Deep-Sea Research, 1970, Vol. 17, pp. 157 to 169. Pergamon Press. Printed in Great Britain.

Sediment redistribution by bottom currents in the central Pacific

DAVID A. JOHNSON* and THOMAS C. JOHNSON*

(Received 30 May 1969)

Abstract--Bottom currents have significantly influenced the sediment distribution pattern in an abyssal hills region of the central equatorial Pacific. Our data indicate that topographic irregularities at the sea floor modify the regional flow of bottom water. This modified flow then interacts with the available sediment supply to create differential sediment accumulation patterns. There is sediment poDding in wide areas between abyssal hills, and relatively slow net accumulation rates on the crest and upper slopes of the hills. At their base there may be erosion or non-deposition on the' upstream' side, and relatively rapid deposition 'downstream '. Both Quaternary sediments and those of the ' transparent layer' are asymmetrically distributed around the hills. Thus, the mean direction of flow of bottom water there has persisted for a considerable period of time, perhaps for much of the Tertiary.

I N T R O D U C T I O N

IN RECENT years there has been considerable study of processes by which sediments are eroded, transported and redistributed on the sea floor along the continental margins of the world. Evidence from both regional surveys (MooRE,1966; UCHUPI and EMERY, 1967) and detailed studies of small areas (LAUGHTON, 1968) suggests that considerable sediment reworking is taking place. Various mechanisms have been proposed to account for the observed distribution patterns, including turbidity currents (HEEZEN, 1963), geostrophic contour currents (HEEZEN, HOLLISTER and RUDDIMAN, 1966) and gravity deposifional processes (DOTT, 1963). By contrast relatively few studies of sedimentation processes have been undertaken in regions of the deep sea where pelagic sedimentation predominates. It is apparent, however, from bottom photographs (LAUGHTON, 1963), sediment cores (RIEDEL and FUNNELL,

1964) and direct current measurements (ISSACS, REID, SCHICK and SCHWARTZLOSE,

1966) that conditions on the abyssal sea floor are far from quiescent. Some general characteristics of the sedimentation patterns present in the central

equatorial Pacific can be inferred from studies of sediment cores. In many of these the Quarternary section is only a few metres thick (RIEDEL, 1967), but a considerably thicker Quarternary section is to be anticipated if sedimentation rates of 1 or 2 cm/1000 years were assumed (ARRHENIUS, 1963). Groups of cores taken within a relatively small area commonly contained markedly different sediment sequences (ARRHENIUS, 1963; MOORE and HEATH, 1967). In addition, sediments of all ages at many localities contain a considerable contamination of microfossils which have been reworked from older deposits (RIEDEL and FUNNELL, 1964). Thus sediment accumu- lation in much of the equatorial Pacific may be patchy, and considerable variations in accumulation rates may occur over horizontal scales of kilometres or less. Some

*Scripps Institution of Oceanography, La Jolla, California

1.57

158 DAVID A. 3OHNSON and TaOMAS C. JOHNSON

attempts have been made to understand the mechanisms whereby pelagic sediments are locally redistributed. In a small region of the central Pacific (MooRE, 1968), tectonic activity may be responsible for exposing old sediments at the sea floor and creating highly irregular sedimentation patterns. Regional bathymetric trends and small-scale changes in slope there indicated that intrusion and block faulting took place about 10 × 106 years ago, when there was a presumed world-wide re- juvenation of sea-floor spreading (EwIrqO and EW~NG, 1967). Another mechanism for redistributing sediments may be the interaction of bottom currents with the sediment supplied to the sea floor. In the deeper central equatorial Pacific a northward flow of bottom water has been inferred from deep hydrographic data (WooSTER and VOLI(M~NN, 1960), and direct measurements there have confirmed the presence of appreciable bottom currents (R[ID, 1969).

During June of 1968, a 150 km = portion of the sea floor in the central Pacific was surveyed and sampled in detail to determine whether small-scale bathymetric features and variations in sediment types or sequences could be related to observed bottom water movements and whether sediment modification by tectonic events (cf. MOORE, 1968) could be demonstrated in another area of abyssal hills.

