Magnetic fabric of sediments from the La Jolla submarine Canyon and Fan, California

M a r i n e Geology -

ElsevierPublishingCompany,Amsterdam- Printed in The Netherlands

MAGNETIC FABRIC OF SEDIMENTS FROM THE LA JOLLA SUBMARINE CANYON AND FAN, CALIFORNIA ANTHONY [. REES 1, U L R I C H VON RAD 2 AND FRANCIS P. SHEPARD

University of California, Scripps Institution of Oceanography, La Jolla, Calif. ( U.S.A. )

(Received August30, 1967)

SUMMARY

The results of measurements of the anisotropy of magnetic susceptibility of 24 box cores from the La Jolla Submarine Canyon and Fan are described and compared with magnetic fabrics produced under controlled conditions and with visible directional sedimentary structures. Many cores have a fabric similar to that produced during deposition from running water in the laboratory, and it is concluded that most of the sediment had a fabric of this type immediately after deposition. Most of the samples from the upper canyon have a "primary" magnetic fabric, characterised by a near horizontal magnetic foliation plane, susceptibility maxima well grouped in this plane and approximately parallel to the canyon axis, and q values ranging about 0.4. The sediments from the fan-valley and from the fan show, at least, traces of a similar type of primary fabric. In general, maximum susceptibility axes trend approximately parallel to the fan-valley axis and to the crests of the adjacent levees. The effects of two types of deformation were distinguished: the first of these, recognized by a tendency of the susceptibility minima to be streaked out toward the horizontal and by variable q values, is explained as being the result of forces tangential to the bedding plane, possibly the component of gravity acting tangentially to the side slopes of the channel. The second type of deformation, reflected in the magnetic fabric by low total anisotropy, high q values, and scattered directions of susceptibility axes, is thought to be due to the randomizing action of burrowing organisms.

INTRODUCTION

The estimation of preferred grain orientation in recent and ancient sediments from the anisotropy of their magnetic susceptibility has recently been developed 1 Presentaddress: Department of Oceanography,The University,Southampton(Great Britain). 2 Present address: Institut ftir Geologie,TechnischeHochschule,Mfinchen,(Germany). M a r i n e Geol.,

6 (1968) 145-178

146

A.I.

R[il!S, U. VON RAI) AN[) ! , P. Sit| PARf~

as a quick and accurate method, promising to have wide application in the ,~tud), of sedimentary fabrics (REEs, 1965). Because of the difficulty, inaccuracy, and tedium of conventional optical methods of fabric measurement, grain orientation has been somewhat neglected though it promises to provide otherwise unobtainable information about the directions, and possibly even the nature and magnitude, of the forces acting during and after the deposition of sediments. Several difficulties, however, arise in interpreting correctly the fabric of ancient rocks: exposures are commonly fragmentary and rocks are frequently altered to varying degrees by weathering, by metamorphism, or by tectonic forces so that the picture derived from the study of even a single stratigraphic unit may well be a confused one. This confusion may be compounded when the unit under study consists of a number of beds each laid down in a different force field. If we study the magnetic fabric of sediments in a contemporary marine environment, we know that the complete spatial environment is available for study, and we have a chance of knowing something about the processes and forces acting during and after deposition. The results of the study of magnetic fabrics may be helpful in the interpretation of the sedimentary structures of ancient rocks and the primary purpose of our study was to obtain such information for one important type of depositional environment, that of the submarine canyon and its associated subsea fan and fan-valley. Although much work has been done recently on this type of environment the mechanisms of transport and deposition of deep-water sands and muds are still very incompletely understood. Another purpose of this study was to contribute to this understanding by comparing the magnetic fabric of sediments from different parts of the canyon-fan system with that found under various controlled conditions in the laboratory. Conclusions drawn from such comparisons must be tentative but they can be useful when taken together with conclusions arrived at independently. A study has been made of the visible sedimentary structures of the material whose magnetic properties have been measured. This will be reported in detail elsewhere (SHEfARD et al., in press), but the results are summarized briefly here in pictorial form so as to provide easy comparison between magnetic and other fabrics.

THE USE OF MAGNETIC METHODS IN THE STUDY OF G R A I N FABRIC

It has been shown (REEs, 1965) that, in favourable circumstances, the measurement of anisotropy of magnetic susceptibility may be used to estimate the preferred grain orientation of sediments. Grains of magnetite or of titomomagnetite are most easily magnetized along their longer dimensions and so any rock containing a small proportion of such grains has directional differences in susceptibility that define a magnetic fabric and that reflect preferred orientation of their long and short axes. Marine Geol., 6 (1968) 145-178

