30. mineralogy and geochemistry of upper …deepseadrilling.org/48/volume/dsdp48_30.pdf · 30....

30. MINERALOGY AND GEOCHEMISTRY OF UPPER CRETACEOUS AND CENOZOICSEDIMENTS FROM NORTH BISCAY BAY AND ROCKALL PLATEAU (EASTERN NORTH

ATLANTIC), DSDP LEG 48

Pierre Debrabant,1 Hervé Chamley,1 Janine Foulon,1 and Henri Maillot1

ABSTRACT

One hundred and thirty samples of various Cretaceous to Pleistocenesediments from DSDP Sites 399 and 400 (north Biscay Bay) and 403 to406 (south Rockall Plateau) were studied for their clay mineralogy andbulk geochemistry. Both geographical areas were compared with eachother, then compared with the area south of Biscay Bay (Site 398, DSDPLeg 47).

In Aptian to Paleogene sediments the clay fraction is dominated bysmectite. In Biscay Bay the mineral is rich in Al-Fe and chiefly expressesthe erosion of soils formed under arid-warm climatic conditions withcontrasted wet and dry seasons. Two stages with fibrous clays occur inAlbian (attapulgite, palygorskite) and Paleocene/Eocene (attapulgite,sepiolite) sediments, suggesting the existence of wet and warm periodsand of nearly closed marginal basins. In the south part of the Iceland Seathe smectite is of a Fe-Mg type and mainly results from thetransformation of volcanic material, probably in subaerial conditions. Inpost-Paleogene sediments the primary minerals (illite, chlorite, quartz,feldspars) and mixed-layers increase irregularly, expressing both periodiccontinental cooling, the major stages of which occurred in late Mioceneand Pleistocene times, an increase in the intensity of the north/southoceanic current. In general, kaolinite and fibrous clays are less abundant,smectite more abundant, and coolings appear to have been stronger inPleistocene time in the more northern drill sites.

The diagenetic changes are unusual. There is no modification withdepth of burial and no basic chemical sedimentation. An in situ evolutionin volcanic environment appears locally in Rockall sites only. The "blackshales" facies does not induce any argillaceous modification in northBiscay Bay and affects only some chemical elements such as Ca, Fe, andMn. Ionic removing and trapping can locally induce the autochthonouscrystallization of cristobalite-tridymite and of some zeolites.

Site 402 of Biscay Bay, not as deep as Sites 398, 399, 400, and 401,seems to be aside from open sea circulations, which might affectespecially the distribution of Mn in Albian to lower Miocene sedimentsand, perhaps, as well, that of fibrous clays. The chemical andmineralogical sequence observed from Site 402 to Sites 399 and 400 inUpper Cretaceous sediments suggests a deltaic paleoenvironment, which,seaward is marked by a geochemical homogenization, a decrease of thecarbonaceous phase, and an increase of small-sized, highly buoyantminerals (smectite, fibrous clays). The sudden arrival of detrital primaryminerals in the Campanian/Maestrichtian suggests the establishment of adeep-sea circulation perhaps related to the opening of the western NorthAtlantic, or to a tectonic event or a marine regression. South of RockallPlateau during early Cenozoic time, the chemical and mineralogicalenvironment was more pelagic than in Biscay Bay, especially at Sites 405and 406 where the volcaniclastic influence was not as strong as at Sites403 and 404. The ending of the major volcanic activity in mid-Eocenewas followed by an increase in the pelagic supply, probably related to thewidening of the eastern North Atlantic.

INTRODUCTIONLeg 48 of the Deep Sea Drilling Project was located in the

'Lille University, 59650 Villeneuve d'Ascq, France. eastern North Atlantic Ocean, partly in Biscay Bay and

703

P. DEBRABANT, H. CHAMLEY, J. FOULON, H. MAILLOT

partly on the Rockall Plateau. Four holes were drilled ineach area, Sites 399 to 402 and Sites 403 to 406,respectively (Figure 1); their general character is asfollows (for detailed description see Site Chapters, thisvolume).

Bay of Biscay

Site 399 (47°23.4'N, 09° 13.3'W; 4399 m water depth;72.5 m sub-bottom depth); located at the foot of theMeriadzek Terrace Escarpment; Pleistocene section.

Site 400 (47°22.90'N, 09°11.90'W; 4399 m water depth;768.5 m sub-bottom depth); located near to Site 399 at thefoot of the Meriadzek Terrace Escarpment; section fromPleistocene to lowermost Aptian.

Site 401 (47°25.65'N, 08°48.62'W; 2495 m water depth;341 m sub-bottom depth); located at the edge of theMeriadzek Terrace on an elevated fault block; section fromPleistocene to Kimmeridgian, with major hiatuses betweenUpper Jurassic and Upper Cretaceous, and between Eoceneand Pleistocene.

Site 402 (47°52.48'N, 08°50.44'W; 2339.5 m waterdepth; 469.5 m sub-bottom depth); located in a canyoncutting a spur on the Armorican slope; section fromPleistocene to Aptian with a major hiatus between Albianand middle Eocene.

Rockall Plateau

Four sites were drilled in two sectors: Sites 403 and 404lie in the southwest margin of the plateau in 2317 to 2322

60°

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Figure 1. Location map.

meters water depths, at 56°08.13'N-23°17.64'W and56°03.13'N-23°14.95'W, respectively. The sub-bottomdepths of the holes were 489 meters and 389 meters,respectively, the oldest sediment cored being upperPaleocene in both cases. A major gap exists from middleEocene to upper Miocene, except at Site 403 where a thininterval of Oligocene sediments is present.

Sites 405 and 406 lie south of the east-west scarp formingpart of the southwest margin of the plateau in 2958 meterand 2907 meter water depths at 55°20.18'N-22°03.49'Wand 55o15.50'N-22°05.41'W, respectively. Sub-bottomdepths are 407 meters and 831.5 meters, respectively, thebottom sediments at each site being of early Eocene age.Site 405 shows a major hiatus between lower Eocene andupper Miocene. At Site 406 only middle Eocene and lowerOligocene are missing.

About 130 samples from the eight sites were studied forthe purpose of comparing the clay mineralogy, bulkmineralogy (locally), and the bulk geochemistry. Resultsare presented on Figures 2 to 8 and will be commentedbriefly upon in stratigraphical order. Comparison first ismade among all sites of each area, then between BiscayBay, Rockall Plateau, and the Vigo Seamounts south ofBiscay Bay (DSDP Leg 47B, Chamley et al., in press).Note that complementary data and comments on mineralogyare given about Biscay Bay by Cassat (this volume) andabout Rockall Plateau by Latouche (this volume).

METHODS OF INVESTIGATION

Clay Mineralogy

All of the samples were submitted to X-ray diffractionanalysis on <2 µ,m decalcified particles. We examinedsome of <8 µm non-calcareous fractions by transmissionelectromicroscopy. Differential thermal analyses were madeon <2 µm non-calcareous fractions of selected samplesoriginating chiefly from sites of Rockall Plateau. In theX-ray diffraction method, the samples were dissociated inwater then decarbonized in JV5 hydrochloric acid; the excessacid was removed by successive centrifugations. A mic-ron omogenization led to deflocculation. The <2 µm frac-tion was collected by decantation (Stokes law), thenoriented aggregates were made on glass slides. A CGR θ 60diffractometer (copper radiation focused by a quartzcurved-crystal monochromator) was used to run the X-raydiffraction scans at 1°2 θ/mn. A receiving slit of 1.25 mmallowed a better determination of mixed-layer minerals.Four passages are carried out as follows: (a) from 1° to 15°θ on natural sample; (b) from 1° to 7° θ on glycolatedsample; (c) from 1° to 7° θ on sample heated for two hoursat 490°C; (d) from 12° to 14° θ on hydrazine-hydratedsample.

Semiquantitative evaluations were based on the peakheights and areas (Chamley, 1971). The height of 001 illiteand chlorite peaks (diagram of glycolated sample) weretaken as references. Compared to these values, smectite,attapulgite, and irregular mixed-layers were corrected inaddition to peak heights whereas well crystallized kaolinitewas corrected in diminution. The balance between chloriteand kaolinite results from peak heights ratio (respectively3.54 Å and 3.58 Å): when this ratio is 1, the amount of

704

UPPER CRETACEOUS SEDIMENTS

chlorite is assumed to be twice as much as that of kaolinite.Final data are given in percentages, the relative error being±5 per cent.

