Marine Geology, 24 ( 1 9 7 7 ) 7 3 - - 9 5 o Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ECHO CHARACTER OF THE EAST BRAZILIAN CONTINENTAL MARGIN AND ITS RELATIONSHIP TO SEDIMENTARY PROCESSES*
JOHN E. DAMUTH and DENNIS E. HAYES
Lamont-Doherty Geological Observatory of Columbia University, Palisades, N. Y~ 10964 (U.S.A.) (Received October 21, 1976)
ABSTRACT Damuth, J.E. and Hayes, D.E., 1977. Echo character of the East Brazilian continental margin and its relationship to sedimentary processes. Mar. Geol., 24: 73--95. The nature and regional distributions of various types of bottom echoes recorded on 3.5-kIIz echograms from the East Brazilian continental margin (8--30°S) provide valuable information about sedimentary processes which have been active on a regional scale. The ten types of echoes observed fall into two major classes: distinct and indistinct. Indistinct echoes have two sub-classes; prolonged and hyperbolic. A qualitative correlation is observed between three types of distinct and indistinct-prolonged echoes and the relative abundance of coarse, bedded sediment (silt, sand, gravel) in piston cores. Regions returning distinct echoes with continuous parallel sub-bottoms contain little or no coarse sediment; regions returning indistinct very prolonged echoes with no sub-bottoms contain very high concentrations of coarse sediment; and regions returning indistinct semiprolonged echoes with intermittent sub-bottoms contain moderate or intermediate amounts of coarse sediment. Thus the regional distributions of these three echo types reflect the dispersal of coarse terrigenous sediment throughout the region. High concentrations of coarse sediment are restricted to relatively small areas which are generally proximal to large deep-sea channels, whereas very low concentrations occur in distal regions such as the lowermost continental rise and adjacent abyssal plain. Moderate concentrations of coarse sediment occur throughout most of the continental rise. Five of the six types of hyperbolic echoes observed are reflected from erosional/depositional bed forms. Although some of these bed forms (especially on the upper continental rise) have probably been produced by gravity-controlled mass flows (turbidity currents, slumps, etc.) the fact that the most extensive and widespread regions of hyperbolic echoes occur in distal regions beneath the present axis of flow of the Antarctic Bottom Water suggests that most of these bed forms are the result of sediment reworking by the contourfollowing bottom currents of this water mass.
INTRODUCTION D u r i n g t h e p a s t t e n y e a r s an i n c r e a s i n g n u m b e r o f s t u d i e s h a v e d e m o n strated that echograms recorded on high-frequency (3.5--12 kHz) precision depth recordings provide a valuable tool for the study of depositional/erosional *Lamont-Doherty Geological Observatory Contribution No. 2489.
74 processes on the sea floor (Ryan and Heezen, 1965; Heezen et al., 1966; Hollister, 1967; Schneider et al., 1967; Heezen and Johnson, 1969; Hollister and Heezen, 1972; Hollister et al., 1974; Damuth, 1975; Embley, 1975, 1976; Jacobi et al., 1975; Jacobi, 1976). Damuth (1975) mapped the distributions of echo types recorded from the floor of the western Equatorial Atlantic (North Brazilian Margin, 20°N to 8°S) and demonstrated that certain echo types reflect the sedimentary processes which have been active in that region. The present study is a southward continuation of this earlier mapping and study and is concerned with the types and distributions of echoes recorded from the East Brazilian Margin (Brazil Basin) between 8°S and 30°S (Fig. 1). A similar study of the echo character of the Argentine continental margin and basins (30--50°S) is also nearing completion (Hoose and Hayes, in prep.). The present study was undertaken as part of a cooperative program for the International Decade of Oceanographic Exploration between Brazilian government agencies (REMAC and PETROBRAS) and Lamont-Doherty Geological Observatory to study the Brazilian continental margin. The 3.5and 12-kHz echograms utilized for this study were recorded during the Lamont-Doherty research cruises of the past fifteen years (Fig.2). The purpose of the study is to classify and map the broad, regional distributions of the various types of b o t t o m echoes and to combine this information with piston-core, bottom-photograph, and near-bottom-temperature data in an attempt to understand the sedimentary processes which have shaped the Brazilian continental margin. A quantitative evaluation of the acoustic properties of the sea floor which give rise to various types of echoes or of the technical complications of acoustic reflectivity (e.g. the effects of changes in ship speed, instrument gain control, or ping length on echo character) is n o t attempted. The present study is thus intended to provide a general background about regional acoustic changes and sedimentary processes and will be used in the future to pinpoint areas in which to concentrate more detailed quantitative studies of acoustic properties and nearb o t t o m processes of the ocean floor. CLASSIFICATION AND DISTRIBUTION OF ECHO TYPES Only 3.5-kHz echograms which were recorded using sound pulses of 2--5 msec were used to classify and map the echo character of the region (Fig.2). Certain portions of 12-kHz records from early cruises on which no 3.5-kHz echo sounder was available were useful for filling in areas where no 3.5-kHz data exist. In general, the 12-kHz records are only useful for distinguishing regions of hyperbolic echoes and cannot be used for distinguishing various types of distinct and indistinct-prolonged echoes because: (1) the sound pulses generally do not penetrate more than 10 m of sediment; and (2) many of these older records were recorded using sound pulses longer than 5 msec. The following echo-character classification for the East Brazilian Margin is similar to that of Damuth (1975) for the North Brazilian
75
PH
OGRAPHIC
- - - - * ' S E A CHANNELS
0
0 15 °
hJ
0
25 °
°
30 ~
30 ° Fig. 1. Physiographic provinces and features of the East Brazilian continental margin. Margin. The areal distribution of each echo t y p e is mapped in Fig.3. B o t t o m echoes recorded with 3.5-kHz sound pulses consist of two principal classes: Distinct echoes (types IA and IB, Figs.4 and 5) and indistinct echoes (types IIA; IIB; IIIA--IIIF; Figs.6--13). Two types o f distinct echoes are recognized on the basis o f the presence or absence of sub-bottom reflectors.