MAGNETIC FABRIC OF LA JOLLA SUBMARINE SEDIMENTS

147

Two types of grain fabric element are commonly recognised in sedimentary rocks (POTTER and PETTIJOHN, 1963). The first, and usually the more obvious, is a planar structure due to the effect of gravity and the second is linear and is due to the action of forces such as the drag of depositing currents, tangential to the bed. The magnetic fabric most commonly observed in undeformed sediments has elements that have been called (REES, 1966) the magnetic foliation and the magnetic lineation. The magnetic foliation is nearly parallel with the bedding and the magnetic lineation lies within the magnetic foliation plane. Susceptibility anisotropy can be measured in a number of ways, the method used here being that described by KING and REES (1962). The measurement defines the directions and magnitudes of three orthogonal principal axes of susceptibility. The orientation of the magnetic foliation, which is a plane of excess susceptibility, is uniquely determined by the direction of minimum susceptibility at right angles to it. The direction of the magnetic lineation is that of the maximum susceptbility. Information about the degree of preferred orientation can be obtained from the magnitudes of the principal susceptibilities. One quantity that has been found useful is the parameter q (REES, 1966), which compares the intensity of the magnetic lineation with that of the foliation, q is nearly independent of the susceptibility of the magnetic material and of its proportion in the sediment and so may be used, with caution, when making comparisons between the magnetic fabrics of different sediments. The susceptibility measurements for this study were made on a number of 2 cm • 2 cm cylindrical specimens cut from each of lhe selected box samples. Two methods of specimen preparation were employed, some of the coarser material being impregnated with polymerizing resin before diamond coring and the remainder being cored wet with a thin-walled corer. The susceptibilities were calculated by computer and the directions of maximum and minimum susceptibilities were displayed, for each box sample, on an equal area projection so as to give a general idea of the grouping of lineations and foliations of the sample. Six factors have been identified as being able to produce or influence magnetic fabric. These are: (1) gravity; (2) the magnetic field acting at the time of deposition; (3) the depositing water currents; (4) the slope of the bed onto which deposition takes place; (5) tectonic compression; and (6) disturbance by burrowing animals. (1) The effect of gravity is to align the long axes of sediment grains, magnetic and non-magnetic alike, as near as possible to the horizontal plane. This produces a planar structure which is reflected in the magnetic fabric as the magnetic foliation. When no other orienting force acts, the longest axes are distributed randomly in the horizontal plane and only a statistical lineation is observed. Values of q, therefore, approach zero (Table I). (2) The m~gneticfield of the earth is thought to be important only for very fine sediments. Its influence has been shown to be important for fine silt (REEs, 1961), but deposition of medium and coarse silts in the laboratory (HAMILTON, in press) showed only vestigial control by the earth's magnetic field, and no sign of this has been detected in artificially or naturally deposited sands. KING and REES (1966) give reasons for Marine Geol., 6 (1968) 145-178

148

A. I. REES, U. VON RAD AND t-. P. SIIEI~AR!~

TABLE I q'VALUES FOR DIFFERENT DEPOSITIONAL ENVIRONMENTS

Experimental depositions experiment

sediment

vahte o1" range of q

reference

Still water, flat bed ar,d Medium sand slopes up to repose angle (Scrippsbeach sand)

0.06-0.22

REES(1966)

Running water, flat bed silt 24-46 a below threshold of movement fine sand

0.13-0.29

HAMILTON(in press)

0.06-0.16

A.I. Rees (unpublished)

Running water, rippled bed fine sand

0.10-0.42

A.I. Rees (unpublished)

Running water, plane and fine silt (magnetic control) rippled bed

0.13-0.67

REES(1961)

Sand flow, in air, on repose angle slope

medium sand

0.50-0.70

HAMILTONet al. (in press)

Sand flow, in water, on repose angle slope

medium sand

0.43-0.67

HAMILTON et al. (in press)

Sand flow, delta foreset

medium sand

0.63 ~0.02

HAMILTONet al. (in press)

Beach (Scripps beach)

medium sand

0.333:0.03

REES(1965)

Desert dune

medium sand

0.404).70

R~rs(in press)

Deformed Appalachian rocks

various

0.40-2.00

after GRAHAM(1966)

Naturally formed material

the assumption that the magnetic aligning forces of permanently magnetized particles are negligible for grains with diameters of more than a few tens of microns and it is to be expected that the magnetic forces aligning particles with an induced shaperelated magnetization will be considerably less. (3) The aligning effect of water currents is well known and has been studied by a large number of workers using conventional techniques. This effect produces a magnetic lineation by the alignment of long axes of magnetic grains and--provided these are not too different in size and shape from the non-magnetic grains--magnetic and non-magnetic particles should behave hydrodynamically in a similar way and become similarly aligned by the same current. The majority of sand-sized sediments have been found to have current-parallel (a) lineation (POTTER and PETTIJOHN, 1963). A numher of cases of double (a and b) lineation--i.e., parallel and normal to the current direction--have been reported (HAND, 1961; BOUMA, 1962; RUKAVINA, 1965). Most magnetic fabric studies (REEs, 1961, 1965; HAMILTON,1963, 1966) show current-parallel (a) type of lineation, though Marine Geol., 6 (1968) 145-178

PP . 149-154

/

/

•

LJF36

• LJF 74

o ~

I

o

)

$.: J>

!...JF?:"J

~,.

,_-' / "

"

• LJF86

o

2!

3 !

;

5

6 I

KI LOMETERS CONTOURS IN

~

o

~

o

'"

A-J

o /

.

-::::II

I;>' \J25 I d magnellca y_ A.o La Jolla Submarine Cany Fig. I. Bathymetry of on and Fan an d 'hdo""on 0 fb""o," ,,'"'"

/

..

FATHOMS

6/
~/

/

I

LA

~:.-

JOLLA _ " . ~~~~,

_

MAGNETIC FABRIC OF LA JOLLA SUBMARINE SEDIMENTS

155

one case of transverse (b) lineation has been reported (REEs, 1965). These studies appear to confirm the view that the alignment of magnetic grains generally reflects alignment in the remainder of the sediment. The grain orientation produced by deposition from flowing water onto a flat bed shows a gravity-produced magnetic foliation with an intensity comparable with that produced in still watcr. The intensity of the lineation produced by the flow probably depends on the strent;th of flow. Observed values of q (Table I) range up to 0.5 approximately. (4) The aligning effect of depositional slopes has been demonstrated in a laboratory-deposited sand by REES (1966) and by HAMILTONet al. (in preparation). When deposition is on slopes with angles less than the angle of rest of the sediment, the distinctive gravity-produced magnetic l oliation is present. The intensity of the lineation varies with the angle of the slope (Table I), q rising to a value comparable with those found for running water deposition as the angle of repose is approached. When the slope is at the angle of repose, matelial is transported downslope by mass movement of an avalanching type. When this happens the gravitational foliation quickly ceases to be the dominant aspect of the fabric though the directions of the principal axes are generally very similar to those found for deposition on slopes on which there is no movement at angles a little less than the angle of repose. The principal axis of minimum susceptibility is approxirnal ely perpendicular to the bedding plane and the maximum is directed downslope. (5) The effect of post-depositional deformation on magnitic fabric and, by implication, on sedimentary fabric in general, has been demonstrated by GRAHAM (1966). He has worked on the Paleozoic rocks of the Valley and Ridge province of the Appalachian Mountains and has demonstrated the existence of a very wide range of fabric types, from a primary fabric similar to that produced in the laboratory by deposition from running water to a secondary fabric dominated by extension perpendicular to the bedding plane. Graham shows that the deformation is best explained as the result of forces acting parallel to the bedding plane, and shows how three phases of deformation may be distinguished by three quite distinct pattern'; of orientation of the susceptibility axes. In the first of these, the strain due to the stress acting in the original plane of magnetic foliation is sufficient to rotate the magnetic lineation directions of individual specimens into a close group at right angles to its line of action. Values of q--expressed by Graham in terms of V, half the angle between planes of zero anisotropy--generally increase, and differential shear is sometimes reflected by a tendency for minima to be streaked toward the direction of the stress. In the second phase of the deformation, shortening along the line of action of the stress is sufficient to cause an interchange in the directions of the minimum and intermediate principal susceptibility axes, and in the final stage the maximum susceptibility becomes oriented perpendicular to the bedding and the least susceptibility parallel to the compression axis. (6) The effect of deformation on sedimentary fabric is usually a systematic modification of a primary depositional fabric. Primary fabrics may be altered by

