Amazon Cone: Morphology, Sediments, Age, and Growth Pattern
NARESHKUMAR™ I Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964
ABSTRACT Oceanographic Exploration between Brazilian government agen- cies (REMAC and PETROBRAS) and Lamont-Doherty Geological The morphology, sediment distribution, and growth pattern of Observatory to study the Brazilian continental margin. the Amazon cone are similar to those of other deep-sea fans; its sed- We utilize the geophysical data (3.5- and 12-kHz echograms, iment, at least during the late Quaternary Period, was deposited in seismic reflection profiles, and sonobuoy data) and sediment sam- response to glacial-interglacial cycles, and its age of formation is ples (piston cores) collected at random during the past 20 years to estimated to be middle to late Miocene. describe the morphology, sediments, and structure of the cone, as Sedimentation on the Amazon cone, at least during Quaternary well as to determine a growth pattern and approximate age for the time, has been climatically controlled. During high sea-level stands, cone. terrigenous sediment is trapped on the inner continental shelf, and only pelagic sediment is deposited on the cone. During low sea- MORPHOLOGY level stands, the Amazon River discharges terrigenous sediment into the Amazon Submarine Canyon, from where it is easily trans- The Amazon cone is elongate and extends northward 650 to 700 ported to the cone by gravity-controlled sediment flows. Wisconsin km from the continental shelf to the Demerara Abyssal Plain at sedimentation rates on the cone were in excess of 30 cm/103 yr. depths of 4,600 to 4,850 m (Fig. 2). The cone is about 380 km wide Average sedimentation rates for the Pleistocene Epoch, based on along the continental shelf and about 600 km wide near its base. the extrapolated age (2.2 m.y.) of a prominent acoustic reflector The longitudinal gradient is 1:150 to 1:200. The boundaries of the within the cone, range from 50 to 115 cm/103 yr. The Amazon cone cone (Fig. 2) were based on subtle changes in gradient and mor- began to form about 8 to 15 m.y. B.P. and is thus about one-tenth phology between the cone and the surrounding physiographic the age of the Equatorial Atlantic. Key words: marine geology, con- provinces as observed on echograms and seismic profiler records. tinental margin, deep-sea fans, Equatorial Atlantic, Quaternary In longitudinal profile (Fig. 3, profile QR) the Amazon cone ap- sedimentation. parently exhibits the following threefold division in morphology, which, according to previous investigators (Normark, 1970, 1974; INTRODUCTION Normark and Piper, 1972; Nelson, 1968; Nelson and others, 1970; Nelson and Kulm, 1973), is characteristic of nearly all deep- The Amazon cone (Fig. 1) is a deep-sea fan that extends seaward sea fans: (1) the upper cone, which has a rugged concave-upward from the continental shelf off the Amazon River to abyssal depths surface and contains a prominent leveed central channel or fan val- (Heezen and Tharp, 1961). The sediments of the cone are largely ley that extends from the continental shelf downslope to the middle derived from the Amazon River, which presently discharges ap- cone; (2) the middle cone or suprafan (Normark, 1970), which has proximately 3.63 X 1011 kg (400 million tons) of sediment a year, a hummocky convex-upward surface and where the central chan- the seventh largest sediment discharge of any river in the world (Holeman, 1968). 60° 50° 40° 30° The term cone was introduced by Ewing and others (1958) to describe the thick fanlike accumulation of sediment on the conti- nental margin off the Mississippi River and was then applied to all large fanlike accumulations such as those off the Mississippi, Congo, Ganges, Indus, and Amazon Rivers (Ewing and others, 1958; Heezen and Menard, 1963). However, most investigators of deep-sea fans contend that cones are actually single large fans that are identical in structure to smaller fans. Thus they apply the term deep-sea fan to all fanlike features regardless of size (Menard, 1955; Menard and others, 1965; Normark, 1970, 1974; Nelson and others, 1970; Curray and Moore, 1971; Nelson and Kulm, 1973). Our observations confirm that the Amazon cone is a large deep-sea fan and not a composite feature, but we retain the name Amazon cone because this is the original name given to this physiographic feature (Heezen and Tharp, 1961). During the past 20 years, reconnaissance surveys by Lamont- Doherty research vessels have periodically collected geophysical data and sediment samples from the Amazon cone. The investiga- Figure 1. Physiographic province map of the western Equatorial Atlan- tions culminated in 1973 with a survey of the cone by R7V Conrad tic showing the location of the Amazon cone (simplified from Damuth, as part of a cooperative program for the International Decade of 1973). Oudined area is shown in detail in Figures 2, 7, 8, and 9.
Geological Society of America Bulletin, v. 86, p. 863-878, 10 figs., June 1975, Doc. no. 50618.
863
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 AMAZON CONE 865
nel or fan valley divides into several meandering, leveed dis- Besides the large central channel, few channels occur on the tributaries; and (3) the lower cone, which has a smooth concave- upper cone (Fig. 2). Small channels (less than 50 m) are rarely ob- upward surface with numerous small distributaries without natural served. A single large channel as much as 100 m deep apparently levees. The upper cone, as shown by Ealey (1969) and to some ex- trends northward from at least the 1,400-m isobath (near 3°50' N., tent by the isobaths of Figure 2, is approximately semicircular in 48°15' W.; Fig. 2). Unlike the large central channel to the east, this plan. Steepest gradients (up to 1:25) observed on the cone occur channel is a broader feature without well-developed natural levees. along the eastern and western edges of the upper cone. The relation of this channel to the central channel and the Amazon Seismic reflection profiles (including profiles EF and GH of Fig. Canyon is uncertain, but the channel appears to be a distributary 3) across the upper and middle cone reveal a convex cross section. from the central channel. A series of at least four prominent chan- Although nearly 4 sec (two-way travel time) of sediment were nels as much as 250 m deep trend downslope along the southeast penetrated, acoustic basement was not reached. Edgar and Ewing edge of the upper cone (Fig. 2). The relation of these channels to the (1968) reported 9 to 13 km of sediment within the upper cone. The central channel and the Amazon Canyon is also uncertain. Their seismic profiles across the upper and middle cone also reveal locations and trends suggest that they emanate from other canyons, numerous discordant and disconformable reflectors that are gener- as yet unrecorded, that occur to the southeast of the Amazon ally laterally persistent for only a few tens of kilometers (Fig. 3; Canyon. profiles BC, CD, DE, EF, FG, GH, and QR). These discordant The large central channel apparently divides into several dis- reflectors mark relict surfaces of the cone, as well as locations of tributaries on the middle cone between the 3,000- and 3,800-m relict channels. isobaths. These channels also have natural levee systems and are 50 The lower cone is smooth to gently rolling (Fig. 3, profiles HI, to 100 m deep (Fig. 3, profiles GH and QR; Fig. 5). Occasionally KL, OP, and QR). Gradients on the lower cone range from 1:300 these channels appear to be braided (Fig. 5B). The leveed distribu- to 1:1,000. In transverse section the lower cone is flat or only tary channels that radiate from the central channel must quickly slightly convex. Although large channels with natural levees are ab- divide into numerous small distributary channels (between 3,500 sent, channels less than 50 m deep are abundant (Figs. 2 and 6). and 4,000 m), because no large channels with natural levees are Reflectors under the lower cone are flat lying and conformable to present on the lower cone. Instead, the lower cone is crossed by an each other. Cone sediment is 1 to 2 sec (two-way travel time) thick, intricate network of numerous small channels that are generally and rugged acoustic basement is often clearly visible (Fig. 3, profile less than 50 m deep (Figs. 2, 6). A 3.5-kHz echogram across the OP). lower cone (profile KL, Fig. 3) reveals approximately 45 small channels, the majority of which are only 2 to 10 m deep. Occa- Channels sional closely spaced groups of five or more channels (Fig. 6E) sug- gest braided channel systems. The spacing of ship tracks does not Channels are the most striking features of the Amazon cone. permit individual channels to be traced for long distances in most Each channel crossed by the various cruise tracklines is shown by cases. Figure 7 is a schematic map showing the probable an arrow in Figure 2. Echograms of the various channel types are configuration of the channel system of the cone. shown in Figures 4 through 6. Nearly all channels apparently be- long to a single large distributary system that radiates outward SEDIMENTS from the Amazon Submarine Canyon (Fig. 7). The Amazon Submarine Canyon extends from at least the 50-m Logs of 26 piston cores raised from the Amazon cone during isobath on the continental shelf to a depth of approximately 1,500 Lamont-Doherty research cruises over the past 20 years are shown m on the cone (Fig. 2). A network