76
45 °
50 °
SHIP
40 °
30 °
35 °
TRACKS
IO o
0°
3.5 KHZ ECHOGRAMS .........
12
KHZ
ECHOGRAMS
15 °
15 o
J i \i i
RTO D E
-
..,"
25 =
25 °
30 °
40 o
35 °
30 °
Fig.2. Location of echogram profiles used to compile echo character map (Fig.3). Shaded areas are regions of rough basement morphology (seamounts, Rio Grande Rise, Sao Paulo Plateau, continental slope, etc. ) which are type IIIA hyperbolae in Fig. 3.
Type IA. C o n t i n u o u s , sharp, b o t t o m e c h o e s w i t h no s u b - b o t t o m reflectors ( t y p e I A o f D a m u t h , 1975) (Fig.4). This t y p e o f e c h o is r e c o r d e d largely f r o m t h e c o n t i n e n t a l shelf (Fig.3). In a d d i t i o n to t h e shelf, t w o areas o f the d e e p - s e a f l o o r also r e t u r n t y p e I A echoes: (1) t h e b r o a d d e p r e s s i o n along t h e s o u t h e r n p o r t i o n o f t h e Sao Paulo P l a t e a u ( 4 0 - - 4 2 ° W ) , and (2) a
77
Fig.3. Echo character map for the East Brazilian Margin. Location of echogram profiles used to compile m~p are shown in Fig.2. large portion of the continental rise adjacent to the flank of the Mid-Atlantic Ridge (24-26°S, 32--35°W) (Fig.3). Type IB. Continuous, sharp, b o t t o m echoes with continuous, sharp, parallel sub-bottom reflectors which persist for tens to hundreds of kilometers (type IB of Damuth, 1975) (Fig.5). Two large regions return this type of echo: (1) the landward portion of the Sao Paulo Plateau and adjacent
78
A
40fro 75m
2km
Fig.4. Echo type IA. Continuous, sharp bottom echoes with no sub-bottom reflectors. continental slope as well as most of the continental rise directly to the south o f the plateau; and (2) a large portion of the lower continental rise and Pernambuco Abyssal Plain between 8°S and 15°S, and 30--33°W (Fig.3). Smaller regions returning IB echoes are also scattered at various locations along the continental rise. Indistinct echoes are subdivided into two sub-classes: prolonged and hyperbolic. Two types of prolonged echoes are observed. Type IIA. Semi-prolonged b o t t o m echoes with intermittent zones of semi-prolonged, discontinuous, parallel sub-bottom reflectors (apparently sound pulses penetrate to sub-bottom reflectors only intermittently) (type IIA-2 of Damuth, 1975) (Fig.6). These are the most widespread echoes observed on the continental rise and are recorded from the base of the continental slope seaward to the flank of the Mid-Atlantic Ridge (Fig.3). Type IIB. Very prolonged b o t t o m echoes with no sub-bottom reflectors (type IIA-1 of Damuth, 1975) (Fig.7). These echoes occur at several scattered localities across the continental rise and are often associated with the large deep-sea channels which meander seaward from the continental slope (Fig.3). Such channels and associated type IIB echoes occur south of the ColumbiaTrindade Seamount Chain (33--38°W, 21--23°S); at about 13°S, 31--34°W; and 9--12°S, 31--35°W. Type IIB echoes are also recorded from the floor of the Vema Channel (38--39°W, 28--30°S). Six types of hyperbolic echoes are recognized. Type IIIA. Large irregular overlapping or single hyperbolae with widely varying vertex elevations above the sea floor (type IIB-1 of Damuth, 1975) (Fig.8). Amplitudes generally range from 10 to > 2 0 0 m and wavelengths are up to several kilometers. These echoes are returned from the rugged morphology of seamounts, continental slope and submarine canyon walls, outer
79
A
.H
B 2kin
12
!