GEOCHEMISTRY

The samples were dried at 105°C, then ground andhomogenized; 0.2 g was submitted to alkaline fusion, thensolubilized by HC1, and diluted to 100 ml. The treatmentallows gravimetric determination of Siθ2, colorimetric de-termination of P2O5, and spectrophotometric determinationof Fe, CaO, MgO, AI2O3 (by atomic absorption). Another 2g of each sample were submitted to fluoroperchloric treat-ment, then solubilized by HC1 and diluted to 100 ml. Thedilution is used for the colorimetric analysis of TiC»2, thespectrophotometric analysis of Na2θ and K2O (by emis-sion) and also for traces Mn, Zn, Li, Ni, Cr, Sr, Co, Cu,Pb, V (by atomic absorption).

The emission and atomic absorption apparatus was aType 503 Perkin-Elmer spectrophotometer using the follow-ing methods—base solution for major elements or com-plex synthetic solution for trace elements to which 5 percent of lanthane in hydrochloric solution is added.

BISCAY BAY—RESULTS

Sites 399 and 400; Figure 2; Table 1

Aptian

Aptian and Albian sediments are carbonaceousmudstone, marly nannofossil chalk, and calcareous clay-stone (lithologic Unit 4). In some levels smectite is nearlyexclusive, accompanied by rather high contents of Fe andMn rather than K. Particular levels locally exist which showreduction (400A-74-1, 45 cm: siderite, rhodochrosite) oroxidation (400A-68-2, 49 cm): irregular mixed-layersillite-smectite and chlorite-smectite, goethite, subamorph-ous iron oxides); both types show an antagonism betweenSiO2/Al2O3 and AI2O3/K2O ratios.

Albian

Smectite is the main clay mineral, as it is in all sedimentsup to those of the Miocene at least. Associate minerals arechlorite, illite, kaolinite, attapulgite (Plate 1; Figures 1,2),quartz, feldspars, zeolites (clinoptilolite). Mn and Ca in-crease whereas the Sr/CaO ratio is variable.

Campanian/Maestrichtian

Four levels (-61-3, 136 cm, -61-1, 26 cm, -60-3, 11 cm,-60-1, 4 cm) studied for clay mineralogy in the nannofossilchalk of Unit 3 show the presence, besides smectite, of 10to 35 per cent illite, and a large amount of quartz andfeldspars. Chlorite is more abundant than below and above;amphiboles are locally present in the <2 µm fraction.

Upper Paleocene, Eocene

Siliceous mudstone, calcareous clay stone, and marlychalk from the base of lithologic Unit 2 contain a largeamount of smectite. Kaolinite increases upwards as well asthe AI2O3/K2O ratio, whereas CaO decreases. Attapulgiteand/or sepiolite are present, especially in middle Eocenesediments. Zeolites occur rarely, then disappear in the

middle Eocene interval. MgO/K2θ and Sr/CaO ratiosincrease, with a maximum at the Eocene/Oligoceneboundary.

Oligocene, Lower Miocene

The siliceous marly nannofossil chalk and mudstone ofthe upper part of Unit 2 show several trends upwards fromthe Eocene/Oligocene boundary: stabilization of Sr/CaOratio; increase of Ca, Mn, illite, and kaolinite; appearance,then development of chlorite; disappearance of fibrous clayson X-ray diagrams, fibers subsisting on electromicrographsin small quantities only (Plate 1; Figure 5); in middleMiocene, end of metallization.

Upper Miocene to Pleistocene

Lithologic Unit 1, nannofossil and marly nannofossilooze, and calcareous mud, is marked by an irregulardecrease of smectite and increase in the abundance of illite,chlorite, followed in turn by irregular mixed-layers, quartz,feldspars, amphiboles, and goethite. Attapulgite appears insmall quantities on micrographs. At the base of the zone is ageochemical break manifested by increase of Ca and Sr, anddecrease of Mn, SiO2/Al2θ3 and AI2O3/K2O. The Pleisto-cene is marked by an increase in SiC»2, Ti, Cr, V, and Zncontents.

Site 401; Figure 3; Table 2

Kimmeridgian

The Upper Jurassic limestone (Unit 4) is poor in clayfraction; it could not be extracted in sufficient amounts forX-ray analysis. The richness of Ca is followed by highMgO/K2θ ratios, low Sr/CaO ratios.

Campanian

Illite and chlorite constitute more than half of the clayfraction (19-1, 125 cm) and are marked by relative highcontents of Al and K. CaCOβ is abundant; Sr/CaO ratiocontinues to be low. Phosphates are locally individualized.A resampling and study of Cores 18 and 19 (Campanian andMaestrichtian: -18-1, 50 cm, -18-2, 49 cm, -18-2, 88 cm,-19-1, 8 cm, -19-1, 50 cm, -19-1, 90 cm) lead to thefollowing average clay mineralogy: chlorite 5 per cent, illite35 per cent, irregular mixed-layers illite-smectite andchlorite-smectite 5 per cent, smectite 45 per cent, kaolinite5 per cent, attapulgite 5 per cent, sepiolite traces; associateminerals are abundant quartz, feldspars and, locally,amphiboles. Illite and chlorite particles are often large sizedand well edged (Plate 1; Figure 3).

Paleocene, Eocene

The nannofossil, marly or calcareous chalks of lithologicUnits 3 and 2 are marked by abundant smectite, with locallyrather abundant attapulgite, sepiolite, and clinoptilolite(resample of -17-1, 50 cm, Plate 1, Figure 4). Illite,chlorite, SiCh/AhCb, and Sr/CaO ratios increase more orless progressively towards upper Eocene. In lowerPaleocene a large increase of Sr and Mn occurs, the latterelement showing a maximum in middle Eocene wheresepiolite content is the larger.

705

BISCAY BAY SITES 399 400 MINERALOGICAL AND GEOCHEMICAL BULK

CLAY MINERALS ASSOCIATED MINERALS SAMPLES

100% O F AmGoPy Ze

CLAY MINERALS: C CHLORITE K[ - | KAOLINITE

VJ|] ILLITE Y/A R 0 U G H VALUE

MIXED LAYERS \_ ~^\ ATTAPULGITE (Low or very low clay content)

SMECTITE W$U SEPIOLITE

ASSOCIATED MINERALS:

Q = QuartzAm = AmphibolesPy = PyriteF = FeldsparsGo = GoethiteZe = Zeolite

• very rareo abundant

O very abundant

Figure 2. Sites 399 and 400, clay mineralogy and bulk geochemistry.


Sample

(Interval in cm)

Site 399

1-4, 95

2-2,68

Site 400

1-1, 102

3-3, 13

5-1,80

7-2, 45

9-1, 11

11-2,5

13-6,45

20-5, 38

23-4, 50

24-6, 107

26-3, 88

31-1,61

36-2, 33

37-1, 82

37-4, 109

39-2, 48

43-3, 48

44-1, 102

45-2, 24

46-5, 59

47-6, 34

48-2, 111

49-1, 79

51-5,74

52-1, 134

53-1, 12

55-2, 77

57-2, 30

59-1,33

62-2, 12

64-2, 56

65-1, 144

68-2, 49

69-1, 106

74-1,45

SiO2

48.35

46.55

34.80

45.90

19.85

26.50

34.40.

21.25

24.15

24.30

29.75

13.65

15.55

4.50

13.10

17.00

37.05

24.45

47.10

22.95

25.90

26.25

57.65

58.25

44.35

46.65

42.65

53.50

30.25

42.10

18.45

28.65

37.15

56.05

46.10

46.95

20.55

A 12O

3

13.74

13.98

9.63

13.10

6.38

9.56

13.79

6.61

7.51

8.78

9.91

4.04

4.98

1.59

4.01

5.19

11.05

6.99

9.56

4.16

4.96

5.94

13.05

14.64

10.00

10.86

11.09

10.58

9.92

12.75

4.48

6.50

9.63

15.71

6.14

13.23

5.76

CaO

14.10

13.82

24.70

14.42

37.15

29.54

22.95

34.48

33.13

33.69

25.38

42.18

39.73

49.18

43.05

39.20

23.06

33.18

15.93

37.92

33.78

31.15

5.60

9.09

17.33

18.82

16.35

18.57

23.65

22.21

36.75

28.61

28.47

4.82

10.46

9.76

20.64

MgO

2.86

2.61

1.67

2.00

1.18

1.42

1.35

1 31

1.37

1.34

1.78

0.91

1.01

0.44

0.90

1.12

1.77

1.36

1.68

0.90

0.99

1.29

1.54

1.67

1.54

1.59

1.78

1.68

1.31

1.65

1.26

0.95

1.62

2.03

1.56

1.79

3.44

%Λ

Na2O

1.65

1.35

1.30

1.33

1.10

1.10

•

:

