10. PETROGRAPHIC AND SEDIMENTOLOGICAL STUDY OF THE CRETACEOUS-PALEOCENE SEQUENCE OF HOLE 398D

Maurice Bourbon, Ecole Nationale Supérieure des Mines de Paris, 60, Boulevard Saint Michel - 75272Paris, Cedex 06, France

INTRODUCTION

The aim of this paper is to report the results of apetrographic study of thin sections of Cretaceous andPaleocene sediments of Hole 398D. An important partof the material for this study was provided by differentpeople (Debrabant, Rehault, Sigal). Special attentionwas directed to certain aspects, such as evidence forbiological activity, and the breccias and the gaps in thestratigraphic column. In view of the restricted numberof thin sections (about 200), divergences with otherstudies and omissions are likely to occur. Figure 1 sum-marizes the petrographic features in thin-sections of theCretaceous and Paleocene sediments in Hole 398D.

MAIN POINTS AND COMMENTS

Organic Matter; Indexes of Biogenic ActivityThe types of organic matter that are or were present

in the sediments include vegetal debris and pyritizedburrows transformed from organic rich material.

The first organic matter type consists of vegetal de-bris, which is especially abundant in Cores 131 to 79,but is also present below (Cores 138 to 131) andabove (Cores 79 to 56) this interval. These vegetalfragments frequently occur in silty material, and bothhave a terrigenous origin (Deroo et al , this volume).The size of these vegetal fragments varies, but is com-parable in grain size to the silt that contains them. Theplant debris is pyritized in some beds. Vegetal debris isalso present in the matrix of the debris flows (Cores124 and 117; see Plate 1), in which sediments of bothpelagic and terrigenous origins were mixed duringtransport.

The second type of organic matter is not directly vis-ible, but may have been responsible for pyritic concen-trations, i.e., the contents of many burrows have beendiagenetically altered to pyrite. In these cases, we as-sume that the pyrite is due to the transformation of or-ganic-rich material. The pyritized burrows are abun-dant in Cores 138 to 56, especially in the less-trans-ported (or "autochthonous") sediments.

Pyritic spherules are generally present in the pre-Se-nonian sequence, but are particularly abundant inCores 79 to 56 (Units IV and V of Sigal, this volume),where they form thin layers (Plate 2, Figures 3 and 4).They possibly result from organic matter (transported,sorted, and accumulated on the sea floor), and latertransformed into pyrite. They might also be condensedlayers or hard grounds, on which the organic matter

was concentrated. Whatever their origin, this abun-dance of pyritic spherules is a characteristic of Units IVand V, perhaps because the terrigenous silty fractiondecreases and the pelagic fraction increases in thesetwo units.

Clastic SedimentsFour main types of clastic sediments occur: (1) silt

and clay, (2) breccia composed of pelagic limestonepebbles, (3) breccia composed of lithoclasts of plat-form carbonate origin, and (4) graywacke.

The first type contains the largest amounts of plantdebris; it is terrigenous.

The three others are of a different origin and at leasttwo of them (2 and 4) probably result from the erosionof submarine strata. The pelagic limestone pebbles arerounded and show internal flow-structures. They appearto result from the transport by slumping of materialthat was not completely hardened. Isolated reworkedcalpionellids in the matrix of Aptian breccias indicatethat occasionally such resedimented oozes may bethoroughly mixed with younger sediments.

The presence of the graywacke layers has importantimplications, because they contain grains of quartz, bi-otite, muscovite, zircon, tourmaline, feldspar, togetherwith carbonate gravels and radiolarians. The mineralgrains obviously resulted from the disintegration ofcrystalline rocks, probably of Variscan basement. Thepresence of all these minerals together indicates poorsorting and a short transport history. The bioclasticfraction is of pelagic origin. Such mixed deposits havebeen described in Alpine sequences (Lemoine, 1967)and were interpreted as being the result of disintegra-tion and erosion of basement rocks, along fault scarpsin a deep marine environment (Figure 2). The pelagiclimestone pebbles and the platform carbonate debris atSite 398 may be derived from the same outcrops.

