Echo character of the western equatorial Atlantic floor and its relationship to the dispersal and distribution of terrigenous sediments

Marine Geology, 18(1975): 17--45 ©Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

ECHO CHARACTER OF THE WESTERN EQUATORIAL ATLANTIC FLOOR AND ITS RELATIONSHIP TO THE DISPERSAL AND DISTRIBUTION OF TERRIGENOUS SEDIMENTS 1

JOHN E. DAMUTH

Lamont-Doherty Geological Observatory of Columbia University, Palisades, N. Y. (U.S.A.) (Received March 13, 1974; accepted for publication July 15, 1974)

ABSTRACT Dumuth, J. E., 1975. Echo character of the western equatorial Atlantic floor and its relationship to the dispersal and distribution o f terrigenous sediments. Mar. Geol., 18: 17--45. The floor of the western equatorial Atlantic Ocean can be divided into several distinct provinces based on detailed characteristics of the bottom echos recorded with short-ping (< 5 msec.) 3.5 and 12 kHz sound sources. Two major types of echos are recorded: (I) distinct echos; and (II) indistinct echos. Indistinct echos can be further sub-divided into (A) continuous prolonged echos; and (B) hyperbolic echos. Each class of echos contains two or more unique echo types. The regional distributions of the various echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about sedimentary processes. In the western equatorial Atlantic, hyperbolic echos are recorded only from small, isolated portions of the continental rise. This contrasts with the continental rise of the western North Atlantic where previous investigators have shown that hyperbolic echos parallel bathymetric contours along the entire rise and thus reflect shaping of the rise by geostrophic contour currents (Heezen et al., 1966; Hollister, 1967). The fact that regions of hyperbolic echos show little or no relationship to bathymetric contours of the continental rise of the western equatorial Atlantic suggests that contour currents have been unimportant in shaping the rise in this region. The three most widespread echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about terrigenous sediment dispersal and deposition in the western equatorial Atlantic. Comparison of the thicknesses and frequencies of coarse (silt- to gravel-size), bedded, terrigenous sediment in piston cores with the echo type recorded at each coring site shows a correlation between echo type and the relative amount of coarse, bedded sediment within the upper few meters of the sea floor. The regional distributions of these three echo types indicate that dispersal of coarse terrigenous sediment has been downslope across the continental rise and Amazon Cone to the abyssal plains via gravity-controlled sediment flows. The Amazon River is the major sediment source and most coarse sediment is deposited on the lower Amazon Cone and proximal portions of the Demerara abyssal plain.

' Lamont-Doherty Geological Observatory Contribution No. 2155.

INTRODUCTION In addition to providing a means of accurately measuring the depths of the ocean, the recent development of high-resolution precision depth recorders (PDR) (Luskin et al., 1954; K n o t t and Hersey, 1956) has provided a new tool for the study of ocean-bottom processes. Several previous studies have demonstrated that b o t t o m echos recorded with short (< 5 msec.), highfrequency (3.5 and 12 kHz) sound pulses (pings) reveal many details of seafloor microphysiography and stratification within the upper one hundred meters of the sea floor as well as much information about erosional and depositional processes (Heezen et al., 1959; Hersey, 1965; Ryan and Heezen, 1965; Heezen et al., 1966; Hollister, 1967; Schneider et al., 1967; Heezen and Johnson, 1969; Hollister and Heezen, 1972). The present study describes the echo character of the floor of the western equatorial Atlantic Ocean (Fig.l) as revealed by short ping (< 5 msec.) 3.5 and 12 kHz sound sources. This study utilizes echograms from this region collected on Lamont-Doherty research cruises during the past 15 years. The different types of ethos observed are classified and the distribution of each type of echo in the western equatorial Atlantic is mapped. For regions such as the continental rise and abyssal plains where variations in echo type apparently are related to sedimentary processes, an a t t e m p t is made to correlate certain widespread echo types with the relative abundance of coarse

Fig.1. Map showing the physiographic provinces and features of the western equatorial AtLantic. (Modified from Damuth, 19 7 3. )

19

(silt to gravel), bedded, clastic detritus present in piston cores in order to discover which sedimentary processes are responsible for the dispersal of terrigenous sediments throughout the region. The purpose of the present study is to demonstrate how echograms can be utilized in conjunction with other data (piston cores, etc.) to evaluate sedimentary processes on the ocean floor. This study is qualitative in nature and is probably best described as an exercise in geological mapping. It is not the purpose of this study to quantitatively evaluate the acoustic properties of the sea floor which cause the observed types and changes of echo character. Nor does this study attempt to evaluate technical problems of acoustic reflectivity such as the effects of changes in ship speed, instrument gain control, or ping length on the character of bottom echos. However, if future investigators propose to use this type of analysis, they should attempt to maintain a constant ship speed as well as the same adjustments to the controls of the precision depth recorder (especially ping length, paper speed, and gain) throughout the survey. Sedimentation in the western equatorial Atlantic

The sediments of the upper few meters of the western equatorial Atlantic floor have been described in detail by Damuth and Fairbridge {1970), McGeary and Damuth (1973), and Damuth (1973). During the Late Quaternary large quantities of terrigenous sediment have been deposited on the continental slope and rise, Amazon Cone, and abyssal plains (Fig.l). Deposition of terrigenous sediment has been modulated by glacio-eustatic sea-level fluctuations. Sea-level lowering during glacial periods such as the Wisconsin permitted rivers to discharge their sediments directly into the heads of submarine canyons where they could easily be transported to abyssal depths by gravity-controlled sediment flows. The terrigenous sediments deposited on the lawer continental margin and abyssal plains are composed largely of gray hemipelagic clays rich in organic detritus. Graded beds up to several meters thick are commonly interbedded with the hemipelagic clays and consist of terrigenous particles ranging from clay to fine gravel in size. The primary structures and composition of the graded beds indicate that deposition was primarily by turbidity currents. Transportation of the terrigenous component of the hemipelagic clays to abyssal depths was also accomplished by some type of gravity-controlled sediment flows although the process of deposition is not entirely certain. Unlike the turbidity-current deposits which were episodic, the hemipelagic clays contain a pelagic component (planktonic foraminifers) which indicates that these sediments accumulated continuously. Possibly upon reaching abyssal depths, the terrigenous clays were entrained in deep contour-following currents which deposited them slowly and continuously throughout the basin. The major source of terrigenous sediments is the Amazon River. Accumulation rates for terrigenous sediment on the continental rise, Amazon Cone,

20

and abyssal plains during the Wisconsin ranged from 4 to > 30 cm/10 :~yr. However, the post-glacial sea-level rise moved the locus of river sedimentation up to 300 km landward of the shelf break and thereby completely cut-off this supply of terrigenous sediments to the lower continental margin and abyssal plains during the Holocene. Presumably terrigenous sedimentation was also interrupted during previous Pleistocene high sea-level stands (interglacials) similar to that of the Holocene. The sediments of the Mid-Atlantic Ridge and smaller topographic highs including the Ceara Rise, North Brazilian Ridge, and isolated seamounts which protrude above the continental rise and abyssal plains are composed of light brown pelagic foraminiferal marls and oozes which accumulated at rates of 1 to 5 cm/10 ~ yr. Deep (> 4500 m) abyssal hill provinces which are remote from terrigenous sediment sources are characterized by low-carbonate (< 10%) brown clays which accumulated at rates of approximately I cm/10 ~ yr (Damuth, 1973).