:

.30

.00

.38

.00

1.30

0.68

0.73

0.65

0.58

0.65

1.08

0.70

1.15

0.65

0.73

0.75

1.28

1.30

1.03

1.18

1.13

1.30

1.15

1.15

0.73

1.18

1.15

1.73

1.35

1.54

0.73

TABLE 1Geochemical Data, Sites 339 and 400

K2O

2.71

2.35

2.01

2.38

1.35

1.55

2.01

1.28

1.50

1.50

1.68

0.78

0.99

0.31

0.76

0.93

1.93

1.21

1.61

0.83

0.95

1.01

1.88

2.281.74

2.01

2.03

1.741.70

2.13

0.78

1.38

1.89

2.50

2.08

0.84

0.99

TiO2

0.58

0.63

0.33

0.63

0.32

0.44

0.55

0.30

0.30

0.22

0.40

0.18

0.21

0.08

0.17

0.19

0.28

0.27

0.31

0.15

0.21

0.25

0.47

0.46

1.43

0.35

0.45

0.30

0.35

0.57

0.17

0.22

0.33

0.54

0.20

0.47

0.21

P2°5

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

Fe S

3.40

4.00

2.43

3 00

1.55

2.08

2.03

1.87

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.14

2.23

1.45

2.47

0.93

1.19

1.90

3.28

3.33

2.38

2.85

3.00

2.45

2.60

3.60

1.53

1.17

2.01

5.05

19.70

3.00

14.03

/ Mn

740

560

570

400

1030

510

275

390

650

400

600

680

400

280

1200

1450

780

1100

750

1340

1470

4100

1450

1720

2050

1680

1650

2250

1950

1350

3050

1750

630

380

500

370

3.73a

Zn

150

160

200

94

180

75

110

126

130

170

88

49

62

40

53

61

160

60

94

165

73

87

156

95

99

170

130

170

200

170

63

154

140

102

120

83

280

Li

37

36

31

38

21

31

41

24

27

30

33

15

19

7

14

17

34

21

34

16

17

21

32

35

24

25

26

33

21

32

14

11

19

24

10

18

12

Ni

35

34

27

22

26

32

34

29

31

34

24

32

19

31

37

83

38

61

47

44

58

67

50

47

51

66

53

57

48

58

36

66

150

44

2931

pgm

Cr

96

110

68

100

42

38

80

36

57

61

45

41

51

35

34

50

64

54

49

33

40

18

92

61

84

69

71

4747

98

33

42

68

110

46

52

43

Sr

200

180

750

430

1210

930

790

1150

1010

1140

900

1200

1300

1370

1300

1150

970

1100

600

1060

930

950

470

410

720

650

670

530

900

460

910

840

500

460

190

470

270

Co

15

13

11

14

11

12

9

16

13

13

13

12

13

7

14

17

26

13

21

19

16

31

24

17

25

19

30

20

24

18

26

9

13

32

6

5

19

Cu

22

18

33

36

28

47

48

63

40

28

48

21

23

17

35

54

37

31

35

16

35

97

67

53

26

63

22

40

49

18

68

92

77

68

8

23

72

Pb

22

23

27

28

26

37

33

39

20

23

34

24

31

39

32

39

26

24

29

23

32

47

18

12

33

18

35

21

45

26

2826

16

26

32

46

39

V ̂

95

110

89

120

58

66

100

49

63

58

59

23

24

5

23

36

63

48

60

16

32

37

63

89

94

68

84

53

37

95

17

53

53

105

210

63

37

aData in %; rhodocrosite is present.

Pleistocene

The marly calcareous ooze of Unit 1 contains illite,chlorite, irregular mixed-layers, relatively abundantassociate primary minerals, accompanied by high values ofSiθ2, K2O, Ti, Cr, and low contents of Sr and Mn.


Aptian, Albian

In the carbonaceous sediments of lithologic Units 3 and2, smectite is abundant and well crystallized, followed byillite and kaolinite. Clinoptilolite occurs in Albian only, asat Site 400. The silty samples show an increase of illite andkaolinite content. Ca is high and variable. Pyrite is presentespecially in the lower Albian "black shales," Geochem-ical changes occur in upper Albian, where smectite is veryabundant, manifested by stabilization of AI2O3/K2O ratio,low content of transition elements (Mn, Co, Cu), and localaugmentation of SiC^/A^Oβ ratios.

Cenomanian

At level 9A-1, 40 cm, where the Albian/Cenomaniantransition is located, clay minerals are rare; silicates

(cristobalite) are abundant, accompanied by relative highvalues of Co and MgO/K2θ. Minimal values occur in Fe,Ni, Cu, Pb and V contents.

Middle Eocene

In the nannofossil chalk to siliceous marly nannofossilchalk of Sub-unit IB, an increase of illite occurs upwards,whereas chlorite, irregular mixed-layers, kaolinite, quartz,and feldspars appear in the clay fraction. Ca and Mnincrease; AI2O3 is little present. Zeolites are absent andfibrous clays are absent or rare.

Pliocene, Pleistocene

The marly calcareous ooze of Sub-unit 1A, as at othersites, shows decreased smectite whereas primary or littleweathered minerals such as illite, chlorite, mixed-layers,quartz, and feldspars increase.

DISCUSSION

General Mineralogical and Geochemical Evolution

No progressive and continuous change is observed alongthe mineral and chemical assemblages of the sediments

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from Upper Jurassic to Pleistocene recovered from Sites399, 400, 401 and 402. This demonstrates the absence ofdiagenetic evolution associated with depth of burial andconfirms the results obtained from other sites in the NorthAtlantic margins (von Rad and Rösch, 1972; Chamley andGiroud d'Argoud, in press; Chamley et al., in press; Pas-touret et al., 1978, etc.).

From Cretaceous to upper Eocene the sediments aremarked by an abundant smectite mineral phase associatedwith Al, Fe, Ti, V, Ca, Li. That phase is virtuallyindependent of lithology, and is not related to volcanicmaterials or chemical sediments. An essentially detritalorigin must be envisaged, which must have been derivedeither from ancient continental rocks subjected to erosion ornewly formed continental soils. Both sources could haveprovided the sediment, but soils would most likely havebeen the principal one on account of the vulnerability ofsuperficial formations and of the broad development of soilsin Cretaceous depressed landscapes.

Such a mainly pedological origin of the smectitesdeposited in Biscay Bay suggests that the continentalclimate was warm and arid with alternating dry and wetseasons (Paquet, 1969). The detrital origin of smectite issupported by the data of accompanying rare earth elementsof typical continental assemblage (analysis of C. Courtois,Paris).

From upper Eocene to Pleistocene the sediments showirregular but continuous increase in primary clayey minerals(illite, chlorite associated with AI2O3 and K2O), togetherwith irregular mixed-layers of low degradation andnon-argillaceous detrital minerals (quartz, feldspars, and,in places, amphiboles). This change, which is particularlymarked from upper Miocene into Pleistocene, suggests ageneral trend to climatic cooling which disfavors soildevelopment and favors direct erosion of rocks. Thisconclusion agrees with the isotopic data of calcareousorganisms for the Leg 48 sites (Létolle and Vergnaud-Grazzini, this volume). Another cause may have been theeffect of Atlantic widening accompanied by development ofmeridian currents and the rejuvenation of European reliefsin the Paleogene with consequent increased continental ero-sion.

Marked geochemical changes occur at various boundariessuch as between Aptian/Albian (Site 402), and Albian/Paleocene, Eocene/Oligocene, and lower-middle Miocene(Site 400). The changes vary in nature, but are not accom-panied by dolomitization or phosphatization. In the hy-pothesis of a warm and wet climate, proposed on miner-alogical grounds, margin tectonism should provokeorganic or inorganic precipitation of phosphate inassociation with upwelling currents on the outer edge ofcontinental shelf (Krauskopft, 1967). Phosphate beingabsent, the geochemical changes may be due to a climatic orcurrent control.

