolomite, rhodochrosite, chlorite, and mixed-layer clays. Amorphous clays, volcanic glass, and biogenic silica arealso present.
Calcite in the form of microfossils is a major component of the Cretaceous and post-Miocene sediment: during thePaleocene to Miocene the sites were below the calcite compensation depth (CCD). Detrital quartz and feldspar increaseupsection, recording the passage of the area from ridge crest, through open ocean, to the vicinity of a volcanic arc. Au-thigenic clinoptilolite may be derived from silicic glass via a smectite intermediate. Opal-CT is present only in Eoceneclays, occurring as lepispheres growing on radiolarians. Palygorskite and dolomite, found in the lowest sediments at Site543, may be related to hydrothermal activity or alteration of basalt.
The smectite content of the sediments strongly affects their physical properties and their behavior during deforma-tion. Clay mineralogy may therefore be important in the formation of décollement zones.
INTRODUCTION
The three Leg 78A sites are located on the northeastside of the Barbados Ridge in the Central Atlantic, wherethe Atlantic crest is believed to be underthrusting theCaribbean Plate (Fig. 1). Seismic reflection profiles inthe area show layered sediments above ocean crust thatdip shallowly westward below a prominent reflector. Abovethis reflector is a discontinuously reflective unit that thick-ens westward to constitute the Barbados Ridge. The sci-entific objectives of Leg 78A included documenting thenature of the discontinuously reflective and layered unitsand their relationship to each other.
This study is concerned with the sedimentology ofthe pre-Pliocene muds recovered at Sites 541, 542, and543, as revealed by X-ray diffraction (XRD) mineralogy.At the oceanic reference Site 543 the layered sequence isa complete section from Campanian to Holocene. Theother two sites contain Miocene to Holocene clays intectonically repeated slices. The majority of the sedi-ments accumulated by pelagic-hemipelagic settling be-low the CCD (calcite compensation depth), though somegravity flow deposits rich in foraminifers occur in partsof the Pliocene section (Hemleben, this volume). Al-though the physical processes of sedimentation are uni-form throughout the Leg 78A sites, there is considerablemineralogical variation. Variations in composition canreveal the source of the sediment and affect its physicalproperties, which in turn determine its behavior duringdeformation.
Biju-Duval, B., Moore, J. C , et al., Init. Repts. DSDP, 78A: Washington (U.S. Govt.Printing Office).
2 Address: Department of Geology, University of Leicester, Leicester, United Kingdom.
METHODSSamples were taken at about 3-m intervals down each core; inter-
vals of poor recovery yielded fewer samples. A few Pliocene and Pleis-tocene samples were analyzed for the sake of completeness.
Experimental TechniqueA bulk sample was prepared simply by grinding about 0.5 g of the
dried sediment, making it into a sludge with deionized water, and spread-ing it on a glass slide. A <2-µm fraction was also prepared for moredetailed study of the clay minerals. This preparation entailed making adeflocculated suspension with sodium hexametaphosphate and lettingthe >2-µm fraction settle out; the remaining suspension was centri-fuged and the resulting sludge spread evenly over a slide. Each samplewas then X-rayed twice—(1) saturated with ethylene glycol and (2)heated to 500°C. Bulk samples were run from 20 = 3° to 32° and<2-µm samples from 3° to 13°. The following instrument settingswere used: Cu Ka radiation, nickel filter, 40 kV/20 mÅ, 1° apertureand receiving slits, goniometer speed 2° 20/min. Minerals were identi-fied from as many peaks as possible on the diffractograms using thetables given in Brindley and Brown (1980).
Analysis of the DataXRD data on peak heights and areas may be used to obtain a semi-
quantitative estimate of mineralogy by applying a correction factor tothe height or area measured for each mineral and assuming that theseweighted amounts add up to 100% of the crystalline fraction. A com-bination of correction factors given by Rex and Murray (1970), Biscaye(1965), and Mann and Muller (1980a, b, c) is used here (Table 1). I fol-low Mann and Muller in not correcting peak intensities for the contri-butions of interfering peaks, because, for a few test samples, the cor-rections were extremely small. The correction interference of illite(003) with the main quartz peak is not quite so small, but does not sig-nificantly alter the trends in abundance reported here. The amount ofamorphous material has not been determined quantitatively. Calcite isdetermined more accurately by carbonate bomb than by XRD (seeWright, this volume), so it was not included in the percentage calcula-tions.
A measure of crystallinity may be obtained by measuring peak widthat half height. Poorly crystalline materials, particularly clay minerals,give a broad fuzzy peak rather than a sharp one. Crystallinity of dif-ferent minerals may be compared using the ratio H/w where H =height and w = width at half height. This approach is not suitable for
325
C. J. PUDSEY
Venezuela
64
Figure 1. Location of Sites 541, 542, and 543 near the seaward edge of the deformation zone of the Barba-dos Ridge.
smectite because of high background radiation at low 20 angles; theratio v/p devised by Biscaye (1965) is therefore used (illustrated inFig. 2D).
Clay minerals are characterized by their strong (001) reflections be-tween 20 = 3 and 14°; in addition (002) peaks occur at higher 20 an-gles. The relative intensity of (002)/(001) may depend on variations incrystallinity and chemistry.
RESULTS
General Comments
The sediments recovered during Leg 78A contain thefollowing minerals, in varying proportions: quartz, feld-spars, amphibole, clinoptilolite, and opal-CT; the car-bonates calcite, rhodochrosite, and dolomite; the clayminerals kaolinite, illite, smectite, chlorite, and palygor-skite, with small amounts of mixed-layer clays (20 from6.8-7.8°, i.e., d1Oi from 11-13Å). These minerals occurtogether with halite as a contaminant from seawater. Ina few samples small amounts of apatite and phillipsitewere tentatively identified. Some examples of diffracto-grams are shown in Fig. 2, and characteristic peaks and
correction factors are given in Table 1. In order that myresults may be compared with those of all similar stud-ies, the raw data of peak heights and areas are given inTables 2, 3, and 4. Semiquantitative percentages of min-erals, with some measurements of peak sharpness and(002)/(001) peak ratios, are plotted against depth for thethree sites in Figures 3 to 8. The main features of thesediagrams are as follows.