C L A S S I F I C A T I O N OF ECHO C H A R A C T E R

Only echograms recorded with short (< 5 msec.) sound pulses were used to classify and map the echo character. Echograms recorded by using a 3.5kHz sound source were utilized whenever available because they provide greater penetration (25--125 m) and better definition of the echo character than do 12-kHz sound pulses which generally penetrate less than the top 20 m of sediment. The classification below is based on 3.5-kHz echograms. Many 12-kHz echograms, especially from earlier cruises, were recorded with sound pulses much greater than 5 msec. which makes them useless except for mapping areas of very rugged topographic relief which are characterized by large irregular hyperbolae (echo type IIB-1, below). Bottom echos recorded with short, 3.5-kHz sound pulses can be readily divided into two principal classes: (I) distinct echos; and (II) indistinct echos. The echos from these two classes can be subdivided further into several discrete types. Three kinds of distinct echos (Class I) are recognized and classified on the basis of the attitude or absence of sub-bottom reflectors. (IA) Continuous sharp b o t t o m echo with no apparent sub-bottom reflectors (Fig.2). (IB) Continuous sharp bottom echo with continuous, sharp, parallel subb o t t o m reflectors {over tens to hundreds of kilometers) (Fig.3). (IC) Continuous sharp b o t t o m echo with continuous, sharp, converging sub-bottom reflectors (which wedge out over several tens of kilometers). Indistinct echos (Class II) can be subdivided into two groups: (IIA) continuous prolonged echos; and (IIB) hyperbolic echos. Continuous prolonged echos (IIA) can be further subdivided into two types:

21

r'MS

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Fig.2. Echo type IA: distinct, continuous, sharp bottom echo with no apparent subbottom reflectors.

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Fig.3. Echo type IB: distinct, continuous, sharp bottom echo with continuous, sharp, parallel sub-bottom reflectors (over tens to hundreds of kilometers).

'2'2 (IIA-1) Very prolonged fuzzy bottom echo with no apparent sub-bottom reflectors (Fig.4, A--C} or an occasional prolonged converging sub-bottom reflector {which wedges out over a few kilometers) (Fig.4D). (IIA-2) Prolonged bottom echo with zones of semi-prolonged, discontinuous, parallel sub-bottom reflectors which alternate with zones of diffuse or intermittent ( " m u s h y " ) sub-bottom reflections (as if sound pulses are reflected from sub-bottom reflectors only intermittently (Fig.5t. Hyperbolic ethos (IIB) are recorded from areas of sea floor with relatively rough b o t t o m morphology. Five separate types are recognized: (IIB-1) Large irregular overlapping hyperbolae to gently rolling single hyperbolae with widely varying vertex elevations above the sea floor (10-->200 m; 5-->100 fm) (Fig.6i. (IIB-2) Regular overlapping hyperbolae with varying vertex elevations above the sea floor (Fig.7). These hyperbolae have amplitudes averaging 10--50 m (5--25 fm) and wavelengths averaging 500--1000 m. (IIB-3) Regular, intense overlapping hyperbolae with vertices approximately tangent to the sea floor (Fig.8). Amplitudes are generally less than 50 m (< 25 fm) and wavelengths are short (100--500 m). (IIB-4) Irregular single hyperbolae with conformable sub-bottom reflectors and varying vertex elevations above the sea floor (Fig.9). Wavelengths range from 1--5 km and amplitudes average 10--100 m(5--50 fm). A . . . .

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,

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Fig.4. Echo type IIA-1 : indistinct, very prolonged, fuzzy bottom echo with no apparent sub-bottom reflectors or an occasional converging sub-bottom reflector (D) which wedges out over a few kilometers.

23

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0

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Fig.5. Echo type IIA-2: indistinct, semi-prolonged bottom echo with zones of semiprolonged, discontinuous, parallel sub-bottom reflectors which alternate with zones of intermittent "mushy" sub-bottom reflections (as if sound pulses are reflected from the sub-bottom reflectors only intermittently). (IIB-5) Very broad low-amplitude single " h y p e r b o l a e " with semiparallel sub-bottom reflectors which wedge out, thin, or are truncated because o f the apparent lateral migration of the sub-bottom reflectors (Fig.10). These echos are apparently reflected from broad (1--10 km), low-amplitude (10--50 m: 5--25 fm), hyperboloid-like features which seem to be migrating dune-like sediment waves. REGIONAL DISTRIBUTION OF ECHO CHARACTER Although echograms from the various cruises display variations in quality, enough high-quality, short-ping echograms (primarily 3.5 kHz) were available to compile a map of the distribution of echo types ( F i g . l l ) .

Distribution o f continuous echo types Type IA. Distinct continuous sharp b o t t o m echos with no apparent subb o t t o m reflectors (IA; Fig.2) characterize the continental shelf ( F i g . l l ) . The shelf is a broad platform of consolidated sediments with an intermittent, thin covering of silt, sand, and gravel and is a very good reflector of sound energy. Because little or no sound penetrates to buried sediment interfaces, s u b- b o tto m reflectors are n o t recorded in echograms.

24

A

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i1

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Fig.6. Echo type IIB-1 : large, irregular overlapping hyperbolae to broad irregular single hyperbolae with widely varying verte× elevations above the sea floor.

25

B

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Fig.7. Echo type IIB-2: regular, overlapping hyperbolae with varying vertex elevations above the sea floor.