Fibrous clay stages occur in the Albian (attapulgite =palygorskite) as well as at Paleocene/Eocene boundary(attapulgite, sepiolite) in the two deeper Sites, 400 and 401.Those minerals with short broken fibers, widely spreadamong detrital minerals in various reworked sediments,have a detrital origin. Their development is favored by aparticularly warm, wet climate favoring ionic mobilization

708


Sample(Interval in cm)

Site 401

1-3,62-1,295-3,796-3, 1128-4,449-2, 14310-2, 13310-6, 10411-3,7313-1,7114-3, 9014-3,11616-2, 13119-1, 125.la

19-1, 125.2a

23-1,7224-1,41

Site 402

1-2,12WlbW2 b

3-1,1435-1,161-4, 1183-2, 1294-2, 814-3, 129-1,38

11-3, 14013-2, 10115-5,7519-4, 10921-1,9023-1,9024-3, 9425-1, 12530-2, 2031-6,78324,47334,6235-5,21

/ s i 0 2

54.0013.0028.2529.1031.8032.8024.7025.9534.5016.2532.3020.3514.857.109.45n.d.n.d.

54.5531.1513.9533.7523.6032.9037.8546.5045.1051.1548.4531.1046.2548.1545.8529.6540.2037.6538.6543.0036.7017.5020.40

A12°3

13.173.034.525.856.026.615.736.2010.514.128.445.191.512.723.50n.d.n.d.

9.334.492.684.434.095.904.706.097.971.068.869.0911.109.0914.704.0211.6912.5211.1016.2413.233.787.20

CaO

7.3143.3830.3329.3830.4526.3431.7631.1722.3538.8330.9236.0548.9746.1745.8254.5754.92

10.1828.5841.2834.7138.8332.3930.6823.1923.9329.6422.2628.3220.5022.0418.7035.9221.1926.0825.0521.2026.4543.0535.99

MgO

2.030.780.991.011.141.010.941.001.650.821.652.650.720.860.860.410.51

1.851.270.910.891.131.161.081.191.240.571.251.731.171.051.200.961.261.351.221.181.171.111.75

%

Na 2O

1.600.800.980.980.901.030.800.881.350.800.940.960.700.390.500.160.14

1.480.940.841.201.150.981.051.201.230.351.241.251.041.041.040.681.101.080.980.950.900.480.48

TABLE 2Geochemical Data, Sites 401

K2O

2.360.630.951.061.251.231.101.231.911.031.481.290.260.941.130.030.03

2.101.050.611.040.981.180.961.281.350.291.431.401.681.401.610.931.451.351.731.801.500.701.38

TiO 2

0.590.150.200.280.310.280.240.280.400.180.050.210.070.100.12n.d.n.d.

0.410.250.140.250.160.270.200.230.280.070.330.360.460.350.500.150.460.470.480.680.570.160.29

P2O5

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.0.83n.d.n.d.

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.0.20n.d.n.d.n.d.

Fè '

3.250.851.211.491.701.651.431.452.711.212.091.400.480.631.35n.d.n.d.

2.401.250.651.191.131.551.311.551.640.271.422.191.712.502.292.023.751.702.482.222.131.101.94

and 402

Mn

440660760780670105011001200460750720990950793001616

34014013015086028026036042080901706911296360100160130250420570320

Zn

9512021011071140100160225207260127852501407282

1409387727612011593130291139713011816067130140130110130120120

Li

288111112129102081596811n.d.n.d.

2317111519201518205

119263531591249534173641142

Ni

272424253238533336293446272026119

1412151325161411139153131327021100463354433434

ppm

Cr

11027415631434433565361321638311313

8956485754335669683968538979140491359584125972849

Sr

2301350920900810800870950580710730780840140160150160

350920

1420720

102092068067061067013401370122011808306709601180960690880460670

Co

1110771416521318813119662415

527484267245666123175

n.d.12102221

Cr

131224151658170192645303429152886

13201613101211775812131019922192523221116

Pb

2431231526182521382919252833323332

7921282402525162627731313231282138373337393332

v \

686368844295115100374295130633753n.d.n.d.

5353374758958916372163849589130531301301201301204279

during hydrolysis and crystallization in a basic chemicalenvironment (Millot, 1964). An enrichment in Mn occursfrom the first appearance of fibrous clays in Albiansediments to their quasi-disappearance after Oligocenetime. The increase in Mn does not affect Site 402 whichcontains few or no fibrous clays. The Mn/fibrous claysrelation, however, is not a direct one, because the sedimentsother than Albian and Paleocene/Eocene, poor in fibrousclays, are marked by manganiferous flow. It is, however, adeep water phenomenon because it is not encountered in theshallower Site 402.

Local Diagenesis

Although the argillaceous and chemical mineralcomponents do not show any geochemical evolution withthe depth of burial, there is suggestion of local andtemporary diagenetic manifestations in the sedimentarysequences of Sites 399 to 402.

The black shales facies of Aptian and Albian age is notcharacterized by a clayey assemblage distinct from moreoxidized facies such as the ones from Paleocene to Eocene.Smectite, in particular, which is often fragile in an organicmedium (Sigl et al., 1978) is as abundant and well

crystallized in them as it is in other facies; irregularmixed-layers are rare or absent. There is consequently noperceptible modification of the argillaceous sequence inorganic-rich surroundings which, themselves, show littlechange (Tissot et al., this volume). However, ionicmobilizations affect certain chemical elements such as Fe,Mn, and Ca, occurring in diffused concentrations, notablyas carbonates (siderite, rhodochrosite, calcite, i.e.,400A-74-1, 95 cm). It is consequently probable that theblack shales facies, which has no effect on clay mineralogy,has a minor diagenetic influence on the dissolution,diffusion, and re-precipitation of carbonates and of someother minerals. Organic acids and decrease of the redoxpotential in specific zones (i.e., immediately in theneighborhood of black shales) favor Fe and Mn dissolutionin bivalent state, and their precipitation in carbonate statewhen the precedent effects disappear.

No evidence of in-situ modification of minerals fromvolcanic origin exists in northern Biscay Bay. Althoughsmectite and fibrous clays are independent of lithology andpetrography, the zeolites and cristobalite encountered mayhave resulted from such an evolution. There is, however,, noevidence that they are not associated with typically volcanicor hydrothermal mineral/geochemical components.

709

o

BISCAY BAY SITES 402 AND 402A CLAY MINERALOGY AND BULK GEOCHEMISTRY

1

oLU

inDO

CJ

<1-LU

α:CJ

LU

<

Cfl

QQ

CO

<

IAL

α_<cca.

L. APT.

tε

B-B

OP

TH

=>£<Λ Q

100-

200-

300-

400-


100% Q F Am Go Py Ze

Sr (ppm)

1000 2000500 1500

Mn (ppm)

2000 40001000 3000

S i O 2 / A I 2 O 3

2 4 6 8

A I 2 O 3 / K 2 O

2 4 6 8

MgO/K 2 O

1 2 3 4

Sr/CaO 1 0 3

When the ratio is superior t o the l imit , this numerical value is indicated.

CLAY MINERALS: C

I I

ML]

SM I

J CHLORITE K j g g KAOLINITE

ityA ILLITE \×^^A ROUGH VALUE (Low or very low clay content)• **• v *J t_— ' I'/^/\ MIXED LAYERS f ~~\ ATTAPULGITE

SMECTITE SEPIOLITE

Figure 4. Site 402, clay mineralogy and bulk geochemistry.



• very rare

o abundant

• rare

O very abundant

GEOCHEMICAL ANALYSES: 1/2: Two analyses for the same sample


The fibrous clays, having a detrital aspect, probablyoriginated in a chemically basic environment (Millot, 1964)in semiclosed marginal basins in Albian andPaleocene/Eocene times. Sediments formed in such basinswere partially reworked owing to the instability of thesubsident margins and thence deposited in the open oceanicenvironment. Some of the clinoptilolite and cristobalite mayhave had a similar origin because they too can form in basicand confined sedimentary environments and underconditions like those governing the formation of somesmectites, attapulgites, and sepiolites (Sommer, 1972;Weaver and Beck, 1977). The increase in zeolites in Site402, which is the innermost and the least deep On theEuropean margin, seems to favor this possibility. It isprobable, however, that some of the zeolites and cristobaliteoriginated in situ under conditions of dissolution andrecrystallization of free silica and associated cations by localdiagenesis (Wise and Kelts, 1972; Stonecipher, 1976;Mitsui and Taguchi, 1977; Kagami, this volume).

