41. MAGNETIC ANISOTROPY AND SEDIMENT TRANSPORT DIRECTIONS IN NORTHATLANTIC EARLY CRETACEOUS BLACK SHALES AND EOCENE MUDSTONES CORED ON

DSDP LEG 48

Ernest A. Hail wood and William O. Sayre, Oceanography Department, University of Southampton, S09 5NH, U.K.

INTRODUCTION

The property of anisotropy of magnetic susceptibility(ams) provides a rapid and potentially precise method forestimating the degree and direction of preferred alignmentof assemblages of magnetic mineral grains. In the case ofsedimentary particles of grain size >50 µm this alignmentis commonly due to a combination of gravitational forces,which tend to make elongate grains lie with their long axesin the horizontal plane, and hydrodynamic forces, whichcause the long axes to be aligned in a direction related to theazimuth of flow. Under favorable circumstances,determination of this direction of alignment frommeasurements of the AMS can provide a precise measure ofthe direction of sediment transport, and place constraints onthe possible magnitude of bottom currents. Suchinformation is of considerable value to paleoenvironmentalstudies.

As part of a wider program aimed at establishinggeneralized paleocirculation trends during the early stagesof opening of the Atlantic Ocean, this paper describes anAMS study carried out on the sequence of Aptian-Albiancarbonaceous mudstones ("black shales") recovered fromHoles 400A and 402A (northeast Bay of Biscay), and thePaleocene-Eocene glauconitic tuffaceous mudstones fromSite 403 (southwest Rockall Plateau), on IPOD Leg 48.

Theoretical Background to AMS

AMS refers to the variation of magnetic susceptibilitywith direction, within an individual magnetic mineral grainor within an assemblage of such grains. The magneticsusceptibility, Xy, represents the relationship between acomponent of induced intensity of magnetization Ji and theinducing magnetic field //•:

Ji = ×ijHf (1)

The susceptibility may be conveniently approximated by atriaxial ellipsoid, with three orthogonal principal axes oflength Xmàx, Xjnt, and Xmjn. AMS in rocks commonlyoriginates from two sources, magnetocrystallineanisotropy, in which the principal susceptibility axes arerelated to the crystallographic axes of the mineral grain, andshape anisotropy which, in elongate mineral grains, may beconsidered due to variation of the self-demagnetizing factorwith direction, such that it is easier to magnetize an elongategrain along its long axis than at right angles to it. One of thecommonest sources of AMS in rocks is the mineraltitanomagnetite, which is crystallographically isotropic andits observed magnetic anisotropy, therefore, due to shapeeffects alone. In this case the maximum susceptibility axis

will correspond with the long axis of the mineral grain, andin a sediment sample comprising an assemblage of suchgrains it will define the direction of preferred orientation ofgrain long axes. As with an individual grain, the AMS of anassemblage of elongate grains may be described by a triaxialellipsoid.

Extensive laboratory redeposition experiments,summarized by Hamilton and Rees (1970) havedemonstrated that in the case of a "primary style" magneticfabric, formed by grain-by-grain deposition, the Xπún axiscommonly lies close to the vertical. This represents thetendency for elongate grains to lie in the horizontal plane,and consequently to define a "magnetic foliation plane"which is approximately parallel to the bedding. Wheredeposition occurs from flowing water, the Xmax axis definesa "magnetic lineation," which normally lies parallel withthe direction of flow.Susceptibility Parameters

A useful parameter which may be calculated from themagnitude of the principal susceptibility axes is the"azimuthal anisotropy quotient," q (Rees, 1966):

q = • (2)

This parameter represents a measure of the relativemagnitude of linear and foliar elements, and ranges in valuefrom zero for a pure foliation to 2.0 for a pure lineation.Studies of natural sediments, together with laboratoryredeposition experiments under carefully controlledconditions, indicate that primary (depositional) style fabricsproduced by grain-by-grain deposition commonlycorrespond with values of q in the range 0.06 to 0.67.Values outside this range frequently represent the effects ofsecondary processes, such as soft-sediment deformation dueto slumping or coring disturbances, or the effect ofbioturbation (Hamilton and Rees, 1970). In the presentstudy q values lying within the above range, together withX ^ directions lying within 15 degrees of the vertical, areconsidered as indicative of a "primary style" magneticfabric, which is potentially capable of providinginformation on bottom transport directions.