Type IC. Distinct continuous sharp bottom echos with converging subbottom reflectors (IC) are rarely recorded and are restricted to the Demerara Plateau, the southern base of the Barracuda Ridge, and a few scattered localities on the upper continental rise (Fig.11). Types IB, IIA-1, IIA-2. Distinct continuous sharp echos with parallel subbottom reflectors (IB; Fig.3) and the indistinct continuous prolonged echo types (IIA-1 and IIA-2; Figs.4, 5) have the most widespread regional distributions. The lower Amazon Cone and the adjacent portion of the Demerara Abyssal plain are characterized by very prolonged echos with no sub-bottom reflectors (Type IIA-1; Fig.ll). This echo type is surrounded by a region of prolonged echos with alternating zones of parallel, prolonged sub-bottom reflectors and intermittent "mushy" sub-bottor5 reflections (Type IIA-2 ). This echo type (IIA-2), in turn, gives way to continuous distinct sharp echos with continuous parallel sub-bottom reflectors (Type IB) which characterize the distal portions of the Demerara abyssal plain (Fig.l 1). Continuous distinct sharp echos with continuous parallel sub-bottom reflectors {Type IB; Fig.3) also characterize most of the Ceara abyssal plain, the lower continental rise and abyssal hills provinces east of Recif~, the upper Amazon Cone, the continental rise east of the Demarara Plateau, the northern abyssal hills province, and the Barracuda abyssal plain.

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D F i g . 8 . E c h o t y p e I1B-3: v e r y r e g u l a r , i n t e n s e , o v e r l a p p i n g h y p e r b o l a e w i t h v e r t i c e s a p p r o x i m a t e l y t a n g e n t t o t h e s e a f l o o r . A. H y p e r b o l a e r e c o r d e d f r o m t h e c o n t i n e n t a l rise e a s t o f T r i n i d a d . B, C, a n d D. H y p e r b o l a e r e c o r d e d f r o m t h e C e a r a R i s e ( F i g . l ) . B a n d C a r e e a s t - - w e s t t r a v e r s e s a c r o s s t h e h y p e r b o l a t e d r e g i o n w h i l e D is a n o r t h - - s o u t h t r a v e r s e . N o t e c h a n g e in w a v e l e n g t h .

27

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Fig.9. Echo type IIB-4: irregular, single hyperbolae with c o n f o r m a b l e parallel s u b - b o t t o m reflectors and varying vertex elevations above the sea floor. A, B, and C. Hyperbolae recorded from the continental rise. D. Hyperbolae recorded from the levee o f a large distributary channel on the A m a z o n Cone.

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29-32 35'

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Fig.l!. Map of echo character of the western equatorial Atlantic floor showing the distribution of the various echo types described in the text and shown in Fig.2 to 10. Location of continuous short-ping echograms (ship tracks) used to compile the map are shown.

flfI<)AO,SINGLE WI "'IGR.l.TING SUS-!lOTTOMSlOUN[51

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33 Continuous prolonged echos with alternating zones of prolonged parallel sub-bottom reflectors and intermittent "mushy" sub-bottom reflections (Type IIA-2; Fig.5) characterize the continental rise landward of the North Brazilian Ridge and to the east of the Barbados Ridge. Very prolonged continuous echos (Type IIA-1; Fig.4) dominate the continental rise and Ceara abyssal plain seaward of the western, topographically-discontinuous portion of the North Brazilian Ridge, as well as the continental rise along the landward base of the eastern, topographically-continuous portion of the North Brazilian Ridge (Fig.ll). In addition, this echo type is recorded from the continental rise around the seamounts at 4°S, and from the floors of the Vidal channel, Equatorial Mid-Ocean Canyon, and smaller deep-sea channels (Fig.ll).

Distribution o f hyperbolic echo types Type IIB-1. The rugged morphology of the continental slope, submarine canyon walls, Mid-Atlantic Ridge, Ceara Rise, North Brazilian Ridge, Barbados Ridge, Barracuda Ridge and isolated seamounts or basement outcrops generally returns discontinuous echos which appear as large irregular overlapping or single hyperbolae with widely varying vertex elevations above the sea floor (Type IIB-1; Fig.6, 11). Seismic reflection profiles across these regions reveal that the morphology is controlled by acoustic basement which either crops out or is buried by thin conformable sediments. Extremely rugged regions such as the crest of the Mid-Atlantic Ridge, steep sides of seamounts, the continental slope, submarine canyon walls, and the North Brazilian Ridge are characterized by numerous, closely spaced, unequal projections above the sea which are recorded on echograms as intensely overlapping irregular hyperbolic echos (Fig.6A). The flanks of the MidAtlantic Ridge, the Ceara Rise, and the Barracuda Ridge, which have less rugged morphology, return more widely spaced overlapping to single hyperbolic echos (Fig.6B). These large, irregular hyperbolae are thus clearly reflections from structurally-controlled morphologies and reveal nothing about depositional processes. The distribution of the large irregular hyperbolae (IIB-1) as shown in Fig.ll is based not only on echograms along the ship tracks shown, but also on echograms from other ship tracks which, because they are long-ping, could not be used for mapping the other types of echos. However, these long-ping echograms can be used to distinguish regions characterized by IIB-1 hyperbolae (the locations of these additional ship tracks are given in Damuth, 1973, fig.III-3). Also, the distributions of the IIB-1 hyperbolae are drawn to conform to the boundaries of the physiographic provinces from which they are recorded (Fig.l). The distributions of all other echo types shown in Fig.ll are based only on the ship tracks shown. The remaining four types of hyperbolic echos are recorded from the continental rise and Amazon Cone where seismic-reflection profiles show that up to several kilometers of gently dipping to flat-lying stratified sediment overlie the rugged basement. Thus these four types of hyperbolae