Fig.5. Echo type IB. Continuous, sharp bottom echoes with continuous, parallel subbottom reflectors.
80
B
4 0 fin
"
/Sin"
.
| nm
C
t
I
.
.
.
.
.
.
Fig.6. Echo type IIA. Semi-prolonged bottom echoes with intermittent zones of semiprolonged, discontinuous, parallel sub-bottom reflectors. portions of the Sao Paulo Plateau, Rio Grande Rise, and the Mid-Atlantic Ridge (Fig.3). Type IIIB. Regular single or slightly overlapping hyperbolae with conformable sub-bottom reflectors (similar to type IIB-4 of Damuth, 1975) (Fig.9). Wavelengths are generally 0.5--2 km and amplitudes are 20--100 m. Several small areas at various locations on the continental rise return this type of echo (Fig.3). The largest region of this echo is just seaward of the Sao Paulo Plateau and northward of the Vema Channel (35--39°W, 23--28°S). Type IIIC. Regular overlapping hyperbolae with varying vertex elevations above the sea floor and no sub-bottom reflectors (type IIB-2 of Damuth, 1975) (Fig.10). Wavelengths are generally less than one kilometer (Figs.10B-10E) although longer wavelengths are occasionally observed (Fig.10A). Amplitudes generally range from 10--100 m. This type of echo is generally recorded only from small scattered localities on the continental rise (Fig. 3). The largest region of type IIIC echoes occurs at the eastern tip of the Columbia-Trindade Seamount Chain (18--21°S, 31--34°W). Type IIID. Regular, intense, overlapping hyperbolae with vertices approximately tangent to the sea floor (Type IIB-3 of Damuth, 1975) (Fig.11).
81
1 nm
~40 frn
2 krn
75m
A
Ci Fig.7. Echo type IIB. Very prolonged bottom echoes with no sub-bottom reflectors. Amplitudes are generally less than 50 m and wavelengths are short (100-500 m). These hyperbolae are recorded from only a few very small, scattered patches across the continental rise and are usually adjacent to and associated with regions of other types of hyperbolae (Fig.3). These hyperbolae are sometimes observed along one or more discrete sub-bottom reflectors (Figs.llC--11E). Type IIIE. This is a complex type of echo which consists of zones of type IIID hyperbolae which are interrupted by zones of type IB distinct echoes with parallel sub-bottom reflectors (Fig.12). The hyperbolic echoes are generally developed at the sea floor as well as along several underlying subb o t t o m reflectors. These echoes are recorded from two large areas in the lowermost continental rise at about 20--24°S and 14--17°S (Fig.3). A region of lesser areal extent is observed at the m o u t h of the Vema Channel (26-27°S 36--38°W) (Fig.3). "Type IIIF. Broad, single, irregular hyperbolae with disconformable subbottoms (Fig.13). This type of echo is recorded from only one small area of the lower continental rise near 25°S and 31--33°W (Fig.3).
82
~
)i-"
40 fm 75m
. . . . .
I
C
lnm 2 km
\ Fig.8. Echo type III/L Large, irregular overlapping or single hyperbolae with widely varying vertex elevations above the sea floor. RELATIONSHIP OF ECHO CHARACTER TO SEDIMENTARY PROCESSES
Distinct and indistinct-prolonged echo types E c h o t y p e IA is r e c o r d e d largely f r o m the c o n t i n e n t a l shelf which is a b r o a d p l a t f o r m o f c o n s o l i d a t e d sediments with an i n t e r m i t t e n t , thin covering o f sand and gravel. T h e shelf is thus a v e r y g o o d r e f l e c t o r o f s o u n d energy and c o n s e q u e n t l y little or no sound p e n e t r a t e s to buried s e d i m e n t interfaces. T h e r e f o r e s u b - b o t t o m reflectors are seldom r e c o r d e d o n e c h o g r a m s from shelf regions. D a m u t h ( 1 9 7 5 ) s h o w e d t h a t a qualitative relationship exists b e t w e e n the relative a b u n d a n c e of coarse (silt, sand, gravel), b e d d e d s e d i m e n t in the u p p e r few m e t e r s o f t h e w e s t e r n E q u a t o r i a l Atlantic f l o o r and the t y p e o f e c h o (IB, IIA or IIB) reflected. He m e a s u r e d t h e relative a b u n d a n c e o f coarse, b e d d e d s e d i m e n t in piston cores using the following three p a r a m e t e r s : (1) t o t a l p e r c e n t a g e o f core c o m p o s e d o f silt/sand beds; (2) thickest silt/sand bed per core; and (3) n u m b e r o f silt/sand beds per 10 m o f core. D a m u t h ' s s t u d y revealed t h a t regions characterized b y distinct echoes with c o n t i n u o u s parallel s u b - b o t t o m reflections ( e c h o t y p e IB o f D a m u t h , 1975 and o f the
83
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H
i. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
i
I
Fig.9. Echo type IIIB. Regular single or slightly overlapping hyperbolae with c o n f o r m a b l e s u b - b o t t o m reflectors.
