I

Id-- -__--

COLUMBIA UNIVERSITY

IN TI*i CITY OF NEW YORK

LAMONT GEOLOGICAL OBSERVATORYPALISADES. NEW YORK

MICROTOPOGRAPHY OF THE BLAKE-BAHAMA REGION

by

G. M. Bryan and R. G. Markl

Technical Repert No. 8

CU-8-6-NObsr 85077

RECEIVED j 1 967AUG 2 4 1%7 UG•1 :36- *,

DECEMBER 1968

for pi•b"-

" w

MICROTOPOGRAPHY OF THE BLAKE-BAHAMA REGION

by

G. M. Bryan-and R. G. Markl

F-

Technical Report No. 8

CU-8-66 Contract NObsr 85077

December 1966

FIr.

h~L1

.qINTRODUCTION

Over most of the North Atlantic the topography 0hich influences

bottom loss is relatively large scale - large enough laterally to be

S.4'lved for the most part by the main beam of the usual 12 echo

sounder (beam width roughly 300), As a result, echo sounder records

have proved useful in predicting bottom loss over the North Atlantic,

v• to a first approximation, on the basis of topography. In the vicinity

of the Blake-Bahama Basin and Outer Ridge, however, it has been found

that relatively small-scale topography plays an important role in bot-

tom loss. -'here are extensive areas of rolling topography with periodic

features that are just resolvable in the main beam, Beyond this resolv-

able range r'ýcords show closely-spveed, overlapping, hyperbolic echoes

extending fox many miles over topography which is relativel- smooth on

a larger scale. In many of these .words the individual hyperbolic

traces are not sharply delineated but have a characteristically fuzzy

or blurred appearance. Occasion-lly this fuýznas I.ers.sts • #Area,-.

where no hyperbolas are discernible and the trace, although fuxr.y, looks

quite flat. Measurements in the area indicate that bottom loss is signi-

r 'ficantly greater over these types of bottom. In areas where the large

scale topography appears quite smooth but the echo sounder trace is

fuzzy, bottom loss can be high.

Therefore a detailed study of the micro-topography of the Blake-

Bahama Region was undertaken. The principal aim was to define the extent

of the zones of fuzzy bottom as shown on 12 ke echo sounder records and

• ii

- A

.1

to include this characteristic as a parameter in the predi.-tion of bot-

tom loss on the basis of echo sounder records. Preliminary results were

reported by Bryan (1964) and Bryan and Ewing (196!.).

As suggested above, it is convenient to classify the flDR data into

four categories: regional topography, periodic topography, hyperbolic

echoes, and incoherent or "fuzzy" traces.

REGIONAL T POGRAPHY

The regional topography of the Blake-Bahama area has been discuss-

ed by Pratt and Heezen (1964). Bathymetric contours shown in Fig. I are

based on all available LGO tracks through November 1966 in addition to a

large number of tracks run by USS COMPASS ISLAND in 1950. Following the

usual terminology we distinguish from west to east, the Blake Plateau,

the steep Blake Escarpment, the Blake-Bahama Basin, the Blake Outer Ridge,

and the Hatteras Abyssal Plain. The tsounding lines involved are shown

superimposed on the bathymetry in Fig. 2 (foldout).

Details within the ridge zone include the crest of the main ridge

which branches off from the Escarpment at about 3230%'N, running roughly

southeast, and a secondary ridge which branches off at the "nose" of the

Escarpment at about 300N, parallels the main crest to about 28%N and then

curves westward, forming the eastern boundary of the Blake-Bahama Basin.

North of the ridge zone at about 3aN-73OW the mouth of the Hatteras Can-

yon opens into the Hatteras Abyssal Plain.

PERIODIC TOPOGRAPHY

Approximately periodic, smoothly undulating topography near the

h

limit of resolution of the sounder beam has been labeled "periodic" in

Fig. 2. In a few striking cases (Fig. f for example) the trace is quite

periodic and appears alm ost sinusoidal; however, the category includes

all approximately periodic topography in the size range and with rela-

tivel•y smooth curvaturE. Apparent wavelengths run roughly from one to

Lsix nautical. miles, amplitudes f'rom ten to forty fathoms.

In several instances differcnt apparent wavelengths obtained from

tracks run on different courses at the same location made it possible to

establish the fact that these are linear features and to estimate their

true wavelength and direction. In general, two tracks specify two pos-

sible directions, each with an associated wavclen•th; a third track is

required to remove the ambiguity. The results show that the axes of

these corrugations tend to parallel the contour lin-s (Fig. 2) and have

true wavelengths of about I to 3.8 miles. This type of topography pre-

dominates on the lower continental rise along the northern border of the

Hatteras Abyssal Plain, where the linear trends are quite marked, and a-

lof the flanks of' th suhr en of tilc ride Directll' South, Of t'let

tip no linear trends are apparent. The almost sinusoidal corrugations

of Fig. 3 occur on the western slope of the smaller ridge (see location

map, Fig. 7).

HYPERBOLIC ECHOES

The most prevalent type of abnormal trace in the area is made up

of closely-spaced, overlapping hyperbolic echoes (Fig. 4b & 14d). For the

most part, there is a fairly high degree of uniformity in the spacing and

shape of the echoes and they tend to be roughly tangent to an apparent

I

sea floor which ig relatively Rmooth. ThIs suggests small regularly-

uptCod linuar features which are being crossed at an angle by the sound-

ing track. The lateral dimensions of the features, in a direction per-

pendicular to their linear trends are too small to be resolved by the

echo sounder beam. Trend directions in this type of topography can be

estimated from a knowledge of the shape of the hyperbolic trace and the

speed of thq ship. Specifically, for a point topographic feqature the

shape of the trace depends on the speed of the ship over the feature.

The problem of computing +• ship's speed in terms of echo shape from

point topographic highlights is discussed by Luskin, Nafe, and Nwing

(1957). If the feature is a line rather than a point, and the ship

E crosses the line at a speed v and an angle 0 (Fig. 5a), then the echo

trace will be sensitive only to the velocity component va a vs inQ per-

pendicular to the feature. Thu. comparison of true ship's speed with

apparent speed computed from the echo trace yields the inclination Q of

the line from the ship's track. The result does not distinguish between

clockwise and counterclockwise senses of the inclination and a second

track is required to remove the ambiguity.