of small tributary canyons, in Figure 8; coring sites are shown in Figure 2. Two cores from the which have up to 150 m of relief, converge on the outer shelf be- adjacent portion of Demerara Abyssal Plain (RC16-53 and tween the 30- and 50-m isobaths to form the Amazon Canyon RC16-54) are also included. The lithostratigraphic relations with- (Zembruski and others, 1971; M. A. Gorini, 1973, personal com- in the upper few meters of the Amazon cone, continental rise, and mun.). A bathymetric study by Ealey (1969), as well as seismic abyssal plains of the western Equatorial Atlantic have been de- profiles in Figure 3 (profiles AB, BC, CD, and DE) and the echo- scribed by Damuth and Fairbridge (1970), McGeary and Damuth grams in Figure 4A and 4B, reveal that the canyon has a maximum (1973), and Damuth (1973). relief of 500 to 600 m and an asymmetric V shape in cross section. The southeast wall of the canyon is much steeper than the north- Sediment Types west wall. A prominent central channel or fan valley meanders northward The upper 30 to 65 cm of nearly all Amazon cone cores are com- from the mouth of the Amazon Canyon to a depth of at least 3,500 posed of very light brown to light orange-tan, pelagic, foraminif- m. A series of large arrows connected by large dots mark the loca- eral marl and ooze of Holocene age (Fig. 8). The Pleistocene- tion of this channel in Figure 2, and several echogram profiles of Holocene boundary is usually marked by a thin (less than 10 cm) the channel are shown in Figures 4C through 4F. These echograms, rust-colored iron-rich crust (McGeary and Damuth, 1973). The as well as seismic profiles EF and GH (Fig. 3), reveal that this chan- sediment underlying the Holocene section of each core is Late Wis- nel is 125 to 225 m deep and is perched on a natural levee system consin in age and is composed of dark-gray to olive-gray clay with that rises as much as 275 to 300 m above the surrounding cone sur- interbedded silt and sand. The two basic sediment types recognized face. This channel is similar in morphology to the large channels of among the sediments older than Holocene are hemipelagic and re- the Ganges cone (Bengal fan) (Curray and Moore, 1971) and the deposited (Damuth, 1973). fan valleys of the smaller fans of the western United States (Nor- Gray hemipelagic clay generally composes the major part of each mark, 1970, 1974; Nelson and others, 1970). core (especially those from the upper cone) and consists largely of
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 —I— —R 100 200 300 400
CRB
50 CM/I000YR
F.flWfV'fra
ÌVì' , .¡W^tò
5A 5B 5C
ic MIDDLE GONE
T —I— T 100 200 300 400 500
DISTANCE N
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 •2 o UJ .4 CO
H
•4
'60 1- Ll) (O u8 100 X 200 115 CM/I000YR CORE RCI3-I83 L I
50 '1000 YR Figure 3. Seismic reflection profiles across the Ama- zon cone. Locations of profiles are shown in Figure 2. Two-way travel time is used for all profiles. Vertical exaggeration is between 1:12 and 1:25. Arrows identify the prominent acoustic reflector discussed in text. The buried structural boundary of the Ceara Rise is in- dicated by CRB in profiles HI and KL. The dark uneven reflector that lies below the 2.2-m.y.-old reflector at a subbottom depth of 1.0 to 1.5 sec in section KL to the east of CRB is not basement but a buried surface of the Ceara Rise. Sonobuoy 178C13 (Fig. 9) indicates that basement is at least 2.0 sec below the sea floor n under section KL. Locations of ^ „ echograms shown in Figures 4A, 4B, 4D, 4F, and 5A through 5D are indicated. C/C indicates course change.
K I L0METERS
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 868 DAMUTH AND KUMAR
terrigenous clay with disseminated organic detritus, hydrotroilite, (Fig. 8). These rates generally exceed 15 to 37 cm/103 yr. Because and silt. In addition, the clay also contains a pelagic component the observed rates are actually a function of the core length, rather (planktonic foraminifera), which shows that clay deposition was than of actual sedimentation rates, the obtained rates do not reflect continuous during Late Wisconsin time. The absence of gray true variations in sedimentation across the cone. However, the high hemipelagic clay from seamounts that protrude above the adjacent rates reflect the high influx of terrigenous sediment to the cone dur- continental rise and abyssal plains suggests that the terrigenous ing the Wisconsin glacial. components have been transported to the deep ocean by gravity- controlled bottom flows rather than by falling from suspension in Sediment Distribution surface waters. The redeposited sediment consists largely of terrigenous silt and The piston cores also reflect dispersal patterns of terrigenous sed- sand beds several meters thick (Fig. 8). Beds are generally graded, iment across the Amazon cone. Only four cores (RC 16-167, with particles ranging in size from clay to fine gravel; however, in RC16-168, RC16-169, RC16-170) were raised from channels. most beds particles range in size from silt to medium sand. Light Cores RC 16—167 and RC16—168 are from the floor of the Amazon minerals of the silt/sand beds are predominantly quartz (greater Submarine Canyon (Fig. 2). The fact that the Z zones of these two than 60 percent), with high percentages (greater than 30 percent) of cores contain gray clay indicate that minor amounts of terrigenous feldspar (Damuth and Fairbridge, 1970). Mica is commonly abun- sediment have reached the upper portion of the Amazon Canyon dant. Heavy minerals compose less than 3 percent of the total min- during Holocene time (Fig. 8). Core RC16-169 is from the floor of eral content. Organic detritus consisting of wood and leaf frag- the large central channel, whereas core RC16-170 is from the floor ments is generally disseminated throughout the beds, and some of a large channel on the eastern side of the cone (Fig. 2). The oc- beds contain discrete layers of organic detritus as much as 20 cm currence of light-brown pelagic marl in the Holocene section of thick. Core RC16-54 contained a piece of wood measuring 4x1 each core implies that the channels have been inactive throughout X 1 cm. The primary sedimentary structures of the silt/sand beds Holocene time (Fig. 8). (graded bedding; sharp, irregular basal contacts; gradational upper Figure 8 shows that the cores from the upper and middle cone contacts; and laminations), as well as the occurrence of displaced (depth less than 3,900 m) contain no thick silt/sand beds; however, shallow-water faunas (coral, mollusc, and bryozoan fragments; some cores contain silt/sand laminae and beds thinner than 5 cm. pteropods; and benthic foraminifera) leave no doubt that the Most cores with abundant silt/sand laminae (such as V18-21) were silt/sand beds were deposited by turbidity currents or related types raised from channel levees. The absence of thick silt/sand beds in of gravity-controlled sediment flows. cores from the upper and middle cone implies that most coarse
Stratigraphy yWV»-
Stratigraphic control was established within the cores by deter- fc-
mining changes in the abundance of foraminifera of the 5 KM' Globorotalia menardii complex down each core using the method of Ericson and Wollin (1956b, 1968). Ericson (1961) has desig- a r nated the G. menardii zones by an inverse alphabetical sequence. The uppermost G. menardii—rich zone, the Z zone, is equivalent to the Holocene (0 to 11,000 yr B.P.); the Y zone does not contain G. menardii and is approximately equivalent to the Wisconsin glacial (11,000 to 75,000 yr B.P.); and the X zone is rich in G. menardii and is approximately equivalent to the last interglacial (75,000 to 125,000 yr B.P.) (Broecker and Van Donk, 1970). All cores from the Amazon cone penetrate only the Z (Holocene) and upper Y (Wisconsin) zones (Fig. 8). The consistent disappear- ance of another temperature-sensitive foraminifer, Pulleniatina ob- liquiloculata, in the middle of the Y zone (Ericson and Wollin, 1956a) provides a useful biostratigraphic datum at about 40,000 yr B.P. (Damuth, 1973). None of the cores penetrated this 40,000-yr datum. Thus the cores record only latest Quaternary sedimentation on the cone. Sedimentation rates were calculated for the Z zone (Holocene) and the Y zone (latest Wisconsin) using a date of 11,000 yr B.P. for the Y-Z boundary (Broecker and others, 1960) and the 40,000-yr-B.P. Pulleniatina obliquiloculata datum. The Holocene sections of most cores accumulated at rates ranging from 4 to 6 cm/103 yr. The highest Holocene rate (13 cm/103 yr) is observed in core RC16-168 from the floor of the Amazon Submarine Canyon. The Holocene sedimentation rates observed for the cone (approxi- mately 4 to 6 cm/103 yr) are similar to the average rate of accumu- lation for pelagic marl and ooze in the deep sea (3 to 5 cm/103 yr). This correspondence implies that significant quantities of terrigen- ous sediment have not reached the cone during Holocene time. In contrast to the low Holocene rates, the Y zone sedimentation rates reveal that enormous amounts of terrigenous sediment ac- cumulated on the cone during the Wisconsin glacial. Because no Figure 4. Echograms at 3.5 kHz (4A through 4D, 4F) and 12 kHz (4E) cores penetrate the 40,000-yr-old P. obliquiloculata datum, only showing the lower Amazon Submarine Canyon (4A and 4B) and the large minimum sedimentation rates for the Y zone could be calculated central channel or fan valley of the upper cone (4C through 4F).
