OCEAN BOTTOM PHOTOGRAPHS IN THE AREA OF THEOLDEST KNOWN OUTCROPS, NORTH ATLANTIC OCEAN*

BY MAURICE EWING AND FELIX MOUZOLAMONT GEOLOGICAL OBSERVATORY, COLUMBIA UNIVERSITY

Communicated September 4, 1968

Ocean bottom photographs have been studied to determine what conditionswill be encountered when the first attempt is made to drill the ocean bottom in thedeep northwestern Atlantic in the area of outcropping of the oldest known sedi-ments in the deep sea,'-4 about 55 miles northeast of San Salvador.

Seismic reflection profiler records made in 1961 on R.V. Vema's cruise 18 hadindicated that this particular area might well be one in which very old sedimentscould be reached by conventional piston coring techniques, even though thepiston corer can normally penetrate only about 20 m. Vema-18 profiler records,and those from subsequent cruises, showed that an acoustic horizon,6' 6 whichover most of the Atlantic Ocean basin is buried beneath 300 to 500 m of sediment,outcrops in this part of the northwestern Atlantic Ocean. Paleontological studiesof cores obtained in the outcrop area showed that the sediment at this acoustichorizon, called horizon A, is of Middle to Upper Cretaceous age.2' 4On both the eastern and western margins of the Atlantic Ocean, beneath hori-

zon A, another zone of reflectors, of which the top reflector is called horizon /3,has been identified and mapped." 3 In our area of interest, horizon /3 appearsonly as a small part within the outcropping area of horizon A. Horizon /3's posi-tion and geophysical characteristics have been discussed previously.3 Five corestaken from an exposure of horizon / contained Lower Cretaceous fossils.4 5On the Joint Oceanographic Institutions Deep Earth Sampling (JOIDES)8

deep-sea drilling expedition now under way on board the ship Glomar Challenger,samples will be taken from much deeper in this outcrop area, hopefully samplesto a depth of 600-750 m beneath the surface of the sediment. If good penetra-tion is indeed achieved, it is expected that the sequences of sediment obtainedhere will provide information on the early geologic history of the ocean basins.

In preparation for this drilling campaign, photographs were studied for evi-dence of conditions on the ocean bottom near the JOIDES drilling sites.Bottom Photography in the Area.-More than 500 Lamont photographs, taken

at 27 camera stations in the horizon A and / outcrop areas, have been examined(Table 1, Fig. 1). These provided examples of massive rock outcrops, areascovered with rock fragments and pebbles (probably debris from the outcrops),areas paved with manganese nodules, and abundant evidence of bottom currents.

All the photographs which show outcrops and rock fragments were takenwithin a relatively small portion of the area under study, the horizon /3 outcroparea. In every case, the topography as shown by the 3.5-kc sediment probe and12-kc precision depth recorder (PDR) records was.that of a relatively flat plainat a depth of about 2800 fin, broken at times by smooth local relief, small gentlehills rising not more than 20-30 fm above the general level of the plain, and smallshallow, channel-like depressions. The PDR records of the gentle hills showedsmall, hyperbolic echoes, quite different from the relatively homogeneous non-

787

Dow

nloa

ded

by g

uest

on

May

21,

202

1

GEOPHYSICS: EWING AND MOUZO

TABLE 1.Camera

Cruise stationRC 10 126

127128129

RC 11 181182183184

V21 56789

125126127128129130

V22 5V24 10

11121314

172173

Lat. N24050'23 4024 4124 5324 5724 4324 4426 1625 3825 2125 0425 1125 2323 4024 5125 0725 1025 0624 2124 3224 3924 4424 4424 4424 4224 2124 52

Long. W74011'72 2073 3973 5072 0973 4773 2974 0271 5972 4173 2073 4074 1573 5173 0273 1773 1072 5673 0172 4472 4473 4173 4273 4573 4471 3473 49

Depth(fM)279928252810281429112804278025532891273527802693269626142868278628602873286828942811283828182799278529152818