Site 541
Figure 3 illustrates bulk mineralogy for Site 541.Quartz shows an overall increase upsection, from about10% at the base of Tectonic Unit B to 20 to 25% in thePliocene and Pleistocene of Tectonic Unit A. Plagio-clase fluctuates widely and rather irregularly, shows nooverall trend, and does not co-vary with quartz. Feld-spar peak sharpness correlates closely with feldspar abun-dance. Amphibole is a sporadic trace constituent. Totalclay (clay minerals in bulk sample) varies mainly from65 to 80% but may be as low as 45% in the Pliocene and30% in the feldspathic near-surface sediment.
326
X-RAY MINERALOGY OF MIOCENE AND OLDER SEDIMENTS
Table 1. Characteristic peaks and correction factors for minerals de-tected in samples from Leg 78A.
Mineral
QuartzPlagioclaseAlkali feldsparAmphiboleClinoptiloliteOpal-CTCalciteRhodochrositeHaliteDolomite
KaoliniteHikeSmectite glycolatedChloritePalygorskiteMixed-layer
clays
Main peak 20
26.7°27.8-28.3°
27.4°-27.6°10.4°9.8°
21.8°, broad29.4°31.4°31.7°30.9°
12.3°8.9°
5.2°, broad6.4°
8.4°, broad6.8-7.6°,variable
Other peaks
20.8°13.8°, 29.4°
11.2°, 22.5°, 30.0°
23.0°
Main peakat 500°C
19.9°, 24.8° Disappears17.8°, 19.9° Stable
19.9° Disappears12.3° Stable13.7° Disappears
Variable
Correctionfactors
Height
1.32.02.02.02.01.5_2.0_2.0
4.0.8.0
10.04.0
12.04.0
Areaa
241232
Note: Feldspar peaks in the range 27.4 to 28.3° commonly interfere with each other, andpositive identification was not always possible; where there were two or more distinctpeaks the highest was measured (see Fig. 2A). Smectite is here used as a blanket termfor all smectites and illite-smectite interlayers that expand to give a 17Å, 5.2°20 peakon glycolation. Calcite and halite were not determined quantitatively using xrd; cal-cite is more accurately determined using carbonate bomb, and halite (probably a con-taminant from seawater) was ignored.
a From quartz through dolomite, this factor was not used.
In Tectonic Unit A, calcite is present in only a fewsamples from the reverse fault at 276 to 200 m sub-bot-tom. At 200 m (lower Pliocene), it increases to about30%. A similar trend is seen in the lower tectonic unit,calcite being absent altogether below 400 m. Rhodo-chrosite shows a positive correlation with calcite, beingvirtually absent below 350 m.
Figure 4 shows the clay mineralogy of <2-µm Site541 samples. The main feature of this diagram is thelarge upward decrease in smectite, with a correspondingincrease in illite. This trend is clearer in Tectonic Unit Bthan in A.
Chlorite increases upward in both units, whereas theproportion of kaolinite stays approximately constant at20 to 25%. The ratio Ka. (002)/(001) shows some up-ward decrease in each unit, and kaolinite has the sharp-est peak of the nonexpandable clays: in general its crys-tallinity (H/w) decreases upsection, and in the lowest100 m, H/w correlates well with abundance. Illite H/wis greatest in the noncalcareous parts of the section. II.(002)/(001) is always rather low: 0.32 ±0.19 in Unit Aand 0.29 ±0.14 in Unit B. Smectite crystallinity {y/p)correlates very clearly with its abundance.
Site 542
Figures 5 and 6 show Site 542 bulk and clay-fractionmineralogy. Trends similar to those in Tectonic Units Aand B at Site 541 are apparent here. Plagioclase contentfluctuates less widely, clinoptilolite is absent, and rho-dochrosite is less abundant. Calcite decreases sharply be-low 240 m sub-bottom but is present in some samplesbelow this depth. Kaolinite is roughly constant at 20 to30%, and smectite decreases rather abruptly above 260 m.
Crystallinity of kaolinite and smectite decrease upsec-tion. IL (002)/(001) is 0.26 ±0.15.
Site 543Figure 7 shows bulk mineralogy for Site 543. Quartz
decreases from 8 to 12% near the base of the section toa constant 6 to 8% from 340 to 180 m. Above this depthquartz increases upsection fairly regularly. Plagioclasecontent fluctuates widely (and also varies in crystallini-ty) from 200 m depth to the seafloor. Below 200 m (Oli-gocene and older) both alkali feldspar and Plagioclaseare trace constituents only. Clinoptilolite occurs from240 to 380 m, a larger depth range than reported fromthe smear slides (see Site 543 report, this volume) andcoinciding with minimum feldspar. The 9.9° 20 clinopti-lolite peak is always sharp and has a rather constantwidth of 2.5 to 3 mm, so H/w correlates well with abun-dance. Opal-CT appears first at 340 m (lower Eocene),where radiolarians also appear, is prominent up to 290 m,and drops to zero at 270 m. Opal-CT and clinoptilolitemaxima correspond to total clay minima. Calcite, inwidely varying amounts, is present in the lowest 30 mcored. Dolomite is also a significant component of thebasal sediments. Calcite is absent from 380 to 80 m sub-bottom and increases upward in the lower Pliocene sedi-ments.
Figure 8 illustrates clay mineralogy of <2-µm Site543 samples. Kaolinite increases from near zero at thebase to more than 50% at 260 m, then shows a decreaseto 12% at 180 m and a steady increase to the surface. Il-lite remains roughly constant at 10 to 20% except in thelowermost 30 m and uppermost 90 m. From 200 m up-ward smectite decreases, whereas illite and chlorite in-crease. Palygorskite has a very uneven distribution be-low 300 m, constituting up to 65% of the <2-µm frac-tion.