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 AMAZON CONE 869
(silt-to-gravel size) detritus bypasses these regions via the distribu- of Quaternary sedimentation on the Amazon cone. Eustatic lower- tary channels. In contrast, the cores of the lower cone depth (grea- ing dropped sea level during the Wisconsin glacial (and previous ter than 3,900 m) and adjacent abyssal plain contain numerous glacial cycles) at least 100 m below that of the present (Flint, 1971). thick silt/sand beds (Fig. 8). The continental shelf off the Amazon River (shelf break at about This distribution pattern for coarse terrigenous sediment is 100 m) was emergent, and the river discharged its sediment load further emphasized by regional changes in the character of the bot- directly into the head of the Amazon Submarine Canyon. Turbidity tom echos across the Amazon cone, as recorded with a 3.5 kHz currents and related types of sediment flows continuously trans- PDR. A detailed study of regional changes in echo character ported terrigenous sediments across the Amazon cone via the dis- (Damuth, 1973, 1974) suggests a qualitative relation between echo tributary channels. Overflow of channels caused terrigenous clay to type and the percentage of coarse- (silt-to-gravel size) bedded ter- be continuously deposited on interchannel areas and formed the rigenous sediment (as revealed by piston cores) within the upper gray hemipelagic clay that characterizes the Y zone of the cores. few meters of the continental rise, Amazon cone, and abyssal plains Most coarse terrigenous sediment (silt to gravel) bypassed the (Fig. 8). The lower cone and adjacent portion of the Demerara upper and middle cone via the distributary channels and was de- Abyssal Plain are characterized by indistinct prolonged echos with posited over the lower cone and proximal portion of the Demerara no subbottom reflections, and 30 to 100 percent of the upper few Abyssal Plain. meters of sediment of these regions are composed of coarse-bedded Sea-level rise at the end of the Wisconsin glacial inundated the terrigenous detritus. In contrast, the upper and middle cone is continental shelf and moved the locus of Amazon River sedimenta- characterized by distinct sharp echos with numerous continuous tion some 300 to 350 km inland from the shelf break and the Ama- parallel subbottom reflections, and coarse-bedded terrigenous de- zon Submarine Canyon. Thus, throughout Holocene time, the tritus composes only 0 to 5 percent of the upper few meters of sed- width and low gradient of the shelf have prevented coarse sediment iment. Regions characterized by indistinct slightly prolonged echos from being carried seaward beyond the inner shelf depth (less than with intermittent subbottom reflections, such as those recorded on 20 to 40 m). The huge quantities of clay discharged by the Amazon the continental rise and abyssal plains along the eastern and west- are also deposited in the river estuary or are transported north- ern edges of the cone, are underlain by sediment that contains 5 to westward along the coast by longshore currents. The clay is even- 30 percent coarse-bedded terrigenous detritus (Fig. 8). The piston tually deposited along the coast as far northwestward as the cores and changes in echo character thus reveal that most coarse Orinoco River (Reyne, 1961; Diephuis, 1966; Allersma, 1971). terrigenous sediment discharged by the Amazon River bypasses the Thus the Amazon cone has been inactive throughout Holocene upper and middle cone (via the large distributary channels) and is time because of a lack of terrigenous sediment. The stratigraphic deposited on the lower cone and on the proximal portion of the relations observed for latest Quaternary time therefore suggest that Demerara Abyssal Plain. the sedimentation on the cone has been cyclic and climatically con- trolled. During glacial cycles, the low sea-level stands allowed large History of Sedimentation quantities of sediment to be deposited across the cone. In contrast, high sea-level stands during interglacial cycles completely shut off The contrasting sedimentary facies and sedimentation rates ob- the supply of terrigenous sediment to the cone, so that the cone be- served for the Z and Y zones of the piston cores reflect the history came essentially inactive, as it is today. It is conceivable that given sufficient time (more than 20,000 yr), the Amazon might be able to extend its delta seaward across the shelf to the Amazon Canyon during a prolonged high sea-level stand, and deposition on the cone could resume despite the high sea level. Studies of the continental shelf off the Amazon River have revealed that during the past 10,000 yr the Amazon has extended its delta to approximately the 30-m contour (Zembruski and others, 1971; M. A. Gorini, 1973, personal commun.), or about one-third of the distance across the shelf. Detailed sedimentologic studies of fans on the continental mar- gin of the western United States also document definite strati- graphic changes from latest Pleistocene to Holocene time for the Astoria fan (Nelson, 1968; Carlson and Nelson, 1969; Nelson and others, 1970; Kulm and others, 1973; Nelson and Kulm, 1973), the Nitinat fan (Carson, 1971), the Redondo fan (Haner, 1971), and the San Lucas, La Jolla, Coronado, Monterey, and Navy fans (Normark and Piper, 1969; Komar, 1969; Piper, 1970; Normark, 1974). Although sedimentation rates were drastically reduced at the beginning of Holocene time because sea-level rise shut off or greatly reduced turbidity-current activity, these fans continued to receive some terrigenous sediment throughout Holocene time. For example, silt/sand beds were deposited on the Astoria fan by tur- bidity currents as often as one every 8 yr during late Pleistocene time, but deposition slowed to only one in 1,000 yr during Holocene time (Nelson and Kulm, 1973). Holocene deposition on most of these fans was largely hemipelagic, although on some fans, channels and fan valleys still received large quantities of turbidites (Nelson and Kulm, 1973). Where documented, latest Pleistocene sedimentation rates apparently were 5 to 10 times Holocene rates (Carson, 1971; Nelson and Byrne, 1968; Nelson and Kulm, 1973). Figure 5. Echograms at 3.5 kHz showing distributary channels that All of these fans off the western United States are situated off a nar- branch off from the central channel at depths of 3,000 to 4,000 m. row shelf and are mostly supplied with sediment by longshore drift.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 870 DAMUTH AND KUMAR
Unlike the fans of the western United States, terrigenous acoustic basement layer could be identified with certainty. Figure 9 sedimentation on the Amazon cone was terminated by sea-level rise also shows the depth in kilometers to the deepest interface mea- at the beginning of the Holocene Epoch. No hemipelagic deposits sured at each sonobuoy station. of Holocene age are observed for the channel or interchannel por- The following equation of Houtz and others (1973) can be used tions of the cone, except for the uppermost portion of the Amazon to calculate the sediment thickness in kilometers of reflection time, Canyon (depth less than 1,000 m; see Fig. 8, cores RC16—167 and 2 H (km) = V0 T + AT /2 , 168), which received gray hemipelagic clay throughout Holocene time. The complete termination of terrigenous sedimentation on where H is the thickness of sediment, and T is one-way reflection the Amazon cone during Holocene time is important because it time in seconds through the sediment column. The values of V0 and 2 demonstrates that even though the cone receives sediment from the A are estimated as 1.77 km/sec and 0.90 km/sec , respectively, for largest river in the world (largest water discharge, seventh largest the Guyana Basin-Amazon cone area (Houtz, 1974). These values sediment discharge; Holeman, 1968), the sediment supply to the of V0 and A are valid up to 2.3 sec in two-way reflection time. deep sea can be abruptly and completely shut off. McGeary and However, we believe that the value of A given by Houtz (1974) for Damuth (1973) also reported