Instrumentation

In this study, all measurements of AMS have beenperformed on a low field torque magnetometer (LFTM) atan applied field of 100 Oe (peak). The value of theparameter X Gi% (King and Rees, 1962) was calculated forall samples, from the deflections observed on the LFTM.The parameter provides a measure of instrumental errors,

909

E. A. HAILWOOD, W. O. SAYRE

and the magnitude of harmonics other than the desiredsecond harmonic in the magnetic torque curves. In thepresent study, anisotropy determinations from sampleswhose X Gi% value exceeded 40 per cent were rejected, onthe grounds that they might reflect significant instrumentaleffects.

Magnetic Constituents

Thermomagnetic analyses have been carried out on anumber of representative samples from Holes 400A, 402Aand Site 403, utilizing a horizontal beam thermobalance; atypical thermomagnetic curve is shown in Figure 1. Thiscurve exhibits a well-defined Curie point at 570°C, andindicates that the dominant magnetic material in thesesediments is titanomagnetite, with a composition close tothat of pure magnetite. Consequently the observed magneticanisotropy may be interpreted as due to the shape anisotropyof crystallographically isotropic magnetite, and is likely toreflect the preferred alignment of elongate magnetite grains.

Orientation of Samples

The paleomagnetic properties of the samples selected forAMS determinations previously had been studied in detail(Hailwood, this volume), and only samples showing awell-defined stable remanence direction were chosen. Byassuming that the direction of stable remanence in eachsample defines the direction of the geographic azimuth atthe time of deposition, it is possible to specify the directionsof the three principal susceptibility axes with respect to thecontemporaneous north pole. This allows the inferreddirections of alignment of magnetic mineral grains to beplotted on paleogeographic reconstructions, and therelevance of the inferred sediment transport directions to bediscussed in the context of the paleoenvironmentalevolution of the continental margins.

The applicability of this technique for providingazimuthal orientation of inferred sediment transportdirections depends upon the assumption that the stablemagnetic remanence is carried by the very fine grainedferromagnetic particles, possibly those approaching singledomain size (—0.5 µm), whereas the AMS is dominated bythe effect of larger grains, possibly those exceeding 50 µm

50 100 150 200 250 300 350 400 450 500 550 600 650 700Temperature CO

Figure 1. Thermomagnetic curve for typical sample fromSite 403.

in size. This assumption is supported by calculations of theapproximate values of magnetic, hydrodynamic, andgravitational couples likely to act on particles of differinggrain size (e.g., Rees and Woodall, 1975). For particles of—(– µm in diameter the couple due to the earth's magneticfield acting on the particle's remanence is dominant, but forincreasing grain size the magnitude of the hydrodynamiccouple increases at a faster rate than that of the magneticcouple. Because of uncertainties in the magnitude ofparameters such as fluid drag force, average grain shape,and intensity of magnetization, it is difficult to calculate thetheoretical limiting grain size at which hydrodynamic forceswill dominate over magnetic aligning forces, but a value ofabout 50 µm is likely for the type of sediments utilized inthis study.

The accuracy of the azimuthal orientation provided by thedirection of stable remanence will also be governed by theprecision with which the remanence is defined, and theextent to which secular variation of the geomagnetic fieldhas been averaged out in a single sample. The stableremanence directions are considered to be defined with aprecision of better than ±5 degrees for the majority ofsamples studied from Site 403, and ±15 degrees for thosefrom Holes 400A and 402A. The extent to which secularvariation has been averaged out in each sample dependsupon the sedimentation rate, assuming the remanence to bedetrital in origin. The time necessary for completeaveraging of secular variation, so that the mean magneticpole calculated on the basis of a centered axial dipole modelshould coincide with the geographic pole, is commonlyaccepted to be at least I04 years. Using thepaleontologically determined sedimentation rates at Holes400A, 402A and Site 403, the time interval represented by asingle 2.5-cm-diameter paleomagnetic sample for each ofthese sites is 0.2 × I04, 0.06 × I04, and 0.03 × I04,respectively. Consequently, relatively complete averagingof geomagnetic secular variation may be anticipated for thesamples from Hole 400A, and less-complete averaging forthose from Hole 402A and Site 403. Since, at this latitude,the amplitude of declination changes due to geomagneticsecular variation is in the order of 20 to 30 degrees, errors ofup to this magnitude may exist in the orientation ofsusceptibility axes for samples from the latter two sites.However, the magnitude and direction of these errorsshould differ randomly from one sample to another, so thatthe mean susceptibility axis orientation derived from anumber of samples at a particular site will be subject to amuch lower uncertainty than the values quoted above.