34 must be reflected from features which were formed by depositional and/or erosional processes. Previous investigators have shown that many regions of hyperbolic echos from the continental rise of eastern North America apparently correlate with areas of contour-current activity and have interpreted these hyperbolae as reflections from series' of linear elevations such as ripples or dunes (Heezen et al., 1966; Hollister, 1967; Schneider et al., 1967; Hollister and Heezen,1972). Contour-current activity may also account for some regions of hyperbolic echos observed in the western equatorial Atlantic. In addition, downslope movement of sediments under the influence of gravity in the form of turbidity currents, slumps, slides, and related types of mass flows can also produce irregular sea-floor morphology which would return hyperbolic echos. Thus many of the regions of hyperbolic echos observed in the western equatorial Atlantic (Fig.11) may reflect gravity-controlled sediment flows and mass movements. Type IIB-2. Several regions on the continental rise and upper Amazon Cone return regular overlapping hyperbolae with varying vertex elevations (Type IIB-2, Fig.7, 11). The features of the Amazon Cone which reflect this echo type are often on or adjacent to natural levees of distributary channels which suggests that these features may be sedimentary deposits formed when turbidity currents overflowed the channels and spread sediments laterally outward across the levees. Slumps or other gravity-controlled mass flows may have formed many of these features on inter-channel areas of the upper cone. Gravity tectonics and gravity-induced sediment flows probably account for much of the rough morphology which returns regular overlapping hyperbolae with varying vertex elevations (IIB-2, Fig.7) on the upper continental rise between the Demerara Plateau and Recif~. Slumping has apparently played an important role in shaping the continental slope and upper continental rise of this region (Ealey, 1969; Moore et al., 1970). Unfortunately, not enough data are available in most cases to definitely determine the origin of the features which return regular overlapping hyperbolae with varying vertex elevations (IIB-2; Fig.7). Sediments of cores from these hyperbolated regions on the continental rise indicate turbidity current deposition; however, very few cores have been raised from these regions. These hyperbolic echos {IIB-2) probably record several different sedimentary processes (i.e., slumps, turbidity currents, contour currents), all of which tend to produce irregular sea-floor physiography. Regular overlapping hyperbolae of varying vertex elevations {Type IIB-2; Fig.7) recorded from the continental rise adjacent to the Barbados Ridge may reflect contour-current activity ( F i g . l l ) . A piston core (V12-91) raised from the northernmost (11 ° N) area of these hyperbolae consists of gray terrigenous clay with 75 silt/fine-sand laminae and beds per 10 m of core. This high frequency of thin silt/sand laminae in continental rise sediments is a characteristic of contourites (Hollister, 1967). In addition, several adjacent parts of the rise (Fig. 11 ) are characterized by regular intense overlapping hyperbolae with vertices tangent to the sea floor (Type IIB-3; Fig.8) which (as described in the following paragraphs) apparently reflect contour-current activity.

35

Type IIB-3. Regular, intense overlapping hyperbolae with vertices tangent to the sea floor (Type IIB-3, Fig.8A)lare largely restricted to the continental rise along the Barbados Ridge and the northwest edge of the Demerara Plateau, and to numerous small isolated areas on the Ceara Rise (Fig.8B--D, 11). The few available echograms across the continental rise to the northwest of the Demerara Plateau suggest that these zones of hyperbolae may parallel the bathymetric contours. In this region the southward-flowing North Atlantic Deep Water may attain speeds of at least 9 to 12 cm/sec. (Defant, 1961). Thus these hyperbolae may represent small sediment waves formed by this water mass as it flows along the continental rise. The intensely overlapping hyperbolic echos (Type IIB-3) recorded from the Ceara Rise are extremely regular and well defined (Fig.8B--D); and are certainly reflections from small, regular, oriented sediment waves or ripples. The southern region of hyperbolic echos is crossed by several closely spaced north--south and east--west track lines (Fig.ll). The hyperbolae are actually recorded from many small (a few kilometers in diameter) isolated patches in valleys and on hillsides but these individual areas had to be mapped as one large area in F i g . l l because of their small areal extents. The hyperbolae have distinctly longer wavelengths (200--300 m) in north--south orientation (Fig.8D) than in east--west orientation {wavelengths = 100--150 m; Fig.8B, C). These observed differences in wavelength suggest that the hyperbolae are reflected from linear sediment waves with north--south orientations. A current meter placed on one region of hyperbolated b o t t o m (at core site RC13-184) for 10 min recorded a b o t t o m current of 11 cm/sec.; although b o t t o m photographs show no convincing evidence of currents (Embley et al., 1972; Embley and Hayes, 1972). However, coarse fraction variations in core RC13-184 (3°52'N; 43°18'W) which was raised from this hyperbolated region, suggest slight current winnowing of the sediments deposited prior to 165,000 yr. B.P. (V zone, 165,000--400,000 yr. B.P.), but no clear evidence of current winnowing was seen in the sediments deposited during the last 165,000 years. The lack of evidence for strong present-day currents over these hyperbolated regions thus suggests that the sediment waves may have formed during the Pleistocene at times when oceanic circulation was faster than at present (Embley and Hayes, 1972). Type IIB-4. Irregular single hyperbolae with conformable sub-bottom reflectors and varying vertex elevations {Type IIB-4; Fig.9) are recorded from the continental rise and Amazon Cone ( F i g . l l ) . The most extensive region is on the continental rise between the Barbados Ridge and the Demerara Plateau. These echos are commonly recorded from natural levees which border distributary channels on the upper Amazon Cone (Fig.9D). The origin of the features from which these hyperbolae are recorded is uncertain except that they are sedimentary features. The restriction of these features to the upper continental rise suggests that they may result largely from deposition by turbidity currents and other types of gravity-controlled mass flows.

36

Type IIB-5. The attitude of the sub-bottom reflectors in the very broad, low-amplitude single hyperbolae with semi-parallel sub-bottom reflectors which wedge out, thin, or are truncated because of apparent lateral migration (Type IIB-5, Fig.10) clearly imply that these echos are recorded from broad, migrating dunes or waves of sediment. These migrating, dune-like features occur on the continental rise to the north of the Demerara Plateau in two isolated localities ( F i g . l l ) . A contour-current origin for these features is supported not only by their dune-like morphology, but also by core RC13-176 which was raised from one of the sediment waves. The core is largely gray clays of Late Wisconsin age which contain approximately 70 very thin (< 1 cm) silt/fine sand laminae and partings per 10 m of core. This high frequency of thin silt/sand laminae in continental rise sediments is characteristic of contourites (Hollister, 1967). The Holocene section of the core (upper 65 cm) consists of light brown foraminiferal marl which shows no evidence of current scour or deposition and thus suggests that the sediment waves formed during the Late Pleistocene when oceanic circulation may have been more intense than at present. Bottom photographs taken on the sediment waves reveal no evidence of present-day current activity (R. W. Embley, personal communication, 1973). The orientation of the sediment waves is uncertain; however, sub-bottom reflectors seem to indicate an apparent westward migration (upslope). THE RELATIONSHIP OF ECHO CHARACTER TO TERRIGENOUS SEDIMENT DISPERSAL

Correlation of echo type with coarse terrigenous sediment o f piston cores The areal distributions of the three most widespread types of continuous echos (IB, IIA-1, IIA-2; Fig.3, 4, 5) suggests that t h e y may reflect the distribution of terrigenous sediment on the continental rise, Amazon Cone, and abyssal plains. To test for such a relationship, 62 piston cores were selected from these regions for which there are high-quality 3.5-kHz echograms at each coring site. The echo type at each coring site was compared with the a m o u n t of coarse (silt- to gravel-size), bedded, terrigenous sediment in each corresponding core (Table I). Three parameters were examined for each core: (1) the total percentage of core composed of discrete silt/sand beds; (2) the thickest silt/sand bed per core; and (3) the number of silt/sand beds per 10 m of core. The results are plotted as a series of histograms relating these parameters to echo type {Fig.12). The relationship of total percentage of core composed of coarse, bedded sediment to the three echo types is shown by the three histograms of Fig.12A. Cores from regions characterized by distinct continuous sharp echos with continuous parallel sub-bottom reflectors (Type IB) have less than 6% of their lengths composed of coarse, bedded, terrigenous sediment {average = 1%), whereas, cores from regions of continuous prolonged echos with alternating zones of parallel sub-bottom reflectors and intermittent