The area selected (03°50'S, 155°45'W) (Fig. 1) was chosen for the following reasons:

~,0 °

AUSTRALIA ~" "~'x - " / ~ . ~ ~ - , ; , ~:j

I R , ' ; _ j . _ . . . . . ~ ,=~ - -~ too. 160"

h':?. ':L': '~ ~,

\ ., ~0 ° I Hawaiian: "% ~,

Islands :~. C:"I':. ..~.'~': :-~ - - - , , - . . . . . o . . . ~

I

SITE OF " ' " ~ DETAILED STUDY .," ~" :.' : ' . , • -I ¢ ' 1

~, ,.~--4OOOm --°'" /

2"]" " ' • ,. r .

la. ' ; 'd s " i - - " ' t

/ Fig. 1. Bathymetry of central Pacific (after MENARD, 1964), showing location of area selected for datailed study. 4000 m and 5000 m contours are shown. Stippled area indicates regions

where water depth exceeds 5000 m.

0 o

Sediment redistribution by bottom currents in the central Pacific 159

(1) Middle Tertiary sediments occur in each of five cores previously collected nearby (RaEDEL, 1967), suggesting that sediment redistribution may also be taking place at the present site. (2) It is in a relatively low region to the south and west of the Line Islands Ridge (Fig. 1). Since there is seemingly northward flow of bot tom water near longitude 170°W (WooSTER and VOLKMANN, 1960), it was anticipated that this deep circulation might extend farther east into the area selected for study. (3) Earlier echo-sounding records indicate that the sea floor there has low relief, with abyssal hills a few kilometers in diameter and 200-400 m high. In such an area, a typical abyssal kill and its surroundings might be surveyed and sampled in sufficient detail to determine the pattern of sediment redistribution.

METHODS OF STUDY

After preliminary surveying and coring in the selected area verified the presence of suitable sea-floor relief and sediment types, two buoys with radar reflectors were anchored approximately 5 km apart to serve as navigational reference points. Fixes on one or both buoys were taken at 3- to 5-minute intervals during the detailed

155*45'W I

T S' K ' ~ - . ,' r R Q' / / ; ,~

I i f I - - f - - _ _ t _ _ i I / ~r . . . . . . . . . . ,

/ , , - - - . . . . . . . . . . . . . . . . . . . . . . . . . -~K I i I I

~0

N

J l . ] 2 . . .

C~k~ inlervo[ 40 meters 155!45,W

Fig. 2. Bathymetry of area selected for detailed study. Dotted lines represent ship's track during detailed bathymetric and seismic reflection survey. Contour interval 40 m.

160 DAVID A. JOHNSON and THOMAS C. JOHNSON

bathymetric survey, and at the beginning and end of all coring and current-meter stations.

More than 200 km of echo sounding and continuous seismic reflection profiling lines were obtained at ship speeds of 5-7 knots (Fig. 2). Water depths were measured using an EDO echo sounder and a Precision Depth Recorder, and have been corrected for the speed of sound in sea water using MATrHEWS' (1939) tables. A migration method (G. G. SHOR, unpublished manuscript) was used for obtaining the true bottom shape from the echo-sounding records. Sub-bottom reflectors were detected by a reflection profiling system which included a 60,000 joule 2-arc energy source, a hydrophone system towed in a neutrally buoyant streamer, and a graphic recorder.

55o45'~,

\ v ~

o Cores o I i ! ~m ~ = Grob Somp/es o ~i ~ ~ ~m / ~ Botlorn Current Measurements

Contour interwl 40 m,~ters [ I

Fig. 3. Location of bottom samples and current meter stations.

Sediment cores and grab samples were taken in the vicinity of the western hill (Fig. 3) to identify those portions of the sea floor where non-deposition or erosion may be taking place and those regions where sediment accumulation may be relatively rapid. Samples were taken at several levels within each core, and age assignments were made, based on RIED•L'S (1968) ranges for diagnostic radiolarian species. In addition, the ages of reworked microfossils and a qualitative estimate of their relative abundance were noted.

Bottom currents were measured at two stations (Fig. 3), using two ' Model 6 ' Free Vehicle Current Meters at each station (ScHICK, ISAACS and SzSSIONS, 1968). The instruments were positioned 3 m and 300 m above the sea floor, and remained anchored at each station for a sufficient length of time to record tidal variations in current speed and direction.