84
A
!
. . . . . .
C
Fig. 10. Echo type IIIC. Regular overlapping hyperbolae with varying vertex elevations above the sea floor.
85
A
C
II
IIIIII I
.g
i IIII I
4DI0~ I
|40fm ,,lnm,
Fig. 11. Echo type IIID. Regular, intense, overlapping hyperbolae with vertices approximately tangent to the sea floor.
86
A
B
1 nm
c
2 km ---
75m I 40 fm
Fig. 12. Echo type IIIE. Zones of regular, intense, overlapping hyperbolae with vertices tangent to the sea floor (echo type IIID, Fig.ll) which are interrupted by zones of distinct echoes with parallel sub-bottom reflectors (echo type IB, Fig.5).
Fig. 13. Echo-type IIIF. Broad, single, irregular hyperbolae with disconformable, migrating sub-bottom reflectors. present study) contain relatively m i nor amounts of bedded silt/sand whereas regions of very prolonged echoes with no sub-bottoms (echo t y p e IIA-1 of Damuth, 1975, and IIB o f the present study) contain very high amounts of bedded silt/sand. Regions returning semi-prolonged echoes with i nt erm i t t ent zones of sub-bottoms (echo t y p e IIA-2 of Damuth, 1975, and IIA of the present study) contain low to m oder a t e amounts of bedded silt/sand. Based on this correlation D a m ut h (1975) was able to show t hat the regional distribution o f these three echo types reflects the paths of terrigenoussediment dispersal across the continental margin and abyssal plains of the
87 western Equatorial Atlantic, at least during Pleistocene time. During the present study the relative abundance of bedded silt/sand was measured in 64 piston cores from regions of distinct (IB, Fig.5) and indistinct prolonged {IIA and IIB; Figs.6 and 7) echoes to determine if the correlation observed for the western Equatorial Atlantic (Damuth, 1975) also holds for the East Brazilian Margin. The three parameters given above plus a fourth parameter, average silt/sand-bed thickness per core, were used to estimate the relative abundance of coarse, bedded sediments in the cores (Fig.14). The histograms of Fig.14 show that a qualitative relationship between echo type and the relative a m o u n t of bedded silt/sand in the uppermost sea floor similar to that observed for the western Equatorial Atlantic (Damuth, 1975, his fig.12) also exists for the East Brazil Margin. Cores from regions of distinct echoes with continuous parallel sub-bottoms (IB; Fig.5) contain minor amounts (less than 5%) of bedded silt/sand. Average bed thickness is generally less than 5 cm and the thickest bed per core does not exceed 5 cm. In contrast, regions of very prolonged echoes with no sub-bottoms (IIB, Fig.7) generally contain high amounts (20--100%) of bedded silt/sand. The thickest beds range from 50--500 cm and average bed thicknesses are 20--500 cm. The histograms for cores from regions of semi-prolonged echoes with intermittent zones of sub-bottoms (IIA, Fig.6) indicate that the abundance of bedded silt/sand in these regions is relatively low to moderate, and therefore intermediate between abundances from IB and IIB regions. Based on this correlation, the distribution of echo types IB, IIA, and IIB apparently reflect the areal distribution and possible dispersal paths of coarse (silt/sand) terrigenous sediment within the Brazil Basin {Fig.3), at least for the Quaternary. The widespread occurrence of semi-prolonged echoes with intermittent zones of sub-bottoms (IIA, Fig.6) suggests that only low to moderate amounts of coarse terrigenous sediment have been spread across the upper and middle portions of the continental rise north of 25°S and across the lowermost rise to the flank of the Mid-Atlantic Ridge between 18°S and 25°S. In contrast, the distribution of distinct echoes with continuous parallel sub-bottoms (IB, Fig.5) indicate that very little or no coarse sediment has reached the lower continental rise and Pernambuco Abyssal Plain between 8°S and 18°S and seaward of 33°W. Also only minor amounts of coarse sediment have reached the upper portion of the Sao Paulo Plateau and the adjacent continental rise to the southwest. The relatively limited distribution of very prolonged echoes with no subbottoms (IIB, Fig.7) indicates that large concentrations of coarse sediment are restricted to a few relatively small areas of the continental rise (Fig.3). Most of these regions are proximal to and follow the trends of large deep-sea channels (Figs.1 and 3; e.g., 38--33°W, 22--23°S; 34--31°W, 13°S; 35--30°W, 9--13°S). The large region of prolonged echoes on the upper continental rise (13--16°S, 34--37°W) is also crossed by several smaller channels which are not shown in Fig.1. The correlation between abundance of coarse, bedded material and echotype does not necessarily imply that the frequency and thicknesses of coarse,
02
0
:L
~['1 50
r,['l'l
tOO
50
'TT
'f'lrlll'l
I00
I00
"
THICKEST ST/SD BED PER C O R E
500
soo
6
0
500
0
B
5
O~
50
so
50
S T / S D BEDS PER I O M E T E R S
0
o
J
;
50
50
50
AVERAGE ST/SD THICKNESS PER
0
0
L
0
)
ECHOTYPE
i .....