37 TABLE I F r e q u e n c y a n d t h i c k n e s s o f coarse, b e d d e d , t e r r i g e n o u s s e d i m e n t s (silt t o gravel) in 62 p i s t o n cores raised f r o m t h e c o n t i n e n t a l rise, A m a z o n C o n e , a n d abyssal plains; a n d t h e e c h o t y p e r e c o r d e d a t each coring site. Core

Echo type

P e r c e n t of core c o m p o s e d of clastic beds

Clastic b e d s p e r 10 m o f core

Thickest clastic b e d p e r core ( c m )

V18-26 V20-227 V22-20 V22-34 V22-36 V22-37 V23-123 V23-125 V24-257 V24-258 V24-259 V25-37 V25-38 V25-39 V25-40 V25-41 V25-48 V25-49 V25-50 V25-51 V25-52 V25-57 V25-58 V25-61 V25-64 V25-65 V25-69 V25-70 V25-71 V25-72 V25-78 V25-79 V26-96 V26-97 V26-98 V26-99 V26-100 V26-101 V26-102 V26-103 V26-104 V26-105 V26-106 V26-107 V26-108 V26-109

IIA-1 IIA-1 IIA-1 IB IIA-1 IIA-2 IIA-1 IIA-1 IIA-2 IIA-2 IB IIA-1 IIA-2 IIA-2 IIA-1 IIA-2 IB IIA-1 IIA-1 IIA-1 IIA-2 IIA-2 IB IIA-2 IIA-2 IIA-2 IIA-2 IIA-2 IIA-2 IIA-2 IIA°2 IIA-1 IIA-2 IB IB IIA-1 IIA-1 IIA-1 IIA-2 IIA-1 IIA-2 IB IB IB IIA-1 IIA-1

90 60 60 0 37 21 25 68 15 16 < 1 88 9 10 11 4 2 9 15 79 0 14 2 12 4 2 2 5 5 5 17 26 0 < 1 0 88 100 32 6 26 3 < 1 < 1 1 16 8

4 16 13 0 12 3 9 14 17 46 2 7 19 44 17 36 12 13 7 2 0 6 1 5 16 8 20 14 67 12 28 24 0 10 0 4 1 4 9 10 10 1 9 3 82 38

285 69 58 0 335 256 35 190 66 27 2 210 47 57 37 9 13 20 37 370 0 45 20 95 6 6 8 20 10 13 25 30 0 < 1 0 188 90 289 36 100 8 3 < 1 6 50 11

38 TABLE I

conti.ued

--

Core

Echo ~_ype

Percent of core c o m p o s e d of clastic beds

V26-110 V26-111 RC8-3 RC9-47 RC9-48 RC13-175 RC13-176 RC13-177 RC18-178 RC13-179 RC13-180 RC13-182 RC13-183 RC13-185 RC13-186 RC13-187

IB IB IIA-1 IB IB IIA-1 IB IIA-2 IIA-2 IIA-2 IIA-2 IB IB IIA-2 IIA-2 IIA-2

2 4 85 3 0 35 6 5 20 10 13 0 0 30 20 6

Clastic beds per 10 m of core

Thickest clastic b e d p e r core ( c m )

7 25 2 3 0 6 70 43 33 18 17 0 0 9 6 6

10 400 20 0 60 9 t0 36 19 90 0 0 167 70 21

"mushy" sub-bottom reflections (Type IIA-2) have up to 30% of their lengths composed of coarse beds (average = 10%). Cores from regions characterized by very prolonged echos with no sub-bottom reflectors (Type IIA-1) have 7--100% of their lengths composed of coarse beds (average = 48%). Thus a definite correlation apparently exists between the total percentage of bedded, coarse (silt to gravel) sediment and echo type. A

c

B

i

ECHO

w,r~ SU~

TYPE

SONTINmlS

I[~

PA~A~Ct~

eOrTOMS

o

a

o F'T"O

"~Pf

[IA

2

, m , l s T , . c ~ ~ E . , PROLONGE0 ~ ¢ ' O

HA

,£

P"~ALL[L

SUS-eOT~OMS

~ND

MUSHY SUe BOTTOM . [ F L [ C ~ I O ~ S

0

7~ . . . .

IO0

z ZIA,I

NalSTIN:' *lT~

~ L ASTIC

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co

o

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eft.

~O SU~

CNOIONr,[D

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CLASTIC BEDS ~E@ , o METg~S OF C O A E

Fig.12. Correlation o f echo t y p e w i t h coarse, bedded, terrigenous s e d i m e n t (silt to gravel). Series o f nine histograms s h o w i n g the relationship o f the frequency and thickness o f coarse clastic beds in piston cores to the e c h o t y p e at corresponding coring sites (Table I). Group A. Total percent o f core comprised o f coarse clastic beds. Group B. Thickest coarse clastic bed per core ( c m ) . Group C. Coarse clastic beds per 10 m o f core.