Sediment redistribution by bottom currents in the central Pacific 161

B A T H Y M E T R Y A N D S T R U C T U R E

The area studied includes two abyssal hills, each of which is about 5 km in diameter and 300 m high (Fig. 2): the western one is a circular knoll with a gently sloping, broad top and steeper side slopes averaging about 11 °, but crossings of the eastern hill frequently showed multiple bottom echoes indicative of a fine-scale relief. There are discontinuous moats around the base of both which merge to form a narrow flat-floored trough in the valley between the two. The deepest moat (up to 60 m) is on the southern side of the western hill. A thick wedge of sediment extends north- westward from the base of the western hill (Figs. 2 and 4).

A sediment layer whose upper surface is nearly transparent to the seismic profiler is present in much of the surveyed area. This "upper transparent layer" has been identified in previous seismic reflection studies in the Pacific, and appears to be distributed over much of the Pacific basin with thicknesses up to about 1.0 see (EwzNo, EWING, AITgEN and LUDWIG, 1968). In our area its thickness varies from less than 0.01 sec to about 0.12 see (Fig. 4), or from less than 10m to 120m, assuming a compressional wave velocity averaging 2 km/sec within the layer. Its distribution

15~'45'W

' ~"" l ' K & ~ 04-or

C,c~ur intervol 40 meters ] ~ ~,10 15~'4~ W

Fig. 4. Distribution and thickness of transparent layer.

and thickness relative to the two abyssal hills appear to be asymmetrical. On the crest and slopes it is thin ( < 0.01 sec) or absent entirely (Fig. 4). It is thickest ( > 0.10 sec) south of the hills, but it rapidly thins near the moats at their base. A " t o n g u e " of sediment of the transparent layer extends northward toward the valley

162 DAVID A. JOHNSON and THOMAS C. JOHNSON

between the hills, but ends abruptly upon entering its southerly end. To the north the layer is relatively thin except on the northwest slope of the western hill, and its thickness increases only very gradually.

A strong sub-bottom reflector which may correspond to the upper surface of the ' opaque layer' described by EWING, EWING, AITI(EN and LUDWIG (1968) underlies the transparent layer (Fig. 5). None could be identified any deeper. The reflector is

O' 0

4 9 0 0 -

m

5100-

5300

~ _ Sea floor

']r3fls~crerit Ioyer 'l

• " - " . " - s "" ".'. , : : . . : . ' . . . ' . . . . . . . . . . . . ,1:.'<.... : . - - . . . . x ' : ' " ' . . ' e " ~ . . . . . . . . . ~: ,qJe o)er . . . . . . . . " , - '

9 ,

-6.5

ECIU~ IrJ#

trovei llme

{s~¢ondsl

70

o ~ "~m F I

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- 6 5

Fig. 5. Line drawings taken from continuous reflection profiles, illustrating the irregularity of the sea floor relative to the supper surface of the ' opaque layer '. Locations of the profiles

(OO' and FF ' ) are shown in Fig. 2. Vertical exaggeration 5"4 x .

nearly horizontal in most of the area, and its relief is commonly less than that of the overlying sea floor, particularly near the base of the hills (Fig. 5, profile FF'). Reflec- tion profiles indicate that it may outcrop in the moats, in the narrow valley between the hills, and over a rather extensive area to the northwest of this valley (Fig. 4).

An equally strong reflector is present at or very near the sea floor on the hills (Fig. 5, profile OO'), one which may be continuous with that underlying the trans- parent layer in the surrounding lows. However, it is difficult to determine the degree of continuity between the two near the base of the hills because of the limited resolution of the profiling method used.

SEDIMENTS

From micropaleontological studies of the sediment cores (Table 1) the sediments of Quaternary age are found to be asymmetrically distributed around the western hill. Near its crest (cores No. 1, 14, 15, 16, 62) the Quaternary section is about 50 cm thick. At its base on the western (cores No. 8, 9, 10), southern (cores No. 17, 18, 19) and eastern (cores No. 54, 66, 67) sides Quaternary sediments are generally thin ( < 20 cm) or absent. They are thickest (> 1 meter) on the northern slope (core No. 11). The thick accumulation there was not cored, where it is presumably several meters thick. The north-south asymmetry of Quaternary sediments is clearly shown in the north-south series of free-fall cores (Fig. 6, profile W ' ) .