RESULTS

Aptian-Albian Black Shales From Northeast Biscay

The Aptian-Albian black shales from Holes 400A and402A comprise a sequence of dark colored, organic-rich,carbonaceous mudstones, rhythmically interbedded withlayers of lighter colored calcareous mudstone and siltyclaystone that are relatively depleted in organic material.

The thickness of one complete dark-light rhythm variesfrom about 30 cm for the Albian sediments at Hole 400A to50 cm for the Aptian sediments at this site, and 100 cm forthose at Hole 402A.

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MAGNETIC ANISOTROPY

Samples for magnetic anisotropy determinations weretaken only from the dark, organic-rich carbonaceous layers.The reasons for this were twofold. First, the intensity ofmagnetization of the interbedded light colored layers wasgenerally too weak to allow an accurate determination ofstable remanent magnetization for orientation purposes.Second, the dark carbonaceous mudstones appear to havebeen deposited under relatively anoxic conditions, hostile tothe development of benthic communities and therefore notbioturbated; consequently the original primary sedimentarystructures are commonly preserved. This low level ofpost-depositional deformation renders the sediments aparticularly attractive target for magnetic fabric studies.

Hole 400AA total of 31 samples was selected from the Ap-

tian-Albian carbonaceous mudstones at this site, and the ma-jority of these exhibited "primary-style" fabrics. Acontoured equal-area upper hemisphere projection of themaximum and minimum susceptibility axes of thesesamples is presented in Figure 2. A tight distribution ofminimum susceptibility axes exists close to the vertical,indicating the presence of a well-developed near-horizontalmagnetic foliation in these sediments. From the clusteringof the maximum susceptibility axes in the northeastquadrant, a well-defined magnetic lineation is alsoidentifiable. This lineation may be interpreted in terms of anortheast-southwest preferred alignment of mineral grainsresulting from bottom transport in this direction. Thesubsidiary grouping of maximum susceptibility axes with anortherly trend may indicate that the direction of bottom

MEAN MAGNETIC NORTH

UPPER HEMISPHERE

Figure 2. Contoured equal area projection of directions ofmaximum and minimum principal susceptibility axes forEarly Cretaceous samples from Hole 400A.

transport oscillated between a northerly and a northeasterlydirection. Alternatively it could represent the dominance ofthe induced magnetic aligning couple over thehydrodynamic couple during deposition of those samplesshowing a northerly Xmax azimuth. Further experiments onthe possible relationship between grain size of the magneticconstituents and Xmax orientation are in progress in thislaboratory to attempt to discriminate between these twoalternative explanations for the northerly Xmax azimuths.

The downhole variation of key anisotropy parameters inthe Lower Cretaceous carbonaceous mudstones at Hole400A is shown in Figure 3. Because the maximumsusceptibility direction is defined by a double-ended axis,there is ambiguity in choosing which end of this axis to plot.In view of the dominant northeast trend of the upwarddirected ends of these axes revealed by Figure 2, theconvention has been adopted of plotting the end closest tothe northeast quadrant in Figure 3. In addition to thedominant north-northeast trend, Figure 3 reveals asystematic, or possibly progressive, shift in azimuth of Xmax

through some 90 to 100 degrees close to the base of thelower Albian.