Marine Geol., 6 (1968) 145-178

156

A.I. RI!I S, [I. ¥ON RAI) ANI) F. J, Sill PAR:~

other influences, the most important of which is probably that of burrowing animal,.. It might be expected that the effect of such animals would be to destroy any systematK: arrangement of grains, but sediments without magnetic anisotropy are comparative!?. rare. More common are sediments in which the randomizing effects of burrowing are noticeable, but which retain a recognizable relic of the primary fabric. A sediment in which the primary fabric has been completely destroyed by randomization will have a susceptibility anisotropy due to statistical alignment. In such a case, the principal axes will have no preferred direction and the expected mean value of q will be two thirds: Kint--gmiu ~

/('max---Kint

MORPHOLOGY AND GEOLOGICAL FRAMEWORK

The morphology of the area investigated is described in the preceding paper (SHEPARO and BUFF~NGTON, 1968) and is shown in general form in Fig. 1. A very narrow shelf, 5-10 km wide, borders the coast off the La Jolla area. This is crossed by La Jolla Submarine Canyon with Scripps Canyon as its northern tributary. The heads of both canyons come very close inshore into waters only a few fathoms deep. The narrow, precipitously rocky inner canyon is cut into Eocene shales, sandstones and conglomerates, as well as Pleistocene and Cretaceous sediments. The canyon changes its character beyond 300 fathoms depth to a wider, sinuous fan-valley, with less precipitous slopes, eroded into the Quaternary sediments of La Jolla Fan. The sinuous course followed by the axis of the fan-valley is indicated in Fig. 1 and on a larger scale in SHEPARD and BUFFINGTON'S chart (1968). Steep slopes are found on the outside of the bends, and the slopes on the inside are often broken by terraces. At the margin of the fan-valley there are discontinuous embankments, called natural levees because of their resemblance to the levees along delta river channels. Locally these rise as much as 10 fathoms above the outer fan. The fan-valley decreases in depth relative to the surrounding fan as it proceeds seaward until it gradually merges with the floor of San Diego Trough at a depth of 610 fathoms. It has no definite distributaries but near its outer terminus there are two adjacent very shallow valleys that may once have been distributaries. To the southeast another valley is also indicated (Fig.l). This is the outer continuation of Loma Sea Valley (EMERYet al., 1952). The central part of San Diego Trough is about 15-20 km wide, more than 100 km long, and has an axial depth of about 500-700 fathoms. It is bordered on the west by the steep escarpment of Thirtymile Bank and on the east by the northward plunging Coronado Bank. It is one of the many structural basins belonging to the "basin and range province" of the southern California "continental borderland" (SHEPARD and EMERY, 1941, p.9; EMERY, 1960; MOORE, 1966). The flat-floored trough Marine GeoL, 6 (1968) 145-178

157

MAGNETIC FABRIC OF LA JOLLA SUBMARINE SEDIMENTS

has here a uniform, axial slope of about 0.75 ~ to the south that is apparently in equilibrium with the deposition of the fine-grained silty and sandy sediments (EINSELE, 1963, p.33). The trough had previously been an elongated basin of deposition, but is now completely filled with sediments up to its sill, so that part of the recent sediment, abundantly supplied from the nearby continent through the canyon-fan-valley

T A B L E II MAGNETIC

FABRIC

PROPERTIES

AND

SCHEMATIC

SKETCHES

OF BOX CORES FROM THE

UPPER-CANYON

SEDIMENTS

Sedimentary fabrie "~ ~

•~ 32 °

V

B

C

52.49

51.5

51.6

117 °

15.37

16.14

16.38

(fathoms)

8

60

90

~

(degrees east from true north) 240 (dip)

-

315

-

320

O

52.3

16.86

168

330

N

52.80

17.20

186

330

Q

52.85

17.25

188

330

(cm)

~ ~ ~ 8

(cm below sea floor ) 2-11

~q

-

0.16 ~ ± 0.04

-

300

-

16: 7 + 9

315 1- 4

0.58 ± 0.11

16-28

17: 4 + 13

180

0.39 ± 0.03

290

7

2- 7

300

0.36 ± 0.05

6

0- 5

10

0.40 --0.05

11

2-13

300

0.25 i_ 0.03

.