39 A similar relationship exists between the thickest coarse terrigenous bed per core and echo type (Fig.12B). Cores from regions characterized by distinct sharp echos with continuous parallel sub-bottom reflectors (Type IB) have coarse beds ranging to 20 cm in thickness (average = 6 cm), whereas cores from regions of prolonged echo with alternating zones of sub-bottom reflectors and intermittent " m u s h y " sub-bottom reflections (Type IIA-2) contain coarse beds ranging to 260 cm in thickness (average = 44 cm). Cores from regions of very prolonged echo with no sub-bottom reflectors (Type IIA-1) contain the thickest coarse beds (average = 143 cm) which range from 10 to 400 cm. The relationship between the number of coarse beds per 10 m of core and echo type is shown by Fig.12C. Cores from regions of distinct sharp echo with continuous parallel sub-bottom reflectors (Type IB) generally contain less than 15 beds per 10 m (average = 9 beds). Cores from regions of prolonged echos with alternating zones of parallel sub-bottom reflectors and intermittent " m u s h y " sub-bottom reflections (Type IIA-2) contain the highest frequency of coarse clastic beds (average = 19 beds). Cores from regions of very prolonged echos with no sub-bottom reflectors (Type IIA-1) generally contain less than 20 beds per 10 m of core (average = 14 beds). The low frequency of beds in this latter echo type (IIA-1) is to be expected because only a few of these thick beds can be contained within the length (5--10 m) of a normal piston core. The histograms of Fig.12 reveal that a definite correlation apparently exists between type of echo and the a m o u n t of bedded, coarse (silt/sand/ gravel), terrigenous sediment within the upper few meters of the continental rise, Amazon Cone, and abyssal plains. Regions characterized by very prolonged echos with no sub-bottom reflectors (Type I I A 4 ) have sediments which contain the hightest concentrations of coarse terrigenous sediments whereas the sediments of regions characterized by distinct sharp echos with continuous parallel sub-bottom reflectors (Type IB) contain little or no coarse sediment. Intermediate concentrations occur in regions characterized by prolonged echos with alternating zones of parallel sub-bottom reflectors and intermittent " m u s h y " sub-bottom reflections (Type IIA-2). This correlation suggests that these three echo types can be used as a qualitative measure of the a m o u n t of bedded, coarse, terrigenous sediment within the upper few meters of the western equatorial Atlantic. The fact that these three echo types reflect relative concentrations of coarse terrigenous sediment does not necessarily imply that echo type is directly caused by the thickness and frequency of the beds. Knowledge of the relationship of reflections observed in echograms to actual sediment composition, texture, and structure is not sufficiently advanced to permit positive correlations between specific acoustic reflectors and specific clastic beds of cores in most cases. Other physical parameters of the sediments such as changes in density caused by fluctuations in calcium-carbonate content or differential sediment compaction can also cause acoustic reflectors on echograms. For example, most regions dominated by pelagic sediments with no

40 silt/sand beds (terrigenous or biogenic) show numerous sharp, parallel subb o t t o m reflectors on 3.5 kHz echograms {echo type IB). Very prolonged echos with no sub-bottom reflectors (Type lIA-1) seem to be caused by reflections (side echos) from regular small sediment waves with wavelengths (< 100 m) and amplitudes too small to be resolved into discrete hyperbolae by the 3.5 kHz sound pulses (Bryan and Markl, 1966; Ewing et al., 1973). If this is the case, then the occurrence of thick, coarse terrigenous beds in sediments of regions characterized by this echo type probably arises because the high current velocities needed to transport these large quantities of sediment produce small, well-developed sediment waves and ripples which, in turn, reflect the prolonged echos. As the currents gradually weaken, sediment waves and ripples become less well developed and finally fail to form. Thus very prolonged fuzzy echos (Type IIA-1) would grade into semi-prolonged echos with alternating zones of parallel sub-bottoms (Type IIA-2) which, in turn, would grade into distinct sharp echos with continuous parallel sub-bottom reflectors (Type IB). Also, strong sediment-laden currents might erode sole marks which could give rise to prolonged echos (IIA-1) (Ewing et al., 1973).

Dispersal of terrigenous sediment inferred from echo character The observed correlation between echo type (IB, IIA-1, and IIA-2) and the relative amount of bedded coarse terrigenous sediment in the upper few meters of the sea floor provides a method for studying dispersal patterns of terrigenous sediments on the continental margin and abyssal plains. However, this method is qualitative and the observed echo t y p e can only reveal whether a specific region has relatively high, low, or intermediate concentrations of coarse, bedded terrigenous sediment as compared to adjacent regions. Nevertheless, this correlation together with the mapped distribution of echo types (Fig.11) for the continental rise, Amazon Cone, and abyssal plains reveal much information about the provenance and dispersal of terrigenous sediment in the western equatorial Atlantic. The largest area characterized by very prolonged echos with no sub-bottom reflections (Type IIA-1) and thus by highest concentrations of coarse terrigenous sediment, is the lower Amazon Cone and the adjacent portion of the Demerara abyssal plain (Fig.l 1). This large areal distribution implies that enormous quantities of coarse terrigenous sediment have been discharged by the Amazon River and have been spread across the lower cone and proximal portions of the [)emerara abyssal plain. A northwesterly extending region of very prolonged echos (Type IIA-1) suggests that coarse sediments are transported northwestward along the trend of the Demerara abyssal plain. In contrast, to the lower cone, the upper Amazon Cone is characterized b y distinct sharp echos with continuous parallel sub-bottom reflectors (Type IB) which imply relatively minor coarse-sediment accumulation, and suggest that most coarse terrigenous sediment bypasses the upper cone via large distributary channels (Fig.11). The portions of the continental rise and abyssal

41 plains surrounding the region of very prolonged echos on the lower cone and Demerara abyssal plain are characterized by prolonged echos with alternating zones of parallel sub-bottom reflectors and intermittent " m u s h y " sub-bottom reflections (Type IIA-2) which indicate intermediate amounts of coarse terrigenous sediment are concentrated in these regions ( F i g . l l ) . The concentric distribution of this echo t y p e (IIA-2) a b o u t the lower Amazon Cone implies that sediments spread radially outward from the cone. Intermediate amounts of coarse sediment have been transported northwestward along the trend of the Demerara abyssal plain to about 12°--14°N where the sediment has apparently been ponded by the rugged basement morphology ( F i g . l l ) . Very prolonged echos (Type IIA-1) from in and around the tributaries of the Vidal channel suggest that coarse sediments are transported through this region to the deeper portions of the Demerara abyssal plain (north of 14°N) and to the Barracuda abyssal plain via the Vidal channel (Fig.l, 11) as was previously suggested by Embley et al. (1970). The fact that distinct sharp echos with continuous sharp parallel sub-bottom reflectors (Type IB) are recorded from these distal portions of the Demerara abyssal plain as well as the Barracuda abyssal plain indicates that only minor amounts of coarse terrigenous sediment have reached these regions. Very prolonged echos with no sub-bottom reflectors (Type IIA-1) are recorded from the continental rise adjacent to the North Brazilian Ridge (Fig.11). West of 38°W this echo type and hence highest coarse terrigenous sediment accumulations occur in and around the topographic gaps and directly seaward of the ridge which suggests that much coarse terrigenous sediment has moved downslope through the gaps in the ridge to the lower continental rise and Ceara abyssal plain. In contrast, east of 38°W this echo t y p e (Type IIA-1) and hence highest coarse terrigenous sediment accumulations are confined to the landward side of the North Brazilian Ridge ( F i g . l l ) . Thus coarse terrigenous sediment has either been ponded against the North Brazilian Ridge or transported around its eastern tip to the Ceara abyssal plain. The occurrence of continuous sharp echos with continuous parallel sub-bottom reflectors (Type IB) on the lower continental rise and the Ceara abyssal plain adjacent to the seaward side of this portion of the North Brazilian Ridge further demonstrates that the ridge forms a barrier to the seaward movement of terrigenous sediment. Very prolonged echos with no sub-bottom reflectors (Type IIA-1 ) characterize the continental rise surrounding the group of seamounts at 4 ° S (Fig.11) and a narrow zone of these echos extends northeastward (following the trend of a large deep-sea channel(s), Fig.l) to the north wall of the Romanche Fracture Zone. This suggests that large quantities of coarse terrigenous sediment have been transported across the continental rise and ponded along the base of the fracture zone wall as far east as 29°W (Fig.11). Very prolonged echos (IIA-1) and hence high coarse sediment concentrations dominate the upper continental rise southward from the seamount group to about 6°S as well as the region in and adjacent to the Equatorial Mid-Ocean Canyon. In contrast, most of the lower continental rise from 4 °