Computation of the apparent speed va was based on an approximation

which makes use of the fact that ship's speed (- .;nters/second in this

case) is small compared with sound speed (1500 m/s). The trace then ap-

proaches an exact hyperbola given by (see Fig. 5b):

(v, - (va t) a (Vm)

where T is the two-way travel time of the pulse, measured when the ship

Li

C~if

s ij., a time t away from uhe point of, clons.t approach P to the feature

at Q. V and % ar. sounld upeed and apparel4t ship's speed rospartively.

Although we could ,qolve this equation for v in terms o1' V, tp T, and TM,

as was done by Luskink et al, it is often difficult to measure t as Ac-

curately as the other quantities. Altornatlvely, therefore, we differ-

entiate the above equation and CiUininate t. The result,

T dT

gives the apparent velocity in terms of V, T, Tm aud Lhe slope dT/dt of

the t.race at T. This method gave consistent results and was found n-peei-

ally useful where only ialf the hyperbola was visible. Trend lines were

computed in this way in sever•al instances where tracks showing hyperbolas

intersected. Once again the trends are consistently parallel to the bat)y-

metric contours, |{yperbolic echoes are found chiefly on both flanks of

the main ridge and on the western flank of the secondary ridge, where

they extend well into the Blake-Bahamn, bsin (see also, Clay & Ilona (l9;h].

FUZZY TRACES

An extmple of the characteristic fuzziness often anco'intered in the

echolrrams of the area is shown in Fig. Ite. This should be compared with

the normal trace shown in Fig. ha, The normal echoes are atrong and well-

aligned, producing a sharp leading and trailing edjfe on a trace whose

width faithfully reflocts the length of the outgoing ping. In the fuzzy

traces the ec'ioot are faint and ping-to-ping alignmont is poor. Although

intermediat c: ",,'.an bo found, ranging t-etween the two extremes shown,

II=

ji

the traces in the arca are more often either good or bad, ;,"ld transition

I ~between good and bad is often remarkably abrupt as in Fig. 6 (4ee Fig. 7

' tr locatJ.on) .i

In the older records, wit'- longer ping lengths, these fuzzy traces

wore often associated with topography that appeared to be quite smooth

I ieand flat, More recently the ping length has been kept as short an pom-

- ible and in neatrly all cases of fuzzinesa in the later records hyperbolas!* are discernible to some extenti Fig. 6 shows traces thkon with two dif-

t ferent ping lengths In the same area. The Increased resolution as the

ping lexieth is varied froee about 18 ms in (a) to about 3 ms in (b) is

quite evident. The fuzzy uneven texture persists to me extent, how-

ever. This dependence on ping length is l&rgely responsible for the In-

oonsistenojee aometimes foiud in Fig. 2 at points where tracks of dir-

ferent oruises cross. Generally speaking, however, regions of concentra-

ted fuzziness tend to coincide roughly with those where hyperbolas are

prcseat. On the basis of the tracks shown in Fig. 2 the area involving

hyperbolic and fuzzy topography has been daeineated in Fig. 8. North of

the prominent nose feauture of the iflake Escarpmeont (,300 1N) this topO-

grt'phy is rather :harpl.v bounded on the west by the 16o0 fathom contour.

This boundary can be correlated with an unconformable change in sedi-

mentary regimes observed on seismic profiler records. ltyperbolic and

fuzzy topography in largely absent from the crest of the main ridge.

The dependence oiv ping length, the tendency for areas of fuzziness

to coincide with ar,Žas of lkyperbolar, nd the lack or pilng-to-ping align-

ment in the fuzzy trace suggest that the fuzzy trace results principally

-I--

froimi small-seale .opography of' Uli water-sediment interl'ace ratb'er than

from unusual acoustical propertiea; of the sediment.

The usual plune-wave reflection coefficient for tile water-sediment iinterface, which is appropriate for a plant wave reflected from a plane

interface, depends only on these material propertitej and the angle of

incidence. For t tiel-planar ih'erface the ihaupe of' the interface must

also be tokeni into account. For examnp-1h relatively gentle curvature will

ijLtuduce a certain amount. ot' focursing or defo;usLtng (urvatlure of tile

reflected wavefront) but the reflection is still. oseontially specular,

On te•e othfer hand a very rough interface will scatter the sound anergy

inconerently in all dir-ctious and no specular reflection will be poss-

ible. Those two limiting coses are tentatively identified respectively

with sharp and fuzzy AMl traces. The sediment material need not differ

in the two cases, although sediment type no doubt is a condition of form&-

tion of the microtopography itself.

In the case of acattering from a rough surface the ext-nt of scat- I~ci~i~~ oil i.,~ VV'Ledi- vind flt( i'±oft-f~- dme-.-nn~ -,f the .irrcguiarli

ties compared with tile wivelength of the ineIdent radiation. If the vertU-

cal dinmonsions arn vary nmal.l. compared to the wavelength the scattering

will, be negligible regardle].s of the horizontal dimensions. If tile verti-

cal dimensions* are of the order of a wavelength the scattering will be

great for horizontal dimensions of the order of a wavelength but small

for horizontal dimensions much larger than a wavelength. Anl opportunity

to observe this dependence on wavelength is afforded to some extent by

Smeasurements made simultaneously with the 12 kc echo sounder and the seismic

reflection profiler (about 100 cps). Results from simultaneous measurements

are shown in Fig. 9. The records have been reproduced to approximately

the same scale. They show a rather typical sudden transition from smooth

flat bottom to fuzzy hyperbolas with peak-to-peak spacing of the order of

500 m. The seismic record (c) was made by a compressed air source at a

repetition rate of about 15 seconds; the DR records by a 12 kc t-.ansducer

at a 1 second repetition rate. Records (b) and (c) were made simultaneously,

just before the ship stopped for a station. During the station the ship

drifted back approximately over its previous track. Record (&) i. the

PDR trace made as the ship left the station on the original course and

before the profiler was back in operation. Records (b) and (c) can thus

be compared feature for feature. However, since the PDR was inadvertently

gated at the time, record (a) is included to permit overall comparison

with an ungated trace.