~

Grain size: Circles = gravel, pebbles, mud pebbles and cobbles; Dots = fine sand (hea'Q- dots, coarse sand); Dashes = clayey silt. Structures: Thin parallel lines = parallel lamination; Curved lines = convolute and current-ripple cross lamination; Leaf symbols = plant (kelp) layers; W o r m symbols -- distortion by burrowing animals. Marine Geol., 6 (1968) 145-178

T A B L E 111 MAGNETIC

FABRIC

PROPERIIFS

AND SCHEMATIC

SKE'IUHES

~

~

~

32 °

117 °

Q (fathoms)

Sc/wmatic Sk~'tch @' m a g n e t i c a n d sedi)~;ct~ tary fabric o f core, pritniti~e horizon~a;

"

"

"~

"~ .~

~ ~" ~ (degreeseastfrom true n o r t h )

F R O M T H E I - A N - ~ v A L I Y Y S E D 1 M I N i'!-

~ "~

~ ~ "-

OF' B O X C O R E S

~ ~, "~ ~

~ = ( cm below sea f l o o r )

susceptibilit r

~ ~

hemisphere

.~ ~ .~. ~

siisceptibility Upper hemi,sphere

~ "~ ~" ~ mean ~ q

"~

---.--5 i - ~ "I 1

10

51.85 32.95 490

1i 51],49 3 3 . 7 4 498

245

220

-

3

250

6: 2

3 . 46

77

45

50.7

33.95 508

50.49 33.85 512

49.84 34.57 514

180

10/190

200

185

?330 240

78

49.15 34.61 531

270[180

190

80

48.35 36.42 545

250

-

52

46.95 38.38 560

165

215

75

45.70 38.82 580

i55 1- 4

19-26 . .

.

. 185

6-8

4

13-15

2

16-19

0.57 :b 0.10

...........

~ [7

-; ': I

0.36 ± 0.07

.:-

,

:

0.36 ± 0.05

,

/~-~" [

~ ......... /

220

A

ii:~ ~: ~ 5 ,'-: ' 1 i~'" .....t _ ~i~: i.... i -zL---° :LJ w - - - - - 7 -' q

180

49.62 34,89 521

210

205

?270

19

7: 3

18-20

270

7: 3 b 2

8-10

2

12-15

4z 0.12

295 3- 6

0.47 ± 0.13

185

7: 4 10-12 + 3 17-19

135

0.47 =[z 0.07

0.57 0.10

10: 6 2- 9 + 4 12-18

175

0.38 ± 0.09

/}

i. . . .

/-

,

i . . . . :~ 1".'-__2"~

-z~r

i .....

]

)

5a :

" %::.

{

'x

255 6- 8

4-

"°

210

7:

,,

0.41 ± 0.07

0- 2

2

/o \~

7: 3 6- 8 +4 18-21

5

.....

0.49 ± 0.16

/

"a

I~......

•

:

T A B L E IV M A G N E T I C F A B R I C P R O P E R T I E S A N D S C H E M A T I C S K E T C H E S OF B O X C O R E S F R O M T H E S E D I M E N T S F R O M T H E C H A N N E L S A D J A C E N T T O T H E O U T E R M O S T F A N - V A L L E Y ~ L O M A SEA V A L L E Y A N D T H E O U T E R L A J O L L A F A N

,~

•-

~2 o

117 °

~ ~

~

.~

-~.~

.~ ~ Cfa,hom~) Cdegreeseast ~ fromtruenorth)

36

50.65 32.68 L

475

45

50

74

45.95 40.18 L

566

165

260

Schematic sketch of magnetic and sedimentary fabric of cores-primitive horizontal

~

• Maximum susceptibility Lower hemisphere > ~ ~. × Minimum susceptibility ~~ ~ Cembe~o~ ~ ~> mean Upper ! ~o hemisphere k. . . . ~ seafloor) ~ ~ q 3

20-23

180

3 + 2

26-29

210

4

22-24

5~

49

48.02 31.18 O

489

-

-

0.44

I.

'

35-38

190

1.26

/

-k 0.26

{

~

0.28 ± 0.09

~ ' i

", I

5"

72

43.5

40.18 O

592

170

290

2

7-9

3

14-17

240

+

'"

I:

84

86

40.00 41.2

42.5

38.5

O

O

645

596

160

2

5: 2 + 3

210

7-10

2-5

210

,/

0.57 ± 0.45

/ I

0.25 -2--0.09

(\ ,

.

~ .

. . . . . I ~ .;

.) ~ i - ~ /'

: ~

_j

8-10

42

52.08 39.56 F

541

290

-

2

10-12

scattered

!' ~ "

51

46.00 33.99 F

536

200

-

2

16-18

scattered

/-->.., /....

i

] i -

I '

160

:x, [. R1ES, [:. \ O N R A I ) , \ N I ) I [' :,,lit t~A]~,I~

system, is not deposited on the fan or in the trough but bypasses it and spills over I,~ the southwest into the adjacent San Clemente Basin (EMLRY, 1960; StiI~PARI) and EINSELE, 1962: HAND and EMERY, 19641.

LOCATION OF SAMPLES

A large number of new cores, including about 90 oriented and undisturbed box cores, have been collected recently from La Jolla Fan and adjacent areas and examined by SHEPARD et al. (in press). The introduction of the box corer (REINECK, 1958; BOUMA and SHEPARD, 1964; BOUMA and MARSHALL, 1964) has enabled us to obtain virtually undisturbed and oriented cores of sufficient width and length (20.3 ~,:: 30.5 cm wide, up to 45 cm long) for sedimentary structures and magnetic fabrics to be studied, even in compacted sands. From the box cores, 24 representing all the different depositional environments from the canyon heads down to the floor of the San Diego Trough, were selected for the study of magnetic properties (Fig. 1). Sketches illustrating the sedimentary features of these are given in Tables II-IV. They can be divided into five groups, according to their location and environment: (a) The first group consists of six box cores (,4, B, C, O, N, Q) from the upper part of the rock-walled La Jolla Canyon with depths ranging from 8-188 fathoms. The steepness of the rock walls and the narrowness of the canyon make it very difficult to take samples exactly from the canyon axis, although this was attempted in each case. (b) The second group of ten samples (LJFllO, 11, 46, 77, 45, 19, 78, 80, 52 and 75) were taken from the channel axis ofLa Jolla Fan-Valley from a depth of 490 fathoms down to the outer part of the channel at 578 fathoms. (c) Two samples (LJF 36, 74) from the levees at 472 fathoms and 567 fathoms make up a third group. (d) Four samples (LJF 49, 72, 84 and 86) may be classified as coming from "other valleys". LJF 49 is from the Loma Sea Valley at 489 fathoms, LJF 72 is from a possible distributary of the main fan-valley at 592 fathoms and LJF 84 is from a depth of 645 fathoms in the "rift valley" at the base of Thirtymile Bank. (e) Finally, two samples (LJF 42 and 51) are from the open fan at 541 and 536 fathoms.