~12 to 8°S is characterized by continuous sharp echos with continuous parallel sub-bottom reflectors {Type IB) which imply relatively minor concentrations of coarse sediment for these regions. In contrast to the indistinct prolonged echo types {IIA-1 and IIA-2) which characterize most of the continental rise south of the Amazon Cone, distinct continuous sharp echos with continuous parallel sub-bottoms (IB) characterize much of the continental rise north of the Amazon Cone to 10°N. Thus relatively minor amounts of coarse sediment have been deposited on this portion of the rise ( F i g . l l ) . SUMMARY AND CONCLUSIONS Examination of short-ping (~ 5 msec.) 3.5- and 12-kHz echograms from the western equatorial Atlantic reveals that b o t t o m echos from this region can be classified into several distinct types on the basis of appearance. The distribution of each echo type throughout the region is mapped (Fig.11). The three most widespread echo types (IB, IIA-1, and IIA-2; Figs.3, 4 and 5) observed on the continental rise, Amazon Cone, and abyssal plains correlate with the frequency and thickness of coarse (silt- to gravel-size), terrigenous beds in piston cores from these regions. Thus these echo types apparently reflect the relative amounts of coarse terrigenous detritus in the upper few meters of sediment throughout these regions and the distributions of the echo types yield much information about dispersal and depositional processes in the western equatorial Atlantic. The regional distributions of these three echo types (IB, IIA-1, IIA-2; Fig.l l ) indicate that dispersal of coarse terrigenous sediment on the continental margin and abyssal plains was in the downslope direction (via gravity-controlled sediment flows). They identify the Amazon River as the major terrigenous sediment source and reveal t h a t most coarse terrigenous sediment bypasses the upper Amazon Cone (via distributary channels) and is deposited across lower cone and proximal portion of the Demerara abyssal plain. Coarse terrigenous sediment concentrations decrease radially outward from the lower cone. Only minor amounts of coarse sediment reach the distal portions of abyssal plains. Smaller coastal rivers (present or past) have also apparently discharged large amounts of coarse terrigenous sediment onto the continental rise south of the Amazon Cone. Sediments have been transported downslope through topographic gaps as well as around the eastern tip of the North Brazilian Ridge to the lower continental rise and the Ceara abyssal plain. In contrast, the echo character of the continental rise to the north of the Amazon Cone implies that the coastal rivers of this region have discharged relatively minor amounts of coarse terrigenous sediment. Isolated regions of hyperbolic echos (types IIB-2 to IIB-5; Fig.7--10) are recorded from the continental rise and Amazon Cone and seem to reflect depositional features of various morphologies and origins. Previous investigators have presented evidence to show that hyperbolic echos (similar to type IIB-2 and IIB-3) recorded from the continental rise of the western

43 North Atlantic and the Mediterranean Sea are small sediment waves or ripples formed by contour-following or other b o t t o m currents (Ryan and Heezen, 1965; Hollister, 1967; Heezen and Johnson, 1969; Hollister and Heezen, 1972). Unfortunately, the wide spacing of cruise tracks and lack of cores, b o t t o m photographs, and current measurements from regions of hyperbolic echos in the western equatorial Atlantic prohibit definite conclusions about the formation of the features that produce these echos. An exception may be the continental rise to the north and west of the Demerara Plateau (Fig.11), where the types (IIB-2, IIB-3, IIB-5; Figs.7, 8, and 10) and distribution of hyperbolic echos plus the high frequency of thin silt/sand beds in piston cores suggest contour-current activity. Another exception is the southern portion of the Ceara Rise, where the orientation of hyperbolae (IIB-3) revealed by closely spaced echograms clearly indicates that these features are small, regular north--south trending sediment waves or ripples which formed during the Pleistocene. In contrast, hyperbolic echos recorded from the Amazon Cone and continental rise to the south record sedimentary features which do not seem to reflect contour-current activity, but rather deposition by turbidity currents, slumps, and related types of gravitycontrolled mass flows. Comparison of the distribution of echo types in the western equatorial Atlantic ( F i g . l l ) with the distribution of echo types mapped in the western North Atlantic by Hollister (1967, fig.III-13) and Hollister and Heezen (1972, fig.10) reveals a striking difference between the two regions. In the western North Atlantic continuous, narrow bands of hyperbolic and indistinct prolonged echo types {like types IIA-1, IIB-2, and IIB-3 of the present study) parallel the bathymetric contours of the lower continental rise and are thought to reflect the southward distribution of sediment by the contour-following currents (Heezen et a1.,1966; Hollister, 1967; Hollister and Heezen, 1972). In contrast, the regional distributions of prolonged and hyperbolic echo types on the continental rise of the western equatorial Atlantic ( F i g . l l ) are largely restricted to the upper continental rise and do not parallel bathymetric contours, except for possibly the portion of the rise to the northwest of the Demerara Plateau. These distributions suggest that contour-current activity has not been an important process on the continental rise in this part of the Atlantic and that the features which reflect the hyperbolic echos in this region were probably deposited largely by gravity-controlled sediment flows and mass movements. This contrast between the distribution of echo types of the continental rise of the western equatorial Atlantic and the western North Atlantic as well as the relationship between echo character and terrigenous sediment distribution presented in the present study demonstrates that echo character studies provide an extremely valuable tool for evaluating sedimentary processes on the ocean floor. Furthermore, the contrasting sedimentary processes revealed for these two regions by the echo character (contour currents in the western North Atlantic versus turbidity currents in the western equatorial Atlantic) demonstrate that the continental rise may be