An unconformity is present at the base of the Quaternary section below which lie

Sediment redistribution by bottom currents in the central Pacific 163

WEST U

4800 111

5200 ~i,~,.i~ I0 9 ~ i /

EAST ! I I

U U I

i s t._._Is l r ? ~ - . . . z

" / - " ~ \ 66 iglh "0 cm -50

--loo

NORTH v

4800 r

SOUTH I

V

5200

1 4 f ? f

Age of Sedimenl H Quolernory M Lower Pliocene Upper Miocene Middle Miocene

I Lower Miocene

Fig. 6. Ages of sediment cores from western hill. Core locations are projected onto west--east (UU' U") and north-south (VV') profiles across hill. Locations of profiles are shown in Fig. 3.

Vertical exaggeration 4"7 x.

generally Upper Miocene or Lower Pliocene sediments on the crest and upper slopes of the hill (Fig. 6). On the lower slopes the Quaternary sediment directly overlies Lower or Middle Miocene sediments (Fig. 6). Lower Miocene sediments outcrop at the sea floor in three cores (Nos. 18, 19, 54) from its base (Fig. 6). Seven other cores (Nos. 4, 8, 9, 17, 53, 66, 67) from the lower slopes penetrated Lower Miocene sediments within approximately 1 meter of the sea floor (Table 1). Since reflection profiles indicate that t h e " opaque layer" may outcrop in the moats and in the valley between the hills, we may thus infer from the cores that the "opaque layer" is Lower Miocene or older, and that the "transparent l ayer" consists of sediments of post- Lower Miocene age. Sediments older than the latter were not encountered, although radiolarian species of Eocene age are present in many of the cores, particularly in the Quaternary and Pliocene sections (Table 1). Their common occurrence in surface

164 DAVID A. JOHNSON and THOMAS C. JOHNSON

sediments may reveal the presence of a nearby Eocene outcrop which is being eroded.

A more likely explanat ion is that, due to their high resistance to corrosion (MooRE,

1968), they have survived several cycles of redeposit ion since the Eocene.

B O T T O M C U R R E N T S

Each of two current meters posi t ioned 300 m above the sea floor, or at a level

slightly above the top of the abyssal hills (Fig. 3), recorded a northwesterly mean flow

of bo t tom water when the t idal componen t is el iminated (Table 2). This flow closely

parallels the nearest major topographic feature, the Line Islands Ridge (Fig. 1).

Ins t ruments posi t ioned 3 m above the sea floor recorded a mean flow in a norther ly

direction, suggesting that the regional northwesterly flow changes direction at levels

near the base of the hills to conform with the local topography. Three hundred meters

above the sea floor at Sta. A (Table 2) the mean and m a x i m u m velocities recorded

were 1.1 and 3.2 cm[sec, respectively. At Sta. B, 3 m above the sea floor, mean and

m a x i m u m velocities were 2.4 and 5.9 cm/sec, respectively.

Table 1. Micropaleon to logyo fsed iment cores

Presence of reworked micro fossils* Core Level Sediment

number sampled age Eocene Oligocene L. Miocene M.-U. Plioeene Miocene

1 0-2 cm Quaternary xx 10-12 cm Quaternary x 28-30 cm Quaternary x 50-52 cm U 76-78 cm U

2 0-2 cm Quaternary x 3 0-2 cm Quaternary xxx

20-22 cm Quatemary xxx 50-52 cm Quaternary xx 80-82 cm Quaternary xxx 99-101 cm L. Pliocene x

4 0-2 cm Quaternary x 25-27 cm Quaternary x 29-31 cm Quaternary xx 41-43 cm Quaternary x 46-48 cm Quaternary x 59-61 cm Quaternary x 70-72 cm L. Miocene 80-82 cm U 90-92 cm U

102-104 cm L. Miocene 7 0-2 cm Quaternary x

8-10 cm Quaternary x 19-21 cm Quaternary 30-32 cm L. Pliocene x 44 46 cm L. Pliocene 60-62 cm L. Pliocene

8 0-2 cm Quaternary xxx 9-11 cm L. Miocene x

30-32 cm U 60-62 cm U 92-94 cm U

X X

XX

XX X

X

XX X X

XXX X

XX XX

XXX X

XX XX

XX XX XXX

XX X

XX XX X

XX

XX X

XXX X

X XX

X

XX

XXX

XXX X

XXX

XX XXX XX XX

* x = rare; xx = common; xxx = very abundant.