In general kaolinite has by far the sharpest peak,H/w exceeding 20 in some samples (arbitrary units), andH/w correlates well with abundance in some parts ofthe section. Illite H/w correlates slightly with abundanceabove 300 m. Below this depth palygorskite is promi-nent and the illite and palygorskite peaks are so close to-gether that they merge and their separate widths are notmeasurable (see Fig. 2). Again smectite v/p correlateswith its abundance. Illite (002)/(001) tends to decreasewith depth:
Depth(m)
II.(002)/(001)
0-100 0.32 ±0.11100-200 0.13 ±0.19200-300 0.22 ±0.2300-410 0.13 ±0.12
Comparison of Age-Equivalent Sections at theThree Sites
Mineralogy of the Miocene sediments recovered atSites 541 (two tectonic units), 542, and 543 is comparedin Fig. 9. There are no major differences, although mi-
327
C. J. PUDSEY
Smectite(001)
Quartz(101)
Kaolinite(002)
Kaolinite(001)
Cri
Cliπoptilolite(010) Palygorskite
(110)
Illitecollapsed smectite(001)
Figure 2. X-ray diffractograms of samples from Leg 78A. (Peak heights and areas are measured in arbitrary units.) A. Sample 542-H2-6, 20 cmbulk sample, glycolated. (Note double Plagioclase peak.) B. Sample 543-34-2, 52 cm bulk sample, glycolated. (Note the very broad opal-CT[cristobalite] peak.) C. Section 543-2.CC (<2-µm fraction, heated to 500°C, showing measurement of H and w for chlorite). D. Section543A-5.CC (<2-µm fraction, glycolated, showing measurement of H, w, v, axidp).
328
X-RAY MINERALOGY OF MIOCENE AND OLDER SEDIMENTS
Table 2. Peak heights and areas for minerals of Site 541 samples.
Sample(core-section,cm level)
2-6,334-7, 140
6-1,488-6, 50
10-6, 5012.CC14.CC
16.CC18-6, 5020-6, 5021,CC22-6, 50
23-4, 5023,CC24.CC25-4, 5025.CC26-2,5026-6,2027-4, 50
27.CC28-2, 5028.CC29-4, 50
29.CC30-4, 50
30-6, 5030.CC31-2, 5031-4, 5031.CC32.CC
33,CC34.CC35-4, 4435,CC36-2, 50
36.CC37-2, 5037-4, 5037-6, 4938-4, 50
38.CC39-2, 5039-4, 5040-2,5240.CC
41,CC42-7, 70
43-3, 5043-6, 5044.CC46.CC47-1, 4947-2, 9547.CC48-1, 48
48-3, 5248-4, 50
48-5, 3648-6, 5048.CC
49-2, 5050-2, 1050-4, 1050.CC
Qz.
2530382223182719262042291847252630242225222728252823262622232017263024313332273436262623322627232822.52719212122262422293162726232725
Plag.
454103151023356471858.55684231.51246.5211.59222121759.5202.53.5435162644101179351433221841282519141212.55.57
A.F. Amp. Clin.
— 9 —_ _ _
_ _ _
— 1 —— 1 —* 1 —(̂ )— 4 —
««• 1.5 —_ _ _— 1 —
_ _ _— 1 —
— 1 —_ _ __ _ _
_ _ _
— 1 —_ _ _
^_ _ _— — —»* — —»«•
— 1 —— 1 —— —
¥
ft
(>
(»
'
-_ _ __* 2* j
»
'
') - 2— 2.5
' — —•') 1.S —
1 —' — —
'*\ 4
K ) 4.5("*) — 4.5
Bulk samples
Ka.Phil. Crist. (001)
— — 5— — 7— — 5- — 8— — 6— — 5— — 6— — 5— — 3.5y — 6— — 14— — 5.5— — 3— — 9.5— — 9— — 9— — 6.5— — 10— — 5— — 5— — 7— — 12— — 9— — 5.5— — 10— — 7.5— — 12— — 5— — 7.5— — 5— — 9— — 5— — 6.5— — 6— — 6.5— — 8.5— — 20— — 11— — 15— — 15— — 15.5— — 7— — 9— — 9— — 6— — 8— — 11— — 9— — 10— — 7— — 9— — 6
6— — 5— — 6— — 11— — 3.5— — 7.5— — 9.5
— — 2.5— — 12— — 8— — 13— — 21— — 27
Ka.(002)
_
3242
523.5394.5257843.5324.56.56452.5543.543.5334.53310.5
6.56.5109.5566.56.57687.56.55.52.54.54.52.592.54.56_1.57.54.591017
11.(001)
3756.56262.5551263.57714683474.544749.555674787617912.512.51478967724653.5355934.510.52.556611118
11.(002)
2
1.51
32224.5213.51.54.5132.5111.51.51.521.5322.512.511.53234.535.54.54.522.5222.53
_
11.511.52221.51.5
2112.52.52
Sm.
1.52312_2.5
1.52641773661069511319571734445751045743.512362.575.558761477119.5176.5997101615108.514
Ch.
1.5
1
2
1251.52
422.5212.5522223.52312.822232.54344324342.54222.52.5
22.52.5
21.5321.5
31.513
Pal.
33
_
_
2.532_
2.5
_
3
3.5
2111
2
_
3_
4
_
2.5
3
_—
1.522
Ca.
4291372662571808278204515
2422_
1881715
68396960856359434618213011.5
118
_
_
_
—
_—
_
—
Rh. Hal.
_ ^
^5 *5 *
4.5 —6.5 *4 —3.5 ."3 —4 ft"
s1 —1.5 —
1.5 —_ _
^2 —5.5 —
1.5 —1.5 —_ _1 •"
4 —1.5 —4 —4 —5 —3 —3 —2 *3 —1 —_ _
1.5 —
— 11 *1 *
_ _
_ _
1 —_ _
s
— *>^s
1.5 —_ _— —4 —
_ _— —
Ka.
4851794939415635352213155
8473
53
392466
66255943
39
28373038453071
6463147
51
3054346338
322318
38
422998
106
< 2-µm samples
11.
5562623941223335332012364
7343
40
491316
19162518
17
10303852601633
3033121
23
163021217
1076
8
12727
11
Sm.
65511474352481241106159271125
300223
222
310205256
283292276248
71
93102588069345166
240121419
246
237307276229260
377373323
337
398476528
382
Ch.
20221513161014121512316
1518
15
2265
8578
11
661123181616
1723—
16
1367610
1067
7
5
—
Pal.