Table 1 summarizes the ages of both the reworkedelements and the matrix of breccias and graywackes.

Hiatuses: Possible Pauses in SedimentationThe major discontinuity, in Sample 56-2, 18 cm, is

the contact between black, gray, or green middle Ceno-manian sediments and Senonian sediments (Sigal, thisvolume). The latter are composed of fine, red clays, in-terpreted as autochthonous, and thin, green layers withcoarser reworked elements (carbonate gravels, radio-larians. This abrupt change in lithology seems to corre-spond to (1) a pause in sedimentation (late Cenomanianto early Senonian) or condensed sedimentation dur-

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Figure 1. Leg 47B, Hole 398D petrographic features in thin sections of the Cretaceous andPaleocene sediments.

420

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398D.

ing the same period; (2) deepening of the bottom(or shallowing of the CCD) based on the fact thatthe lower, Cenomanian autochthonous sedimentscontain some organisms (radiolarians), while theyounger red clays do not; and (3) a complete changeof redox conditions, from reducing before and duringthe middle Cenomanian, to oxidizing later.

Such a sedimentary discontinuity is well known atapproximately the same period at other localities in theAtlantic (Hollister, Ewing et al. 1972) and also in theTethyan paleo-ocean (Bourbon, in press a). It may berelated to a major tectonic (and/or climatic ?) event.

A minor discontinuity may be present at the bound-ary between the Cretaceous and the Tertiary. The ship-board book describes a strongly burrowed interval at

this level, together with a thin clastic bed. I have notstudied thin-sections from this interval, but I think itmay correspond to a hard ground and a short intervalof non-deposition. Such a pause in sedimentation iswell established for this period in different parts of theworld (Pacific, Atlantic, Tethys; van Andel et al., 1976;Lemoine, 1953; Bourbon, in press b). Nevertheless,this possible hiatus is not perceptible from the micropa-leontological study (Sigal, this volume).

ACKNOWLEDGMENTS

I would like to thank the Deep Sea Drilling Project andespecially William B.F. Ryan and Jean-Claude Sibuet for giv-ing me the opportunity to work on the sediments of Leg 47B,

TABLE 1Petrography and Age of the Main Reworked Fades in Hole 398D

Sample(Interval in cm)

33, CC

54-1, 127-129

104-1,56-58

105-1, 16

106-1,8-10

106-1, 100-115

112, CC

116-1,104-106

117-4,52-56117-4,76-79118-2,72-74118-2,144

124-3, 56-72

125-3, 13-15

Petrography ofReworked Elements

Grainstone withoncolites, pellets

Grainstone withoncolites, pellets

Graywacke(see below)

Micrite

Micrite

Micrite

Micrite

Micrite

Graywacke : Qz,Feldspar, biotite,Muscovite,Tourmaline, zirconcarbonate gravels

Micrite

Grainstone withoncolites, pellets

Fauna in the Elements

Echinoids, benthic foramin-ifers, etc.

Echinoids, benthic foramin-ifers, etc.

C. alpina

C. alpina (dominant)C. parvula, T. carpathica

C. alpina (dominant)T. carpathica, C. boneti

C. alpina, T. carpathica(dominant), T. longaC. simplex ?, C. oblonga,A. subacuta ?

C. alpina

C. alpina (dominant)T. carpathica

Echinoids, benthic foramin-ifers, etc.

Age of theElements

?

?

Variscan ? (ageof the basement)

Late Tithonian-Berriasian

Late Tithonian

Late Tithonian

Late Berriasian

Late Tithonian-Berriasian

Variscan ?

Late Tithonian-Berriasian

9

Age of the Matrix(from Sigal,this volume)

Early Eocene

Senonian

Late Aptian

Late Aptian

Late Aptian

Late Aptian

Late Aptian

Aptian

Aptian

Early Aptian

Early Aptian

Age Difference BetweenElements and Matrix

(m.y.)