DESCRIPTION OF THE MAGNETIC FABRICS

Sediments from the upper rock-walled canyon ( A, B, C, O, N, Q) This group consists of six samples from the upper part of the Scripps and La 1 N u m b e r s given in the order o f descent d o w n t h e valley axis.

Marine Geol., 6 (1968) 145-178

MAGNETIC FABRIC OF LA JOLLA SUBMARINE SEDIMENTS

16l

A

B Fig.2. Sedimentary fabric of box core O. A gravel layer (grains up to 4 mm) consisting of plutonic and volcanic rock fragments, shell debris, quartz and plagioclase grains has a sharp contact with the over-lying fine sand. Dark laminae consist of heavy minerals. For location of core, see Table II. A. Photograph of 5 x 5 cm thin section. B. Detail of A. Marine Geol., 6 (1968) 145-178

/62

A.I. REES, t.. VON RAIl)AND t. P, StlIiPARt~

Jolla Canyons between 8 and 188 fathoms, representing the zones of the "'shallo~ sandy bowls", the "intermediate steep rocky gorge", and the "deeper rocky g o r g e with lower average gradient including nearly horizontal platforms separated by ~'ock steps (SIJEPARDet al., t 964). Because of the relatively shallow water and the comparatively good navigation so close to shore, the location of our cores along the canyon is quite accurately known, We cannot be entirely sure that they are from the central deepest part of the 3-30 m wide canyon floor, but there is little doubt that they come from some part of the floor. The positions of the six upper canyon samples are shown in Fig.3 together with the estimated mean directions of maximum magnetic susceptibility. In general it can be seen that the directions correspond with the likely direction of transport. Of the six samples, three (A, N and Q) are composed of fine sand and three (B, C and O) contain layers of very coarse sand and fine gravel (Fig.2, 4). [t seems likely that the poorly sorted gravel and coarse sand layers of these samples came, at least in part, from the talus slopes which have been observed to constitute the base of the canyon walls in much of this area (SHE]'ARD et al., 1964). The magnetic and sedimentary fabric properties of these samples are summarized in Table II and the directions of principal axes of maximum and minimum susceptibility of all the individual specimens are shown in the stereograms of Fig.5. These are illustrated in full here because they represent most of the fabric types found in the whole environment and because they can be interpreted with a fair degree of confidence in terms of the canyon morphology. With the exception of small groups of specimens from cores B and C, all have primary fabric showing a strong gravitationally produced magnetic foliation. Sample A comes from a depth of 8 fathoms at the head of the Sumner Branch of the Scripps Canyon. It is parallel laminated sand, very rich in mica and plant fragments and represents the results of the accumulation of debris recently swept into the head of the canyon. The sea floor was observed to slope at an angle of 15':" at the locality of collection (R. F. Dill, personal communication). The directions of magnetic lineation of specimens from this sample are widely scattered in the horizontal plane and the mean value of q is low (q =: 0.16 ± 0.04). This property of dominant gravity-produced foliation is typical of material deposited in a situation where stresses parallel to the bedding produced either by currents or the slope of the bed are not important. Much of the material entering the canyon system probably does so in the same manner as this and has a similar magnetic fabric. With the exception of the two small groups of specimens mentioned above, the magnetic fabric of the remaining five samples of this group shows very little variation. All have a strong gravity-produced foliation and mean values of q in the range 0.25-0.4. This is remarkable in view of a very wide range in grain size and composition, both within and between samples (SHEI'ARDet al., in press). These properties are all characteristic of current-deposited material (Table l) and the general parallelism between magnetic lineations and the canyon axis lends support to such an explanation. The most obvious alternative that the magnetic ,VIarine Geol., 6 (11968) 145 -178

INNER FAN~L

SCRIPPS ,. INSTITUTION OF OCEANOGRAPHY

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Marine Geol., 6 (1968) 145-178

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fabric might have been produced during an avalanche type of mass flow of sediment is not supported by the results of experimental work on angle of rest slopes (HAMILTON et al., in preparation). The possibility of some alternative method of mass movement, perhaps assisted by moving water cannot, however, be excluded on the magnetic evidence. The first of the two groups of specimens with anomalous magnetic properties comes from the topmost layer of sample B (Fig.4L This layer is of fine sand and has parallel and contorted lamination, the fold axes of the contortions being approximately parallel to the mean direction of magnetic lineation. This lineation is in the same direction as that of the rest of the core but the other magnetic fabric properties are very different, q being high and very variable (range 0.4-1.3) and the susceptibility minima being tilted toward the horizontal. These properties may be explained, after GRAHAM (1966), as being the result of deformation by a force acting in a direction at right angles to the lineation and at a large angle to the vertical. The most likely origin of such a force is in the component of gravity acting tangentially to a sloping bed and it seems very probable, especially in view of the mixed nature of the core material, that the slope here was the surface of the talus heaped against the canyon wall. The contortion of the bedding supports an explanation based on deformation. The magnetic fabric of the four specimens in the anomalous group from core C appears to provide additional evidence of the importance of transverse slopes. The fabric in this case is very like that produced by an under water sand flow (HAMII_'rON et al., in preparation) such as might occur on a steep talus slope dipping at right angles to the canyon axis.