—57—563————
8—
13
———
4
——
—
—67———4
4514
—
—————
3——
—
—6
—
Mixed
—2314512———8——
——
7
10——
——9—
—
————8—6
——9
—
4———
——
—
5
—
Note: Units are arbitrary (i.e., chart-paper squares). ** indicates present but not quantifiable; (s) indicates probably present; Qz. = quartz; Plag. = Plagioclase; A.F. = alkali feldspar;Amp. = amphibole; Clin. = clinoptilolite; Phil. = phillipsite; Ka = kaolinite; II. = illite; Sm. = smectite; Ch. = chlorite; Pal. = palygorskite; Ca. = calcite; Rh. = rhodochrosite;Hai. = halite. — indicates not present. Mixed = mixed-layer clays, 29 mainly between 7° and 8°.
nor fluctuations are apparent in feldspar and total clayin the bulk samples.
DISCUSSION
General Remarks
The sediments cored during Leg 78A contain bothdetrital and authigenic components. Detrital minerals arebiogenic (calcite) and terrigenous (quartz, feldspar, am-phibole, kaolinite, illite, chlorite, mixed-layer clay min-erals, and some smectite). Authigenic minerals includezeolites, cristobalite, dolomite, some smectite, and paly-gorskite. Overall, the section cored at Site 543 records
the passage of this site from ridge crest, through openocean, to its present position close to an island arc. Theshorter sections recovered at Sites 541 and 542 providesome more detail on the upper part of the succession.There is little variation in the Miocene and lowest Plio-cene between sites (Fig. 9). This is to be expected, be-cause the sites are relatively close to each other and werefar from island arc or continent sources during the Mio-cene and early Pliocene (see Fig. 1).
The mineralogy of deep ocean sediments approach-ing a subduction zone has potentially important effectson their physical properties and hence their behavior dur-ing deformation.
329
C. J. PUDSEY
Table 3. Peak height and areas for minerals of Site 542 samples.
Sample(core-section,
cm level) Qz.
Bulk samples <2-µm samples
Plag. A.F. Amp. Clin. Phil. Crist.Ka.
(001)Ka.
(002)II,
(001)II.
(002) Sm. Ch. Pal. Ca. Rh. Hal. Ka. II. Sm. Ch. Pal. Mixed
Al.CCAH2-6, 201-2, 50l.CC2.CC3,CC4,CCA2.CCA3-4 50A3.CCA4.CCA5.CCA6.CCA7-4, 50A8-2, 50A9.CCA10-4, 64
2316
2227273728
1819.5222225232622
3.5 *19.5 —
2 —7.5 —1.5 ^6 (s)3.5 S
6.5 —5 —3 "3 —5 •"8 •-3.5 "6 *
1.53
1___
1—2_1.5_1.51
107.5
8.59
106.58
4368
141616.510
74.5
45.5656.5
3.52.52.54.59
11.5104.5
13.56.5
96.597
11
323.554.58.5
104.5
4.51.5
31.51.52.53.5
2_1_11.521.5
7.510
5454.55.5
5.5646.58.5
1211.59.5
—1.5
32.53—3.5
—232.52.53.53.5
—
—2.5_
_
_22.5485.54
4438
714842—_
5_—
2__13
— t*
— S
3 •*2.5 >-2 —— s1.5 —
— —1.5 *1.5 ^_ _— —
4831201926325872c i
1621172654495723
27147
126
25393521
6323
109
115
115232
978289
125251267332244202174157257311311251
613—
6114
16147
1525
—101268
— 2
— 5
3 —
Note: See Table 2 note for explanation of symbols and abbreviations.
Detrital and Authigenic Minerals
Calcite
The majority of Paleogene and Miocene sedimentsare carbonate-free, having been deposited below the CCD.However, calcite in the form of nannofossils and fora-minifers is a major component of the Pliocene andyounger sediments. This distribution is a simple conse-quence of an abrupt drop in the CCD in the area at theend of the Miocene (Hemleben, this volume); it doesnot imply any significant vertical movement of the sea-floor at that time. Calcareous sediments do occur in thepredominantly clay sections of the Miocene at Sites 541and 542. This may be a consequence either of gravitysliding or tectonic repetition of units that are too thinand of too short a time span to detect using biostratigra-phy. There is at least one minor repetition at the base ofTectonic Unit A at Site 541: a nannofossil zone is re-peated (Bergen, this volume), some calcite appears inthe sediment, and the upward decrease in smectite is in-terrupted (Figs. 4, 9).
Calcite forms up to 50% of the basal sediments de-posited near the ridge crest, again mainly in the form ofnannofossils showing evidence of dissolution (Hemleben,this volume). The transition from calcareous to noncal-careous sediments reflects the cooling of the crust andits subsidence below the CCD. Backtracking the site us-ing the method of Berger and Winterer (1974) yields apaleo-depth of about 4000 m for the Eocene CCD in theCentral Atlantic.
Quartz
Terrigenous quartz may be derived from three sources:the Lesser Antilles arc, the South American continent,and the African continent. The increase in quartz upsec-tion at Site 543 implies that arc sources were the mostimportant, with quartz content increasing as the site ap-proached the arc. During the deposition of the basalsediments (Campanian), the Atlantic Ocean was onlytwo-thirds of its present width, and it is possible thatwind-blown continental dust from North Africa con-tributed to the sediments. However, the Sahara Desert,
an important present-day source of airborne dust, didnot develop before the late Miocene (van ZinderenBakker, 1978). Another source of quartz may be recrys-tallization of opal-CT (see later discussion).
Feldspar and Amphibole
The decrease in Plagioclase and (less clearly) amphi-bole downsection at Site 543 may be attributed to thegreater distance of this site from the volcanic arc in thePaleogene and perhaps to a less explosive style of vol-canism (or 2 more tholeiitic sequence of differentiation)at those times. Thin ash beds are common in the Neo-gene (see Natland, this volume) and are mixed with thesurrounding sediment to a varying degree by biotur-bation and drilling disturbance. Wide variations in feld-spar and amphibole content at all sites may result partlyfrom our having taken small samples. If the samples hadbeen composited from a whole core or section, the lo-cal effects of ash beds might have been smoothed out.Small amounts of alkali feldspar in the basal sedimentsmay be related to ridge-crest volcanism or to alterationof calcic Plagioclase (see later discussion). In contrast toquartz, feldspar is not a common component of wind-borne pelagic sediment, being mechanically and chemi-cally less stable.