In spite of the much slower repetition rate, the profiler produces

sharper traces of individual features than does the PDR. Note especially

the second hill from the left, which is quite evident in the profiler re-

cord but barely vxLsible in the PDR. This suggests smaller scale topo-

graphy superimposed on the features seen in the records. That such micro-

topography is present is shown by bottom photographs taken during the sta-

tion. Small, sharply-crested ripples (Fig. 10) were apparent in 16 of the

20 frames comprising the station. In several frames parallel ripples are

separated by about 2 meters as in the photographs shown. The features

appear to be several centimeters high.

The effect of such topography in each case can be estimated very

roughly by assumi.ng equally-spaced parallel ridges 2 m apart with relief

of perhaps 10-15 cm. Th.n for vertically incident radiation at 12 kc

II!

-9-

(wavelength Xýl/8 m) the reflected radiation will be split up into vari-

ous orders of diffraction with directions from the vertical given by the

Bragg formula:

L 16

where L u 2 m is the ridge separation. The specular reflection is given

by n = 0 and is of course vertical. The first order diffraction peak

(n= i) is about 3-1 from vertical, and is therefore offset some 300 mi

from the transducer in a water depth of 5000 m. Thus only the specular

term contributes. If the relief is approximated by a sinusoid with ampli-

tude a, and the sediment is considered a fluid, the intensity of this term

can be approximated, for X<< L, by (see for example, Officer, 1958)

~2I I jo (4 r

where 10 is the intensity returned from a plane surface (a=O) of the same

impedance contrast, and Jo is the Bessel function of order zero. For

La P 1/8 m the envelope of J (4 " ) is about .23 whence I $ 0.05 1o.

Thus the intensity returned to the transducer is only 5% of that expected for

a plane surface.

For 100 cps X = 15 m and all non-specular directions are imaginary

since )>)L. In this case the non-specular or scattered portion is con-

fined to the vicinity of the boundary and is lost to the specular term.

In the long wavelength limit, X >>L, the sinusoidal model with amplitude =

12 cm and L = 2 m gives, to the same approximation, negligible loss due to

the microtopography.

-I0II -10O-

In summary, three types of abnormal or unusual FDR trace have been

considered: periodic, hyperbolic, and fuzzy traces. In the area studied

all three types are caused principally by bottom corrugations of different

wavelengths. Hyperbolas generated by isolated point sources and rough,

steep slopes such as the Blake Escarpment are not of interest here. The

approximate range of wavelengths for each type is as follows:

periodic: 1000 - 7000 m

Shyperbolic: 100 - 1000 m

fuzzy: 10 - 100 m

The sources of the majority of the periodic and hyperbolic echoes

shown in Fig. 2 are of two types: (1) longitudinal ripples formed by bot-

tom currents, as exhibited in the eastern Blake-Bahama Basin, (2) minor

slumping and compressional folding such as is frequently seen on the east-

ern flank of the main ridge, for example.

The detailed study of topography in the 10-100 m wavelength range in

deep water requires considerably more resolution than is possible with con-

ventional surface-mounted echo-sounders and a much greater field of view

than is afforded by bottom photography. One possibility is the use of a

deep-towed echo-sounder, or at least a deep-towed pinger working into a

ship-mounted receiver. Preliminary work with a deep-towed pinger in the

area gave evidence of micro-relief but a detailed study was hampered by

a large drift in the pinger time base. It is expected that future pinger

lowerings will enhance our knowledge of the microtopography and its rela-

tion to bottom loss.

BOTTOM LOSS

As mentioned in the introduction, measurements in the Blake-Bahama

area indicate that bottom loss is significantly greater over hyperbolic

and fuzzy bottoms. A ship in this area, using a 4 kc system, found a 10-

15 db increase in bottom loss while its !2 kc echo sounder (ping length

e.19 ms) simultaneously noted a gradual change from fairly coherent bot-

tom trace to a very fuzzy or incoherent one (Weaver, 1957). This run of

approximately 30 miles was made Just east of the Outer Ridge (29030'N;

72040'W) at about three knots over what appeared to be practically smooth

topography. The Lamont ship CONRAD was subsequently directed to duplicate

the track with its 12 kc short ping sounder (ping length Ov5 ms). The

CONRAD data, taken at 10 knots, disclosed microtopography indicated by

hyperbolas and small periodic corrugations with wave-length and amplitude

increasing in the direction of increasing fuzziness noted by the first

ship. Calculation of the orientation of the corrugations at several points

indicated that they are parallel to the regional contours. Subsequently

the vertical exaggeration of the record made at 3 knots was increased by

means of a flow camera, thereby accentuating details of topography which

were not obvious in the original record. Fig. 11 shows (a) a portion of

this modified record and (b) its equivalent CONRAD section.

OUTER RIDGE

In the process of investigating the echo sounder records in the area

and plotting the extent of the fuzzy zones, it became apparent that these

zones are related to the large .cale structure of the Blake-Bahama Outer

VSIi

-12-

Ridge, and therefore probably result frum the processes forming the ridge.

The Blake Outer Ridge is an elongate feature over 300 miles in length

which trends southeast from a point about 320 30'N, 76030'W where it merges

with the northern portion of the Blake Plateau. At this point the ridge

is rather broad; it tapers down toward the southeast, The eastern flank

is consistently steeper than the western.

The following discussion of the structure of the Outer Ridge is based

upon detailed study of the character and behavior of the reflectors obscrv-

ed on continuous seismic reflection profiles along approximately 13,000

miles of ship's track. Three diagnostic sections are shown as Fig. 12

and oriented in Fig. 13. Both comprossed air (sections AB & CD) and ex-

plosive (section EF) sound sources were used. Typical penetration in this

area with these methods is 2 seconds of reflection time; the maximum pene-

tration (3.5 sec) was achieved using explosives.

It is known that the Outer Ridge is a depositional feature (Ewing &-

Ewing, 196 4 ) built upon Horizon A, an essentially flat-lying reflector

of Cretaceous age which is present throughout the area of Fig. 2 east of

tihe Blake Escarpment (see also, J. Ewing, et al, 1966). Above "A" we dis-

tirnguish two major sequences divided by a reflector labelled x in Fig. 12.