Laboratory experiments on sediments deposited from adilute suspension (summarized by Hamilton and Rees,1970) have demonstrated that a well-developed magneticlineation accompanied by low q values, typically less than0.42, may be produced either by strong and persistentbottom currents (e.g., Hamilton, 1967) or as a result of theshear produced by deposition on a slope (Rees, 1968).There is some suggestion that it might be possible todiscriminate between these two mechanisms from thedirection of imbrication of the maximum susceptibilityaxes. In the case of grain-by-grain deposition from bottomcurrents, the maximum susceptibility axes are commonlytilted through a small angle (—10°) away from thehorizontal, with the upward directed ends pointing in adownstream direction. This appears to correspond with themost stable configuration of grain long axes, and is referredto as a negative imbrication. In contrast, initial experimentssuggest that deposition on a sloping interface in still waterresults in the maximum susceptibility axes being tiltedupwards in a direction upstream, i.e., the production of apositive imbrication.

The direction of the magnetic lineation at Hole 400A isplotted on an Early Cretaceous paleogeographicreconstruction of northwest Europe in Figure 4a. In thisreconstruction the orientation of the continental margin hasbeen determined from continental paleomagnetic data(Smith et al., 1973). It is clear from this diagram that thedirection of the magnetic lineation at Hole 400A isapproximately perpendicular to the contemporaneouscontinental margin. Because it is unlikely that the sedimentwas transported in an upslope direction, it is reasonable toassume that the observed magnetic lineation was producedby sediment movement toward the southwest, rather thannortheast. Consequently, the northeast grouping of theupward directed ends of the AMS (Figure 2) indicates theexistence of a positive imbrication. This conclusionsuggests that the strongly developed magnetic lineation atthis site may have been produced as a result of deposition ona slope.

911

E.A. HAILWOOD, W.O. SAYRE

650260 280 300

×maxMEAN MAGNETIC NORTH

320 340 t 20 40 60 80

675 -

q

0 .4

ΣG%

0 10 20 30 40 50

700 -

725 -

750 L

AGE

LATEALBIAN

EARLY-MIDDLEALBIAN

LATEAPTIAN

LU

E×<

ACCEPTABLE RANGE

• PRIMARY STYLE FABRICO PROBABLY PRIMARY STYLE FABRICX SECONDARY STYLE FABRIC OR HIGH ΣG\% VALUE- INDICATES UPWARD DIRECTED END OF AXIS IS PLOTTED+ INDICATES DOWNWARD DIRECTED END OF AXIS IS PLOTTED

Figure 3. Downhole variation of key susceptibility parameters at Hole 400A. For explanation of symbols see text. "Probablyprimary style" fabric refers to samples whose susceptibility parameters lie close to, or just outside the accepted range fortrue primary style fabrics, yet do not show clearly developed secondary style characteristics.

The laboratory experiments discussed above refer todeposition from dilute suspension, in which intergranularinteractions are unimportant. Experiments in whichsediments were deposited from concentrated dispersions,such as turbidity currents, appear to reveal more complexmagnetic fabrics. For example, Rees and Woodall (1975)describe two redeposition experiments in which sand andsilt, respectively, were deposited from a density currentonto an interface dipping at 6 degrees downstream. A strongpositive imbrication was produced for the sand, and aweaker negative imbrication for the silt. Full assessment ofthe relevance of these, and other experiments forinterpreting the Leg 48 anisotropy data, must await detailedgrain-size determinations on both the magnetic andnon-magnetic fractions, and the outcome of furtherlaboratory redeposition experiments. However they serve toindicate that the positive imbrication of the Xm a x axes in theHole 400A black shales could have been produced not onlyby deposition onto a slope from relatively still water, but

possibly also by deposition from density currents flowingdown the continental slope toward the southwest.

Although magnetic anisotropy determinations were notcarried out on the interbedded light-colored calcareousmudstones and silty claystones, these sediments (above adepth of 700 m) show abundant evidence of slumping,including flow structures, microfaulting, and slump folding,and may well have been emplaced by mass downslopemovement. This supports the suggestion of deposition of theinterlaminated dark carbonaceous mudstones on, or near to,a slope.