Detrital Clay MineralsKaolinite, illite, chlorite, and the few mixed-layer clays
are probably derived both from volcanic-arc and conti-nent sources. The Tiburon Rise has partly shielded theLeg 78A area from South American continental sedi-ment (see also Wright, this volume), but these clay min-erals are also present in weathered tropical soils on vol-canic islands and in sediment aprons derived therefrom(e.g., Lonsdale, 1975). The generally low proportion ofchlorite is consistent with the low-latitude setting (Bis-caye, 1965). The abundant and well-crystallized kaolin-ite found in the Oligocene and Eocene at Site 543 maybe partly authigenic or neoformational in origin. Fur-ther discussion of this point occurs in the chapter by La-touche and Maillet (this volume). Low (002) peak inten-sity of illite compared with (001) intensity implies that
330
X-RAY MINERALOGY OF MIOCENE AND OLDER SEDIMENTS
Table 4. Peak height and areas for minerals of Site 543 samples.
Sample(core-section,
cm level)
Al-4, 501-6,502,CC3.CC4.CC5.CC6.CC7.CC8,CC9,CC10.CC11,CC12-2, 5012.CC13.CC14.CC15.CC16.CC17.CC18.CC19.CC20.CC22.CC23.CC24-7, 12225-2, 5125.CC26-2, 5026-4, 5026-6, 5026.CC27-2, 5027-4, 5028-2, 5028-4, 5028.CC24-2, 10029-4, 10029-6, 10030-2, 5530-4,2430-4, 8630.CC31-2, 0831.CC32-2, 5032-4, 6232.CC33-1.033-2, 5333.CC34-1, 034-2, 52AH 1-2, 50A2-1, 0A2-1, 10A3-1.0A3.CCA4-1.0A4.CCA5-1.0A5.CCA6-1, 0A6.CCA7-1, 0A7.CCA8-1.49A8-1, 120A9-1, 0A9.CCA10-1, 35
Qz.
24291826222220263320177
111718191321161216181521.52633222226302325202117189
1978
189
101357
106.58.5878
1020
811102818.5302034261822271519161323
Plag.
7143669
1143.5
131620
7.512.553999.53.5656222232.531.5372.52———————————————
1——1.5——21—1.53.522_
—
—
A.F.
—
_
—
—
——
( ' )
——
———
(")(')(")(")(")
—
—_————————4—2
———————
———
364.584.538
Amp.
1.51
_
1.51——
1——_—
—————1.511.5————
_—
__—_———1———
—
———_—0.5——_
_————1——
_
—
Clin.
_
_
___
_—
_
_—
——
___—
—3.545434684.587.54.53.54.5596.54
3745.543.5
1144.5
13111486.56.5——
_—
_
—
Phil.
_
_
_———
_—
_
_—
_———_
———_
_
_
_——_———————
__
———_—
—
—
_———
_
—
_
—
Crist.
_
_
___
_—
_
_—
_—
_———_
_
_
__
5.53584486
195.59
13.512172312939.5
175_
_4——
——
_
—
Ka.(001)
8678967897623699756677
14201520252523262838193430181339151726141321
512554.5
1242
1721
294.5
128
205
169
118
124.561.512.5
Bulk samples
Ka.(002)
3.5343.54.534.547.544.5_
855224.547.51.5
12.515.515171718172414.52318.5121026.5
9111998.5
152.59441.5641.5
1118252.587
145
108.565.5722.5_
2
11.(001)
5785546587422
232.53334.525866
11669674964244293.545.5227.532.5431.5671538472
129
104
116
1069
12
11.(002)
2.52.52.51.51.511131.52_
1_
11
1
21.5141.51.521.51.5121.512
1_2.5_12
_
_
1.51.5
2121.51.51331.51.5111_
—
Sm.
33323356598
241012229
1115135
38291221171225161517182223139
144
2514665
1451.551.5524.53
66122
204
244.5
212.54_5
3
3
Ch.
_
2
2231.5222_
13.5
_
_
32_2
—
_
_
_
—
_
_
_
2_21.52.5
22
12—
Pal.
__
_1.5_
_—
_1.5
(^)
——2.5
312_1.5171.56.5
1332413
1943.5533.533342.523129
1017478
21122121
Ca.
828
1.532
13(•×)
_—
_
_—
__
___
_—_
_
__
_—
_——
—_——
__
——
__
_
_
__——
_——
217—
Rh.
_
_
1.5
_1
1.52
_
_—
__2.5———6——_
_
_
_——__1.5——_————1
———_——1.5
_
_———————
_
—
Dol.
_
_—_—
__
_
_—
__————
——_
2
_
__—_————_————_
——
_—
——_
——————
3964
18—
Hal.
_
—__
__
s
_
—
——_
_
__
_——_———
—
—
I*
yf
—
—
_
_
—
—
—
—
—
( " )
—
—
_
—
—
Ka.
38707056837880829959806
835973572932281731688598
120
129
101
109
95
30
27151
3017
153395
1455653685427149
14814519865332155
15
11.
23434531463548526135362
151724151011886
152535
23
18
8
11
8
5
217
94
21417
729121123
46
21815410623851343788
<2-µm samples
Sm.
7012193
123204190245186153153425301
502315231350232221231234309254291391
361
302
328
131
152
72
86302
7849
55102344
2221424026937724036619526717258
1667328—
—
Ch.
211813201312221715145
_142417—
355
——_
—
_
14
—
——
——
———
—————————10——
5439
—
Pal.
———————————
————————————
—
—
—
—
—
2
—18
713
635
6
111725
———
619
147110
1545637980
111131
Mixed
8————_4———_—
_
———_——————
—
—
—
—
—
—
—9
——
5——
—7—8
—4———————————
Note: See Table 2 note for explanation of symbols and abbreviations.
the Al:Fe ratio must be high, between 3:1 and 4:0 (Brind-ley and Brown, 1980, p. 338). It is difficult to estimatethe amount of disordering of kaolinite in the presenceof other clay minerals, because the relevant peaks atabout 19.8° 20 overlap. In a few samples the 19.8° peakis strong and asymmetrical, and the kaolinite group min-eral may in fact be nearer structurally to halloysite thankaolinite sensu stricto.