Samples from the La Jolla Fan-Valley ( LJF 10, 11, 46, 77, 45, 19, 78, 80, 52 and 75) The cores of this group are from the axis of the La Jolla Fan-Valley. Except for LJF 10, a mud core, they consist of well sorted fine and medium sands interbedded with thin layers of clayey silt. The sand layers are usually parallel- or current ripple cross-laminated and are only graded in part (SHEPARD et al., in press). The fabric properties of these samples are summarized in Table III. Since the channel in the inner fan-valley winds quite sharply (SHEPARD and BUFFINGTON, 1968) partly within the confines of the fan-valley, the local trend of the channel axis can only be estimated. However, it seems quite probable from their relationship with other sedimentary properties and from what we know of the bottom topography that the magnetic lineations are, in most cases, roughly parallel with the channel axis. Several of the sand samples show a typical primary magnetic fabric with wellgrouped near-horizontal susceptibility maxima, well-grouped, near-vertical, minima and mean values of q of about 0.4 (Table III, Fig.6). These have one or two odd specimens of atypical character, but these may be ascribed to accidents in sampling or to the local action of burrowing organisms. Marine Geol., 6 (1968) 145-178

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Marine Geol., 6 (1968) 145-178

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Fig.7. Sedimentary fabric of core LJF46. Graded, (cross-) laminated sand layer, clayey silt on

top. Photograph of box core slice. U.S. Navy photograph (LSF 368-6-65). Good examples of typical primary sedimentary and magnetic fabric, probably produced during deposition from flowing water, are found in cores L J F 46 and 75 (Fig.6, 7, 8a). The sand layer of core L J F 46 which is approximately 30 cm thick and is undisturbed is parallel- and cross-laminated on a millimeter scale in its top third and unlaminated in its basal, coarser-grained part (SHEPARD et al., in press). Crosslamination foresets dip toward 190 ° approximately. The magnetic fabric shows a dominant horizontal foliation with magnetic lineation directions closely grouped about a north south axis. Six of the seven specimens in this core have an up-valley imbrication. The values of q (q ~ 0.36 ~- 0.05) are comparable with those found for deposition from flowing water (Table I). Core L J F 75 (Fig.8a) has a similar magnetic fabric with lineations well-grouped along an axis 75°-255 ° approximately parallel with the direction of maximum crosslamination foresets. Minima are vertical and the mean value of q is 0.49 -5_ 0.16. The material is a well-sorted fine sand with excellently preserved parallel and crosslamination. Cores 52 and 80 have fabrics very similar to that of the anomalous portion of core B, which is thought to have suffered deformation by shear acting transverse to the line of sediment transport (Fig.6). Both have lineations which are near horizonMarine Geol., 6 (1968) [45-I 78

Fig.8a. Sedimentary fabric ol box core L,II- ]5, parallel- and cross-laminated fine sand, unconformably overlain (channel) by clayey silt (photograph of a lacquer peel). b. Sedimentary fabric of box core LJt: 19: convolute lamination within the top part of a thick graded sand layer. Lamination caused by thick biotite layers. X-radiograph shows that convolute lamination developed from oversteepened ripple-drift lamination. For location of core see Table II1, U.S. Navy photograph (LSF 368-6-65). c. Sedimentary structures of box core LJI:" 72. Resedimented mud pebbles and cobbles in coarse sand. Thick graded sand layer, cross-laminated toward the top and overlain by homogeneous clayey silt. The mud pebbles were probably derived from steep channel walls cut into Pleistocene semi .consolidated sediments (SHI-PARD et al., in press). U.S. Navy photograph (LSF 359-6-65). Marine GeoL, 6 (1968) 145-178

M A G N E T I C F A B R I C OF L A J O L L A S U B M A R I N E SEDIMENTS

173

tal and in the estimated down-valley direction, but the minima in both cases are streaked toward the horizontal at right angles to this. Values of q are not very high (0.38 ± 0.09 and 0.57 ~z 0.10, respectively) but have a wide range. Five cores from the fan-valley (LJF 11, 19, 45, 77 and 78~ have magnetic fabric intermediate between these two extremes. Fig.6 and 8b show the effects of disturbance on the sedimentary and magnetic fabrics of one of these. The magnetic properties of core LYF 77 appear to have been strongly influenced by small-scale disturbances during coring that produced the effects seen in the sedimentary fabric. The directions of the susceptibility axes are very scattered (Fig.6) and show very little relationship with the geometry of deposition. Values of q are high and variable (Table Ill). LJF 10 is of fine silt grade and has magnetic fabric similar to the deformed fabrics of cores LJF52 and 80. The magnetic fabric of the cores of this group is generally of a "current-deposited" type, but shows some modification, probably by deformation of a fairly homogeneous nature. The effect of gravityin producing a sub-horizontal foliation is noticeable in all the cores, though in some of the cores with more disturbed fabric this is only so bacause the original foliation is reflected in the horizontal deformationproduced lineation. The magnetic fabric of the undeformed cores is very similar to that of the bulk of the material discussed in the preivous section and, for the same reasons, is probably the result of current deposition. The effects of small-scale disturbance by burrowing organisms, though observed, are not seen to be important in this group of samples.

Samples from the levees (LYF 36, 74) Levee core LJF 36 has a primary magnetic fabric of the "current-deposited" type. The other levee core, LJF 74, consists of clayey silt with layers of sitt and sand. The magnetic fabric of the clayey silt shows evidence of deformation, while that of the coarser-grained material, which has well preserved cross-lamination, is of a primary type with strong gravity-produced foliation (Fig.9). The properties of these cores are summarised in Table IV. Because of the winding character of the valley, we cannot be quite certain of the alignment'of the levees at the points sampled. However, the indications are that the levee is aligned north-south at the position of LJF 36 and northeast-southwest at LJF 74. Since the primary magnetic lineations of the two samples are parallel with these directions the magnetic evidence would suggest that most sediment movement was along the levee rather than across it at these points.

Samples frorn "other valleys" (LJF 49, 72, 84 and 86) The properties of these cores are summarised in Table IV. Core LJF 49 from Loma Sea Valley is of clayey silt with a coarse silt layer which was found to have indistinct laminations partially disturbed by burrowing.