The lower lies directly on "A" and represents the major period of differ-

entiaJ. deposition which gives thle ridge its relief; the upper has a more

complex structure.

Post "A" sedimentation took the form of a mound of unevenly distri-

buted acoustically transparent sediment. The poorly-defined reflector

which tops this sequence, reflector x, can be traced throughout the Blake-

Bahama Basin and secondary ridge provinces as well as over most of the

71~

-13--

Outer Rlidge. In section AD of Fig. 12 (which is a representative cross-

section of this portion of the ridge) this layer thins abruptly toward

the Hatteras Abyssal Plain from an estimated thickness of 1.5 sec at the

ridge axis to only .5 sec in less than 50 miles; it thins more gradually

toward the Blake Escarpment. 'The secondary ridge is displayed by this

reflector, though its axis lies slightly to the east of tde present topo-

graphic feature. In effect, the gross topography at the time reflector

x was the sea floor was virtually the same as the present-day topography.

Post x sedimentation begins with .5 see or more of acoustically trans-

parent material which gradually gives way to series of thin parallel re-

flectors at about .5 sec below the bottom. Post x sedimentation is com-

plicated north of 30ON in the ridge area by a reflector (y in Fig. 12)

intermediate between reflector x and the sea floor. In the area of Fig.

2 this reflector is visible only under the crest of the main ridge (roughly

delineated by the 2000 fm contour) and over an extensive area of the contin-

ental rise northeast of Cape Hatteras. Where y is present interpretation

is made more difficult by its excellent reflecting characteristics which

commonly prevent deeper reflectors from being seen. Reflector y appears

occasionally to cut across bedding planes while maintaining its usual

position about .6 sec below and parallel to the sea floor (Fig. 12 AB &

EF). It is never clearly visible deeper than 5.5 see below the water

surface, a depth which corresponds, perhaps fortuitously, to the depth

of principal change in calcium carbonate content (Peterson 1966).

Post x sedimentation is also complicated by the existence of the

steeply-dipping reflector shown midway between the two ridges in Fig. 12

F 4-section AB and Just to the right of the point where AB crosses on section

EF. This reflector forms a plane with a dip of about 20 to the south.

The plane, striking essentially east-west, has been observed only between

74030, and 75'15'W, outcropping approximately along the 30 0 15'N latitude

line. It is not present on the western flank of the southward extension

of the main ridge (section CD); therefore it seems that unless this re-

flector is regarded as a fault plane, one of small throw and within the

post x sequence, the post x evolution of the ridge may be considerably

different to the north and south of latitude 300 15'N.

The ridge sediments above Horizon A both thin and dip graduall3y

to the southeast along the ridge axis as shown in Fig. l4 (a structure

section along the ridge axis derived from all available profiler cross-

ings of the ridge crest). The sediments also thin and dip sharply to

the east (from the axis) along the entire length of the ridg4. On the

west flank north of the "nose" the sediments have to a large degree filled

in the area between the Escarpment and the ridge crest. This pattern of

sediment thicknesses suggests the Blake Plateau as the principal sediment

source for the construction of the Outer Ridge. This appears to be sup-

ported, at least for recent times, by the carbonate studies of Pratt &

Ileezen (1964) which indicate the Blake Plateau sediments as containing

over 80% of carbonate material, the Blake Outer Ridge about 60%, the

Blake-Bahama Basin 40-60%. North of Cape Hatteras the amount of calcium

carbonate is small, rarely more than 3% (Stetson, 1939).

The two principal current systems available as media for trans-

portation of sediment to the Outer Ridge are (1) the Florida Current,

I'

augmented by the Antilles Current, moving north and then turning north-

east across the shallow Blake Plateau, and (2) the deep western boundary

undercurrent which flows southward along the lower continental slope and

upper continental rise in the vicinity of Cape Hatteras (Swallow & Worth-

ington, 1961, and Barrett, 1965). Additional evidence Lor a source on

the Blake Plateau is the presence of marked bottom scour on all *eho

sounder traverses across the axis of the Florida Current. This scour

zone (Fig. 8) begins to widen at about 30 0 30'N where the combination of

the gradual increase in bottom slope in the direction of the Blake Eacarp-

ment plus the coastal control exerted by the protrusion of the coast at

Cape Hatteras induces a marked easterly component in the bottom current

and scour patterns. The east-west "shoulder" at 31°N; 78-79°W also as-

sists in promoting this tendency. Actual bottom current measurements

(Pratt, 1963) indicate strong bottom currents of 25-40 cm/see moving east

and northeast at about 31 0 N;780 W. According to tl.ý bathymetry of reflector

11 (Ewing, J., et a!, 1966), it seems probable that the principal flow in

the past crossed the Escarpmient slightiy farther north, even nearer the

root of the ridge than at present.

The existence of the two oppositely directed currents, together

with the overall shape and structure of the Outer Ridge, suggests a mechanism

for the formation of the ridge which is based on the assumption that the

two currents interact to create the conditions necessary for the observed

pattern of deposition. It is postulated that sediment-laden bottcm water

of the Florida Current, flowing roughly northeast, meets the undercurrent,

flowing roughly southwest, at the edge of the Blake Plateau, both streams

then being diverted to a generally southeast direction. The surface water

p.I

-16-

of the Florida Current, passing over the undercurrent, continues north-

east as the Gulf Stream. In the vicinity of the Junction the interactingWcurrents are slowed down and duposition takes place along the Patti of the

resultant combined flow.

This picture is developed along quantitative lines in a separate

note included as an appendix; the results obtained are in reasonable

Iagreement with the main features of the depositional pattern. Since the

major differential deposition occurred between Horizon A and reflector x

(or possibly y)p the model may not be as applicable to more recent sedi-

ments, including present day deposition. This would hardly be surprising

since the model does not take into account the effect on the flow pattern

of the changing topography as tho ridge is being formed. In the limit

the ridge would build up into the level of interaction and dominate the

flow pattern. The original flow pattern may, of course, have been dis-

turbed by processes unconnected with the building pro-ca itself, such

as the previously mentioned possible southward shift of the region where

the Florida Curront bottom water leaves the Escarpment. A change of

this sort would have a direct effect on the flow pattern and could ex-

p- 1. •s' b t ,Ahe presence of Lhe se~ondnry ridge. In any case there are a

number of secondary features which cannot be explained by the first , order

theory invoked here. Among these are the bench-like structures of Fig.

