Magnetic fabric of sediments from the shelf at La Jolla (California)

Marine Geology Elsevier PublishingCompany - Amsterdam Printed in The Netherlands

Letter Section Magnetic fabric of sediments from the shelf at La Jolla (California)

N. HAMILTONand A.I. REES University of Southampton, Southampton {GreatBritain] (Received April 13, 1970)

SUMMARY The magnetic fabric is described of seven oriented box cores taken in water depths less than 100 m. All are fine sands and except one, a relict beach sediment, all appear to have been in sedimentary equilibrium with present conditions. They show signs of burrowing. The present magnetic fabric of all samples show signs of disturbance. Four of the seven retain sufficient primary fabric for a current direction to be estimated. There is evidence that the fabric disturbance is a result of the organic activity. INTRODUCTION This note reports the results of measurements of the anisotropy of magnetic susceptibility of specimens taken from seven large oriented samples of sediment from the continental shelf immediately north of the Scripps Submarine Canyon. Susceptibility anisotropy in sands containing a small fraction of magnetite grains reflects the preferred orientation, if any, of the sediment grains in general and therefore can provide information about the forces responsible for grain alignment. This has been used to estimate the directions of depositing currents (e.g., Galehouse, 1968) and to demonstrate the effects of tectonic deformation on grain fabric (Graham, 1966). The usefulness of the method in the study of ancient rocks can be increased by the study of sediments deposited in present day natural environments or under controlled conditions in the laboratory. Several laboratory investigations have been reported (Hamilton and Rees, 1970), but the only substantial body of work on a contemporary environment has been on the sediments of the La Jolla Canyon and Fan (Rees et al., 1968). Much more is needed and this note is intended to contribute toward the accumulation of the required body of knowledge. THE MAGNETICFABRIC OF SEDIMENTS The current state of knowledge about magnetic fabric in sediments has recently been reviewed by Hamilton and Rees (1970). It is possible to distinguish a number of fabric Marine Geol,, 9 (1970) M6-M11

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types which may be either primary, that is produced by the processes of deposition, or secondary. The magnetic fabric of many sediments can be fitted quite closely by a symmetric second rank tensor and it is customary to adopt a convention that specifies such a tensor. The convention used here specifies the tensor by stating the orientation of the principal axes of maximum susceptibility (the magnetic lineation) and minimum susceptibility and the values ofKint, h and q where:

h_Km~ -Kmin Kmt and: Kmax - Kint q - (Kmax + Kint)/2 - Kmin Kmax, Kint and Kmin being the values of the three principal susceptibilities. Values of the inclination I of the Kmin-axis and o f q have been found useful in distinguishing between primary and secondary fabrics (Hamilton and Rees, 1965; Crimes and Oldershaw, 1967).

,/

~

32*

c~ ~°~

Scripps Pier

o Meters

~

.oo \ \

Locstion of t h e s s m p l e s lEstpmsted m e a n lineatiOn

i

Fig.1. The area studied with samplinglocalities and estimated mean lineation directions.

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THE SEDIMENTS Seven box samples (Bouma and Shepard, 1964) were taken at depths less than 100 m from the shelf immediately to the north of the Scripps tributary of the La Jolla Submarine Canyon (Fig. 1). The shelf is about 2 km wide and has a uniform seaward slope of about 4%. It is believed to be of Tertiary rocks with a thin veneer of modern sediments (Wimberley, 1955). Details of the mechanical composition of the samples are given in Table I. Sample 2 is atypical being probably a relict beach sediment. The others are moderately well-sorted micaceous fine sands containing a variety of organic debris. X-radiographs show that all the samples have been disturbed to some degree by organic activity. TABLE I MECHANICAL ANALYSES Sample

Percent less than 4 ~

Median diameter of coarse fraction (9)

Standard deviation of coarse fraction (9)

29.5 26.6 22.1 6.7 22.8 25.9

3.58 3.49 3.44 3.22 3.48 3.53

1.14 0.33 0.35 0.30 0.31 0.34

1

2 3 4 5 6 7

THE MAGNETIC FABRIC A summary of the magnetic fabric properties is given in Fig.2 and details of the fabrics of the seven samples are given in Table II and in Fig.3. The results are mixed. All samples have some proportion of specimens whose fabric appears to be secondary. However, apart from the elimination of sample 6 the

TABLE II MAGNETIC FABRIC PROPERTIES Sample

No. of specimen

Mean Kintl

Mean h (%)

Mean q

No. of specimens with q > 0.67 I <~60 °

1 2 3 4 5 6 7

6 10 10 9 7 8 10

505 112 155 194 147 154 144

9.05 3.19 4.87 4.04 4.04 4.31 4.97

0.55 0.60 0.35 0.51 0.46 0.58 0.47

1 3 1 3 2 2 1

0 2 1 1 0 5 1

1Approximately times 107 emu/cm 3. Marine Geol., 9 (1970) M6-M11

LETTER SECTION 50, = m c

M9 20.

D i s t r i b u t i o n of value~ of Kin t

Distribution

of

40-

~- 30" 10

Q. 20

o--

q

j,--,---,F/-4

O Kint/10-6

700

Oo

5

10

h%

per specimen Distribution of fq~.