Hole 402A

The directions of the maximum and minimum principalsusceptibility axes of the 61 samples of Lower Cretaceousmudstone studied from Hole 402A are shown on acontoured equal area projection in Figure 5a. As with Hole400A, a well-developed near-horizontal magnetic foliationis indicated by the steeply inclined minimum susceptibility

912

CLAY FRACTION MINERALOGY

a) EARLY CRETACEOUS(APTIAN-ALBIAN)

V///400A '

(b) EARLY CENOZOIC(PALEOCENE-EOCENE)

1,1 ',»»,

1V* ->'

Figure 4. üαr/j; Cretaceous and early Cenozoic paleogeographic reconstructions of Biscayand Rockall areas, with inferred sediment transport axes superimposed. Orientation ofcontinental margins is derived from land-based palaeomagnetic determinations (e.g.,Smith et al, 1973).

axes of a large proportion of the samples. The samedominant shallow inclination Xmax trends (northeast andnorth) are evident at this site, as at Hole 400A, but inaddition there is some stringing into the northwest andsouthwest quadrants, and approximately 25 per cent of thesamples show near-vertical Xmax axes. The latter mayrepresent the effect of regional or drilling deformation or theinfluence of sample shape (Kent and Lowrie, 1975).Consequently, the anisotropy data have been replotted inFigure 5b after exclusion of all samples with high Xmax

inclinations. The resulting plot shows many similarities tothat for Hole 400A (Figure 2), and in both cases thedominant feature of the Xmax distribution is thenortheast-southwest and north trends.

A conspicuous difference between the magnetic fabric atthe two sites is the lack of a clear imbrication at Hole 402A;as with Hole 400A, the inferred axis of sediment transport atthis site is approximately perpendicular to thecontemporaneous continental margin (Figure 4a).

It seems probable that the difference in degree ofimbrication is related to the differing environment ofdeposition. Seismic profiles indicate that a difference inrelief of some 2000 meters existed between these two sitesduring Aptian-Albian times (Roberts, this volume), andpaleontological evidence suggests that the carbonaceousmudstones at Hole 400A were probably deposited at waterdepths of only a few hundred meters, whereas those at Hole402A were deposited in a bathyal environment (Hailwood etal., this volume).

At Hole 402A there is lithological evidence that thesediments in the depth range 270 to 385 meters sub-bottommay have been deposited in a "delta front" environment

characterized by deep channels, levees, and pools. Withinthis sequence, sediments from the range 335 to 385 meterssub-bottom contain mud gravels, wood fragments, andevidence for local slumps, and may have been deposited insome form of deep open channel. In contrast, sedimentsfrom the interval 270 to 335 meters sub-bottom showcharacteristics similar to modern "tidal flat" deposits,including laminations suggestive of variation in flowregime, slight to intense bioturbation (mostly of theinterbedded marly limestones), presence of pelecypod andgastropod fragments, and well-rounded glauconite grains.

The boundary between these two facies, at approximately335 meters sub-bottom, corresponds with an importantchange in the magnetic fabric characteristics (Figure 6).Below this depth the inferred "channel deposits" showrapidly varying xmax azimuths, and a higher incidence of"secondary style" fabrics, perhaps reflecting the influenceof local slumps. Above this level the inferred "tidal flat"deposits show less variability in both xmax azimuth and qvalue.

The important contribution of the magnetic fabric studiesis in the recognition of a well-defined preferred alignment ofgrain long axes, particularly in the latter deposits, which isoriented approximately perpendicular to the contemporane-ous continental margin (Figure 4a). This is consistent withthe proposed model of a "delta-front" environment at Hole402A in Aptian-Albian times, with sediment being activelytransported across the continental shelf, and down the conti-nental slope towards Hole 400A.

Comparison between the xmax azimuths at these two sites(Figures 3 and 6) suggests the possible occurrence of asystematic shift of magnetic lineation trend through some 80

913

E. A. HAILWOOD, W. O. SAYRE

MEAN MAGNETIC NORTH402A ALL DATA

MEAN MAGNETIC NORTH

UPPER HEMISPHERE

402A PRIMARY-STYLE FABRICSMEAN MAGNETIC NORTH

b.

MEAN MAGNETIC NORTH

UPPER HEMISPHEREmiπ

12% \•<:•:'• 16%

Figure 5. Contoured equal area projections of maximum and minimum principal susceptibility axes for Early Cretaceoussamples from Hole 402A.

914

MAGNETIC ANISOTROPY

Xmax q Xmin 'NC.