]]A
""-
~--
'"
SUB-BOTTOMS
IB
-:~
BED CORE
500
~"z~T~-3f,~'~
W/
ECHOTYPE
PROLONGED
IO0 I00
tO0
NO
.._
TrB
.....
.
~
SUB'BOTTOMS :-
"
S E M I - PROLONGED W / INTERMITTENT SUB-BOTTOMS
I00
__ ._
CONTINOUS
ECHOTYPE ?ISTINCT W /
eig. 14. Correlation o f e c h o t y p e s IB (Fig.5), IIA (Fig.6), and IIB ( F i g . 7 ) w i t h the a b u n d a n c e o f coarse (silt, sand, gravel), b e d d e d errigenous s e d i m e n t in p i s t o n cores ( s h o w n as histograms). The f r e q u e n c y and t h i c k n e s s e s o f coarse beds in each piston core were neasured using the f o l l o w i n g parameters (A) total percent o f core c o m p o s e d o f silt/sand beds; (B) t h i c k e s t silt/sand bed per core; C) n u m b e r o f silt/sand beds per 10 m o f core; and ( D ) average silt/sand bed thickness per core.
BEDS
PERCENT OF C O R E C O M P O S E D OF S T / S D
0
o r,l,i,
50
5
5f
;-]
r d n
D J E D D
o
:O
15
i0 f
15
2O
A
10 10
89 bedded sediment in the uppermost sea floor causes the echo types observed. Other physical parameters such as changes in density caused by fluctuating calcium-carbonate content or differential sediment compaction can also give rise to acoustic reflectors on echograms (Embley, 1975). For example, most regions of the Brazil Basin which contain only homogeneous pelagic sediments (foraminiferal ooze, brown clay) and no silt/sand beds return distinct echoes with numerous parallel sub-bottom reflectors (IB, Fig.5). Recent studies utilizing near-bottom sound sources suggest that very prolonged echoes with no sub-bottoms (IIB, Fig.7) are caused by reflections (side echoes) from regular, small erosional/depositional bed forms which have wavelengths ( < 1 0 0 m) that are t o o small to reflect discrete hyperbolae (Ewing et al., 1973; Embley, 1975). Thus the correlation between this echo type (IIB) and the occurrence of thick, coarse, bedded sediment apparently occurs because the high-velocity currents needed to transport such large amounts of coarse sediment produce small, regular bed forms such as sediment waves, sole marks, erosional furrows, etc. which, in turn, give rise to the prolonged echoes (Ewing et al., 1973; Damuth, 1975; Embley, 1975). As these highvelocity currents weaken downslope, these bed forms become less well developed and eventually do not form. Thus very prolonged echoes (IIB, Fig.7) which are observed in proximal areas, gradually grade into semiprolonged echoes with intermittent zones of sub-bottoms (IIA, Fig.6) which, in turn, grade into distinct echoes with continuous parallel sub-bottoms (IB, Fig.5) in distal regions (Damuth, 1975).
Hyperbolic echo types Large irregular hyperbolae with varying vertex elevations (IIIA, Fig.8) are returned from the rugged morphology of seamounts, continental slope and submarine canyon walls, outer portions of the S~o Paulo Plateau, Rio Grande Rise, and the Mid-Atlantic Ridge (Fig.3). Low-frequency seismic reflection profiles across these regions reveal that the morphology is controlled by rugged acoustic basement (generally basaltic Layer 2) which either crops out or is buried by nearly conformable sediments. Extremely rugged regions such as the crest of the Mid-Atlantic Ridge, certain portions of the continental slope (Fig.8A), steep sides of seamounts (Fig.8B), and submarine canyon walls are characterized by numerous, closely spaced, unequal projections above the sea floor which are represented on echograms as irregular overlapping hyperbolae. The less rugged morphology of the flanks of the MidAtlantic Ridge (Fig.8C) and portions of the Rio Grande Rise, Sao Paulo Plateau, and continental slope return more widely spaced overlapping to single hyperbolae. The five other types of hyperbolic echoes (IIIB--IIIF) are recorded from regions underlain by one or more kilometers of relatively flat-lying, often well-stratified sediments. Thus these hyperbolic echoes must be reflected from bed forms created by erosional/depositional processes. Previous detailed studies on the continental rise and Blake-Bahama Outer Ridge off the eastern