1

15 0?

25-35

35

35

25

20-30

150?

20-30

1

421

M. BOURBON

and the Centre National pour 1'Exploitation des Oceans for thefinancial support.

I am also grateful to A. Rehault, P. Debrabant, and J. Sigalfor the thin sections they loaned me, and to all my colleaguesfor our fruitful discussions.

My thanks are also due to Daniel Bernoulli and Edward L.Winterer for review of this text.

REFERENCES

Bourbon, M., in press a. Reconstitution palémorphologiquede fonds marins sur la marge Nord-Tethysienne: le Ju-rassique supérieur et le Crétacé Briançonnais externes prèsde Briançon. Colloque ATP "Géodynamique de la Médi-terranée et de ses abords."

, in press b. Nature et repartition des minerali-sations liées aux lacunes dans le Mésozoique et le Pa-

léocène pélagiques Briançonnais (region de Briançon). Es-sai de comparaison avec certaines mineralisations desoceans actuels. Colloque ATP "Géodynamique de laMéditerranée et de ses abords."

Hollister, C. D., Ewing J. I., et al., 1972. Initial Reports of theDeep Sea Drilling Project, v. 11: Washington (U.S. Gov-ernment Printing Office).

Lemoine, M., 1953. Le problème de la transgression desmarbres en plaquettes dans la zone Briançonnaise. C. R.Acad. Sci., Paris, v. 236, p. 1056-1058.

, 1967. Brèches sedimentaires marines à la fron-tière entre les domaines Briançonnais et piémontais dansles Alpes Occidentales, Geol. Rundschau, v. 56, p. 320-335.

van Andel, T. H., Thiede, J., Sclater, J. G., and Hay, W. W.,1976. Depositional history of the South Atlantic oceanduring the last 125 million years, Journal of Geology.

422

M. BOURBON

PLATE 1

Figure 1 Thin section of calcareous breccia: the pebblesconsist of pelagic white limestone with radiolar-ians; the matrix shows fluidal deformation andconsists of marly limestone, with radiolariansand vegetal debris.Sample 398D-128-4, 37-39 cm (Barremian).

Figure 2 Thin section of slumped bed of calcarenite,marly limestone, and silt: the marly limestoneand the silt contain vegetal debris.Sample 398D-126-6, 100-102 cm (lower Ap-tian).

Figures 3, 4 Thin sections of calcareous breccia resultingfrom slumping: the pebbles are made up of pe-lagic white limestone with radiolarians; the ma-trix consists of pelagic marly limestone with ra-diolarians and vegetal debris.Sample 398D-125-4, 66-68 cm (lower Aptian);Sample 398D-124-3, 56-60 cm (lower Aptian).

Figure 5 Thin section of graywacke: probably slumpedand with internal unconformities.Sample 398D-118-2, 144 cm (Aptian).

Figure 6 Thin section of slumped bed of marly and siltylimestone with vegetal debris.Sample 398D-112-5, 36-38 cm (upper Aptian).

424

PETROGRAPHIC AND SEDIMENTOLOGICAL STUDY

PLATE 1

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PLATE 2

Figure 1 Thin section showing accumulation of vegetaldebris in a pelagic limestone.Sample 398D-129-6, 105-107 cm (Barremian).

Figure 2 Thin section with pyritized burrows, calcitizedand partially pyritized radiolarians in a micriticpelagic limestone.Sample 398D-96-5, 76-78 cm (lower Albian).

Figure 3 Thin section showing microspherules or smallmasses of pyrite, partially pyritized radiolariansand pyritic infilling of foraminifers in a slightlycalcareous claystone.Sample 398D-65-7, 8-10 cm (upper Albian).

Figure 4 Thin section of sinuous bed of microspherulesor small masses of pyrite in a marly limestone.Sample 398D-92-4, 103-105 cm (lower Al-bian).

Figure 5 Thin section of siliceous radiolarians or moldsof radiolarians in a claystone: bed of pyritic mi-crospherules at bottom of photograph.

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