MEAN MAGNETIC NORTH260 280 300 320 340 jf 20 40 60 80 0 .4 .8 1.2 1.6 0 20 40 60 80

Σ G %

0 10 20 30 40 50 60

AGE

300 -

ACCEPTABLE RANGES

PRIMARY STYLE FABRICO PROBABLY PRIMARY STYLE FABRICX SECONDARY STYLE FABRIC OR HIGH SGi% VALUE- INDICATES UPWARD DIRECTED END OF AXIS IS PLOTTED+ INDICATES DOWNWARD DIRECTED END OF AXIS IS PLOTTED

Figure 6. Downhole variation of key susceptibility parameters at Hole 402A. Symbols and notation as in Figure 3.

to 100 degrees near the base of the lower Albian at bothsites. Since the recognition of this shift at Hole 400A isbased on only three samples, it must be regarded as highlytentative at the present time. However, if further moredetailed studies bear it out, the possible occurrence of a

synchronous event recorded in the preferred alignment ofmineral grains at these two sites is suggested. Furtherspeculation on the possible cause of this feature isconsidered premature until additional magnetic fabricstudies have been completed.

915

E. A. HAILWOOD, W. O. SAYRE

Significance of Magnetic Anisotropy Determinations for Holes400A and 402A: Origin of the Black Shales

Laminated mudstones with high organic carbon content,frequently referred to as black shales, have been identifiedin DSDP cores of Jurassic, and Early and Late Cretaceousage from the Atlantic, Pacific, and Indian oceans; threecontrasting models have been put forward to account forthese unusual organic-rich sediments. The first (referred tohere as Model 1) entails episodes of basin-wide abyssalstagnation, such as that occurring at the present time in theBlack Sea (Degens and Ross, 1974) where a markedpycnocline at about 200 meters water depth separates thewarm, low-salinity, well-oxygenated surface waters fromthe denser saline anoxic deep water masses. The resultantstable density stratification inhibits the overturning of watermasses, and the oxygenation of the deeper regions. Similarepisodes of basin-wide abyssal stagnation are known tohave occurred in the Mediterranean Sea during thePleistocene, and may well have occurred in the Atlantic andIndian oceans in Cretaceous times, when these oceansoccupied narrow and restricted seaways, during the earlystages of fragmentation of Gondwanaland. An alternativemodel for the origin of black shales (Model 2) involves theirdeposition beneath the oceanic mid-water oxygen-minimumdeveloped beneath highly productive surface water massesalong certain continental margins, particularly thoseassociated with the upwelling systems of eastern boundarycurrents. This model appears to adequately account for thedistribution of Late Cretaceous euxinic sediments in theSouth Atlantic (Thiede and van Andel, 1977) and also thefact that anoxic sediments of this age in the Pacific appear tobe restricted to the flanks of seamount chains, whilewell-oxygenated conditions existed in deeper water(Winterer, Ewing, et al., 1973).

A third possible explanation for the origin of organiccarbon-rich sediments (Model 3) is the occurrence ofunusually high inputs of terrestrial organic material,particularly plant remains, derived from coastal swamps andlagoons, or transported by rivers from the continentalinteriors, and carried into the offshore region by submarinecurrents. The preservation of this material would be duemore to the high rate of supply than to the existence of ananoxic environment, but the decay of such large volumes oforganic material would tend to rob the bottom waters ofoxygen and, perhaps, produce local euxinic conditions.

The magnetic anisotropy studies of the Aptian-Albianblack shales recovered from Holes 400A and 402A havesignificant bearing on ascertaining which of the threealternative models is most applicable to these particularsediments. Models 1 and 2 (deposition in a stagnant basin,or beneath a local mid-water oxygen depleted zone) imply alow level of oxygen replenishment in bottom waters and,hence, sluggish bottom water movement. Model 3 (highinput of terrestrial organic material) requires an extremelyactive downslope bottom transport mechanism.