that some cores from the Ganges the Amazon cone area will not significantly change for reflection cone (Bengal fan) and Mississippi cone have lithic and stratigraphic times slightly more than 2.3 sec, and as an approximation, the relations identical to those observed on the Amazon cone. There- thickness of sediment can be. estimated by assuming a mean sedi- fore, at least portions of these large fans were also inactive during ment velocity of 2 km/sec; in such a case, 0.1 sec of two-way Holocene time, even though the fans are situated off major rivers reflection time equals 100 m of sediment. (the Ganges River has the second largest sediment discharge of any river, whereas the Mississippi has the eighth largest; Holeman, Sediment Thickness 1968). The fact that some of the largest deep-sea fans off major rivers The sediment thickness of the lower cone (depth greater than became inactive during the Holocene Epoch whereas many small 3,900 m) ranges from 1 to 5 km. The western portion of the Ceara fans off small rivers continued to receive sediment (although in re- Rise has been buried by sediment of the cone (Figs. 3, 9). The in- duced quantities) thus indicates that the quantity of sediment dis- ferred structural boundary of the Ceara Rise under the cone has charged by the river is not necessarily the most important factor in been shown as a dashed line in Figure 9 and as CRB on sections HI fan growth. Other factors, such as glacio-eustatic sea-level and KL in Figure 3. Before this buried portion of the Ceara Rise fluctuations, shelf width, and longshore current activity can be was inundated by Amazon sediment, the structural boundary (CRB more important in controlling sediment supply and, hence, growth of Fig. 3) served as a barrier to the sediment and deflected them to of deep-sea fans. Thus a fan may receive terrigenous sediment dur- the northwest. Hence, the sediment column at equal water depths is ing high sea-level stands similar to that of the present only if (1) the shelf is absent or narrow; (2) headward erosion of the submarine canyon that channels sediment to the fan keeps pace with sea-level rise, so that the canyon head stays within the surf zone; or (3) the river is able to extend its delta across the shelf to the submarine canyon. None of these conditions has been met in the case of the Amazon cone, and hence sediment supply to the cone was com- pletely cut off during Holocene time and presumably during previ- ous high sea-level stands of similar magnitude.
STRUCTURE AND AGE OF FORMATION
An isopach map was constructed to show the total thickness of sediment across the cone (Fig. 9). Contours are in seconds of two- way travel time. On the upper cone, the sediment column is too thick for the sound energy to reach acoustic basement. Hence no isopachs have been shown for the shallowest part of the cone. The sediment-isopach map was constructed by using the techniques of Ewing and others (1969, 1973) and Houtz and others (1973). Only a brief description of technique is given here. Depth to acoustic basement was measured at five equally spaced points along each profiler record. At a speed of 9 knots, each profiler record represents 56 km of ship's track, whereas at a speed of 5 knots, each profiler record covers only 31 km of track. Thus at a speed of 9 knots, the data points were approximately 11 km apart, whereas at 5 knots, they were only 6 km apart. All the tracks used for constructing the map of Figure 9 are shown in Figure 2. After all the data points were marked on the map, contouring was done at an interval of 0.2 sec. Approximately 30 sonobuoy measurements are also plotted on F Figure 9. The sonobuoy data were reduced using the methods of Le | J -4300 Pichon and others (1968) and Houtz and others (1968). Depths to various acoustic interfaces were calculated in seconds as well as -4400 kilometers. Velocities ranging from 4.5 to 5.5 km/sec were consid- ered to represent acoustic basement (Raitt, 1963; Ewing, 1969). -4500 The profiler data are supplemented with measurements of depth to Figure 6. Echograms at 3.5 kHz of small distributary channels (less than acoustic basement in seconds from those sonobuoys where an 50 m deep) that occur on the lower cone.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 AMAZON CONE 871
locities characteristic of the oceanic basement (Layer 2), while on the other hand, the 4.26- to 4.55-km/sec layer is too thick (4 to 9 km) to be entirely composed of Layer 2 material. The 4.26- to 4.55-km/sec layer is thus probably a combination of lithified or consolidated sediment and oceanic basement (Layer 2) material (J. Ewing, 1973, personal commun.). The underlying layer (6.33 to 6.67 km/sec) has velocities similar to, but generally lower than, those of oceanic Layer 3. Edgar and Ewing (1968) estimated the total thickness of the upper Amazon cone to be 11 to 13 km. Cochran (1973) also estimated a thickness of 11 to 12 km for the upper cone on the basis of gravity data. Section EF (Fig. 10) is a south-to-north structural section of the cone. It is possible that the 4.2- to 5.4-km/sec layer of the Ceara Rise, which is about 2 km thick, extends into the lower part of the 4.26- to 4.55-km/sec layer under the Amazon cone. If the lower 2 km of the 4.26- to 4.55-km/sec layer under the upper cone is base- ment, then the total thickness of sediment within the upper cone would be 9 to 11 km.
Sedimentation Rates
As described previously, cores from the cone penetrate sediment less than 40,000 yr old and yield minimum sedimentation rates for latest Quaternary time. In this section, average rates of sedimenta- tion for the cone over a significantly longer time are calculated by estimating the age of a prominent acoustic reflector that is visible under much of the cone. This prominent acoustic reflector is observed at approximately 100 m (0.1 sec two-way reflection time) below the sea floor on the Ceara Rise at core site RC13-183 in section LM (Figs. 2, 3). Core RC13-183 is composed of pelagic foraminiferal marl and ooze. Biostratigraphic zonation of the core based on the abundance of foraminifera of the G. menardii complex reveals a continuous sedimentary record for approximately the past 200,000 yr. The V/W climatic zone boundary (about 165,000 yr B.P., Broecker and Figure 7. Sketch map based on data in Figure 2 showing the possible Van Donk, 1970) occurs at 750 cm in this core, and thus the aver- pattern of the distributary system of the Amazon cone. Because of the wide age rate of sedimentation for the past 165,000 yr is approximately spacing of the ship tracklines, it is generally not possible to trace individual 3 channels for long distances, especially on the lower cone. Hence, this map 4.5 cm/10 yr. Assuming this is the average rate for the entire represents only a schematic view of the channel system of the cone. Quaternary Period on the Ceara Rise, the 100-m-deep reflector would be about 2.2 m.y. (approximately the Pliocene-Pleistocene approximately 200 m (0.2 sec) thicker under the northwestern boundary). lower cone than under the northeastern lower cone. Acoustic This prominent reflector (marked by arrows on sections KL and basement is clearly visible under the lower cone. LM) lies at the base of an acoustically transparent layer and can be The thickness of sediment overlying the buried portion of the traced across the buried portion of the Ceara Rise on section KL Ceara Rise ranges from 0.8 to 3.0 sec, or approximately 1.0 to 3.0 (see arrow on profiler sheet 1541, section KL, Fig. 3). At the site of km. The lowermost sequence of the sediment must be pelagic be- profiler sheet 1541, this reflector is 1.1 km (1.1 sec two-way travel cause only pelagic sediment could have been deposited on the time) below the sea floor. Assuming that the reflector is 2.2 m.y. Ceara Rise prior to burial by Amazon-derived turbidites. However, old, then the apparent average sedimentation rate at this site is 50 the thickness of pelagic sediment is uncertain but is probably only cm/103 yr. minimal. The sonobuoy data from the Ceara Rise indicate a layer Sections KL and HI (Fig. 3) are similar except that the former is with velocities ranging from 4.2 to 5.4 km/sec at a depth of 1.0 to located farther down the cone than the latter. A prominent reflector 2.0 km below the sea floor. This depth range matches fairly well (marked by an arrow) occurs at a depth of 2.5 sec (2.5 km) on sec- with the depth to basement measured from the profiler records near tion HI and is apparently the same 2.2-m.y.