Comparison Between the Three Sites of North Biscay Bay

From the shallowest Site 402 to the deepest Sites 399 and400 an increase occurs in relatively small sized minerals ofhigh buoyancy potential (smectite, fibrous clays) to thedetriment of more rapidly deposited minerals (illite,chlorite, kaolinite, feldspars) (Figure 5). This indicates adifferential settling offshore evoking a deltaic environments. I. (Millot, 1964; Chamley, 1971; Gibbs, 1977), which isin agreement with other sedimentological observations(Auffret, this volume). Another cause of the scarcity offibrous clay minerals at Site 402 could be a supply byrelatively deep meridian currents originating from lowerlatitudes where these minerals are abundant on continents(Millot, 1964; Chamley, 1971; Chamley et al., in press).The difficulty to supply these minerals from the adjacentland masses must also be envisaged: fibrous minerals arepoorly represented in the Armorican basins (Estéoule-Choux, 1967), and the flat morphology of the large Basin ofParis where they are frequent (Sommer, 1969; Trauth et al.,1969; Trauth, 1974) does not favor their erosion and theirtransport toward the open sea.

From Site 402 to Site 400, the differentiation between thethree sites also manifests itself from a chemical point ofview by decrease of carbonates and increase of transitionelements (Mn).

ROCKALL PLATEAU — RESULTS


Upper Paleocene (?), Lower and Middle Eocene

Lithologic Units 3 and 2 consist of arkosic sandstone,lapilli tuff and conglomerate, tuffaceous mudstone, andsiliceous nannochalk. Smectite is predominant and wellcrystallized. Illite and kaolinite locally occur in traces. Thegeochemistry is characterized by the predominance of thesiliceous phase (Si, Al, Fe, Mg, Ti) in contrast with thecalcitic one (Ca, Sr). An exception exists in -40-1, 5 cmwhere Ca is present in noticeable amounts, associated withMn and P2O5. The ratios are as follows: SiCWAhOa low,MgO/K2O and TiO2/Al2O3 high, Sr/CaO unstable. Thelower Eocene sediments are rich in Mg, Fe, V, and Cr.

Alkaline and reducing sediments are suggested by thepresence of rhodochrosite, carbonate-apatite, calcite,siderite, and glauconite.

Oligocene

In the upper part of lithologic Unit 2 the clay fraction ispoor; slight increase in illite and kaolinite and theappearance of chlorite, zeolite, and irregular mixed-layersoccur. Mn and AI2O3/K2O and TiO2/Al2θ3 ratios decrease,whereas that of Siθ2/Al2θ3 increases; Sr carbonate andminor phosphate appear.

Upper Miocene

The base of Unit 1 (nannofossil ooze, marly ooze,calcareous mud) is poor in the clay fraction, and consistschiefly of smectite. The chemical composition is marked byhigh contents of Ca and Sr, and a decrease of the Si phaseand Mg. A major change occurs between -9-5, 80 cm and-9-4, 129 cm, with abrupt decrease of AI2O3, Si, Na, andquartz contents.


In the upper part of Unit 1, a marly foram nannofossilooze, the clay fraction becomes abundant and is marked bya steady upward increase in illite, chlorite, irregularmixed-layers, quartz, feldspars, amphiboles. Siθ2 alsoincreases, whereas a general decrease affects CaO, Sr,SiO2/Al2θ3, TiO2/Al2θ3, and MgO/K2O.

Site 404; Figure 7; Table 3The same mineralogical and geochemical successions

occur as in Site 403. A local enrichment in Mn (rhodo-chrosite) and P2O5 (apatite) contents is noted in Eocenesediments.


Although sparsely sampled, the site shows results close tothose obtained for Site 406. Sediments of lower Eocene arepeculiarly rich in SiO2 and also contain fairly high amountsof Mn.


Eocene

Calcareous claystone, followed upward by marlylimestone and siliceous and/or calcareous chalk, constitutesUnit 5 and the base of Unit 4.

The clay mineral is nearly exclusively smectite, and isoften accompanied by cristobalite. A geochemical breakoccurs between lower Eocene and middle/upper Eocenesediments (Cores 45 to 42); the lower Eocene has (1) highvalues of Si, Sr/CaO, AI2O3/K2O, TiO2/Al2O3, (2)moderate to low Ca content, (3) locally, simultaneousabundance of Fe and Mg (presence of hypersthene in -43-6,47 cm). Middle/upper Eocene sediments are high incarbonates, show a decrease of the above-mentioned ratios,an end to Mn metallization, and simultaneous disappearanceof high Fe and Mg values.

Oligocene, Early Miocene

In the calcareous chalk of the top of Unit 4 and in thesiliceous calcareous chalk and calcareous diatomite of Unit

711

SITE 403 ROCKALL PLATEAU

LUCS<

PLEIST.

PLIO.

O

MIO

CE

NE

EO

CE

NE

ST

AG

E

CH

a.Q_

D

3CENE

MIDDLE

LXLU

o

SU

B-B

OT

TO

MD

EP

TH

(m

)

100-

200-

300-

400-

1 S

AM

PL

ED

Z]

C

OR

ES

T4

•

17

21

22

2526

34

l | 3 5

^ 3 9

m 41

-

I G

EN

ER

AL

IZE

D:_

L

ITH

OL

OG

Y- 1 _ - 1 -

_L_ _ 1 _

_ 1 _ _L

_ l _ _ 1 _

1 1

* '/ ~\ U ** " 1/ ^ \ \ \ *

| U

NIT

Sr|

B

U

NIT

1

t A

ZCN

r>

<

UN

IT3

CO

u

CLAY MINERALS ASSOC

10 20 30 40 50 60 70 80 90%

m.r•:•:v//λ iiiiiiini i HUE

no clay enough

no clav enouqhno clay enough.no clay enough

I' / / / / /SM- / / / /V.nr> Hay enmigh

'I / / / / / SM/ / / / ,K

iltllllltllllllllllltll1llll1ll|lllll1lllll1l1ll1lllllllllll|lllll1 / / / / ^SM / / / /- K•

Illlllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllffllllllll|NIIIIIIUIllllllll|llllllllllil|llllll|llllllllllll|lllll

10 20 30 40 50 60 70 80 90%

ATEDMINE

Q Fe Am C

i i i

i

RA

oP

LS

y z

!

{

CLAY MINERALOGY AND BULK GEOCHEMISTRY

SAMPLES

B

--2-6,2"~-3-2, 73"-4-1, 104*-5-1, 134

8-1,269 4 , 1289-5, 79

11-1,88

14-1, 130

17-1, 108

21-2, 1221-3,6622-3,23

) 25-1, 3126-4, 41

34-1, 1335-2, 28

39-2, 6140-1, 241-1, 139

1 0 3 T i O 2 / A I 2 O 3

40 8020 60 100140 180

Ill ll

CaO (%)

10 20 30 401 ' ' 1

51

U54

53

51

-

Sr (ppm)

1000 2000500 1500

1 ' 1 1

1 .1

ll.

Mn (ppm)

2000

1000 3000

|

||

J.

M

I

..1

.

.1

S iO 2 /AI 2 O 3

2 4 6 8m^mml 1—"

MgO/K2O

2 4 6' ' '

AI 2 O 3 /K 2 O

1 2 3 4I ' ' 1

Sr/CaO I 0 3

2 4 6 8_ Å — I 1 1

-

When the ratio is superior to the l imit , this numerical value is indicated.


CHLORITE SM

••i ILLITE K

MIXED LAYERS

I SMECTITE

KAOLINITE

ROUGH VALUE (Low or very low clay content)

Q = QuartzAm = AmphibolesPy = PyriteGo = GoethiteZe = ZeoliteF = Feldspars

1: Two analyses for the same sample.

• very rareo abundant• rareO very abundant


ILLITE, CHLORITE, KAOLINITE, FELDSPARS


Differential

NNE

Armorican Shelf

Shamrockcanyon

12 km

Figure 6. Origin and distribution of clay fraction components on the north of Biscay Baymargin.