Currentevidence*RSS-R

RemarksBig rock bouldersSoft bottom

Soft bottom

R

LL

S-LR-S-LLpR-SS-L

pRpR-S-LpRpR-SR-S

SSRR

MnO2 nodules

MuddyPebbles, bouldersMuddyMnO2 nodulesMuddy

Rock frag., soft coverMassive outcropRock bouldersRock outcrop

it I" & frag.Soft bottomRock outcrop

* R, ripples; pR, parallel ripples; S, scour; L, lineation.

reflecting surface layers of the plain. The rock outcrops shown in the photo-graphs are certainly associated with the topographic irregularities.At station V-24-K-11 (Fig. 2) the massive character of the outcrop can be ap-

preciated. Four successive pictures, of the 27 made at that station, clearly il-lustrate the conditions at the outcrop. One of these (Fig. 2A) shows part of anoutcropping rock surrounded by soft, fine-grained sediment like that recovered ina core at the same station. The boundaries of the rock show evident signs of scourand the sediment has been marked by an animal track. In three other photo-graphs (Fig. 2B, C, D), the entire area shown, 4-6 m2, is occupied by a mass ofrock. Other photographs at this station show a rippled, soft sediment, indicatinga definite current action.The topography in the outcrop area is controlled by elevations in the base-

ment. The small gentle hills which show on the PDR records as hyperbolicechoes are places where the basement emerges with little or no sediment cover.The profiler records show similar basement irregularities which do not emergeabove thick layers of sediment. The profiler redord shown in Figure 3 illustratesthe general case in which we would expect that rock outcrops would be encoun-tered.The core taken at the same station as V-24-K-11 penetrated only 115 cm, and

788 PRoc. N. A. S.

Dow

nloa

ded

by g

uest

on

May

21,

202

1

GEOPHYSICS: EWING AND MOUZO

FIG. 1.-Area covered by the study. The principal characteristics of the camera stations areindicated. Topography modified after Ewing et al., 1966.1

the cutting edge of the corer was smashed. The lowermost semiconsolidatedpart of the core was dated by its fauna as Lower Cretaceous.3Photographs taken at stations near V-24-K-1 1 are shown in Figure 4, in which

boulders and pebble-sized rock fragments with sediment showing ripples andscour marks may be seen. These and other camera stations where rock outcropswere seen are indicated by asterisks in Figure 1. Within the relatively small areadescribed in this paper, it is suggestive that this rather rough, hilly bottom to-pography, where the camera showed rock outcrops and fragments, occurs onlyin the area of outcropping of horizon F.Camera stations V-21-K-125 and 130, although in different morphological zones

and widely spaced, showed very similar features. Here we see pebble-sized,subangular to rounded rock fragments covering the whole area (Fig. 4E and F).These features could be related to a sediment distribution pattern like that foundin cores at both stations, which contained alternating lutite and foraminiferalsands with rounded limestone pebbles. The fossil contents of these sedimentcores were studied, and ages of Middle Miocene and Lower M1iocene, respectively,were determined for them.3 Turbidites are abundant.

VOL. 61, 1968 789

Dow

nloa

ded

by g

uest

on

May

21,

202

1

GEOPHYSICS: EWVING AND MIOUZO

*S =:o

i Cd g ;4;~ -4

0 c

be~C e c

4C

790 PRoc. N. A. S.

Dow

nloa

ded

by g

uest

on

May

21,

202

1

VOL. 61, 1968 GEOPHYSICS: EWING AND MOUZO 71

V24|iv.K Ad- _*laA'.2400 fim.

1 ____ 2800 fi

07 at0 _ i-4 'X 3E ;4 -.IA¢.tX G

0 1070011 2500 1 230027 JANUARY 152

FIG. 3.-Reproduction of the seismic profiler record showing the outcrop where the photographsin Fig. 2 were taken. The gap between 0920 and 1340 hr is the time elapsed during the station.