12 (8F), the related westward protrusion from the Outer Ridge at 30O30'N

(well-displayed by the 1900 fmn. contour), and the very sinusoidal periodic

topography associated with some portions of the west flank of the secondary

ridge (Figs. 3b and 12(AB)). A more exact hydrodynamic theory, coupled

with more detailed knowledge or' the boundary conditions, might explain

- --

- 17-

some of theos featur(l'; post•-di-puzitional, ,iJtui•plo or othcr tectonic

meohanitms might. explain uthers, Such as the aforementioned benc hee,

T'he q,:t1on trl.iseo!; whether th.e two cuirrents can in fact meet at

the swae level and interact as required. Estimates regarding the upper

level of the. k' d-rC uL'rVnt are confl ijg but Barrett (1965) gives evidence

that it is as shallow as 500 m aLt the inshore boundary near Cape flatterar.

Since the floor of the Blake Plateau Is typically 800 m deep, considerable-

overlap would exist ,even if both urruents were constrained by ttmpcrature

stratirication to remain at the respective levels dictated by the upstream

boundary conditions. On the other hand, Swallow and Worthington suggest a

figure of close to 2000 m for ths top of the undercurrent farther south t.t

about 33 0 N which would preclude any interaction if the streams remained in

perfect tharmal stratif'i.eiAtion, However, since the stream velocities are

relatively high some disturbance of the stratification due to turbulence

may be expected. Thus, the sediment-laden bottom wator of the Florida Cur-

rent way iti effcct Aink upon crossing thv LscarpmenL. through the meehanism

of downward turbulent transfer" of both sediment nnd momentum. For suffi-

ciently Larjedit Wrains. gravitatio, ul .,ettltng could account for thv

;illnking oif the , dim,•n even without ,eddy di fi'u.5io11 the more important

question is whether or not the eddy vs.osity 1s large eliouiih to effect

the required momentum trnnt ler against the temperature gradient.

Wnatevor th.- det-ails of the iinteraction, and In tpite of the many

secondary o;lpect: * tchu 1j('.J,; ':.tUCLtU.? whiCh r'1main unexpIlal ud 1y t|ii;_

flrst order th,:o il , theoveri. patt-c-n ol •'p io" is udquaely cx-

plailned in t..o thr. hydrtoIdyflif[i,'.t of 3i mTv, nt- cl-rryiing curren.t;.

I--I

Appendix: lkydrodynadie Model of the Formation of the blakc-LahamaOuter Ridge

G. M. Bryan

Ii The proposed picture of the formation of the Blake-Bahama OuterRidge by aurrentn can be developed quantitatively on the basis of hydro-

dyinaic theory. It is postulated that sediment-laden bottom water of

the Florida Current, flowing roughly northeast, encounters the western

boundary undercurrent, flowing roughly southvret, vt the edge of the

Blake Plateau. both streams then being diverted to a generally s4outh-

east direction. The surface water of the Florida Cuirent, passing over

the underourretxt, continues northeast as the Gulf Stream.

In the vicii'ity of the Junction 4 hydrodynamically stable flow

pattern can develop in which the flow velocity decreases to a minimum in

the turn, then inoreases as the flow straightens out on the new course.

If one or both of the streams is carrying sediment, deposition will take

place in the region of lowered velocity, and the depositional pattern

will be governed by the flow pattern. The relationship between flow

pattern and depositional pattern will, in general, be quite cmplex and

will depend to a large extent on the r,•nge of velocitias end parti.cle

sizes involved. Relatively heavy particles, with settling velocities

comparable to the flow velocities, will tend to deposit along isotaehs

corresponding to appropriate throshold velocities for deposition. On

the other hand very light particles, which tend to stay in suspension

for long periods of time, will tend to be distributed along streamlines,

to the extent that they are deposited at all.

in order to procecd toward a quantitative theory it it necessary

-19- !

flirst te esltabiIlh tA ruasonabie flow pattern fitting tile known tounda'y

eonditiomu and second to develop a suitable mechaniam for deposition as

a function of flow velocity, and particle sigze. iDoth spects of the pro-

blem require rather severe simplifying assumptions.The 1-kydr:lynamic equa!tions nre wu.tal!y Iine-arizcd by¢ (L per'tur'bation,

method in which the unporturbed solution represonts the case of static

t'quil1br-iull. FiV.-At ordder porturbation for free steady motion with plane Ilevel surfaces and non-isentropic stratification then yields two-dimen-

sion.al st:ean:linc: 'ho" of an incomlres sible fluid (see for example, Eck- Uart, 1960). Thu4 first order velocity v then satisfies

V V 0 and V . VVX 0

where X is the •ravitational altitude,

In this approximation the flow is also irrotational and the methods

of potential tbeory are applicable. In particular for two-dimensional

motion we introcducu th complex potential IW(z) W + J

where-i z = -- i.y and the velocity potentLal # and the stream function

X are onnJuGat.e solutions of Laplace's equation.

,itn• f'or" the velocity V c VX+iVY x qs 6 we have,

V ~ - a ndV

froi ,Whi.h it follows that

* Al

S! • -:

_ _ j

i -20-Iwhere v* qe io is the complex conjugate of the flow velocity.

Sj In Fig. 15a the x-axis is a rigid boundary representing the Blake

Escarpment from Cape Hatteras on the left to about latitude 30030'N on

4 the right, Two streams of widths wI and w2 - w,, following the boundary,

meet at the origin and are diver+ed to a new direction. This problem is

eqiuivalent to that shown in Fig. 15b in which two streams of widths 2w1

fl and 2w2 meet head on without the influence of rigid boundaries. The line

of symetry in Fig. 15b can be replaced with a rigid wall to give the pro-

blem of Fig. 15a. The problem of impinging streams in two dimensions has

been treated in considerable detail in classical hydrodynamics (see for

example Milne-Thompson, 1955, chapter XI). A stationary flow pattern re-

quires that the two streams have the same speed at infinity, and this is

assumed without further justification. The complex velocity v = qeiQ

is then related to the position z = x + iy by

J- 2 W• In ( - PI

IU IUI• -(w,÷..,),,BIn (, I •*, )- (w,÷ w.) 4r',,I (, I v - e,

where U = upstream speed at infinity, P= asymptotic direction of final

flow. An isotach is the locus of z traced out by varying Q at fixed q.