Sample(Interval in cm)/

Site 403

14,672-6,33-2, 744-1, 1055-1, 1358-1,2794, 1299-5, 80

11-1,8914-1,13117-1, 10921-2, 1321-3,6722-3, 2425-1,32264, 4334-1, 1535-2, 2839-2, 6340-1,5414, 140

Site 40411-2,764-2, 1027-1, 20

10-1,6516-1,11517-5, 1821-2, 132

SiO2

29.1522.9014.3022.6020.753.253.553.701.702.601.903.751.903.05

16.358.95

48.5056.8049.5012.7547.45

7.401.80

57.5047.4043.2031.1525.25

A12O3

7.324.962.784.804.380.580.460.350.590.440.330.900.650.992.131.18

48.5014.2911.57

3.0710.75

1.320.634.966.618.276.384.72

CaO

30.8534.6741.8334.6535.0050.5852.1550.5853.3853.9052.8551.4551.9850.7540.3645.50

7.828.93

10.0042.64

8.85

46.2052.8516.4221.6720.6127.1132.51

MgO

1.781.610.981.281.310.430.450.520.450.370.350.400.360.501.740.656.703.325.751.487.31

0.590.421.783.664.703.833.66

%

Na2O

3.182.181.531.731.651.000.901.080.930.930.900.980.750.941.501.253.553.682.950.884.45

1.450.932.602.302.281.581.73

K2O

1.501.280.601.000.930.160.150.150.160.140.130.190.150.231.110.191.503.251.730.652.38

0.340.140.650.431.301.001.08

TiO2

0.360.350.190.280.290.050.040.040.040.040.030.050.040.060.140.051.550.651.300.281.04

0.100.040.641.050.630.190.15

p2°5

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

0.89n.d.n.d.n.d.n.d.n.d.n.d.

n.d.n.d.n.d.n.d.n.d.1.133.72

Fe \

2.092.220.941.451.620.290.280.360.310.180.150.310.250.372.480.617.804.337.002.149.68

0.470.252.883.704.555.003.80

/Mn

560450510380430330460390400210360280400280380960800470

10104750

750

410400450950670

92507300

Zn

11075545466335342343037474634

140469971

12086

110

34368479

17011691

Li

16145

109233321323

208

223327

87

23

2418171515

Ni

342918252619272621151625182489316338472456

17214552893644

pj>m

Cr

65463026382524251916272327295215

14013011026

110

33237991

1305771

Sr

72010301050900970

15001220102013001350123012401300103010301050

160460240330140

11201190330450460180310

Co

141911121613151416111115111535135221361844

10121633352932

Cu

38482224251414131414128

13242816

10028611794

18183544476534

Pb

23462621274131323232413637333344

497

267

39332014171118

V ^

8979304551115998

1113159

794

32012027068

300

1212

170190195190130

713

CHLORITE SM ||||||||j||| SMECTITE

ILLITE K \• | KAOLINITE

MIXED LAYERS \//\ ROUGH VALUE (Low ot very low clay content)




• very rare

o abundant• rareO very abundant


AG

E

PLEIST.

PL

IOC

EN

EM

IOC

EN

EE

OC

EN

E

ST

AG

EU

PP

ER

MIDDLE

LO

WE

R

SU

B-B

OT

TO

MD

EP

TH

(m

)

100-

200-

300-

SA

MP

LE

DC

OR

ES

4

7

10

16

17

21

GE

NE

RA

LIZ

ED

LIT

HO

LO

GY

Δ ΔΔ ΔΔ

Δ f* „ - « ss

v~

UN

ITS

<

CD

Z=>

Z 1 N 1

<

ml -zD

CD

MINERALOGICAL AND GEOCHEMICAL BULK


10 20 30 40 50 60 70 80 90% Q F Am Go Py Ze

cs / / / Λ y / / / / \a

V / / / / SM/ / / / /K,

IlllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllππππmiπiiπππππiiiiiiiiiiiiiHiHii Minimum

10 20 30 40 50 60 70 80 90%

1-2,75

4-2, 101

7-1, 19

10-1,64

16-1, 11317-5, 17

21-2. 130

I 0 3 TiO2/AI2O3

40 80 120 16020 60 100140

CaO (%)

10 20 30 40

5 3

Sr (ppm)

1000 2000500 t 1500

• —

Mn (ppm)

1 000 f 3000 f

-

SiO2/AI2O3

2 4 6 8

11.6

AI2O3/K2O

2 4 6 8

15.4

MgO/K2O

1 2 3 4

8.5

Sr/CaO 10 3

2 4 6 8


AG

E

MIO.

EO

CE

NE

ST

AG

EU

PP

ER

LO

WE

R

SU

B-B

OT

TO

MD

EP

TH

(m

l

100-

200-

300-

400-

ISA

MP

LE

DC

OR

ES

• •

M

6

8

32

34

36

4041

s

GE

NE

RA

LIZ

ED

LIT

HO

LO

GY

_ J _ _ L _

uΔµ-H

P-~-

UN

ITS

UN

IT1

B ,

A

<

m

h•zD

u

a

CLAY MINERALOGY AND BULK GEOCHEMISTRY


100% Q F Am Go Py Ze

no clay enough

llll!lllllllimiMjMIM1MMI|lll|[lllllJIIII|llll)lll IlllllllllI i

6-1, 135

» 8-1,1

32-1, 109

34-1,81

36-1, 116

40-2,6741-6,8042-4 4

I 0 3 TiO2/AI2O3)

20 60 100 140180

CaO (%)

10 20 30 40

Sr (ppm)

1000 2000500 1500

Mn (ppm)

2000 40001000 3000

-

SiO2/AI2O3

2 4 6 8

AI 2 O 3 /K 2 O

2 4 6 8

MgO/K2O

2 4 6 8

Sr/CaO 10 3

2 4 6 8

CLAY MINERALS: ASSOCIATED MINERALS:

CHLORITE SM

ILLITE K

fl SMECTITE

| KAOLINITE

MIXED LAYERS ] / / \ ROUGH VALUE (Low or very low clay content)

QAmPyFGoZe

= Quartz= Amphiboles= Pyrite= Feldspars= Goethite= Zeolite

• very rareo abundantO rare• very abundant




Sample(Interval in cm)

Site 405

1-3,436-1, 1398-1,1

32-1, 10934-1,8136-1,11640-2, 6741-6,80424,443-6,47

Site 406

1-1,3224, 1254-1,707-1,308-5,5610-1,2413-2,7114-1,11716-1,8522-1,2523-2, 13024-1,9527-1, 2028-2, 7294, 130.1

a

294, 130.2a

30-2, 1431-3,55324, 70334,6735-2, 3036-2,5337-3,13738-3,77394, 1340-3, 33414,8042-3, 101454, 3846-1,847-1,6348-1,4049-1,7549-2,31

xS i O

2

39.551.40

14.7053.1578.7070.4055.6067.4580.4550.60

41.6554.75

1.104.203.902.453.156.05

2.506.4012.3526.5048.4541.3527.5032.9513.7011.3019.0013.505.5018.255.7012.3532.8513.858.159.45

7.6540.3039.2039.8037.0537.95

A 12O

3

8.500.352.687.322.783.567.444.372.56

15.12

12.9914.230.321.151.020.600.911.560.581.072.315.160.890.921.752.601.421.602.242.920.933.320.960.834.131.751.501.001.068.279.218.74

8.267.32

CaO

19.3249.7037.6318.334.558.68

11.038.404.626.82

12.715.43

52.5049.7048.3749.7048.6545.3350.5846.5541.6528.6023.0027.1334.6529.2243.0543.7535.7039.5549.0036.9347.9543.5829.6142.8046.3745.5046.3714.8819.4317.5022.3321.18

MgO

1.87

0.371.121.740.540.75

1.451.010.672.98

2.532.200.31

0.410.370.330.360.460.320.410.621.200.290.320.530.650.470.560.660.85

0.411.090.460.501.040.580.530.410.412.491.821.641.76

1.45

%

N a2O

2.48

0.681.481.850.751.03

1.731.73

0.862.45

2.492.490.760.700.700.650.630.640.630.810.841.111.11

1.910.981.280.860.781.05

0.630.460.790.400.400.710.430.380.460.501.73

1.451.391.23

1.19

K2O

1.73

0.130.85

0.750.400.460.790.620.500.60

2.582.73

0.130.280.240.160.190.340.160.240.461.040.200.200.380.490.310.300.400.460.190.680.210.140.560.240.230.150.161.230.780.700.61

0.63

TiO2

0.450.030.280.090.250.430.940.420.230.55

0.750.750.030.070.060.040.040.080.030.060.110.280.040.040.080.120.080.080.130.150.050.190.040.050.180.070.070.060.050.640.710.710.480.67

p2°5

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.0.30n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.0.17n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

F e ^ '

2.260.202.573.501.352.064.002.661.874.95

3.57

4.310.170.390.400.240.300.500.240.490.831.850.300.310.670.920.480.570.861.120.452.290.520.451.110.600.600.420.287.143.603.623.503.29

' Mn

480

270600

770

270

5401080730290340

510360

320

540600510550490

420120010002806897

270210

310570

600610

680

520660

570

380540

630

74035009301400150016003400

Zn

42

552

180n.d.1604734

160

16

74491571466663562

150

823

1201107311645

191667

658

13092

123

42

6274

48

8613564

Li

1529724954

17

29

32144346349262395559838664

130

683222712765

PRNi

33

40

514579

494159

280

61

385944

4650

49515542

5563

95151936

5544

6637

52

49

8832

32

11043

45

34

46

6870

587663

TI

Cr

73

3553

48

6164

7362

57

55

969134

42

49174839

253848

5826482747

2927

3547215524

4047

2825

2414589197

9142

Sr

450

1470900510

170

260

380310

160

270

320290

1480150014201430148015301450149015401200830900108012301090

UIO105011001270890

1030710770

9201130109012101000940930

980

930

Co

15161639

52

12122149

n.d.