20-

]

Sample shaded

1

i 10"

6 Z O

O

1I'D

2'0

Fig.2. Distribution of magnetic fabric properties. rejection of specimens with anomalous values o f / a n d q (I = 60 °, q = 0.67) does not have any very marked effect on the appearance of the results. Four of the samples (1, 4, 5, 7) have lineations grouped closely enough for a common mean direction to be estimated. The mean directions are plotted as bars over the appropriate sample locations in Fig. 1. The two near to the rim of the La Jolla Canyon are nearly parallel to it. The other two are nearly perpendicular to the strike of the sea floor. The nature and origin of the secondary fabric in the rejected specimens is of some general interest. The slope of the sea floor is so small and the sediments are so comparatively coarse that mass movement is unlikely to have caused deformation. Furthermore, the fabric style does not obviously resemble that shown by Graham (1966) to be the result of tectonic deformation or that suggested by Rees et al. (1968) to have been brought about during local deformation of soft sediments. The presence of animal and plant remains suggests that the secondary fabric may have been produced by organic disturbance. It has been confirmed by X-ray photography that considerable disturbance by plants and animals has taken place. The number of obviously deformed specimens is small, but the nature of the deformation can be demonstrated by a sample analysis of the results. It has been shown by Hamilton and Rees (1965) and by Graham (1966) that a systematic relationship can frequently be demonstrated in deformed fabrics between the deviations of the susceptibility axes from their primary directions and the relative magnitudes of the principal susceptibilities. In this study the former can be represented by I and the latter by h and q. The total and partial correlation coefficients between these three quantities, for all specimens except those from sample 1, are given in Table III. Marine Geol., 9 (1970) M6-M11

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2

.... ~

•

3

.

•

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•

4

Lower

Hemisphere

• Maximal Accepted Minima • •

•

Maximal Rejected Minima I

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~: IA:I&i\ ~

~x

x

Fig. 3. Directions o f susceptibility axes of the seven samples.

T A B L E III C O R R E L A T I O N COEFFICIENTS F O R THE PARAMETERS I, h AND q OMITTING SAMPLE 1 N = 54

Correlation coefficients for I, h and q

lh= 0 . 1 9 7 ; p > 1 0 % lq = - 0.205; p > 10% hq= - 0.467; p < 0 . 1 %

lh,q= 0.117 lq, h = - 0.130 hq,l = - 0.444 Marine GeoL, 9 (1970) M 6 - M l l

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These show no evidence for any correlation between directional and magnitude parameters. However, there is a very significant negative correlation between h and q. This correlation does not appear if the values from sample 1 are included, but it seems likely from its anomalously high values of Kint and h (Fig.2) that the susceptibility o f this sample is derived from a mineral assemblage different from that found in the others- perhaps in the ferromagnesians that are abundant in shallower water (Wimberley, 1955; Rees, 1965). h is a measure o f the total susceptibility anisotropy while q is the ratio of the strength of the magnetic lineation to that of the magnetic foliation, q increases therefore as a result o f decreasing strength o f foliation and does so without any great deviation of lineations from the horizontal. This can be achieved by a process that destroys preferentially components o f preferred orientation in a horizontal plane. The predominantly vertical activity of plants and burrowing organisms may provide such a process. CONCLUSION Of the seven samples all have some secondary fabric but four retain sufficient of a primary fabric for a probable current direction to be estimated. The directions appear to be controlled by sea floor topography. ACKNOWLEDGEMENTS This work was carried out in collaboration with Professor E.L. Winterer. We wish to thank him for his advice and support. We wish also to thank Professor F.P. Shepard for the loan o f the box corer and Neff Marshall and Larry O'Mara for help at sea and in the laboratory. The work was supported by ONR Contract 2216(05) and NSF Grant GP 3373. REFERENCES Bagnold, R.A., 1962. Autosuspension of transported sediment; turbidity currents. Proc. Roy. Soc. (London), Set. A, 265: 315-319. Bouma, A.H. and Shepard, F.P., 1964. Large rectangular cores from submarine canyons and fan valleys. Bull. Ant Assoc. Petrol. Geologists, 48: 225-231. Crimes, T.P. and Oldershaw, M.A., 1967. Palaeocurrent determinations by magnetic fabric measurements on the Cambrian rocks of St. Tudwal's Peninsular, North Wales. Geol. J., 5: 217-232. Galehouse, J., 1968. Anisotropy of magnetic susceptibility as a palaeocurrent indicator: a test of the method. Geol. Soc. A ~ , Bull., 79: 387-390. Graham, J.W., 1966. Significance of magnetic anisotropy in Appalachian sedimentary rocks. In: J.S. Steinhart and T.J. Smith (Editors), The Earth beneath the Continents- Am. Geophys. Union Monograph, 10: 627-648. Hamilton, N. and Rees, A.I., 1965. The anisotropy of magnetic susceptibility of the Franciscan rocks of the Diabio Range, central California, Tech. Mere., Marine Phys. Lab., Univ. Calif., San Diego, Calif., 46 pp., unpublished. Hamilton, N, and Rees, A.I., 1970. The use of magnetic fabric in palaeocurrent estimation. In: S.K. Runcorn (Editor), Palaeogeophysics. Academic Press, New York, N.Y., pp.445-464, in press. Rees, A.I., 1965. The use of anisotropy of magnetic susceptibility in the estimation of sedimentary fabric. Sedimentology, 4: 257-271. Rees, A.I., Von Rad, U. and Shepard, F.P., 1968. Magnetic fabric of sediments from the La JoUa Submarine Canyon and Fan (California). Marine Geol., 6: 145-178. Wimberley, C.S., 1955. Marine sediments north of Scripps Submarine Canyon, La JoUa, California. J. Sediment. Petrol., 25: 24-37. Marine GeoL, 9 (1970) M6-Mll