The complex potential W, whose imaginary part is the stream function, "•

also most easily expressed in terms of v*

IV

L

-21-

"The final flow direction is related to the initial upstream widt"3 at

infinity by

COS fl: • -W I +Wl+ W,

and the final downstream width of the combined flow is w= W] + w2

if the widths and velocities of the two streams were known the final L'low direction 0 could be computed and isotachs and streamlines mapped

in the x,y plane. In the present case the parameters of the incoming

streams are not that well known; the angle 0 was inferred from the known

topogk aphy and used to compute isotachs and streamlines in dimensionless

form. The resulting patterns were then adjusted in scale t.n fit the top-

ography, after which the stream widths could be predicted. (Since we are

assuming deposition on a nearly level Horizon A, the topographic contours

correspond approximately to isopachs.)

The streamlines and isotachs computed for • =660 are shown in dim-

ensionless form in Fig. 16. There is a stagnation point at the origin,

where the streamlines coincident with the boundary change direction dis-

continuously as the flow velocity vanishes. A dividing streamline, -;,hioh

marks the boundary between the two bodies of fluid in the combined flow,

is perpendicular to the rigid boundary at the stagnation point, curves

downstream away from the wid- current, and appi-ach'cz asymptotically the

final flow direction 9 . Velocity is continuous across this -1viding

%rea•mii; uhe isotachs approich concentric circles about the -tagnaLion

point at the low velocity limit and approach the streamlines as the velo-

tLy approaches the u,... tr.am velocity. The outer edges of the flow

constitute free streamlines with constant velocity U. The isotachs in

'K

-22-

Fig. 16 are plotted at velocity intervals of 0.lU.

In Fig. 17 the flow pattern is compared with the bathymetry of the

ridge, the scale having been adjusted to obtain the best fit between the

dividing streamline and the ridge axis. The resulting stream widths are

74 km on the north, 31 km on the south and 105 km for the combined down-2--

stream flow. Available experimental determinations of the undercurrent

are variable and sometimes show considerable structure. According to

Barrett (1965) the width off Cape Hatteras is probably between 20 and

80 kim. Farther south ( ' 330N) Swallow and Worthington (1961) show sev-

eral sections with peak velocities in the range of 10-18 cm/sec and widths

of 90-100 km to the 1 cm/sec points at the bottom. Still farther south

(--32030'N), and well into the pattern of converging streams, their

Section I shows peak velocity down to about 7 cm/sec and stream width

more than doubled.

Similarly detailed measurements do not seem to be available for

the Florida Current in the neighborhood of the current junction. For

what it is worth, however, the scour pattern on the Blake Plateau Just

north of the Florida Straits is about 30 km wide, in reasonable agree-

ment with the predicted upstream width of 31 km.

Further support for the hydrodynamic model. is found in the close

fit between the curving ridge crest and the dividing streamline of the

flow pattern, along which maximum deposition should take place. The

crest appears to divide the ridge into two distinct depositional regimes:

on the east flank the topographic contours are nearly parallel to the

crest, tending to follow the streamlines of the flow pattern (Fig. 17)

-23.

while on the west the contours make large anxles with the crest and tend

to follow isotachs (Fig. 18). As discussed qualitatively above, this

suggests that relatively fine sediment is moving south in the under-

current, and relatively coarse sediment is eming north in the Florida

Current.

I.

A-

-24-

ACK•0WLEDGMTS

This work was supported by the U. S. Navy under Contract NObsr

85077 with the Bureau of Ships. Thls publication is for technical infor-

mation only and does not represent recommendations or conclusions of the

sponsoring agency. Reproduction of this document in whole or in part is

permitted for any purpose of the United States Government.

The authors wish to acknowledge many helpful discussions with

J. Ewin and R. Houtz as well as the contributions of the many Lamont

personnel involved in obtaining the data at sea.

The cooperation of the staff of Hudson Laboratories of Columbia

University in making available their flow camera facilities is gratefully

acknowledged.

The investigation of microtopography grew out of work performed

in cooperation with H, Weaver and J. Geary of the U. S. Navy dnderwater

Sound Laboratory.

It

REFERENCES

Bari .tt, J.R. ., Jr., Subsurface currents off Cape Hatteras; Deep Sea Research,v. 12, p. 173-1814 (1965)

Bryan, 0. M., PRoceedings of Seminar on Reflection of Sound by the OceanBottoti; USNUSL, May 25, 26 (1964)

Bryan, G, N. and J. Ewing, Proceedings of the 22nd U.S. Navy Symposium onUnderwater Acoustics; October 26-28 (1964)

Clay, C.S. and P. A. Rona, On the existence of bottom corrugations inthe Blake-Bahama Basin; Journal of Geophysical Research, v.69,No. 2, P. 231-234 (1964)

Eckert, C., Hydrodynamics of Oceans and Atmospheres, Pergamon Press Inc.,New York (1960)

Ewing, J., M. Ewing and R. Leyden, Seismic-Profiler Survey of Blake-Plateau; Am. Assoc. Petroleum Geologists Bull., .v 50, No. 9,p. 1948-1971 (1966)

Ewing, J., J. L. Worzel, M. Ewing, C. Windisch, Ages of Horizon A andthe Oldest Atlantic sediments; Science, v. 154, No. 3753, P. 1125-1132 (1966)

Ewing, M., and J. Ewing, Dist:ibution of oceanic sediments; Studies onOceanography, p. 525-537 (1964)

Luskin, B., J. E. Nafe and M. Ewing, The Use of Topographic Highlightsfor the Measurement of Ship's Ground Speed; TR No. 15 (CU-41-57-NObsr 64547-Geol.) (1957)

Miine-Thompson, L. M., "Theoretical Hydr-dynamics', 3rd Edition, MacmillanCo., New York (1955)

Officer, C. B., "Introduction to the Theory of Sowid Transmission" McGraw-Hill Co., New York (1958)

Peterson, M. N. A., Calcite: Rates of dissolution in a vertical profilein the central Pacific; Science, v. 154, No. 3756, p. 1542-1544(1966)