S e d i m e n t r e d i s t r i b u t i o n b y b o t t o m c u r r e n t s in t h e c e n t r a l Pac i f i c 1 6 5

Table 1--(contd.)

Presence of reworked microfossils* Core Level Sediment

number sampled age Eocene Oligocene L. Miocene M.-U. Pliocene Miocene

9 0 - 2 c m Q u a t e r n a r y x x x x x x x x 1 0 - 1 2 c m L . P l i o c e n e x x 3 0 - 3 2 c m U . M i o c e n e 5 5 - 5 7 c m M . M i o c e n e 7 5 - 7 7 c m L. M i o c e n e x

10 0 - 2 c m Q u a t e r n a r y x x x x x x x x xx 4 - 6 c m Q u a t e r n a r y x x x

1 1 - 1 3 c m U 3 0 - 3 2 c m U 5 0 - 5 2 c m U 66--68 c m M . M i o c e n e 7 4 / 7 6 c m M . M i o c e n e

11 0 - 2 c m Q u a t e r n a r y x x 6 - 8 c m Q u a t e r n a r y x x

2 0 - 2 2 c m Q u a t e m a r y x x x x x 5 0 - 5 2 c m Q u a t e r n a r y x x x 8 0 - 8 2 c m Q u a t e r n a r y x x x

1 0 1 - 1 0 3 c m Q u a t e r n a r y x x x x

13 0 - 2 c m Q u a t e r n a r y x x x x x x x 6--8 c m Q u a t e r n a r y x x x

3 8 - 4 0 c m Q u a t e r n a r y x x 6 8 - 7 0 c m Q u a t e r n a r y x x 8 3 - 8 5 c m Q u a t e r n a r y x x x 9 8 - 1 0 0 c m U . M i o c e n e x x x x x x

14 0 - 2 c m Q u a t e r n a r y x x x 1 1 - 1 3 c m Q u a t e r n a r y x x x x 4 6 - 4 8 c m Q u a t e r n a r y x x x 5 3 - 5 5 c m Q u a t e r n a r y x x x 60--62 c m U . M i o c e n e x x x x x x 7 2 - 7 4 c m U . M i o c e n e x x x x x x x

15 0 - 2 c m Q u a t e r n a r y x x 8 - 1 0 c m Q u a t e r n a r y x

2 8 - 3 0 c m Q u a t e r n a r y x x x 4 8 - 5 0 c m L . P l i o c e n e x 6 8 - 7 0 c m L . P l i o c e n e x x 8 6 - 8 8 c m L . P l i o c e n e x x x x

16 0 - 2 c m Q u a t e r n a r y x x x x 1 0 - 1 2 c m Q u a t e r n a r y x 3 0 - 3 2 c m Q u a t e r n a r y x x x x 3 7 - 3 9 c m U . M i o c e n e x x 5 8 - 6 0 c m U 8 6 - 8 8 c m U

17 0 - 2 c m Q u a t e r n a r y x x x x x x x 1 0 - 1 2 c m L. M i o c e n e 3 6 - 3 8 c m L . M i o c e n e 6 6 - 6 8 c m L . M i o c e n e 9 0 - 9 2 c m L . M i o c e n e

18 0 - 2 c m L . M i o c e n e 3 0 - 3 2 c m L . M i o c e n e 6 8 - 7 0 c m L . M i o c e n e

1 0 2 - 1 0 4 c m L . M i o c e n e

19 0 - 2 c m L . M i o c e n e 3 0 - 3 2 c m L . M i o c e n e 6 0 - 6 2 c m L . M i o c e n e

1 9 3 - 9 5 c m L . M i o c e n e

*x = r a r e ; x x = c o m m o n ; x x x ---- v e r y a b u n d a n t .