-old reflector of section the sonobuoy stations on the lower cone and thus suggests that the KLM. The morphology of the reflectors and the distance between basement under the northeastern lower cone (or under the buried the reflectors and the basement (approximately 1.0 sec) is similar in part of the Ceara Rise) has a velocity range of 4.2 to 5.4 km/sec. sections KLM and HI. Because section HI is located in a shallower The only available measurements of the total sediment thickness depth and is located closer to the Amazon Canyon, the prominent for the upper cone (depth less than 3,900 m) are from seismic re- reflector of section HI is buried under thicker sediments. If this is fraction data of Edgar and Ewing (1968; unpub. data). The the same reflector on both sections, then the apparent average sonobuoy measurements of the present study penetrate only the sedimentation rate during the past 2.2 m.y. under profiler sheet upper 5 km of sediments of the upper cone. Section CD of Figure 1932 (section HI, Fig. 3) is 115 cm/103 yr. The 2.2-m.y.-old 10 (from unpublished data of Edgar and Ewing) reveals that the reflector is also present in section FG (Fig. 3) and lies (profiler sheet sedimentary sequence of the uppermost cone consists of four layers 1917) at a depth of approximately 2.0 sec (2.0 km). Thus the ap- with the following velocity ranges: (1) 1.68 to 1.86 km/sec, (2) 2.33 parent average sedimentation rate is 90 cm/103 yr under FG. If the to 2.65 km/sec, (3) 2.94 to 3.60 km/sec, and (4) 4.26 to 4.55 prominent reflector is time stratigraphic at all of these locations, km/sec. The nature of the 4.26- to 4.55-km/sec layer is uncertain then apparent sedimentation rates averaged 50 to 115 cm/103 yr on because on the one hand, this velocity range is very close to the ve- the cone during the past 2.2 m.y.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 1000 M 3000M 3900M AMAZON 4800 M UPPER CONE MIDDLE ABYSSAL CANYON CON E LOWER CONE PLAIN
CLIMATIC > >io ZONES METERS > 6M 3226 3404 3679 3682 3730 3835 3930 3935 3968 4024 4104 4165 4344 4485 O—i * * « « * * * * rn * 'S" A A 2 (HOLOCENE) „X ,/n * - G \ R«** II 000 YBP 0 G SE G G G G G i ES 3- G G
4- tu-: i an
5- zn G Y (WISCONSIN) 6- G - i« 7- Ili 8- i
9-
10-
II-
DISTINCT SHARP BOTTOM ECHO WITH INDISTINCT PROLONGED BOTTOM ECHO 40000YBIR f CONTINUOUS PARALLEL SUB-BOTTOMS WITH NO SUB-BOTTOMS
KEY DISTINCT SHARP BOTTOM ECHO WITH LT. BROWN PELAGIC F0RAMINI FERAL MARL 8 OOZE CONTINUOUS PARALLEL SUB-BOTTOMS K RUST-COLORED IRON-RICH CRUST
GRAY HEMIPELAGIC CLAY
SILT/SAND PARTINGS OR LAMINAE <« I CM THICK) INDISTINCT PROLONGED BOTTOM ECHO SILT/SAND BEDS (1-5 CM THICK) WITH NO SUB-BOTTOMS
MASSIVE SILT/SAND BEDS (> 5 CM THICK)
INDISTINCT SLIGHTLY PROLONGED BOTTOM ECHO WITH. INTERMITTENT ZONES GRADED SILT/SAND BEDS OF PARALLEL SUB-BOTTOMS
IRREGULAR HYPERBOLIC (BASEMENT OUTCROPS)
Figure 8. Graphic logs of the lithology of piston cores from the Amazon cone. Cone locations shows the echo character of the Amazon cone and adjacent regions as recorded on 3.5-kHz are shown in Figure 2. Depth of core site is indicated (corrected meters) under core number. The echograms (simplified from Damuth, 1973, 1974). Numbers indicate Y-zone (Late Wisconsin) Holocene-Pleistocene boundary is correlated from core to core by solid black line. Inset map sedimentation rates obtained from the piston cores in centimeters per thousand years.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 10° N FLANKS OF V- MID-ATLANTIC RIDGE \ . rfrrv,,...
o n SUB MARINE ^p WA/ ^ CANYON ^ t
S. n/
A 50° 45°W Figure 9. Sediment-isopach map for the Amazon cone area. Contours are in seconds of two-way travel time. Boundaries of physiographic provinces are indicated by dotted lines, and basement outcrops are indicated by diagonal line shading (see Fig. 2). Dashed line through center of figure indicates the extension of the southern boundary of the Ceara Rise, which is now buried by Amazon cone sediment. Lines AB, CD, and EF show locations of cross sections of Figure 10. Short dashes mark the locations of sonobuoy measurements in this area. Various symbols associated with sonobuoy results are as follows: D, depth to deepest recorded reflector in kilometers. D, basement observed in seismic profiler record at depth, D. Vp (see below) at this station is the highest velocity measured in the sediment. Vp, velocity of the deepest layer measured by the sonobuoy in kilometers per second. Wherever there was some doubt in recognizing the basement by its velocity, the deepest two or three velocities and corresponding depths have been entered. V„ measured by wide-angle reflection method. D at this station gives the depth of the base of the layer with the given Vp. No asterisk means that the V„ was measured by the refraction method. D at this station gives the depth to the top of the layer with the given Vp. (), assumed velocity. 221C15 and so forth, Lamont number of the sonobuoys.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 874 DAMUTH AND KUMAR
Age of the Cone (1963), a layer with a velocity of 1.8 km/sec would have a velocity of 50 percent. We therefore estimated the porosity of each buried Although very few estimates for the age of the cone have been layer of the cone from its velocity by using the Nafe-Drake curve. published (Damuth and others, 1973), a rough age for the cone can We then expanded the thickness of each layer to conform to an ini- be calculated by simply dividing the total thickness of the cone by tial porosity of 50 percent. the sedimentation rates calculated in the previous section. Thus 9 With these corrections the total sediment thickness of 7 to 9 km to 11 km of sediment for the upper cone, deposited at rates of 90 to for the cone is expanded to 9.7 to 13.7 km. This corrected thick- 115 cm/103 yr, would yield an age for the cone of 7.8 to 12.2 m.y. ness of the sedment column yields a minimum age ranging from 8 This age implies that the cone is a young feature in comparison to to 15 m.y. (middle to late Miocene) for the initiation of cone for- the age of the Equatorial and South Atlantic (approximately 130 mation. These calculations indicate that the cone is a relatively m.y., Larson and Ladd, 1973). youthful feature compared to the age of the Equatorial Atlantic In this calculation and those that follow, uniform rates of (approximately 130 m.y.). sedimentation were assumed for the entire sediment column of the An independent indication of beginning of cone formation can Amazon cone. Although there is no direct evidence to suggest that be inferred from the estimates for the time when the Amazon River pre-Pleistocene sedimentation rates were the same as those of Pleis- began to flow into the Atlantic. Based on studies of the geology of tocene time, it is now well known that glacial-interglacial oscilla- Brazil, de Oliveira (1956) suggested that prior to middle Miocene tions were not restricted to the Pleistocene Epoch but have oc- time, drainage from the Amazon Basin was into the Pacific Ocean. curred since at least Miocene time (Flint, 1971; Hayes and others, The uplift of the Andes blocked this drainage in middle Miocene 1973). Hence in our calculations we have assumed that the overall time, and immense lakes formed within the Amazon Basin. As the rate of sedimentation for the cone has been fairly constant Andean Cordillera continued to rise, the dammed waters eventu- throughout its entire history. In addition, our inferred rates for the ally forced a drainage into the Atlantic to form the Amazon River past 2.2 m.y. (Pleistocene) represent average rates through several during late Miocene or earliest Pliocene time. According to Cob- complete glacial-interglacial cycles. Hence these average rates take bing (1972), the