20

1912141715141614131832

4479912913132111111781212926

3125

35

23

Cu

23

61872

4163646

29

35

2520

64128117793025162026

435

1018268952

1614111834

68

82

86

53

Pb

33

35

316627

171330

10

29

213335

35

6336

3334

343226

251848

32

26

33

2528

3327

32

28

22

32

25

32

14023

25

25

64

28

V ^

58

n.d.29

1202163

28047

32100

130

150

1116n.d.11n.d.16551663

n.d.n.d.323716161621542

16n.d.42

2121516205

170

130

120

97

Heterogeneous sample, two analyses.

3, clay is rare and is characterized by smectite, locallyaccompanied by traces of illite and kaolinite. Thegeochemistry is heterogeneous, with a high S1O2/AI2O3ratio due to free silica (diatoms); transition elementsdisappear.

Middle and Upper Miocene

The nannofossil chalk and nannofossil ooze of Unit 2contain few clay minerals. Smectite is in excess of chloriteand illite, but the latter minerals increase upwards. Therather homogeneous chemical composition is marked bylittle Siθ2 and Mn, and much carbonate. A local phosphateoccurrence and an increase of Mn content occurs at theearly/middle Miocene boundary.


In the marly foraminiferous nannofossil ooze of Unit 1the clay assemblage becomes diversified and includeschlorite, illite, irregular mixed-layers (chiefly illite-

smectite, little chlorite-smectite), smectite (fairly well crys-tallized), kaolinite, accompanied by quartz and feldspars.Siθ2 increases whereas CaO decreases.

DISCUSSION

Because of the limited sampling of Sites 404 and 405, thefollowing discussion is based mainly on data from Sites 403and 406.

Origin of the Principal Mineral and Chemical Phases

In the sediments deposited from Paleocene to lowerMiocene time nearly all of the clay is usually well-crystallized smectite. Fe and Mg are relatively abundantand are commonly associated with cristobalite/tridymiteand sometimes zeolites which suggest a source richin the vitreous and mineral elements of volcanic origin(Harrison et al., this volume; Harding, this volume;Latouche, this volume; see also Site Chapters, this volume).Fe and Mg do not generally occur as oxides, sulfides,

716

SITE 406 ROCKALL PLATEAU CLAY MINERALOGY AND BULK GEOCHEMISTRY


10 20 30 40 50 60 70 80 90% Q F AmGoPy Ze

45A3846-1,847-1,63

(49-1, 73[49-2,31

103 TiO2/AI2O3

60 10040 80 120

CaO <%)

10 20 30 40

Sr (ppm)

5001000

Mn (ppm)

1000

SiO2 /AI2O3

2 4 6 8

AI 2O 3 /K 2O

2 4 6 8

MgO/K2O Sr/CaO 10J

2.5 5 II

CLAY MINERALS:

| CHLORITE S | | | SMECTITE

foj ILLITE K r = l KADI INITF

^Z\ MIXED LAYERS Y/A ROUGH VALUE (Low or very low clay content)



Q = QuartzAm = AmphibolesPy = PyriteF = FeldsparsGo = GoethiteZe - Zeolite

very rareabundant

O very abundant

GEOCHEMICAL ANALYSES: 112: Two analyses for the same sample


carbonates, or ferromagnesian sandy minerals andconsequently are incorporated mainly in the smectite latticeas shown by differential thermal analysis curves. Suchsmectites suggest, at least partially, a volcanic origin. Sucha hypothesis is supported by titanium enrichment (Rankamaand Sahama, 1950), especially in the Eocene sediments ofthe Rockall area (Figure 10). Furthermore, the rare-earthelements examined by C. Courtois (University of Paris XI)are marked by a noticeable poverty in light elements(lanthanium, cerium, neodyme, samarium) in comparisonto heavy elements (europium, terbium, yterbium, lutetium).Such a composition, which is not in equilibrium with that ofseawater and cannot be of purely sedimentary origin, isclose to the composition of a tholeiitic basalt. It is thereforehighly probable that the smectite and other associatedchemical elements resulted from the transformation ofvolcanic ash and glass rather than being derived as detritusfrom the weathering of sedimentary rocks.

An indeterminable part of the smectite, however, may beof remote origin, either pedologic or sedimentary, as shownby the occasional presence of detrital minerals such askaolinite, illite, and chlorite. A non-volcanic origin isprobable in those Eocene and Oligocene levels that aremarked by a decrease in Fe, Mg, Ti (Table 4).

Whether the smectite formed in a submarine or subaerialenvironment is questionable. The composition of therare-earth elements (in Sample 405-43-6, 47 cm) is not inequilibrium with that of typical seawater which does notfavor a submarine genesis. The active volcanism during thePaleogene, and particularly in the Eocene, probably

produced volcanic areas exposed to subaerial alteration. Theauthors are inclined to favor a subaerial genesis for thesmectite, followed by its erosion and deposition in the seaalong with the other components, including pelagiccarbonates, silica, and/or volcaniclastics.

Sites 405 and 406 are marked generally by morecarbonates, less clay, and larger amounts of smectite andcristobalite/tridymite than at Sites 403 and 404. Thissuggests that sedimentation was generally more pelagic inthe Paleogene in the Site 405-406 area than at Sites 403 and404. In Figure 11 the mean compositions for Sites 403 and404 are more closely related to the MgO and F2O3 polesthan are those of Sites 405 and 406; this suggests closerproximity of Sites 403 and 404 to volcanic sources which isthe sign of a strong volcanic influence.

Climates and Currents

Mineralogical and chemical changes, announced inOligocene time, develop in the Miocene and particularly inthe Pleistocene: primary detrital minerals or those frommoderate continental alteration (mixed-layers) graduallyreplace smectite whereas carbonates and associate chemicalelements increase. That change indicates a decrease involcanic activity accompanied by a general modification ofclimatic conditions. The step-by-step cooling took placefrom late Paleogene onwards (see also Létolle andVergnaud-Grazzini, this volume) and favors directcontinental erosion of rocks, and the development of largemeridian currents and calcareous planktonic communities.Pelagic sedimentation developed after the sinking of

Figure 10. Rockall Plateau and Biscay Bay: TiO2 = f(Al2O3).

718


MgO

AI,O,

Fe9O2 U 3

Figure 11. Geochemical average data of each Leg 48 site.

emerged volcanic areas located between Greenland andEurope, which followed the major phase of the opening ofNortheast Atlantic (Laughton, Berggren et al., 1972). In thePleistocene the spread of glaciers provoked an increase interrigenous supplies which diluted the planktoniccarbonates.

GEOGRAPHICALCOMPARISONS—CONCLUSIONS

Comparison Between North and South Biscay Bay (Leg48, Sites 399,400,401, 402; Leg 47B, Site 398, Chamleyet al., 1978)

Common Characteristics

1) Late Cretaceous to Paleogene sediments in both areascontain abundant crystallized aluminoferrous smectite withisochronous quantitative variations. The smectite wasprobably derived from a deeply weathered continentalsource subject to a warm and arid climate with alternatingwet and dry seasons.

2) Two intervals of fibrous clay with associated Mndeposition occur in both areas in Albian time (attapulgite =palygorskite) and at the Paleogene/Eocene boundary(attapulgite, sepiolite). Derivation was probably from achemically basic setting (Millot et al., 1957; Millot, 1964)established on the continental margin under hydrolysingclimates. Simultaneous development of fibrous clays isknown from the South Atlantic (Chamley, unpublisheddata, DSDP Leg 40). Fibrous clays in all these DSDP sitesare detrital and reworked due to the marginal unstability.

3) Abrupt mineralogical change occurs in theCampanian/Maestrichtian period, manifested by a suddenincrease in the quantity of primary minerals. The causecould be a major stage in the North Atlantic widening(Laughton, Berggren et al., 1972; Berggren and Hollister,1974), favoring the mineralogical supply from high lati-tudes. Additional causes could be a pre-alpine rejuvena-tion and/or a continental cooling.