The inferred existence of well-defined preferredorientations of magnetic mineral grains approximatelyperpendicular to the continental margin support thesuggestion that the Aptian-Albian organic-rich sediments atshallow water Hole 402A were deposited in a "delta front"

environment in which large volumes of terrestrial organicmaterial may have been transported into the offshore region.The magnetic fabric of the deep water sediments at Hole400A is consistent with deposition on a slope, and possiblyalso with the periodic influx of organic-rich material fromturbidity currents. Thus, although the exact depositionalenvironment cannot yet be established from magneticanisotropy studies, existing data from both sites are morecompatible with the dynamic environment implied byModel 3, rather than the relatively static environmentrequired for the first two models. Further evidence that theAptian-Albian black shales at Holes 400A and 402Aresulted from rapid input of terrigenous material is providedby the shipboard pyrolysis studies (Tissot et al., thisvolume).

Paleocene-Eocene Tuffaceous Mudstones From Rockall

Magnetic fabric determinations have been completed on atotal of 76 samples from the early Paleogene sediments ofdeltaic aspect recovered at Site 403, near the rifted westerlymargin of the Rockall Plateau. Of these, 56 per cent show adefinite primary style magnetic fabric; 28 per cent showXGi% values close to 40 per cent or q and λVin inclinationvalues that are just outside the "primary style" limits, andhave been classified as "probably primary style." Theremaining 16 per cent have been rejected, either becausethey show clear "deformational style" fabrics, or %& percent values >40 per cent, that could reflect significantinstrumental errors.

A contoured equal area projection of the maximum andminimum susceptibility axes for the samples from this siteshowing "primary" and "probably primary" style fabricsis shown in Figure 7. Although the orientation of xmax

a x e s

extends over a wide range of azimuths, the highestconcentrations occur within or close to the southeastquadrant. This effect is also apparent in the downhole plot(Figure 8), from which it is clear that no systematic changewith depth in xmax azimuth can be identified.

The overall average Xmax azimuth is plotted on apaleomagnetic reconstruction of the northeast Atlanticregion for early Paleogene times in Figure 4b. The inferredaxis of bottom transport is approximately perpendicular tothe incipient continental margin between Greenland andRockall. It is noteworthy that the xmax axes are directedupwards in a southeast direction, i.e., the direction ofupward imbrication is consistently away from the Greenlandmargin. The overall mean q value for these sediments is0.29 ±0.03, which value lies within the range normallyencountered in laboratory experiments for grain-by-graindeposition from running water. There appear to be nogeological reasons for suspecting that these shallow watersediments may have been deposited from concentratedsuspensions such as turbidity currents. Thus, assuminggrain-by-grain deposition from bottom currents, the resultsfrom Site 403 are consistent with transport of the earlyPaleogene sediments from the northwest (Greenland) ratherthan from the southeast (Rockall). At first sight this seemssomewhat surprising, in view of the present location of thesite on a margin which slopes to the northwest. However,paleomagnetic measurements indicate that the entire

916

MAGNETIC ANISOTROPY

Data passing MEAN MAGNETIC NORTH

UPPER HEMISPHERE

Figure 7. Contoured equal area projection of maximumand minimum principal susceptibility axes for earlyPaleogene samples from Site 403.

200-meter section of early Paleogene sediments at Site 403is reversely magnetized (Hailwood, this volume). Acomparison with the magneto-biostratigraphic record fornearby Sites 404 and 405 indicates that these sedimentsacquired their magnetization during the long reversepolarity interval that preceded anomaly 24, the oldestmarine magnetic anomaly in the Northeast Atlantic.Consequently these sediments were probably depositedduring the very early stages of rifting of Rockall from EastGreenland, possibly before Site 403 had been detached fromthe East Greenland margin. The inferred early Paleogenedirection of sediment transport towards the southeast at Site403 is consistent with this proposed tectonic history.

CONCLUSIONS

Detailed magnetic anisotropy determinations from theLower Cretaceous black shales cored at Holes 400A and402A in the Bay of Biscay reveal a well-developedmagnetic lineation. It is proposed that this lineation wasproduced by a strong and persistent bottom transportmechanism, oriented perpendicular to the contemporaneouscontinental margin. The existence of such an activetransport mechanism makes it unlikely that the high organiccarbon content of these sediments results from theirdeposition in a stagnant basin, and supports the alternativeview that their organic-rich nature results from an unusuallyhigh influx of terrigenous plant material, perhaps derivedfrom coastal swamps and lagoons.