ZeolitesClinoptilolite is the only zeolite positively identified.
One might have expected phillipsite as an alteration prod-
uct of vitric basaltic ashes, but it is difficult to detect inthe presence of Plagioclase and kaolinite. It may bepresent in some samples (see Figs. 3 and 7). Clinoptilo-lite, the high-silica member of the heulandite group, isa characteristic alteration product of rhyolitic glassesin an alkaline environment (Hay, 1966; von Rad andRösch, 1972). The initial alteration of glass to smectitemay be an important factor in providing the chemicalenvironment (high pH and high SiO2) suitable for subse-quent zeolite formation (Hay, 1966, p. 81). In DSDPcores clinoptilolite is more common in calcareous sedi-ments, though it is not unknown in clays (Stonecipher,
331
C. J. PUDSEY
o-
Quartz Plag.
0 10 20 0 10 20
100-
200-
3 0 0 -
400-
Plag.
{H/w)0 10 20
A m p C l i n •
0 10 0 10Total clay
20 40 60 80
Calcite Rh. Dol. Age
0 20 40 0 10 0 10
Figure 3. Site 541, mineralogy of bulk samples (%). (Horizontal lines represent sample positions. Calcite percentages are from carbonate bomb.Percentages of remaining minerals assume no calcite [i.e., totalled to 100%]. Plag. = Plagioclase, A.F. = alkali feldspar, Amp. = amphi-bole, Clin. clinoptilolite, Phil. = phillipsite, Rh. = rhodochrosite, Dol. = dolomite. For alkali feldspar and phillipsite, a bar across thecolumn indicates presence [these minerals could not be determined quantitatively]. A half bar indicates minor presence [tiny peaks].)
332
100-
200-
300-
400-
K(002)(001)0 1
I(002)(001)0 1
X-RAY MINERALOGY OF MIOCENE AND OLDER SEDIMENTS
Clay mineralogy Crystallinity
(Hjw) {v/p)
60 80 100 2 4 6 8 020 40i i I r
\
' /4
I 7
77
-C-IK
C —I K-\
C I-K-
C—I K-\
C— I— KC - l - K-
C-l r K-
C-l—KI
C—} ×
- S —
- S -
-s-- 8 -
- s -— s—s--s—
-s—
i•i
Figure 4. Site 541, mineralogy of <2-µm samples. (Horizontal lines represent sample positions.(002)/(001) peak ratios calculated from bulk mineralogy diffractograms. K = kaolinite, I =illite, S = smectite, C = chlorite, P - palygorskite, M = mixed-layer clays. H, w, v, andp aredefined in the text and shown in Figure 2D.)
333
C. J. PUDSEY
2 0 0 -
3 0 0 -
Quartz Plag. Plag. ul Amp.(W/W) <
20 0 20 0 10 0 10 0π r
Total clay Calcite Rh. Age
20 40 60 80—r~—r
20 40 0 10π 1 1 r
Figure 5. Site 542, mineralogy of bulk samples. (See Fig. 3 for explanation of symbols.)
K Iα> (002) (002)<3 (001) (001)
0 1 0 1
Clay minerals Crystallinity Age
vlp)
200-
300-
Figure 6. Site 542, mineralogy of <2-µm fraction samples. (For an explanation of the symbolssee Fig. 4.)
1976): it is very commonly associated with opal-CT(Stonecipher, 1976; Fig. 6). With increasing time clino-ptilolite may eventually be converted to K-feldspar, whichdoes in fact appear at the very base of Hole 543A (Hay,1966; Fig. 7). However, K-feldspar is also an authigenicreplacement of calcic Plagioclase phenocrysts in basaltsimmediately beneath these sediments (see Natland, thisvolume; and Bougault, et al., this volume) and thereforemay reflect diagenetic conditions that also modified thesediments.
Opal-CT
Opal-CT is formed by crystallization of amorphoussilica that is biogenic (or volcanic) in origin. On X-raydiffractograms it is characterized by a broad peak cen-tered at 21.8°20 (Fig. 2D), commonly with a small"shoulder" on the low 20 side resulting from some tri-dymite stacking in the opal. X-ray amorphous opal-Acan give a very broad hump in about the same position(Jones and Segnit, 1971), but this was not seen in the
334
X-RAY MINERALOGY OF MIOCENE AND OLDER SEDIMENTS
Quartz Plag. Plag. A. F. Amp. Clin. Clin. Opal-ct Total clay Calcite Rh. Dol. Age(Hlw) (H/W)TÚ
0 10 0 10 0 10 0 10 0 10 0 10 0 10 £ 10 20 0 20 40 60 80 0 20 40 0 10 0 101 ' ' T~I r~i r~i r~i r 1 1 1 r "I 1 r ~π—r
100-
r
30 0 -
400-
"I—I
EPaleo
cene
Figure 7. Site 543, mineralogy of bulk samples. (For an explanation of the symbols, see Fig. 3. Alkali feldspar was determined quantitatively below380 m only; above 380 m only its presence was recorded.)
Leg 78A samples. Biogenic amorphous silica transformsin due course to opal-CT and then to quartz (e.g., Pi-sciotto 1981). The controls on the time taken for thesediagenetic reactions include temperature, pH, and theconcentration of magnesium and hydroxyl ions (Kast-
ner, Keene, and Gieskes 1977; Kastner and Gieskes, inpress).