Pratt, R. M., Bottom currents on the Blake Plateau; Deep Sea Res., v. 10,p. 2145-249 (1963)

Pratt, R. M. and B. C. Heezen, Topography of the Blake Plateau; Deep SeaRes., v. 11, p. 721-728 (1964)

I:

I

2-6-

Stetson, H.C., Summary of Sedimentary condition, on the continental shelfoff the east coast of the U. S. in "Recent Marine Sediments",P. D. Trask, Ed., p. 230-244 (1939)

Swallow, J.C. and L. V. Worthington, An observation of a deep counter-current in the Western North Atlantic; Deep Sea Res., v. 8,i• p. 1-19 (1961)

Weaver, H. R., Proceedings of Seminar on Reflection of Sound by theOcean Bottom; USNUSL, May 25, 26 (1964)

Li

F

A

lz

!I

1

L,.I�

.1

I +I -�

V IK

I

*-1

* *1

I -

'L

IFig. 1. Dathymetric and province map of the B1ake-Bahazr�a region.

Ii

800 778°S. .. I ... I ., ,I .. .i •

36°-

MICROTOPOGRAPHY of the BLAKE-BAHAM

CONTOUR INTERVAL: 100 FATHOMS

S---TRENDS OF CORRUGATED TOPOGRAPHY

TYPES OF ECHO- SOUNDER RECORDS :INCOHERENT or "FUZZY" RECORDS

• "-*4-s44VERY "FUZZYT " RECORDS.,HYPERBOLIC ECHOES

•,.•'#N-N PERIODIC TOPOGRAPHY

NORMAL COHERENT RECORD

?34 "

S. , ,C14ARLESTON

SAVANNAH

%go-4

r ~ 3 0- SAVAN NAH-~ -- - - ~---

JACKSONVILLE

,.ST. AUGUSTINE

v

1.*)ST. AUGUSTINE\ ' I J

*4 *

44

00

IAM

800 780

76* 740

BAHMAREGION

IPt FEARy

00I

, If

ArO 'POOV

so 4

v Ir

v f. T :/

80 -

- -I--

76I a74

720- 70

ZY0OO

00

f/0

/0

210

/0ol

PT -� ___________________

_______ ______ ______________________________ ___________________________ ___________ ___________ ________ ������1��*---F- -- - _

000

'4-b

o -V -

$

S

1' - -- - -�V

'U

I--

K00

�poe4

I/

I.

-

I/

II

\II$I

/ I

tip

--

I

I$

I

I--

£

a N

-A.,

- �- S�AMouNt

II

I

/

//7

77

7

///

//

720 TOo

..Al

"40

44

.' L 0

N44

p4

. 44

I 2: * -

Itt

4.

1411714)

04

sopa. - V . d- --

} i Z o dMM

§~~~V. .4 .-- .~

,L I

S~Fig. S. Construction of hyperbolic echo assuming a Hinear feature.

}2

I

L

1-...I i�uA I

'iIiTII -J ' I II

__ � V�. -v

U

.4

%J0

[9 p0

__ � U

M�J S4.'

U

0

04.'

S4 �wII

.4

I.

*msum

V 01�

4

60 - - .

al,1;

201

~~2I

Fig. 7. Location map for Figures 3, 6, 9, and 10.

1-- - ý-T

7000

3'-7

Aw w

Fig. 8 Outline of the area exhibiting fuzzy and hyperbolic PDR tracescaused by microtopography. The axis of the Florida Currentand the zone of scoured bottom on the Blake Plateau are alsoindicated.

--- - --

NWTfIC*A MILas -

"0WNSDICA. NOLGO

0 P 4-0

F ig. 9. Typical sharp transition from coherent to hyperbolic echotrace. (a.) ungated PDR (b.) gated PDR trace made simul-taneously with c. (c.) an enlarged section of seismic pro,-ilerecord.

Fig. 10. Bottom pIhotogral)IS .3howillg long9itudinal cur rentripples with ap~pr,-,xmiately Z In \'V.high

Qzf.

- 9T

t

St

*1~ * jt

*

p,

REFLECTION TIME IN SECONDS

I I-

41 sea$

ik-

uc omoss.. 4

Fig 1. oninou sisirelconpflr rejord fro tiihe;Blk-aai Oue ig9ra

'-P

7 Fig 3 oainmpfrFg 2

00

09 tA

wC

N.04 -04

311NI3-13N 00O03

y /I

2W 2 W2 - x

Fig. 15. Flow patterns resulting from impinging currents.a. rigid boundatry along x-axisb. equivaleait pattern for unrestricted currents

I"IiI

I

Fig. 16. Streamlines and isotachs computed for a - 660. Isotachsare plotted in intervals of 0, IU from zero at the stagnation point toU at the free streamrlines,.

I I it I

I It it Ito I

I I

so 1 I,

: j IIis 10

'IIf

It Ili

Fig. 17. Comparison of computed streamnlines with bathymetric contours.

I iIN

of IPol I

I 0 t . 1

AP/ /

Fig.~ ~ ~ 18 Co p r s no o p t di o a h ih bt/in t i o t u s

UNCLASSIFIEDS~euraly GaseeficaeaiL

i ~DOCUMENT MOITROL DATA - R&D

0. ONGINAr*WO A4 i " OT SSCVM'iV CLAISI,.CASiO10Iunclassified

Lamont Geological Observatory ,,-u -Palisades, New York 10964

Microtopography of the Blake-Bahama Region

4 bfeICIfIV oti u Novas .e yn d~n4M

Technical Report

Bryan, George M. and Markl, Rudi G.

"a. W DSP Y T _V&! 'iVAL NO, OF A2 LNorFAo

December 1966 44 17*& CONTRACT OR ONANT NO. 946 @NII6NATONI MNPORT MUWSfts)

NObsr 85077& PHRO•TT AND YW O•N. Tech. Report No. 8, CU-8-66

4. DO& IMlSNo? *. Ov (Awl o vst Am &W U

0. AVA-ij;AOlTLM1YTIAf1~iTIM NO

Di'stribution of this document is unlimited.

0. "VPLKMpwYAftV *OTC$ " SeowaMIND MIMITA00 ACTIwayy D.