MEAN MAGNETIC N E S W

20 40 60 80 100 120 140 160 180 200 220 240 260 280

250 -

275 -

•B 300 -

UJ 325 -

0 .2.4 .6.81.0

Σ G %

0 10 20 30 40 50 60 70 80 90 100

AGE

350 -

375

400 -

425 -• PRIMARY STYLE FABRIC

O PROBABLY PRIMARY STYLE FABRIC

X SECONDARY STYLE FABRIC OR HIGH ΣG!% VALUE

UPWARD DIRECTED ENDS OF AXES PLOTTED IN ALL CASES ACCEPTABLE RANGES

Figure 8. Downhole variation of key susceptibility parameters at Site 403. Symbols and notation as in Figure 3.

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E. A. HAILWOOD, W. O. SAYRE

The magnetic anisotropy of the thick sequence ofglauconitic mudstones and volcaniclastic siltstones cored atSite 403, near the rifted westerly margin of the RockallPlateau, also reveals a strong preferred grain alignmentapproximately perpendicular to the present continentalmargin. However the imbrication of the xmax axes isconsistent with transport from the northwest, rather than thesoutheast, and supports the view that these sediments werelaid down within a "deltaic" complex which drained thesouthern part of East Greenland in late Paleocene and earlyEocene times. Final separation of the Rockall Plateau,including part of this deltaic complex, from East Greenlandoccurred in the early Eocene, and this tectonic event isprobably represented by the hiatus between the early andmiddle Eocene sediments at Site 403 (Figure 8).

ACKNOWLEDGMENTSWe are very grateful to Pat Copland for her invaluable

assistance with anisotropy measurements of the Site 403 samples.We would also like to thank Tony Rees and Norman Hamilton fortheir helpful reviews of the manuscript. Financial support has beenprovided by the Natural Environment Research Council.

REFERENCESDegens, E.T. and Ross, D.A. (Eds.), 1973. The Black Sea: Its

geology, chemistry and biology: Am. Assoc. Petrol. Geol.Mem., v. 20, p. 1-663.

Hamilton, N., 1967. The effect of magnetic and hydrodynamiccontrol on the susceptibility anisotropy of redeposited silts, J.Geol., v. 75, p. 738-743.

Hamilton, N. and Rees, A.I., 1970. The use of magnetic fabric inpaleocurrent estimation. In Runcorn, S.K. (Ed.),Palaeogeophysics: New York (Academic Press), p. 445-464.

Kent, D.V. and Lowrie, W., 1975. On the magnetic susceptibilityanisotropy of deep-sea sediment, Earth Planet. Sci. Lett., v.28, p. 1-12.

King, R.F. and Rees, A.I., 1962. The measurement of theanisotropy of magnetic susceptibility of rocks by the torquemethod, J. Geophys. Res., v. 67, p. 1565-1572.

Rees, A.I., 1966. The effect of depositional slopes on theanisotropy of magnetic susceptibility of laboratory depositedsands, J. Geol., v. 74, p. 856-867.

, 1968. The production of preferred orientation in aconcentrated dispersion of elongated and flattened grains, J.Geol., v. 76, p. 457-465.

Rees, A.I. and Frederick, D., 1974. The magnetic fabric ofsamples from the Deep Sea Drilling Project, Legs I to VI, J.Sediment. Petrol., v. 44, p. 655-662.

Rees, A.I. and Woodall, W.A., 1975. The magnetic fabric ofsome laboratory-deposited sediments, Earth Planet. Sci. Lett.,v. 25, p. 121-130.

Smith, A.G., Briden, J.C., and Drewry, G.E., 1973. PhanerozoicWorld Maps. In Organisms and Continents Through Time:Systematics Association Publication 9, Special Papers inPalaeontology No. 12. London (Palaeontological Association),p. 1-42.

Thiede, J. and van Andel, T.H., 1977. The paleoenvironment ofanaerobic sediments in the Late Mesozoic South AtlanticOcean, Earth Planet. Sci. Lett., v. 33, p. 301-309.

Winterer, E.L., Ewing, J.I., et al., 1973. Initial Reports of theDeep Sea Drilling Project, v. 17: Washington (U.S.Government Printing Office).

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