Andean mountains attained their present altitude into account the drastic changes in sedimentation rate from glacial in a phase of intense uplift during Pliocene time. James (1971) also to interglacial phases. supported the idea that compression began in the Andes in the The rough estimate of the age of the cone given above fails to Miocene Epoch and culminated in Pliocene time. Thus, if the take into account sediment compaction or sedimentation on the drainage from the Amazon Basin was formerly into the Pacific, the continental margin prior to cone formation. Thus, corrections must blockage in the west and subsequent breakthrough in the Atlantic be introduced to take each of these factors into account when cal- (initiation of Amazon River flow and cone formation) must have culating the age of the cone. The thickness of upper continental rise taken place during late Miocene or earliest Pliocene time. sediment under the Brazilian margin just to the south of the Ama- zon cone is approximately 4 km (Edgar and Ewing, 1968; unpub. GROWTH PATTERN data; see also sonobuoy 104 V 24 in section AB of Fig. 10). Of course, this thickness includes sediment deposited during the dep- In recent years several researchers have presented models for osition of the cone. If the average rate of sedimentation on the rise deep-sea fan growth based primarily on studies of the morphology during the past 10 m.y. (the approximate age of the cone) was simi- and sediments of the relatively small deep-sea fans off western lar to average rates (approximately 20 cm/103 yr) obtained for North America. Normark (1970, 1974) has described several of the latest Quaternary time by Damuth (1973), then approximately 2 deep-sea fans off southern California and the Baja Peninsula, in- km of sediment have accumulated on the rise during the formation cluding the La Jolla, San Lucas, Navy, Coronado, and Monterey of the cone. This implies that the remaining 2 km of sediment were fans, and he has proposed a model for deep-sea fan growth based deposited prior to initiation of cone sedimentation. Thus, 2 km of on the morphology and sediment distribution observed for these sediment are subtracted from the 9 to 11 km of sediment of the fans (see also Normark and Piper, 1972). Normark recognized cone to account for sedimentation prior to the formation of the three distinctive morphologic divisions of the fan surface — upper, cone. The true thickness of the cone is therefore estimated to be 7 middle, and lower fan — and postulated that these divisions re- to 9 km. No correction for compaction in this subtracted 2 km of sulted from the normal pattern of fan growth. In Normark's model, sediment was made because the sediment was deposited over a deposition of coarse sediment and hence fan growth is largely re- period of at least 100 m.y. Hence, the change in velocity with depth stricted to the middle or "suprafan" portion of the fan at the end of in these sediments may reflect not only compaction but also the central channel or fan valley. As the central channel migrates lithification and other diagenetic changes. across the upper fan, the suprafan bulge migrates across the midfan Because the cone sediment was apparently deposited within a region and thus builds the low cone shape of the fan. The inter- short span of time (approximately 10 m.y.), we have assumed that channel portions of the upper fan and the entire lower fan receive the change in sound velocity with depth observed for the Amazon only minor amounts of fine sediment. Haner (1971) proposed a cone is solely the result of compaction, and we have attempted to model for fan growth based on studies of the Redondo fan off Los apply a correction factor for the 7 to 9 km of cone sediments. The Angeles. Haner postulated that the morphology of the Redondo thicknesses of layers in profiles 40 and 41 of section CD (Fig. 10) fan developed as a result of tectonic displacement and found that have been used for this correction. The following thicknesses of large-scale sediment deposition is not limited only to the middle fan layers are considered to constitute the 7 to 9 km of Amazon cone but occurs on the lower fan as well. However, Normark (1974) has sediment: profile 41: (1) 1.5 km of 1.86 km/sec layer, (2) 1.5 km of recently re-evaluated Haner's data and conclusions and suggested 2.65 km/sec layer, and (3) 4.0 km of 3.60 km/sec layer (total thick- that Haner may have confused tectonic segmentation of the fan ness: 7.0 km); profile 40: (1) 1.5 km of 1.86 km/sec layer, (2) 1.5 surface with a normal growth pattern. In particular Normark ar- km of 2.65 km/sec layer, (3) 1.5 km of 3.56 km/sec layer, and (4) gued that Haner's middle and lower fan actually are both part of 4.5 km of 4.53 km/sec layer (total thickness: 9.0 km). The porosity the middle or suprafan division of his model, and thus the Redondo of each layer can be estimated using the porosity-velocity curves of fan actually has a morphology and growth pattern similar to other Nafe and Drake (1963). These curves attempt to predict porosity in fans of the California borderland. a sediment layer whose compressional wave velocity is known. The Nelson and co-workers (Nelson, 1968; Nelson and Byrne, 1968; original thickness of buried layers within the cone can be estimated Nelson and others, 1970; Nelson and Kulm, 1973; Nelson and by assuming that these layers had an average initial velocity similar Nilson, 1974) also proposed a model for deep-sea fan growth to that of the uppermost layer of cone sediments (1.8 km/sec, Fig. based on studies of the Astoria fan off the Columbia River. They 10). According to the velocity-porosity curve of Nafe and Drake recognized a threefold division of fan morphology similar to that of
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 Amazon Cone B Okm 1 l(j>0 200 300 400 500 6
2.30 ——! 3.10 — 2.31 _ 2.30 3.67 —? » ? 2.55
I
n nr Figure 10. Structural cros¡ss sections across the Amazon cone based on seismic data. Sections AB reversed. Section CD is from Edgar and Ewing (1968) and Edgar and Ewing (unpub. data). All and EF are based on single-shihip unreversed radio sonobuoy data, whereas section CD is based on velocities are in kilometers per second. two-ship refraction data. Sections 40, 41, and 43 of CD were reversed, whereas section 42 was un-
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 876 DAMUTH AND KUMAR
Normark; however, in contrast to Normark's observation of no sediment from reaching the Amazon Submarine Canyon. Thus channels on the lower fan, they observed numerous braided dis- pelagic sedimentation prevails across the entire cone. tributaries on the lower Astoria fan. They also found that thick de- In summary, the growth pattern observed for the Amazon cone is posits of coarse sediment occur across the lower fan, which suggest in many respects quite similar to the growth patterns observed for that deposition, and hence fan growth, is not restricted to the mid- the smaller deep-sea fans off western North America, although dle or suprafan as Normark proposed. there are some inherent differences, as outlined above. Such differ- The Amazon cone exhibits a threefold morphologic division of ences, however, should be expected because, as Normark (1974) the fan surface similar to that observed by both Normark and Nel- points out, all fans need not exhibit identical growth patterns. Fac- son and co-workers (see Fig. 3, profile QR). The rugged upper cone tors such as grain size, sediment supply, basin setting, and tectonic has only a single large leveed central channel or fan valley (Figs. 2, activity can modify growth patterns (Normark, 1974; Nelson and 3, 4). The middle cone, which corresponds to Normark's suprafan Kulm, 1973). region, is a convex-upward bulge that contains several leveed dis- Although numerous and detailed studies of small fans have been tributary channels (Figs. 2, 3, 5). The smooth lower cone is crossed made (many of which are cited throughout this text), large deep-sea by numerous small distributary channels without natural levees fans comparable in size to the Amazon cone have not received (Figs. 2, 8). The