Marine Geol., 6 (1968) 145-178

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The magnetic fabric strongly suggests deformation. The lower part of core L.IF 72 (Fig.Be) grades upward f r o m m e d i u m to c o a r s e sand, with embedded mud pebbles, at the base, to parallel and cross-laminated tim: sand in the middle. Then there is an abrupt change and the upper part of this c o r e consists of clayey silt with layers of fine sand and sandy silt. Despite this contrast in texture and sedimentary structures there is no apparent difference in magnetic pro-° perties. The scatter in directions is somewhat larger and the mean values of q a little less, but the magnetic fabric is similar in general to the primary fabrivs of the cores described previously. Magnetic lineations are approximately parallel with the direction of maximum dip of the foresets and with the regional slope (Fig.9). The other possible distributary core (LJF 86) has an intermediate type of magnetic fabric. Core LJF 84 is from the valley along the east base of Thirtymile Bank. It has very variable magnetic properties and appears to have suffered deformation, possibly due to coring. The presence of convolute lamination supports the view that deformation has taken place.

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MAGNETIC FABRIC OF LA JOLLA SUBMARINE SEDIMENTS

175

Samples.from the openfan (LJF 42, 51 ) These two cores have susceptibility anisotropy that is below the instrumental sensitivity employed (5 × 10 -v e.m.u./specimen). Their mean susceptibilities are low but not exceptionally so (10-4 e.m.u./5 cm "~ specimen). This small anisotropy has probably been brought about by post depositional randomization. Both are homogeneous clayey silt and show evidence of organic disturbance.

DISCUSSION

The primary purpose of this study was to describe the magnetic fabric of a selection of cores taken from a major submarine canyon and its associated fan and fan-valley. We have also indicated briefly which sedimentary structures are associated with different types of magnetic fabric. These will be discussed in detail elsewhere (SnEPARD et al., in press). The secondary purpose was to find out if information gained from the susceptibility measurements can tell us something about the sedimentary processes taking place in this system. With the exception of three small groups of specimens, all the material from the floor of the inner rock-walled canyon has what has been described as a primary magnetic fabric. This is a fabric similar to that described for material known to have been deposited from running water onto a horizontal bed and is characterized by a near horizontal magnetic foliation plane produced by gravitational settling, susceptibility maxima well grouped within this plane, and values of q ranging around 0.4. The mean directions of the maxima of this material are all, approximately, in the direction of transport down the canyon. One sample (A) with different fabric was taken in 8 fathoms from the slope at the head of the Scripps Canyon. The mean value of q of this was low and there was no preferred orientation of maxima within the foliation plane.!The interpretation advanced here is that this is a case, essentially, of deposition from still water onto a sloping bed, the fabric properties being quantitatively very similar to those observed under such circumstances in the laboratory. The anomalous properties of the other two groups (from samples B and C) were explained as being due to deformation by pressure acting transversely to the canyon axis. Movement down transverse slopes of talus heaped against the walls of the canyon was suggested as the cause of these anomalies. More surprising than the anomalous properties are the simi'arities between the magnetic properties of sand grade material and material containing rock fragments up to 2 cm in diameter. Those of the former are not difficult to explain in terms of the effects of transporting currents. Such an explanation is supported by the good sorting and the presence of parallel and cross-lamination in the sands, but it is difficult to see how the coarser materia! could have retained its present very poor sorting if it had been transported by currents for any distance. A possible explanation is that Marine Geol., 6 (1968) 145-178

176

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the coarser material was deposited relatively slowly, not moving any great distance after falling from the valley walls, and that each grain was oriented b5 currents flowing along the axis as it lay on the surface. Alternatively, the magnetic fabric ma S have been developed during some kind of movement en masse of the sediment bed. The only experimental evidence relevant to the second possibility is that produced in laboratory experiments on sand flowing down slopes at the angle of repose. The fabric produced in these experiments is distinguishably different from that observed here. The sediments from the fan-valley and from the fan all show some trace at least of a primary type of sedimentary magnetic fabric and a few of them appear to have retained their primary fabric quite unaltered. The sediment of the fan-valley axis is predominantly sandy and sampling was confined, in general, to the coarsergrained parts of the cores from the outer fan, so that the conclusion reached can be applied only to these coarser materials. Most of the cores show at least some sign of deformation of a type thought to be due to pressure acting at right angles to the direction of transport. This is recognized by a tendency of the susceptibility minima to be streaked toward the horizontal whi!e values of q become very variable. Magnetic lineations, meanwhile, remain in the direction of transport. Evidence from other sedimentary features and from the morphology of the environment is generally in support of this view. Deformation of the magnetic fabric can sometimes be seen to be associated with deformation of bedding. It is suggested tentatively that this type of systematic deformation is caused by the component of gravity acting tangentially to the sloping surfaces at the sides of the channel. Some cores, particularly those consisting of clayey silt, show signs of disturbance by animals. This is reflected in the magnetic fabric by a low total anisotropy, scattered and high values of q, and scattered directions of susceptibility axes. The most obvious feature of the magnetic fabric of the canyon-fan system as a whole is the high degree of homogeneity displayed over a wide range of depositional environments and sediment types. Fabric of a type, named here the primary type, is common to all three sections of the system, as is modification of the fabric by deformation. We might expect that most of the sediment was laid down initially with a fabric of primary type, and that this was later modified by deformation where the conditions were appropriate and sufficient time was available. Deformation by burrowing animals in the environment of the outer fan is sufficiently intense in places to lead to complete randomization of the magnetic fabric. Such animals may be expected to prefer the uppermost layers of sediment on the sea floor and to be able to achieve complete randomization of the fabric only when deposition is sufficiently slow. With our present knowledge about the production of magnetic fabrics it is inevitable that an account of this kind should be largely descriptive. It is possible, however, to recognize uniquely the influences responsible for some of the fabric features and to state a limited number of possible causes for others. As our experience Marine Geol., 6 (1968) 145-178

MAGNETIC FABRIC OF LA JOLLA SUBMARINE SEDIMENTS

177

of the production of fabric under controlled conditions increases we can hope to become more confident in attaching causes to the observations. Meanwhile, observations of the type recorded here may be helpful in the understanding of fossil environments with similar characteristics.