90 United States have shown that hyperbolic echoes are reflected from erosional/depositional bed forms which were created by contour-following b o t t o m currents (Heezen et al., 1966; Hollister et al., 1974; Flood and Hollister, 1975). In contrast, deposits from gravity-controlled mass flows (turbidity currents, slumps, slides, etc.) can also give rise to hyperbolic echo types (Damuth, 1975; Embley, 1975 and 1976; Jacobi, 1976). Thus, it is generally difficult to determine from echograms alone the exact type of depositional or erosional process that has given rise to the hyperbolic echoes observed at any specific location. Often only studies with sophisticated deep-towed instruments (e.g. Hollister et al., 1974) can resolve the true origin of the bed forms which give rise to specific types of hyperbolae in a given region. On the East Brazilian Margin bed forms which give rise to hyperbolic echoes have apparently been formed by both gravity-controlled mass flows (turbidity currents, slumps, etc.) and by deep thermohaline-generated b o t t o m currents (contour currents). The thick, silt/sand beds observed in many of the piston cores (Fig.14) show that turbidity-current deposition has been widespread throughout the basin. However, the Antarctic B o t t o m Water (AABW) flows northward over much of the basin floor (Fig.15) and is known to transport and deposit sediment within the basin (Ewing et al., 1971; Jacobs et al., 1973; Eittreim and Ewing, 1974; Biscaye and Eittreim, 1974). In addition most photographs of the sea floor beneath the AABW show erosional/depositional bed forms such as ripples, scour, lineations, etc. (Fig.15). Thus widespread portions of the East Brazilian Margin have also apparently been affected by contour-current activity. In the following discussion the relationship of each hyperbolic echo type to these sedimentary processes is considered. Type IIIB. Regular single hyperbolae with conformable sub-bottom reflectors (Fig.9) are apparently some type of large sediment waves although the sub-bottom reflectors show no evidence of lateral wave migration. Most of these echoes are recorded from the lower continental rise (Fig.3). The largest area of these echoes is located just north of the Vema Channel and east of the Sao Paulo Plateau {23--27°S, 36--39°W) and lies beneath the coldest (<0.25°C) portion or axis of the AABW (Fig.15). At this location the hyperbolae may represent depositional/erosional bed forms created by the AABW. A large area of type IIIB hyperbolae also lies beneath the AABW near 16--18°S, 33°W. Three piston cores raised from these regions of type IIIB hyperbolae which lie beneath the AABW consist of brown clay with a few (2--4) thin (<30 cm) beds of redeposited diatoms and calcareous silt/ sand. Two large areas of type IIIB hyperbolae occur on the upper continental rise near 12--15°S. A core from one of these regions consists of gray hemipelagic clay with four silt/sand turbidites. Thus these echoes from the upper rise may represent bed forms deposited by turbidity currents and related types of mass flows, whereas on the lower rise they may represent bottomcurrent deposition. Damuth (1975) observed type IIIB hyperbolae on the levees of deep-sea channels of the Amazon Cone.
91
45 °
50 °
NEAR-BOTTOM
40 =
POTENTIAL
35*
30 °
TEMPERATURE
FOR ANTARCTIC BOTTOM WATER (C °) I0 •
~]
o.oo-0.25
~;"~ o.5o- ,.oo
No--o.,o ITllTl,.oo-,.oo..,...._./j HYPERBOLIC / 5°
ECHOES
~,
LARGE IRREGULAR HYPS.(ITrA) i
I
FROM BASEMENT HIGHS, ETC.
~
REGULAR HYPS. (~IB-1TrFI FROM/ OEPOSITIONAL/EROSIONAL FEATURES JJ
B O T T O M - CAMERA
STATIONS CURRENT EVIDENCE: SCOUR, LINEATIONS, SMOOTHING,ETC. f
• 20 °
•
CURRENT EVIDENCE: RIPPLE MARKS
•
TRANQUIL BOTTOM
CO
20 °
r
/ J
JA , ' v ' E t , ~ ' ~ .... ".,~ "">~" . ~ ......... J
25 °
& T
30 ° 50 °
\
Z
25 °
~
30 °
Fig. 15. Distribution of hyperbolic echoes (shaded areas) recorded from depositional/erosional features in relation to the flow of Antarctic Bottom Water through the Brazil Basin. Isotherms for Antarctic Bottom Water (cross-hatched areas) are from an unpublished map of Feducowicz. Bottom photographs were collected during Lamont-Doherty research cruises.