4) Irregular increase in primary minerals and irregularmixed-layers from Oligocene upwards, in relation with aworld cooling the major stages of which occur in the upperMiocene and in the Pleistocene (see also Pastouret et al.,1978; Chamley and Giroud d'Argoud, in press). At thesame time the marine circulation increased, leading to thedevelopment of calcareous plankton and carbonated sedi-ments. In the Pleistocene the Glacial/Interglacial alternationdetermined an increase in erosion of northern continentalrocks, rich in silicated components.

5) The chemical composition of the sediments at Sites398 and 400 are comparable (Figure 12). This is particularlywell demonstrated in the mean compositions (Figure 11)and indicates a common origin for the detrital minerals. Thechemical successions with time also follow one another inthe same manner at the two sites.

6) Study of geochemical relations confirm the analogiesbetween Sites 400 and 398. In both cases the siliceousphase, well defined by close linkages between Siθ2, AI2O3,Fe2θ3, MgO, and K2O, is marked by the same group oftraces (Ti, V, Cr, Li). This fact defines not only identicaldetrital contributions but also identical sources for thesecontributions. Moreover in these two sites there exists an

719


AloO2 U 3

+ 398. 400

MgO•

+ 98-2,34(2)+ 118-6,4^

•74-1,45

68-2

Fe2O3

Figure 12. Triangular diagram, Al2O3 - Fe2O^ - MgO. Comparison between south andnorth of Biscay Bay (Sites 399 and 400 and Site 398).

independent metalliferous phase, characterized by transitionelements (Mn, Ni, Co). This third phase begins anddisappears at the same time as the fibrous clays, that is, itpersists from Albian to Oligo-Miocene.

Unique Characteristics

1) The sediments of the north Biscay Bay sites containless kaolinite and fibrous clays than those in the south. Thisevokes the climatic zonation of sedimentary rocks and soilson land masses, favoring kaolinite and fibrous clays inlower latitudes. The occurrence of terrestrial paleoclimaticbelts seems to be reflected in sediments as today (Griffin etal., 1968; Pedro, 1968), with major influence of suppliesfrom adjacent continents.

2) Smectite and MgO increase northward, probably inrelation with soil and sediment supplies from the Paris Basinrather than with submarine volcanic activity.

3) More primary minerals (and, consequently, associatedchemical elements) occur in the Neogene and Pleistocenesediments in the north than in the south. Probably this pointsto a manifestation of lower temperatures in the source areathat supplied the northern sites, as is the case today (Griffinet al., 1968).

4) In the Albo-Aptian "black shales" in the north dia-genesis of organic matter is less marked and concernsmobile chemical elements only (Mn, Fe, Ca), showing thevariability of mineral and organic components in this litho-logical facies (Ryan and Cita, 1977a,b; Deroo et al., 1978;Deroo and Herbin, in press; Tissot et al., this volume).

Comparison Between North Biscay Bay and RockallPlateau (Sites 399 and 400 to 402 and Sites 403 to 406)

Common Characteristics

The main similarity between the two areas drilled duringLeg 48, and which is equally common to the area located offthe Iberian peninsula, invokes the unsteady increase ofprimary minerals and the occurrence of mixed-layers fromlate Paleogene onwards and particularly during Neogeneand Pleistocene time. That change is illustrated in particularby the decrease of the smectite-Mg couple and the increaseof the illite-Siθ2 couple. It is a product of general cooling inMiddle and Upper Cenozoic, influencing the conditions oferosion, continental pedogenesis, and oceanic circulation.

Unique Characteristics

1) The geochemistry of the sediments of Paleocene,Eocene, and Oligocene age in the Rockall area are distinctlyferromagnesian and titaniferous (Figures 10, 11, and 13).Figure 11 demonstrates that, for the Rockall sites, the cloudof points is close to the MgO and Fe2θ3 poles and thedispersion is greater than for the Biscay Bay sites.

2) The analysis shows, for Biscay Bay, there is a closerelation in the Al2θ3/Tiθ2 bond (due to Ti in the smectites);the relationship is linear (Figure 10). For the Rockall area asimilar relationship occurs, but differs in that 80 per cent ofthe points fall above the line. This is particularly true for theEocene section of the Rockall sites. The Ti is concentrated

720


AUO

• 399, 400, 401, 402 BISCAY BAY+ 403, 404, 405, 406 ROCKALL PLATEAU

MgO

Figure 13. Triangular diagram,and Rockall Plateau.

e 2 u 3

Oo - MgO. Comparison between Biscay Bay

in the Fe- and Mg-rich smectites (Caillère and Henin, 1963;Deer et al., 1962), and suggests, together with rare earthand V, Cr, Co data, that the smectite chiefly originated frombasic volcanic material.

3) In the Rockall sites, Mn is not present consistentlyas a metalliferous phase. We find it in the Eocene asrhodochrosite, associated with other minerals (siderite,glauconite, phosphate; see also Odin, this volume). Thissuggests an environment reduced by an anaerobic decay oforganic material of animal origin (Riviere, 1941). In thesites of the Bay of Biscay, rhodochrosite does not appearafter the Albian and especially is not associated withphosphorous. Effectively the reduced conditions areinduced by decay of vegetable organic material of detritalorigin, suggesting a very different environment.

4) A lesser clay fraction and more abundant carbonate,siliceous and volcaniclastic material occurs in the north,indicating a more oceanic and volcanic environment. Thistrend increases from Sites 403 and 404 toward Sites 405 and406.

5) Kaolinite and fibrous clays are less abundant in theclay fraction northward, due to remoteness from continentalsources and from less hydrolyzing climatic zones (lowerlatitudes).

6) The cooling expressed in clay assemblage occurs laterand stronger in Rockall area (increase in illite, chlorite,mixed-layers content). The principal causes are theadditional influence of volcanic activity to the climaticchanges in the setting up of the mineralogical composition,

and the vicinity to the cooling sources during thePleistocene (see Moyes et al., 1974; Chamley, 1975).

ACKNOWLEDGMENTS

We are grateful to the National Science Foundation (USA) andto the shipboard team of Leg 48 for allowing our shore-basedinvestigations. Geochemical and mineralogical studies receivedthe financial support of C. N. E. X. O. (France) through grants76/5319 and 76/5320. The mineralogical study profited by thetechnical assistance of G. Giroud d'Argoud. Scientific commentsand assistance were given especially by L. Montadert, G. A.Auffret, and P. C. de Graciansky. We gratefully acknowledgeG. A. Auffret and J. Usher for reviewing the manuscript.

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Chamley, H., 1971. Recherches sur la sedimentation argileuse enMéditerranée: Sci. géol., Mem., Strasbourg, no. 35.

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Chamley, H., Debrabant, P., Foulon, J., Giroud d'Argoud, G.,Latouche, C , Maillet, N., Maillot, H., and Sommer, F., inpress. Mineralogy and geochemistry of Cretaceous andCenozoic Atlantic sediments off the Iberian peninsula (Site

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Deroo, G., Herbin, J.P., Rouchaché, J.R., Tissot, B., Albrecht,P., and Dastillung, M., 1978. Organic geochemistry of someCretaceous claystones from Site 391, Leg 44, western NorthAtlantic. In Benson, W.E., Sheridan, R.E., Initial Reports ofthe Deep Sea Drilling Project, v. 44: Washington (U.S. Gov-ernment Printing Office), p. 593-598.

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PLATE 1Electromicrographs

Figure 1 Sample 400A-64-2, 56 cm (×25,000). Upper Albian.Attapulgite ( = palygorskite) in short and straightfibers. Smectite in fleecy particles. Well-shaped illite.

Figure 2 Sample 400A-64-2, 56 cm (× 39,000). Upper Albian.Attapulgite, smectite, hexagonal kaolinite.

Figure 3 Sample 401-19-1, 125 cm (×6800). LowerCampanian. Large-sized and well-shaped illite andchlorite, with fleecy smectite.

Figure 4 Sample 401-17-1, 50 cm (× 20,000). EarlyPaleocene. Local increase of straight or flexuousfibrous clays (sepiolite, attapulgite) in a smectite-richassemblage.

Figure 5 Sample 400A-43-3, 148 cm (×39,000). UpperMiocene. Smectite, illite, kaolinite, fairly rareattapulgite, fragments of dissolved diatoms.

Figure 6 Sample 400A-20-5, 38 cm (×39,000). UpperMiocene. Smectite, illite, kaolinite, rare attapulgite.

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PLATE 1

1 µm 1 µm

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