ACKNOWLEDGEMENTS

We wish to thank Professor Victor Vacquier for his interest in this work. Our thanks are also due to Dr. J. R. Curray, Dr. R. F. Dill and Dr. E. L. Winterer for their helpful criticism. Size analyses were made largely by Nell Marshall. One of us (U. Von Rad) is grateful for the award of a NATO postdoctoral fellowship by Deutscher Akademischer Austauschdienst, Bad Godesbelg (Germany), which enabled him to work for two years at the Scripps Institution of Oceanography. A. 1. Rees was employed by Natural Environment Research Council, Great Britain, during the manuscript preparation. Financial support for the research was provided under office of Naval Research Contracts Nonr 2216(01) and Nonr 2216(05) and NSF Grants GP-111 and GP-3373.

REFERENCES BOUMA, A. H., 1962. Sedimentology of Some Flysch Deposits. Elsevier, Amsterdam, 168 pp. BOUMA, A. H. and MARSHALL, N. F., 1964. A method for obtaining and analyzing undisturbed oceanic sediment samples. Marine Geol., 2 (1/2): 81-99. BOUMA, A. H. and SHEPARD, F. P., 1964. Large rectangular cores from submarine canyons and fan-valleys. Bull. Am. Assoc. Petrol. Geologists, 48 (2) : 225-231. EINSELE, G., 1963. Uber Art und Richtung der Sedimentation im klastischen rheinischen Oberdevon (Famenne). Abhandl. Hess. Landesamtes, Bodenforsch., 43 : 60. EMERY, K. O., 1960. The Sea off Southern California. Wiley, New York, N.Y., 366 pp. EMERY, K. O., Bt:TCHER, W. S., GOULD, H. R. and SI4EPARD, F. P., 1952. Submarine geology off San Diego, California. J. Geol., 60 (6) : 511-548. GRAHAM, J. W., 1966. The significance of magnetic anisotropy in Appalachian sedimentary rocks. In: STEINHARTand SMITH (Editors), The Earth beneath the Sea--Am. Geophys. Union, Monograph, 10 : 627-648. HAMILTON, N., 1963. The Magnetic Properties of some Fine Grained Sediments. Thesis, University of Birmingham, Birmingham, 162 pp., unpublished. HAMILTON,N., in press. The effect of magnetic and hydrodynamic control on the susceptibility anisotropy of redeposited silts. J. Geol. HAMILTON, N., OWENS, W. and REES, A. I., in press. Laboratory experiments on the production of grain orientation in shearing sand. J. Geol. llANo, B. M., 196l. Grain orientation in turbidites. Compass, 38 (3) : 133-144. HAND, B. M. and EMERY, K. O., 1964. Turbidites and topography of north end of San Diego Trough, California. J. Geol., 72 : 526-542. KING, R. F. and REES, A. I., 1962. The measurement of the anisotropy of magnetic susceptibility of rocks by the torque method. J. Geophys. Res., 67 (4) : 1565-1572. KING, R. F. and REES, A. I., 1966. Detrital magnetism in sediments: an examination of some theoretical models. J. Geophys. Res., 71 : 561-571. MOORE, D. G., 1965. Erosional channel wall in La Jolla Sea-Fan Valley seen from the bathyscaph Trieste 1L Bull. Geol. Soc. Am., 76 : 385-392.

Marine Geol., 6 (1968) 145-178

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~. [. Rlil S, t . \ON RAI) ANI) J. | ' ~;ttI_PAi(!

MOORE, D. G., 1966. Structure, Lilh-oroge,ic Units and Postorogenic Basht Fill by R~flec:ion Prt{/ili,,,,: California Continental Borderland. Thesis, Univ. Groningen -U.S. Nav.v Electronies. ,S~'~. Publ., 151 pp. POTTEr, P. E. and Pf.TTIJOHYS,1::..I., t963. Paleoetn'rents and Bas#l Analysis. Springer. Berlin -G~ttb~gen-Heidelberg, 295 pp. RF.ES, A. I., 1961. The effect of water currents on the magnetic remanence and anisotropy of susceptibility of some sediments. Geophys. J., 5 : 235-251. REES, A. I., 1965. The use of anisotropy of magnetic susceptibility in the estimation of sedimentary fabric. Sedimentology, 4 (4) : 257---271. REES, A. I., 1966. The effect of depositional slopes on the anisotropy of magnetic susceptibility of laboratory deposited sands. J. Geol., 74 : 856 867. REES, A. I., in press. The production of preferred orientation in a concentrated dispersion of elongated and flattened grains. J. Geol. REINECK, H. E,, 1958. Kastengreifer und Lotr6hre "Schnepfe." Senekenbergiana Lethaea, 39 (1--2) : 45-48. RUKAWNA, N., 1965. Particle orientation in turbidites, theory and experiment, ln: Abstracts, 1965 Meeting, Geol. SoL. Am., Kansas City, pp. 141-142. SHEr'ARD, F. P. and BUFFIN(;TON,E. C., 1968. La Jolla Submarine Fan-Valley. Marine Geol., 6 (2) : 107-143. SHEPARD, F. P. and DILL, R. F., 1966. Submarine Canyons and Other Sea Valleys. Rand MeNally, Chicago, Ill., 381 pp. SHEPARO, F. P. and EINSELF, G., 1962. Sedimentation in San Diego Trough and contributing submarine canyons. Sedimentology, 1 (2) : 81-133. SnEPArD, F. P. and EMERY, K. O., 1941. Submarine topography off the California Coast. Geol. Soc. Am., Spec. Papers, 31 : 171 pp. SHEPARD, F. P., CURRAY,J. R., INMAN,D. L., MURRAY,E. A., WINTERER,E. L. and DILL, R. F., 1964. Submarine geology by diving saucer. Science, 145 (3636) : 1042-1046. SHEPARD, F. P., DILL, R. F. and VON Rat), U. Physiography and sedimentary processes of La Jolta Submarine Fan and Valley. Marine Geol., in press. VON RAP, U., in preparation. Possible non-turbidite origin of some deep-sea sands, Cretaceous flysch (Bavarian Alps, Germany) and Recent San Diego Trough (California).

Marine Geol., 6 (1968) 145-178