T y p e IIIC. R e g u l a r 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 a r y i n g v e r t e x e l e v a t i o n s a b o v e t h e sea f l o o r ( F i g . 1 0 ) are e c h o e s f r o m r e g u l a r l y s p a c e d e r o s i o n a l / depositional bed forms. The occurrence of such bed forms and associated
92 IIIC echoes on the upper continental rise, deep-sea fans such as the Amazon Cone, and levees of deep-sea channels (Damuth, 1975) as well as on large slump deposits (Embley, 1975) indicates that in some cases these bed forms are the result of deposition and/or erosion by turbidity currents and related types of mass flows. In the Brazil Basin, however, most areas of type IIIC hyperbolae occur on the lower continental rise in regions remote from terrigenous sediment sources but beneath the AABW (Figs.3 and 15; 25°S, 36--39°W; 18--22°S, 31--34°W). The six cores which have been raised from these regions generally consist of brown clay or foraminiferal marl with no silt/sand beds. Thus the bed forms in the regions which reflect the type IIIC hyperbolae are probably features formed by the AABW. However, four cores from the small scattered areas of type IIIC echoes on the upper rise consist of gray hemipelagic clay with silt/sand beds. One core has several thick turbidites. Thus at least some of the bed forms which reflect the IIIC hyperbolae on the upper rise may have been formed by downslope movement of sediment. Types IIID and IIIE. Regular, intense, overlapping hyperbolae with vertices tangent to the sea floor ( F i g s . l l and 12) are recorded from small, regular, often directionally oriented, bed forms which are formed by contour-current activity. Such features have been assumed to be small sediment waves or ripples {Clay and Rona, 1964; Heezen et al., 1966; Hollister, 1967; Hollister and Heezen, 1972; Embley and Hayes, 1972; Damuth, 1975); however, a recent study of such features on the Bahama Outer Ridge using a deep-towed instrument package {Hollister et al., 1974) revealed that at least in that region the features are erosional furrows. In the Brazil Basin type IIID and especially type IIIE hyperbolae occur on the lower continental rise beneath the axis (<0.25°C) of the AABW flow (Figs.3 and 15). Six piston cores have been raised from areas returning IIIE type echoes. These cores consist of homogeneous brown clay or foraminiferal clay. Three cores have a few (2--8) thin (<10 cm) silt/sand beds composed of mineral grains, diatoms, or foraminifera. Thus the IIID and IIIE types of echoes apparently represent small erosional and/or depositional bed forms which were created by the northward-flowing AABW. Type IIIF. The only region returning broad, single, irregular hyberbolae with disconformable sub-bottoms (Fig.13) is a small portion of the lowermost continental rise adjacent to the Mid-Atlantic Ridge flank (Fig.3; 25°S, 32--34 ° W). The disconformable, migrating sub-bottom reflectors in this region indicate that lateral transport and redeposition of sediment has occurred through time. In addition, the present sea floor clearly truncates some of those sub-bottom reflectors, thus indicating that subsequent erosion has occurred. The stratigraphic relationships observed on the echograms plus the fact that this region lies beneath the axis (<0.25°C) of the AABW (Fig.15) imply that the AABW has actively eroded and redeposited sediments in this region. The total areal distribution of echo types IIIB to IIIF is shown in Fig.15 in comparison to the spreading of the Antarctic Bottom Water (<1.50°C)
93 throughout the Brazil Basin. Most of the hyperbolic echoes form two large regions on the lower continental rise. One region extends from the Vema Channel near 30°S northward to 24°S whereas the other region extends from 25°S to 13°S between 30°W and 35°W. As shown by the isotherms in Fig.15, the major portion of each region lies under the coldest (<0.50°C) flow of the AABW. This relationship suggests that a large proportion of the hyperbolic echoes from the continental rise between 8°S and 30°S represent bed forms which were created by contour-current erosion and deposition, and that the Antarctic Bottom Water was responsible for the sediment redistribution. CONCLUSIONS Ten discrete types of b o t t o m echoes are recorded from the floor of the East Brazilian Margin using 3.5-kHz echograms (Fig.3). Most of these echo types have also been previously observed on the North Brazilian Margin (Damuth, 1975) and in the Argentine Basin to the south (Hoose and Hayes, in prep.). On the East Brazilian Margin a qualitative correlation is observed between the relative abundance of coarse (silt, sand, gravel), bedded sediment in piston cores and three types of b o t t o m echoes (IB, IIA and IIB). Distinct echoes with sub-bottoms (echo type IB; Fig.5) correlate with regions containing little or no coarse, bedded sediment whereas prolonged echoes with no sub-bottoms (echo type IIB; Fig.7) are recorded from regions with high concentrations of coarse, bedded sediment. Regions which return semiprolonged echoes with intermittent zones of sub-bottoms (echo type IIA; Fig.6) contain low to moderate amounts of coarse sediment. The areal distributions of these three echo types reflect the dispersal of coarse terrigenous sediment throughout the basin. High concentrations of coarse sediment are restricted to several isolated regions