presence of numerous channels on the lower much attention in the geological literature. The Ganges cone or Amazon cone suggests that the channel system of the cone most Bengal fan is the only deep-sea fan of comparable size to the Ama- closely resembles the model of Nelson and others. This differs from zon cone for which some data on the growth pattern exist (Curray Normark's model, in which channels are absent from the and Moore, 1971; Moore and others, 1974; Normark, 1974); lower fan. however, Normark (1974) has observed that the morphology and The sediment distribution pattern observed for the lower Ama- growth pattern of this largest of deep-sea fans is different from zon cone also most closely fits the model of Nelson and co-workers. those of the small fans off western North America, possibly be- The sediment of the lower Amazon cone contains numerous thick, cause the Ganges cone receives no large supply of coarse sediment. widespread sand beds and is thus quite similar to the sediment re- Hopefully, our observations for the Amazon cone will contribute ported for the lower Astoria fan (Nelson and Kulm, 1973; Kulm to the understanding of growth processes of large deep-sea fans and and others, 1973; Nelson and Nilson, 1974). In contrast, Normark will stimulate more detailed studies of these interesting features. (1970, 1974) found that thick sand beds are generally absent from the lower cone because nearly all coarse material is deposited on SUMMARY AND CONCLUSIONS the middle fan. For the interchannel and levee areas of the upper Amazon cone, The Amazon cone is one of the largest deep-sea fans in the world our observations more closely coincide with those of Normark and has a morphology, sediment distribution, and growth pattern (1970, 1974) who observed a general absence or low concentration similar to smaller deep-sea fans that have been described in the of coarse sediment for these regions. The interchannel regions of geological literature. The cone has the threefold division of mor- the upper cone consist largely of silty clay. Thin silt/sand interbeds phology characteristic of other fans, and like most fans, it has one are frequent on or near channel levees. These observations contrast large, leveed, central channel or fan valley that divides on the mid- with Nelson and Nilson's (1974) and Nelson and Kulm's (1973) dle cone into numerous distributaries that radiate outward across observations of abundant thick sand beds on levees and interchan- the lower cone. During relatively low glacio-eustatic sea-level nel areas of the upper Astoria fan. stands, such as those during the Wisconsin glacial, terrigenous sed- Both Normark and Nelson and co-workers have suggested that iment discharged by the Amazon River is spread across the cone via sedimentation is confined to only a specific radial segment of a fan the distributary system. Coarse sediment largely bypasses the upper at any given time; however, new segments of the fan become pro- cone, as well as most of the middle cone, via the distributary chan- gressively active and previous segments become inactive as the cen- nels and is deposited on the lowermost middle cone and lower tral fan valley and distributary system migrate across the fan cone. Deposition on the interchannel areas of the upper and middle through time. In contrast, the piston cores suggest that, at least dur- cone is limited largely to clay-size particles derived from overbank ing Late Wisconsin time, the entire Amazon cone was active spilling of the channels. Rates of sediment accumulation for all por- and receiving large quantities of sediment at rates in excess of 25 tions of the cone during Late Wisconsin time were in excess of 30 cm/103 yr. cm/103 yr. In contrast, during periods of relatively high sea level Our observations suggest the following growth pattern for the (interglacial periods) such as today, the Amazon cone is inactive Amazon cone. During periods of relatively low sea level (glacial because the wide continental shelf and longshore currents prevent phases) large amounts of terrigenous sediment are continuously Amazon River sediment from reaching the cone. Thus sedimenta- discharged by the Amazon River into the Amazon Submarine Can- tion on the cone is entirely pelagic. Seismic reflection and refraction yon. Turbidity currents and related types of mass flows transport data indicate that the cone ranges in thickness from 1 km on the coarse sediment across the upper cone via the large leveed central lowermost portion to 14 km on the uppermost portion. Based on 3 channel or fan valley. Overbank spilling spreads clay and minor • inferred sedimentation rates of 90 to 115 cm/10 yr for the upper amounts of silt and fine sand to interchannel areas of the upper and cone, the age for the beginning of formation of the cone is esti- middle cone; however, the bulk of coarse sediment bypasses the mated to be 8 to 15 m.y. B.P. or middle to late Miocene. This age upper and middle cone. Deposition of most coarse sediment occurs agrees with independent evidence from the land geology of Brazil, on the lowermost portion of the middle cone and across the entire which suggests that the Amazon River began to flow into the Atlan- lower cone as an intricate network of distributaries transport sedi- tic during latest Miocene time. Thus the Amazon cone is a rela- ment radially outward from the end of the central channel. The tively youthful feature compared to the age of the Equatorial Atlan- amount of coarse material deposited apparently increases tic (approximately 130 m.y.). downslope because the sand beds are thickest and most frequent on the lowermost cone and adjacent abyssal plain (Fig. 8). Deposition of coarse silt/sand beds seems to result from overflow of the ACKNOWLEDGMENTS numerous small distributary channels. During relatively high sea-level stands (interglacial phases) such Financial support for this research was provided by National as that of today, the Amazon cone is inactive because the wide con- Science Foundation Grants GX—34410 and GA—27281 and by the tinental shelf and prevailing longshore currents prevent Amazon Office of Naval Research under Grant N00014-67-A-0108-
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 AMAZON CONE 877
0004. The sediment cores used for this study are from the raphy of the Gulf of Mexico, in Weeks, L., ed., Habitat of oil: Tulsa, Lamont-Doherty Geological Observatory Core Library, which is Am. Assoc. Petroleum Geologists, p. 995-1053. financially maintained by a grant from the Office of Naval Re- Ewing, M., Eittreim, Stephen, Truchan, Marek, and Ewing, J. I., 1969, Sed- iment distribution in the Indian Ocean: Deep-Sea Research, v. 16, p. search (N00014-67-A-0108-0004) and National Science Foun- 231-248. dation Grant GA-35454. WethankJ. I. Ewing for providing unpub- Ewing, M., Carpenter, George, Windisch, Charles, and Ewing, John, 1973, lished seismic data and S. Connary and D. E. Hayes for providing Sediment distribution in the oceans: The Atlantic: Geol. Soc. America unpublished bathymetric data. R. E. Houtz provided the reduced Bull., v. 84, p. 71-88. sonobuoy data. G. M. Bryan, Houtz, Ewing, and R. Embley re- Flint, R. F., 1971, Glacial and Quaternary geology: New York, John Wiley viewed the manuscript and provided helpful suggestions and criti- & Sons, 892 p. cism. Technical assistance was provided by B. Batchelder, B. Haner, B. E., 1971, Morphology and sediments of Redondo submarine fan, Hautau, and H. Cason. southern California: Geol. Soc. America Bull., v. 82, p. 2413-2432. Hayes, D. E., Frakes, L. A., and others, 1973, Leg 28 deep-sea drilling in the southern ocean: Geotimes, v. 18, no. 6, p. 19-24. REFERENCES CITED Heezen, B. C., and Menard, H. W., 1963, Topography of the deep-sea floor, in Hill, M. N., ed., The sea, Vol. 3: New York, Wiley Intersci- ence, p. 233-280. Allersma, E., 1971, Mud on the oceanic shelf off Guiana, in Symposium on Heezen, B. C., and Tharp, Marie, 1961, Physiographic diagram of the investigations and resources of the Caribbean Sea and adjacent re- South Atlantic Ocean, the Caribbean Sea, the Scotia Sea, and the east- gions: Paris, UNESCO, p. 193-203. ern margin of the South Pacific Ocean (with explanation): New York, Broecker, W. S., and Van Donk, J., 1970, Insolation changes, ice volumes Geol. Soc. America, scale 1:10,000,000. and the O18 record in deep-sea cores: Rev. Geophysics and Space Holeman, J. H., 1968, The sediment yield of the major rivers of the world: Physics, v. 8, p. 169-198. Water Resources Research, v. 4, p. 737-747. Broecker, W. S., Ewing, M., and Heezen, B. C., 1960, Evidence for an Houtz, Robert, 1974, Preliminary study of global sediment sound velocities abrupt change in climate close to 11,000 years ago: Am. Jour. Sci., v. from sonobuoy data, in Hampton, L., ed., Physics of sound in marine 258, p. 429-448. sediments: New York, Plenum Press, p. 519-535. Carlson, P. R., and Nelson, C. H., 1969, Sediments and sedimentary struc- Houtz, Robert, Ewing, John, and Le Pichon, Xavier, 1968, Velocity of tures of the Astoria submarine canyon-fan system, NE Pacific: Jour. deep-sea sediments from sonobuoy data: Jour. Geophys. Research, v. Sed. Petrology, v. 37, p. 1269-2182. 