At Site 543 the occurrence of opal-CT is clearly re-lated to that of radiolarians, in that the deepest level ofboth is at 330 m. However, the stratigraphically higher
335
C. J. PUDSEY
o-
K I(002) (002)(001) (001)0 1 0 1
Clay minerals
100-
E 200-
I
300-
400-
20 40 60
Crystallinity
(H/w) W/p)
0 2 4 6 8 10 12 14 16 18 20 22 0 2
Age
Figure 8. Site 543, mineralogy of <2-µm fraction samples. (See Fig. 4 for an explanation of the symbols.)
levels of the radiolarian muds do not contain opal-CTthe transformation from opal-A is very slow at low tem-peratures and has apparently required the 40 Ma sincethe late Eocene to take place. Scanning electron micro-graphs reveal opal-CT in the form of lepispheres grow-
ing on radiolarian tests (Plate 1, Figs. 1 and 2). Theseradiolarians do not seem significantly dissolved, thoughmany are broken (N. Renz, personal communication,1982); they may be important as sites of nucleation aswell as sources of silica. The silica in the lepispheres
336
Quartz
20
Plagioclase< 20
Amp.
0 10 0 20
Total clay
40 60 80
Calcite
20 40
Rhod.
10 0 20
Clay minerals
40 60 100
\\/
.—~Λ•
\
Pliocene
Miocene
X -
•
± 541 Tectonic Unit A 541Tectonic Unit B
R i
O X
o •
Ka + l|. Sm.
Ka+ll. Sm.
542
A
ii/ n
/ \.
\
Figure 9. Comparison of mineralogy of Miocene sections cored at Sites 541, 542, and 543. (Plag. = Plagioclase, Amp. = amphibole. In the clay minerals column only smectite is plottedseparately. Kaolinite + illite are on the left, chlorite + palygorskite + mixed-layer clays are on the right.)
- J
Onwz>zσorσaCΛ
ma
zGO
C. J. PUDSEY
may have been derived from very delicate parts of theskeletons such as spines or from less robust species ofradiolarians that have now completely dissolved.
Opal-CT is most abundant where there are both radi-olarians and palygorskite. Palygorskite is a magnesium-rich clay, and whiskers of it may have been additionalsites of silica nucleation. Further transformation of opal-CT to quartz is suggested by the increase in quartz con-tent of the sediments below 330 m. The presence of car-bonate below 380 m has no particular effect on the pro-portions of silica minerals (contrast Lancelot, 1973).
DolomiteDolomite, conspicuous as scattered rhombs in smear
slides as well as in diffractograms of the basal sediments, iseuhedral and almost certainly authigenic (Plate 1, Fig. 3).Calcareous nannofossils in these lowest cores show strongdissolution (Bergen, this volume) and probably acted asa source of calcium. The extra magnesium source in thisnear-ridge setting could have been hydrothermal or aresult of alteration of basalt.
Authigenic Clays—Smectite and PalygorskiteWhereas the poorly crystalline (low v/p) smectite in
the near-surface sediments may be detrital, the abun-dant and well-crystallized smectite in the Miocene andolder deposits is probably authigenic. It may have beenderived both by neoformation of detrital smectite (Du-noyer de Segonzac, 1970) and by alteration of unstablevolcanic debris.
Palygorskite was first reported from deep-sea sedi-ments by Bonatti and Joensuu (1968) and has since beendescribed from several Atlantic DSDP sites, for ex-ample, Site 12 (Peterson, Edgar et al., 1970), Site 119(Laughton, Berggren, et al., 1972), Sites 135 to 140,(von Rad and Rösch, 1972), Sites 327 and 328 (Barker,Dalziel, et al., 1977), Sites 332 to 335 (Aumento, Mel-son, et al., 1977), Site 398 (Sibuet, Ryan, et al., 1979),Sites 415 and 416 (Lancelot, Winterer, et al., 1980) andSites 417 and 418 (Mann and Muller, 1980c). At Sites398, 415, and 416 palygorskite was thought to be detri-tal but it has otherwise been considered authigenic. Anunusual occurrence in a dredge haul quite near Site 543was interpreted by Bowles et al. (1971) to have been pre-cipitated directly from hydrothermal solution. Althoughpalygorskite has not been synthesized and little is knownof its genesis, it is believed to form during the alterationof volcanic glass, via a smectite intermediate:
Smectite + silica + (Ca, Mg) palygorskite+ clinoptilolite (Bonattiand Joensuu, 1968)
Silica may be derived from (1) alteration of silicic ashes,(2) dissolution of biogenic opal, and (3) rivers drainingregions of lateritic weathering (von Rad and Rösch,1972).
Calcium and magnesium are derived from seawater:magnesium may also be hydrothermal, or derived fromalteration of serpentine. The formation of palygorskiterather than sepiolite implies that (Mg+ +) and (Al+ + +)
must have been particularly high. An origin from de-graded smectite is consistent with the observed antithet-ic relation in the abundance of the two minerals (Fig. 8).Scanning electron micrographs reveal the typical fibroushabit of palygorskite (Plate 1, Figs. 3 and 4).
Evolution of the Sedimentary SequenceThe sedimentary and diagenetic history of Site 543 as
it passed from ridge crest through open ocean to a near-arc setting is summarized in Fig. 10.
Effect of Mineralogy on Physical PropertiesAn interesting feature is apparent from a plot of phys-
ical properties against mineralogy for Site 543 (Fig. 11).The proportion of smectite has a strong effect on densi-ty, porosity, water content, and shear strength. This ef-fect is not surprising, as smectite is an expandable min-eral, able to take up a variable quantity of water be-tween the layers: thus the smectite-rich muds are lessdense, more porous, wetter, and weaker than the under-lying radiolarian muds. None of the other minerals hasa very marked effect on physical properties, except thatquartz-rich sediments at the top of the section are rela-tively dense. It is curious that the opal-CT-rich mudshave such similar physical properties to the lithologiesabove and below. Core recovery was rather patchy in thecristobalitic interval, which may be linked with the in-cipient formation of chert.
At Sites 541 and 542 the variation in amount ofsmectite is related to the amount and style of deforma-tion noted in the cores (see Cowan et al., this volume).In particular, the smectite-rich intervals below 380 m atSite 541 and below 250 m at Site 542 correspond exactlyto the zones of pervasively sheared, scaly clays. Simi-larly, brecciation in Tectonic Unit A of Site 541 corre-sponds to the increase in smectite below 230 m. It is pos-sible that very high pore fluid pressures in subductionzones may affect the amount of water within the crystalstructure, and the presence of expandable clays may becrucial in the formation of décollement zones.