NAVSHIPS (16ZZK), Washington,

ti A$T•AA •

Sea floor microtopography has been shown to have an adverse

effect on bottom loss. An attempt has been made through analysisof 1Z kc PDR echograms to define (1) the rangc " -,wavelengths and(2) the origin, of corrugations observed in the Blake-Bahama

Basin and Blake Outer Ridge provinces, as well as to delineatetheir areal extent.

- JAN 44

UNCLASSIFIEDsetwicy Claafisc•uiDo

tJNCLASSM ED

119yC "NO LINX A 1.1141018 U

Effect of Microtopography onIBottomn Loss.

Blake -Baharna RegionBottom LossMarine GeologyMicrotopograpihxiz kc echo sounders

INSTRUCTIONS

I. ODIGINATINC ACTIVITY: Friter the name avid sIdtt.. 16. AVAILADILITY/lIMITATION NOTICE&i Emo, mayp leni.of the cofltroetor, sobront racta#, 1gtontea, Department of 4titons on further dioteminoktion .1o the rop~rt. ther than the..Defer.., alCtiVity or Other OO-g5fljLtiOl, (Cooperate aotiop) Imsvo by security clateioiication, "IRSn ottausd *ed Aateusht.ttit'vift thM report. suhno.:

2.IMEPORT SEC URITY Cl-ASSIICATIONt Enter the o-er. (I) "Qualified requester may obtain copies of 4bLollseeuit -ln~lctono k e.11. Inhdicate whether report from, 1)C."Reotricted IDoma" is included, Murkiing is to bei jaccord. (2) "Fereign announcement andl diasetsemntion of thislicc with o~peporolste security fegal.aiton. repoot by DDC is Wo autlnouired."

26. GROUP11: Autontutic downpnidmna in sepcified it, Dal) (3) "U. 51 Government VgeClOC6 may tobtalti copiers ofDirecttive 32DO. 10 and Alretd I'ances')nduumiial Manual. thin repoft directly from 3DIC. loote qualified amDEnter the -opnmto, Also, when. alplicabl-c. sow that user% shall requoAln through I"opionl ehIngo have been ated tor lootp 3 andJ Group 4 1"w~tr61ktatoeised. ____

(4)"U S mliaryeencies mnsaytobtin copeofteI.MRPORT TITLE: Voiter the complete report titll* in all rq ot directly Irom, l)1X. Oher qualliIf=daecapital letters. Titles in all esses athouldl be unclsassified.,hftsqis hoIt a mCamfir~tful title cannot be selected without cloatiaica.tole. ehow title cleaaification in all capitals ink parenthelsilimenjistely followin the title. (3) "All distributiont of thin repeat in ccontrolld. -Quail.L DES§CRIPTIVE NOTES: If appropriate. enten the tyre of lied DDlC users slhall request throughreoart. e*p** interim, progress. 61a1Mmsiy. annual. of linse_____ ____

cover.ýd. It.~ ncO j5it eotu ~t 5 lthe rp-ot ha.. been furnialted to the Office of TachemitelServices, Dtpartffnuet of Commerce. for *sit to he public. hadl.

S. AUrTl1OMS).- Enter the atameta) of mahton(s) as shows em cate ibis fact and etnter the price, of know&.ar in tho mrpnt Ewe~rr st name, first name, mniddle initial.If military. Nho rthank n breach of service. The sweme of 11. SUPPLEMPATA fY NOTES- Uine foe additiiamal explaa*.the principal author is an abuolyie minimum requirement. Sooy note..

6. RPOR DAP: nte th dat ofthere~ asdit, I1. SPONSOflANG Ml~IIARIY ACTrIVITYi Enter tho mamam ofaot. yewOR ofE monter thea dftoe ofthen repat dat appar* the deparuroe.,aI fn'oicel office or laboratory epeenerung lpd.memit. ear orirceth yer. I neo tatiomodawappars ins f"ri the resarach and development. Incluade address.

as the* rapoot. *so date of publication.Ta. TOT1AL NUMHtEI OF PAGES. The total pag count i.. ABISTRlACT: Entter an, abatract Siving a bWief sor4 factualshould foillw nortmal papuilation procedures, i.e.. enter the suot yo h lcann ntamof.erpreecamber of pilgee evatainttR infornatton. niouhoa it ma atlso appear elsewhere is thc body of the took.

reot.I dilit ioral spaec is required, a continuation7b. NUMBPR OF 1EFEFRENCES: Eraitr the total sathnker of oheet sahll be attached.eclranas ime tnthereprt.It is highly desirable that the abstract of classified ro

U. CONTRACT OR GRANT NUXIDlEN: If ppotpeioote. enter prent ho unclossified. Each paragcraph of the abstrac. shallthe applicable nmbnier of the contract or peoat u odr which ond wuith at, indiiation of the military security classificationiAth rep-ort wee written. of the information in the Parlagrapih, fepresented an 031). (SA.84, Sc. 41 Sol PROJICT NUMBFR: Enite the appropriste Cn(i.wilw depanrtment identification, ouch no project nmbmler. There in nc. limitatioa on the lengtlh of the abstract. Hew.subproject number. sysitem numabers. task number. etc. oere, the suggested length is from M5 to 235 wordl..Pa. ORIGINATOR'S RFI'ORT NJUMDERMS: Enter theof at(- 14. KhEY WORM)S Key word3 -te technically meanmiagful ten*tQlal repeat mumbor by ahich the document will kbe identified or shmo plarane thast characterize a report mild mtay be used as*tA# controlled by the originating activity. This number muist index entries for catalogling the repot. Key words mast behe unique to this report, selected so that no security classification its required. Idesli.M4 OTHER REPPORT NUMBER(s): If tha repent hall botn liens, suc, as equipment model ataigtaaaion.. trade 4mom, mlill.- -isaalgrood amy other report n..rnbena (eja~rr by~ M, ocigiftasor tar! project cede name, rpi location, my he mood as

or ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~e wyMrq ~ .oe ne hsam~.IA ords hut will be forlovel byaon indicatien of tecliaicaltar y 'A dymeer. alo eter hiscoaltent The sa erdof link.s rales. sad weLot. Is

IJNCIAS S!PED

NND ~tatio

a;