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Amazon Cone: Morphology, Sediments, Age, and Growth Pattern

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Amazon Cone: Morphology, Sediments, Age, and Growth Pattern 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).
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