44 shaped b y d i f f e r e n t processes f r o m region t o region. Thus, the f o r m a t i o n and shaping o f the c o n t i n e n t a l rise o f t h e western N o r t h Atlantic by geostrophic c o n t o u r - f o l l o w i n g currents (Heezen et al., 1966; Hollister, 1 9 6 7 ) is a m o d e l which does not necessarily a c c o u n t for the f o r m a t i o n of the continental rise t h r o u g h o u t the world. Each p o r t i o n of the c o n t i n e n t a l rise must be e x a m i n e d o n its o w n merits in o r d e r t o ascertain its particular m o d e of formation. ACKNOWLEDGEMENTS This research was a c c o m p l i s h e d at the L a m o n t - D o h e r t y Geological O b s e r v a t o r y and represents a p o r t i o n o f a c o m p r e h e n s i v e , basin-wide s t u d y of the m o r p h o l o g y and sediments o f the western equatorial Atlantic Ocean which was s u b m i t t e d b y t h e a u t h o r as a d o c t o r a l dissertation t o C o l u m b i a University. The a u t h o r is grateful t o his advisor, Professor Bruce C. Heezen, for advice and critical suggestions which were invaluable t o the d e v e l o p m e n t o f this study. Helpful discussions and critical suggestions b y R. W. E m b l e y , Dr. D. E. Hayes, and Dr. G. M. Bryan were very valuable. T h e s e d i m e n t cores used for this s t u d y are f r o m the L a m o n t - D o h e r t y Geological O b s e r v a t o r y Core Library which is m a i n t a i n e d financially b y grants f r o m the Office o f Naval Research ( N 0 0 0 1 4 - 6 7 A - 0 1 0 8 - 0 0 0 4 ) and the National Science F o u n d a t i o n ( N S F - G A - 3 5 4 5 4 ) . T h e collection and m a i n t e n a n c e o f t h e precision d e p t h recordings used for this s t u d y were s u p p o r t e d financially b y t h e Office o f Naval Research ( N 0 0 0 1 4 - 6 7 A - 0 1 0 8 - 0 0 0 4 ) and t h e National Science F o u n d a t i o n ( G A - 2 7 2 8 1 ) . Hester H. Cason kindly t y p e d the original m a n u scripts and David J o h n s o n d r a f t e d the maps. T h e a u t h o r was s u p p o r t e d t h r o u g h o u t this research b y grants f r o m the Office o f Naval Research ( N 0 0 0 1 4 - 6 7 A - 0 1 0 8 - 0 0 0 4 ) and the National Science F o u n d a t i o n ( G A - 1 0 7 2 8 and G A - 2 7 2 8 1 ) . Dr. D. E. Hayes, R. W. E m b l e y , and Dr. W. B. F. R y a n critically reviewed this manuscript. REFERENCES

Bryan, G. and Markl, R., 1966. Microtopography of the Blake-Bahama region. (Tech. Rept., 8, CU-8-66 Contr. NObsr 85077 to Lamont-Doherty Geol. Observ., 26 pp.) Damuth, J. E., 1973. The Western Equatorial Atlantic: Morphology, Quaternary Sediments, and Climatic Cycles. Thesis, Columbia Univ., Palisades, N.Y., 602 pp. Damuth, J. E. and Fairbridge, R. W., 1970. Equatorial Atlantic deep-sea arkosic sands and ice-age aridity in tropical South America. Bull. Geol. Soc. Am., 81: 189--206. Defant, A., 1961. Physical Oceanography, 1. Pergamon Press, London, 726 pp, Ealey, P., 1969. Marine Geology of Northern Brazil: a Reconnaissance Survey. Thesis, Univ. of Illinois, Urbana, Ill., 70 pp. (unpublished). Embley, R. W. and Hayes, D. E., 1972. Site survey report for site 142. In: D. E. Hayes, A. Pimm and co-workers, Initial Reports of the Deep-Sea Drilling Project, 14. U.S. Gov. Print. Off., Washington, D.C., pp. 377--388. Embley, R. W., Ewing, J. I. and Ewing, M., 1970. The Vidal deep-sea channel and its relationship to the Demerara and Barracuda abyssal plains. Deep-Sea Res., 17: 539--552.

45 Embley, R. W., Hayes, D. E. and Damuth, J. E., 197 2. The Ceara Rise: western equatorial Atlantic. Trans. Am. Geophys. Union, 53(4): 408. (abstr.) Ewing, M., Embley, R. W. and Shipley, T. H., 1973. Observations of shallow layering utilizing the pinger-probe echo sounding system. Mar. Geol., 14:55--63. Heezen, B. C. and Johnson, G. L., 1969. Mediterranean undercurrent and micro-physiography west of Gibraltar. Bull. Inst. Oceanogr., Monaco, 67(1382):95 pp. Heezen, B. C., Tharp, M. and Ewing, M., 1959. The floors of the oceans, I. The North Atlantic. Geol. Soc. Am., Spec. Pap., 65:122 pp. Heezen, B. C., Hollister, C. D. and Ruddiman, W. F., 1966. Shaping of the continental rise by deep geostrophic contour currents. Science, 152:502--508. Hersey, J. B., 1965. Sedimentary basins of the Mediterranean Sea. Colston Res. Pap., 1 7 : 7 5 - - 8 9 pp. Hoilister, C. D., 1967. Sediment Distribution and Deep Circulation in the Western North Atlantic. Thesis, Columbia Univ., Palisades, N.Y., 368 pp. Hoilister, C. D. and Heezen, B. C., 1972. Geologic effects of ocean bottom currents: western North Atlantic. In: A. L. Gordon (Editor), Studies in Physical Oceanography. Gordon and Breach, London, pp.37--66. Knott, S. T. and Hersey, J. B., 1956. Interpretation of high-resolution echo-sounding techniques and their use in bathymetry, marine geophysics, and geology. Deep-Sea Res., 14:36--44. Luskin, B., Heezen, B. C., Ewing, M. and Landisman, M., 1954. Precision measurements of ocean depth. Deep-Sea Res., 1 : 131--140. McGeary, D. F. and Damuth, J. E., 1973. Postglacial iron-rich crusts in hemipelagic deep-sea sediment. Bull. Geol. Soc. Am., 84:1201--1212. Moore, T. C., Jr., Van Andel, Tj. H., Blow, W. H. and Heath, G. R., 1970. Large submarine slide off the northeastern continental margin of Brazil. Bull. Am. Assoc. Petrol. Geologists, 54 : 125--128. Ryan, W. B. F. and Heezen, B. C. 1965. Ionian Sea submarine canyons and the 1908 Messina turbidity current. Bull. Geol. Soc. Am., 76:915--932. Schneider, E. D., Fox, P. J., Hollister, C. D., Needham, H. D. and Heezen, B. C., 1967. Further evidence of contour currents in the western North Atlantic. Earth Planet. Sci. Lett., 2:351--359.