Manganese Nodules.-To the northeast of the rock outcrop shown in V-24-K-11, at four stations within a radius of 30 miles, manganese nodules were seen.Camera stations V-21-K-6, 7, 8, and 127 showed large fields of manganese nod-ules, always associated with current evidences such as scour marks and ripples.In photograph G of Figure 4, the nodules are seen surrounded by soft sedimentsand deep scour marks. Figure 4H shows manganese nodules, scour marks, and asessile organism bent in the direction of the current. Due to improper illumina-tion of the compass (lower right), it was not possible to determine the currentdirection. The lack of tone contrast between the nodules and the surroundingsediment is remarkable, and can best be explained by the presence of a thin coverof sediment on the nodules' surface.

Current Evidence.-With few exceptions, the photographs studied showedsome evidence of bottom current and we can now affirm conclusively that a sys-tem of currents is acting in the whole A and # outcrop area. The presence oftrails, burrows, and other marks in the soft sediments indicates periods of quies-cence after the current structures formed. All this suggests fluctuations or peri-odicity in the current action.

Figure 5 illustrates various current structures. Camera station V-21-K-6,taken at 2735 fm, shows strong current action in a southerly direction. Figure5C, at the same station, shows scour, ripples, and several marks of unknown origin

Dow

nloa

ded

by g

uest

on

May

21,

202

1

_ ........... .. ........ .: ..X. a__ ........... .X ......... 'sA '';,',' ..a5 N ' tS' .' .. ' ......

::

S' *

:. "s.... se

.. " a!M:: .. ...

-

tw;SE 'EA'# A;:^ _

-'S

*1F.' Aptv N

.~~~AV

FIG. 4.-Ocean bottom photographs. (A, B) V-24-K-10, (C) V-24-K-13, (D) V-24-K-12, (E)V-21-K-125, (F) V-21-K-130, (G, H) V-21-K-8.

Dow

nloa

ded

by g

uest

on

May

21,

202

1

GEOPH YSICS: EWING AND MOUZO

FIG. .5.Photographs~showing current struttures. In (4) they evidence a southerly flowingcurrent. (4) and (IC) are from V-21-K-6; (B) and (D), from V-21-K 130.

in soft sediment. Figure 5B and D, from station V-21-K-130, show large parallelripples. Some lineation parallel to the ripple crests suggests that these are longi-tudinal ripples.Summary.-Photographic evidence of massive consolidated outcrops., fields of

manganese nodules that could form a relatively hard surface, and of currentaction will give useful guidance to those participating in the first deep-sea drilling.The importance of knowledge of these characteristics and the fact that they mayperhaps be deduced from inspection of the PDR records is evident, since it wouldbe difficult to drill into the bottom where only a thin cover of soft sediment over-lies the hard rock.

* Lamont Geological Observatory, Columbia University, contribution no. 1255. This workwas supported by the U.S. Office of Naval Research under contract N000014-67-A-0108-0004,and by grants from the National Science Foundation. F.c ouzo's presence and work at LamontGeological Observatory were made possible by a fellowship granted by Consejo Nacional deInvestigaciones Cientifies yTehnicas, Republica Argentina.

1 Ewing, J., J. L. Worzel, M\. Ewing, and C. Windisch, Science, 154, 1125 (1966).2 Saito, T., L. H. Burckle, andcM. Ewing, Science, 154, 1173 (1966).Windisch, C., R. L. Leyden, J. L. Worzel, T. Saito, and J. Ewing, Science, in press.

4Habib, D., Micropaleontology, in press.I Habib, D., Science, in press.6Ewing, M., and J. Ewing, J. Geophys. Res., 68, 2.51 (1963).7Ewing, M., in Man, Science, Learning, and Education, ed. S. W. Higginbotham (Houston,

Tex.: William Marsh Rice Univ., 1963), p. 41.8JOIDES, Am. Geophys. Union Trans., 48, 817 (1967) and Am. Assoc. Petrol. Geol. Bull.,

51, 1787 (1967).

VOL. 61, 1968 793

Dow

nloa

ded

by g

uest

on

May

21,

202

1