73, p. 2615-2641. Carson, Bobb, 1971, Holocene-Pleistocene sedimentation in Cascadia Houtz, Robert, Ewing, Maurice, Hayes, Dennis, and Naini, Bhoopal, 1973, Basin, northeast Pacific Ocean: Internat. Sedimentol. Cong., 8th, Prog, Sediment isopachs in the Indian and Pacific Ocean sectors (105° E to with Abs., p. 16-17. 70° W): Am. Geog. Soc. Antarctic Map Folio Ser. 17, p. 9-12. Cobbing, E. J., 1972, Tectonic evolution of Peru and the evolution of the James, D. E., 1971, Plate tectonic model for the evolution of the central Andes: Internat. Geol. Cong. 24th, Montreal 1972, sec. 3, p. Andes: Geol. Soc. America Bull., v. 82, p. 3325-3346. 306-315. Komar, P. D., 1969, The channelized flow of turbidity currents with appli- Cochran, J. R., 1973, Gravity and magnetic investigations in the Guiana cation to Monterey deep-sea fan channel: Jour. Geophys. Research, v. Basin, western Equatorial Atlantic: Geol. Soc. America Bull., v. 84, p. 74, p. 4544-4558. 3249-3268. Kulm, L. D., von Huene, R. E., and others, 1973, Initial reports of the Deep Curray, J. R., and Moore, D. G., 1971, Growth of the Bengal deep-sea fan Sea Drilling Project, Vol. 18: Washington, D.C., U.S. Govt. Printing and denudation in the Himalayas: Geol. Soc. America Bull., v. 82, p. Office, 1077 p. 563-572. Larson, R. L., and Ladd, J. W., 1973, Evidence for the opening of the South Damuth, j. E., 1973, The western Equatorial Atlantic: Morphology, Atlantic in the Early Cretaceous: Nature, v. 246, p. 209-212. Quaternary sediments, and climatic cycles [Ph.D. thesis]: New York, Le Pichon, Xavier, Ewing, John, and Houtz, R. E., 1968, Deep-sea sedi- Columbia Univ., 522 p. ment velocity determination made while reflection profiling: Jour. 1974, Echo character of the western Equatorial Atlantic and its rela- Geophys. Research, v. 73, p. 2597-2613. tionship to the dispersal and distribution of terrigenous sediments: McGeary, D.F.R., and Damuth, J. E., 1973, Postglacial iron-rich crusts in Marine Geology, v. 17. hemipelagic deep-sea sediment: Geol. Soc. America Bull., v. 84, p. Damuth, J. E., and Fairbridge, R. W., 1970, Equatorial Atlantic deep-sea 1201-1212. arkosic sands and ice-age aridity in tropical South America: Geol. Soc. Menard, H. W., 1955, Deep-sea channels, topography, and sedimentation: America Bull., v. 81, p. 198-206. Am. Assoc. Petroleum Geologists Bull., v. 39, p. 236-255. Damuth, J. E., Kumar, Naresh, and Gorini, M. A., 1973, Morphology, sed- Menard, H. W., Smith, S. M., and Pratt, R. M., 1965, The Rhone deep-sea iments, and the age of formation of the Amazon Cone: Geol. Soc. fan, in Whittard, W. F., and Bradshaw, R., eds., Submarine geology America, Abs. with Programs (Ann. Mtg.), v. 5, no. 7, p. 591. and geophysics: London, Butterworths, Colston Papers 17, p. de Oliveira, A. I., 1956, Brazil, in Jenks, W. F., ed., Handbook of South 217-286. American Geology: Geol. Soc. America Mem. 65 Moore, D. G., Curray, J. R., Raitt, R. W., and Emmel, F. J., 1974, Diephuis, J.G.H.R., 1966, The Guiana Coast: Koninkl. Nederlansch Aar- Stratigraphic-seismic section correlations and implications to Bengal drijksk. Genoot., Tijdschr., v. 83, p. 145-152. fan history, in von der Borch, C. C., Sclater, J. G., and others, Initial Ealey, P. J., 1969, Marine geology of northern Brazil: A reconnaissance reports of the Deep Sea Drilling Project, Vol. 22: Washington, D.C., survey [Ph.D. thesis]: Urbana, Univ. Illinois, 70 p. U.S. Govt. Printing Office, p. 403-412. Edgar, Terence, and Ewing, John, 1968, Seismic refraction measurements Nafe, J. E., and Drake, C. L., 1963, Physical properties of marine sedi- on the continental margin of northeastern South America: Am. ments, in Hill, M. N., ed., The sea, Vol. 3: New York, Wiley Intersci- Geophys. Union Trans., v. 49, p. 197-198. ence, p. 794-815. Ericson, D. B., 1961, Pleistocene climatic record in some deep-sea sediment Nelson, C. H., 1968, Marine geology of Astoria deep-sea fan [Ph.D. thesis]: cores: New York Acad. Sci. Annals, v. 95, p. 537-541. Corvallis, Oregon State Univ., 287 p. Ericson, D. B., and Wollin, Goesta, 1956a, Correlation of six cores from Nelson, C. H., and Byrne, J. V., 1968, Astoria fan: A model for deep-sea the Equatorial Atlantic and the Caribbean: Deep-Sea Research, v. 3, p. fan deposition: Geol. Soc. America, Abs. for 1968, Spec. Paper 121, p. 104-125. 218. 1956b, Micropaleontological and isotopic determinations of Pleis- Nelson, C. H., and Kulm, L. D., 1973, Submarine fans and channels, in tocene climates: Micropaleontology, v. 2, p. 257—270. Turbidites and deep-water sedimentation, Lecture notes for Society of 1968, Pleistocene climates and chronology in deep-sea sediments: Sci- Economic Paleontologists and Mineralogists short course (May, ence, v. 162, p. 1227-1234. 1973), p. 39-78. Ewing, John, 1969, Seismic model of the Atlantic Ocean, in Hart, P. J., ed., Nelson, C. H., and Nilson, T. H., 1974, Depositional trends of modern and The earth's crust and upper mantle: Am. Geophys. Union Mon. 13, p. ancient deep-sea fans, in Dott, R. H., and Shaver, R. H., eds., Modern 220-225. and ancient geosynclinal sedimentation: Soc. Econ. Paleontologists Ewing, M., Ericson, D. B., and Heezen, B. C., 1958, Sediments and topog- and Mineralogists Spec. Pub. 19, p. 69-91.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021 878 DAMUTH AND KUMAR
Nelson, C. H., Carlson, P. R., Byrne, J. V., and Alpha, T. R., 1970, De- Jolla deep sea fan, California: Marine Geology, v. 8, p. 211-227. velopment of the Astoria Canyon-Fan: Physiography and comparison Raitt, R. W., 1963, The crustal rocks, in Hill, M. N., ed., The sea, Vol. 3: with similar systems: Marine Geology, v. 8, p. 259—291. New York, Wiley Interscience, p. 85-102. Normark, W. R., 1970, Growth patterns of deep-sea fans: Am. Assoc. Reyne, A., 1961, On the contribution of the Amazon River to accretion of Petroleum Geologists Bull., v. 54, p. 2170-2195. the coast of the Guianas: Geologie en Mijnbouw, v. 40, p. 219-226. 1974, Submarine canyons and fan valleys: Factors affecting growth Zembruski, S. G., Gorini, M. A., Palma, J.J.C., and de Ataide Costa, M. P., patterns of deep-sea fans, in Dott, R. H., and Shaver, R. H., eds., 1971, Fisiografía e distribuido dos sedimentos superficias da Modern and ancient geosynclinal sedimentation: Soc. Econ. Paleon- plataforma continental norte Brasileira: Bol. Téc. Petrobrás, v. 14, p. tologists and Mineralogists Spec. Pub. 19, p. 56-68. 127-155. Normark, W. R., and Piper, D.J.W., 1969, Deep-sea fan-valleys, past and present: Geol. Soc. America Bull., v. 80, p. 1859-1866. 1972, Sediments and growth pattern of Navy Deep-Sea Fan, San Clemente Basin, California borderland: Jour. Geology, v. 80, p. MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 18,1974 198-223. REVISED MANUSCRIPT RECEIVED JULY 29,1974 Piper, D.J.W., 1970, Transport and deposition of Holocene sediment on La LAMONT-DOHERTY GEOLOGICAL OBSERVATORY CONTRIBUTION NO. 2193
Printed in U.S.A.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/6/863/3429521/i0016-7606-86-6-863.pdf by guest on 02 October 2021