which are proximal to and follow the trends of deep-sea channels. Most of the upper and middle portions of the continental rise contain low to moderate concentrations of coarse sediment whereas distal regions such as the lower continental rise and abyssal plain contain little or no coarse sediment. Five of the six types of hyperbolic echoes recorded from the East Brazilian Margin (echo types IIIB to IIIF; Figs.9--13) are reflected from bed forms which were formed by erosional and/or depositional processes. Although in some regions of the upper continental rise gravity-controlled mass flows (turbidity currents, slumps, etc.) probably have created these bed forxns, the fact that the most extensive and widespread regions of hyperbolic echoes occur on the lower rise beneath the coldest flow or axis of the Antarctic Bottom Water suggests that most of these bed forms represent sediment erosion and redistribution by deep contour-following b o t t o m currents of this watermass. This study demonstrates the usefulness of high-frequency echograms for studying sediment processes and distribution on the sea floor. Although the widely spaced data lines can only define various sedimentary processes and
94
bed forms on a broad regional scale, such data indicate locations for concentrating more detailed and quantitative studies in the future using closelyspaced surveys and sophisticated near-bottom instrument packages (e.g. Hollister et al., 1974). Such studies may be able to answer problems raised b y the present study. For instance, the morphology and structure of the various bed forms which reflect the hyperbolated echoes (IIIB and IIIF) and the indistinct-prolonged echoes (IIA and IIB) and the erosional/depositional processes which created these various bed forms. ACKNOWLEDGEMENTS
Financial support for this research was provided by the National Science Foundation through the Office for the International Decade of Oceanographic Exploration under grant No. IDO 72-06426 and by the Office of Naval Research under Grant No. N00014-75-C-0210. The sediment cores are from the Lamont-Doherty Geological Observatory Core Library which is maintained financially by the Office of Naval Research (N00014-75-C-0210) and the National Science Foundation (GA-35454). Collection and maintenance of b o t t o m photographs were also assisted financially by the Office of Naval Research (N00014-75-C-0210). B. Tucholke and N. Opdyke critically reviewed the manuscript. H. Cason and J. Dykstra provided technical assistance. REFERENCES Biscaye, P.E. and Eittreim, S.L., 1974. Sedimentation budget for the western basins of the Atlantic Ocean. Geol. Soc. Am., Abstr. Progr., 6(7): p.656. Clay, C.S. and Rona, P.A., 1964. On the existence of bottom corrugations in the BlakeBahama Basin. J. Geophys. Res., 69: 231--234. Damuth, J.E., 1975. Echo character of the western Equatorial Atlantic floor and its relationship to the dispersal and distribution of terrigenous sediments. Mar. Geol., 18 : 17--45. Eittreim, S.L. and Ewing, M., 1974. Turbidity distribution in the deep waters of the Western Atlantic trough, In: R.J. Gibbs (Editor), Suspended Solids in Water. Plenum, New York, N.Y., pp.213--225. Embley, R.W., 1975. Studies of Deep~Sea Sedimentation Processes Using High-Frequency Seismic Data. Thesis, Columbia University, Palisades, N.Y., 334 pp. Embley, R.W., 1976. New evidence for occurrence of debris flow deposits in the deep sea. Geology, 4: 371--374. Embley, R.W. and Hayes, D.E., 1972. Site survey report for site 142. In: D.E. Hayes, A. Pimm, et al., Initial Reports of the Deep-Sea Drilling Project, 14. U.S. Government Printing Office, Washington, D.C., pp.377--388. Ewing, M., Eittreim, S.L., Ewing, J. and Le Pichon, X., 1971. Sediment transport and distribution in the Argentine Basin. Part 3. Nepbeloid layer and processes of sedimentation. In: Physics and Chemistry of the Earth, 8. Pergamon Press, New York, N.Y., pp.49--77. 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. Flood, R.D. and Hollister, C.D., 1975. Studies and significance of deep-sea bed forms in the North Atlantic. Geol. Soc. Am., Abstr. Progr., 7(7): p.1076.
95 Heezen, B.C. and Johnson, G.L., 1969. Mediterranean undercurrent and microphysiography west of Gibraltar. Bull. Inst. Oceanogr., Monaco, 67(1382): 95 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. Hollister, C.D., 1967. Sediment Distribution and Deep Circulation in the Western North Atlantic. Thesis, Columbia University, Palisades, N.Y., 368 pp. Hollister, 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. Hollister, C.D., Flood, R.D., Johnson, D.A., Lonsdale, P. and Southard, J.B., 1974. Abyssal furrows and hyperbolic echo traces on the Bahama Outer Ridge. Geology, 2 395--400. Hoose, P. and Hayes, D.E., in prep. Echo character of the Argentine Basin. Jacobi, R.D., 1976. Sediment slides on the northwestern continental margin of Africa. Mar. Geol., 22: 157--173. Jacobi, R.D., Rabinowitz, P.D. and Embley, R.W., 1975. Sediment waves on the Moroccan continental rise. Mar. Geol., 19: M61--M67. Jacobs, M.B., Thorndike, E.M. and Ewing, M., 1973. A comparison of suspended particulate matter from nepheloid and clear water. Mar. Geol., 14:117--128. Ryan, W.B.F. and Heezen, B.C., 1965. Ionian Sea submarine canyons and the 1908 Messina turbidity current. Geol. Soc. Am. Bull., 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.