CONCLUSIONS AND SPECULATIONSThe sediments cored at Site 543 correspond quite well
to the distribution patterns of deep North Atlantic sedi-ments described by Thiede et al. (1981). The only differ-ences are in the Neogene sediments, and these are readi-ly explained by the increasing proximity of the deposi-tional sites to volcanic arc sources at that time. Thiedeet al. report a high proportion of terrigenous compo-nents from 70 to 50 Ma and 30 to 10 Ma, high biogenicsilica from 50 to 40 Ma, and moderately high calcitefrom 25 Ma onwards, for basins of about 5.5 km depth.These variations are related to organic productivity, dis-solution rates of calcite and opal (largely functions oftemperature and dissolved oxygen), and sediment sup-ply from the continents (a function of climate, tectonicprocesses, and sea level).
The décollement zone, which divides subducted fromaccreted sediment, is at the boundary between smectite-rich and radiolarian-rich, smectite-poor muds. Thus inthe Leg 78A area the sediments mainly derived from the
338
X-RAY MINERALOGY OF MIOCENE AND OLDER SEDIMENTS
Mid-AtlanticRidge
Sea level
75 Ma
No terrigenous input
Sea level
40 Ma
Ash falls
'
4 Ma
Figure 10. Evolution of Site 543. (Oceanic sediments shown in stipple, Site 543 in black. Vertical exaggeration 1000 × . Insets show Atlantic recon-structions [from Berger and Winterer, 1974] with the site marked. A. Campanian, 75 Ma. (Ridge-crest setting; iron and manganese-rich precipitates,hydrothermal activity, alteration of basalts.) B. Late Eocene, 40 Ma. (Open ocean [red clay facies] below CCD. Formation of zeolites. Deposi-tion of opaline silica.) C. Pliocene, 4 Ma. (Glomar Challenger in her future position. Diagenesis of amorphous silica to opal-CT, zeolites to al-kali feldspar. Neoformation of clays, alteration of volcanic glass.)
339
èDensity (g/cm3)
1.4 1.6 1.8
Porosity (%) Water content (wet wt. %) Shear strength (kPa)
50 60 70 30 40 50 0 50 100
Quartz Clin. Cristobalite
0 20 0 10 0 20 0
Clay minerals O
100
200
300
400
•
••
••
•
•••
•
•
Ka + ll
100%
Figure 11. Physical properties and principal mineralogy of sediments from Site 543. (See text for discussion. Ka. = kaolinite, Sm. = smectite, II. = illite, and P = palygorskite.
X-RAY MINERALOGY OF MIOCENE AND OLDER SEDIMENTS
arc, that is, the Neogene ashy muds, are not recycled inthe subduction zone. It is the muds, derived from open-ocean, continent, or ocean-crust sources, that go down,perhaps to be partially melted and contribute to arc mag-mas. In general, if the composition of arc magmas is tosome extent related to that of subducting oceanic sedi-ment, it might be possible to link temporal variations inarc chemistry to changing hydrographic-tectonic condi-tions in the ocean.
ACKNOWLEDGMENTS
I am grateful to The Natural Environment Research Council(NERC) for financial support and to Professor J. Tarney for labora-tory facilities during the shore-based study. I thank Rob Wilson andGeorge McTurk for technical assistance and my colleagues at Leicesterfor useful discussion. The manuscript was improved by commentsfrom Drs. H. G. Reading, J. D. Hudson, J. R. Hein, and J. Schoon-maker. Dr. M. Kastner advised on silica diagenesis. Janet Wood typedthe final manuscript.
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Bonatti, E., and Joensuu, O., 1968. Palygorskite from Atlantic deep-sea sediments. Am. Mineral., 53:975-983.
Bowles, F. A., Angino, E. A., Hosterman, J. W., and Galle, O.K.1971. Precipitation of deep-sea palygorskite and sepiolite. EarthPlanet. Sci. Lett., 11:324-332.
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, 1975. Origin of Horizon A: Clarification of viewpoint. Sci-ence, 188:1221.
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Jones, J. B., and Segnit, E. R., 1971. The nature of opal. I. Nomen-clature and constituent phases. J. Geol. Soc. Aust., 18:57-68.
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Kastner, M., Keene, J. B., and Gieskes, J. M., 1977. Diagenesis of sili-ceous oozes—1. Chemical controls on the rate of opal-A to opal-CT transformation—an experimental study. Geochim. Cosmochim.Acta, 41:1041-1059.
Lancelot, Y, 1973. Chert and silica diagenesis in sediments from theCentral Pacific. In Winterer, E. L., Ewing, J. I., et al., Init. Repts.DSDP, 17: Washington (U.S. Govt. Printing Office), 377-405.
Lancelot, Y., Winterer, E. L., et al., 1980. Init. Repts. DSDP, 50:Washington (U.S. Govt. Printing Office).
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Lonsdale, P. F., 1975. Sedimentation and tectonic modification of theSamoan archipelagic apron. Am. Assoc. Petrol. Geol. Bull., 59:780-798.
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, 1980b. Mineralogy of the sedimentary sections encounteredon Leg 55 (Sites 430 through 433) based on X-ray diffraction. InJackson, E. D., Koisumi, I., et al., Init. Repts. DSDP, 55: Wash-ington (U.S. Govt. Printing Office), 857-859.
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Date of Initial Receipt: May 27, 1982Date of Acceptance: March 3,1983
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Plate 1. Scanning electron micrographs showing authigenic minerals. 1. Part of a radiolarian test from the Miocene. (Diagenesis has not signifi-cantly affected this sample. Sample 541-50.CC. Scale bar is 1 µm.) 2. Part of a radiolarian test from the upper Eocene. (Extensive silica precipi-tation in the form of lepispheres of opal-CT. Compare the corroded radiolarian with that shown in Fig. 1. Sample 543-31-2, 8 cm. Scale bar is5 µm.) 3. Authigenic dolomite (rhombs) and palygorskite (fibers) from the Cretaceous. (Sample 543A-9-1, 38 cm. Scale bar is 1 µm.) 4.Authigenic palygorskite fibers from the Cretaceous. (Curved object is part of a calcareous microfossil on which some of the fibers have nucleat-ed. Sample 543A-9-1, 38 cm. Scale bar is 1 µm.)
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