166 DAVID A. JOHNSON and THOMAS C. JOHNSON

Table 1--(contd.)

Presence of reworked micro fossils* Core Level Sediment

number sampled age Eocene Oligocene L. Miocene M.-U. Pliocene Miocene

53P 15-17 cm L. Pliocene x 165-167 c m L. Miocene 292-294 cm L. Miocene 442 A,A,4 c m L. Miocene x 590-592 c m L. Miocene x

54 0 -2 cm L. Miocene

59 0 -2 cm Qua te rnary xxx 19-21 cm Qua te rna ry xxx

62 0 -2 cm Qua te rnary x 20-22 c m Qua te rna ry x 39-31 c m Qua te rna ry x 28-40 c m Qua te rna ry xx 50-52 c m L. Pliocene x 65-67 c m L. Pliocene 98-100 cm L. Pliocene x

132-134 cm L. Pliocene x

65 0 -2 cm Qua te rnary xx

66 0 -2 cm Q u a t e m a r y xx 12-14 cm Qua te rna ry x 28-30 c m U 38--40 cm U 54-56 cm U 70-72 c m L. Miocene x 95-97 cm L. Miocene x

67 0 -2 cm Qua te rnary xxx 10-12 cm U. Miocene 20-22 c m U. Miocene 30-32 c m M. Miocene 60-62 c m M. Miocene 90-92 cm M. Miocene

120-122 cm L. Miocene

xx

x

xx xx xx x xx x

x

XX

XXX XXX

X

XX

XX

X X X

XXX

XXX XXX XXX

* x = rare; xx = c o m m o n ; xxx = very abundan t .

Table 2. Summary o f current meter observations

Station Height above Duration of Mean flow Mean speed Maximum speed sea floor measurement direction (cm/sec) (cm/sec)

(m) (hr)

A 3 40 346 ° * * A 300 40 318 ° 1.1 3.2 B 3 22 001 ° 2.4 5.9 B 300 22 310 ° * *

* In format ion no t available. I n s t r u m e n t s at these localities recorded flow direction only.

D I S C U S S I O N

A m o n g t h e s i g n i f i c a n t b a t h y m e t r i c f e a t u r e s i n t h e a r e a a r e t h e m o a t s w h i c h

p a r t i a l l y e n c i r c l e b o t h h i l l s , a n d t h e t h i c k s e d i m e n t d e p o s i t o n t h e n o r t h w e s t s l o p e o f

t h e w e s t e r n h i l l . T h e o r i g i n s o f s i m i l a r f e a t u r e s e l s e w h e r e h a v e b e e n a t t r i b u t e d t o

b o t h t e c t o n i c a n d e r o s i o n a l p r o c e s s e s . M o a t s e n c i r c l i n g l a r g e g r o u p s o f s u b m a r i n e

Sediment redistribution by bottom currents in the central Pacific 167

volcanoes (e.g., Hawaiian Islands, Line Islands, Emperor Seamounts), result from subsidence causing a depression of the sea floor around the group (MF.~qARD, 1964). Most hills and seamounts on the abyssal plains of the Gulf of Alaska have moats around their bases, "where a hill occurs within a channel . . , there may be a small depression on one side and a ponding effect on the opposite side " (HAMmTON, 1967). There are also moats surrounding small knolls in the Atlantic which are attributed to the scouring action of bottom currents (HEEZEN and JOHNSON, 1963; LOWRIE and HEEZEN, 1967).

A tectonic origin of the moat in our area can be ruled out on the basis of the sub- bottom structure. If either subsidence or faulting had taken place, there would be a structural dip or a discontinuity in the sub-bottom reflector beneath the moat. This is not the case (Fig. 5, profile FF'). The sub-bottom reflector is nearly horizontal there and is disconformable with the overlying sea floor. The moat is probably due to processes of erosion and/or differential accumulation near the base of the hill.

The thick sediment deposit on the northwest slope of the western hill may represent a region of gradual, particle-by-particle accumulation in its ' lee '. Alternatively it may have formed by episodic mass movements such as gravitational slumping. Such movements would be especially likely where tectonic processes such as faulting have produced oversteepened slopes. No irregularities in slope were seen on the echo-sounding profiles, although small-scale irregularities may well have been present and remained undetected.

The mean flow of bottom water toward the northwest is consistent with that required to account for the observed bathymetric, structural and sedimentary features. The limited data suggest that near the base of the hills the prevailing currents are modified in direction and perhaps in magnitude to conform with the local topography. An abrupt acceleration of the currents as they are diverted around the base of the hills causes sediment erosion or non-deposition, creating a moat and exposing middle Tertiary sediments at the sea floor.

We cannot state with confidence whether the currents measured during our survey are of sufficient magnitude to cause significant sediment redistribution. The effect of bottom currents upon pelagic sediments is unknown, because traction studies in flumes have never been performed upon such materials. Studies of sands, silts and clays of terrigeneous origin have demonstrated, however, that many different charac- teristics of a sediment in addition to particle size determine its behavior in a fluid; the behavior of pelagic sediments is no doubt equally complex. An additional complication is introduced by the activity of bottom-dwelling organisms. Although no bottom photographs were obtained during our survey, analyses of photographs from many environments indicate the widespread presence of benthonic fauna, even in depths in excess of 5 km (EWING and DAvis, 1967). The activity of these organisms might be responsible for the continual re-suspension of sediment particles at the sea floor, facilitating their transport by any bottom currents present. Consequently, in the absence of relevant experimental data, and lacking sufficient knowledge of the effect of benthonic organisms at the boundary layer, quantitative predictions of the behavior of pelagic sediments under the influence of bottom currents are hazardous.

The striking asymmetry of the distribution of both Quaternary sediments and the transparent layer, overlying a reflector of Lower Miocene age or older, suggests that

168 DAVID A. JOHNSON and THOMAS C. JOHNSON

the observed northwesterly direction of bottom-water flow in the area has persisted during the past, and perhaps for much of the time since the early Miocene.

I t is likely too that the magnitude of bot tom water flow in the area has fluctuated considerably during the past. We measured variations by a factor of 5 in bot tom current speeds during a tidal period. Seasonal fluctuations may also exist. During the glacial epochs of the Pleistocene there were undoubtedly significant changes in the circulation patterns of the atmosphere and surface waters. Perhaps these fluctuations occurred in the flow of bot tom water as well. I f so, significant sediment redistribution may have taken place episodically, during brief periods of high current flow.

SIGNIFICANCE IN RELATION TO OTHER AREAS

Abyssal hills topography is characteristic of about 8 0 ~ of the Pacific Ocean floor (MENARD, 1964). Hence, the sedimentation pattern described in this report may be applicable to other regions of the Pacific. In a similar area (MOORE and HEATH, 1967) sediment redistribution tends to have a smoothing effect on pre-existing topography; deep reflectors were seen to be generally conformable with the sea floor, but with significantly more relief. Within our area sediment accumulation seems to have been more rapid in broad regions between hills than on the crest and slopes of the hills. Since the sea floor often shows considerably more relief than an underlying reflector, sediment redistribution may create new topographic irregularities in addition to smoothing out older ones. There is no evidence for faulting as in other similar regions (MooRE, 1968; SVIESS, LUYENDYK and MUDIE, 1968). The presence of old sediment at or near the sea floor in our area is due to the presence of currents of sufficient magnitude to prevent sediment accumulation. Tectonic processes may also have modified the sediment distribution patterns, but the effects may be on too fine a scale to be delineated by conventional surveying techniques.

Acknowledgements Observations at sea were conducted on the R.V. Agassiz of the Scripps lnstitution of Oceanography. Members of the Data Collection and Processing Group of the Scripps Institution of Oceanography obtained the current measurements. The assistance of T. J. WALSH and other members of the scientific party who participated in the work at sea is gratefully acknowledged.

We thank T. C. MOORE, G. R. HEATH, H. W. ]V[ENARD and W. R. RIEDEL for help during the planning of the project. W.H. BEGGER, B. P. LUYENDYK, H. W. MENARD, F. B PHLEGER, J. L. REID, JR., W. R. POEDEL and F. N. SPIESS assisted in discussing the results and in reviewing the manuscript. H. W. SmRLEY prepared the illustrations. This research was supported by grants from the National Science Foundation (GA-1189) and the Socony-Mobil Oil Company to W. R. RIEDEL.

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