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The deltaic nature of Amazon shelf

C. A. NITTROUER \ S. A. KUEHL* > Department of Marine, Earth and Atmospheric Sciences, State University, Raleigh North Carolina 27695 D. J. DEMASTER J R. O. KOWSMANN CENPES/PETROBRAS, Rio de Janeiro, Cep. 21910, Brazü

ABSTRACT deposits. This fine-scale stratification resem- mouth. Despite the supply of more than a billion bles deposits of (respectively) the - tons of each year, prograding subaerial Despite the annual of more than mouth , distal bar, and prodelta observed deposits near the Amazon mouth are a billion tons of sediment by the Amazon for the Mississippi and other deltas. much subdued relative to many other . In River, the sedimentary environment near the The has built a subaqueous fact, the physiographic expression at the Amazon has little subaerial expression feature which stretches for hundreds of River mouth closely resembles that of an . and thus does not meet the classic definition kilometres offshore and alongshore from its This paper describes recent stratigraphic - of a delta. The river mouth, however, is not mouth. The feature is prograding seaward servations of the at the mouth an estuary, eithei. These observations raise a and accreting upward, and it contains fine- of the Amazon River. Strata are evaluated on msyor question as to what type of sedimen- scale stratification typical of classic deltas. vertical scales ranging from tens of metres (seis- tary environment the Amazon river mouth The feature forming at the mouth of the mic stratigraphy) to millimetres (sedimentary represents. Amazon is a subaqueous delta; it differs from structures). The objectives are to examine strati- Seismic stratigraphy has been examined on classic deltas primarily in its lack of subaerial graphic relationships at the Amazon River the continental shelf at the mouth of the expression. Subaqueous deltas, such as the mouth, to compare them with sedimentation at Amazon River using high-frequency (3.5- Amazon, represent the general case of a other river mouths, and to consider deltaic kHz) seismic records from about 6,000 km of major river entering an energetic oceanic sedimentation in an environment with little sub- ship track. These records demonstrate three regime. aerial expression. regions. (1) <4l0-m water depth—topset strata composed primarily of muddy sediment INTRODUCTION BACKGROUND with sandy interlieds that dip gently and di- verge seaward; stratification (sandy inter- The geologic and economic importance of Deltaic Sedimentation beds) that becomes less predominant north- ancient deltaic deposits has led to extensive eval- westward along the dispersal system. (2) uation and synthesis of knowledge about modern Gilbert (1885) introduced the concept of -40- to ~60-m depth—foreset strata that deltas (see, for example, van Straaten, 1964; delta : relatively steeply dipping dip relatively steeply (but <1°) and converge Shirley and Ragsdale, 1966; Morgan, 1970; foreset deposits prograde outward over thi i, - seaward. (3) -6(1- to -100-m water depth— Broussard, 1975; American Assoc. Petroleum lying, finer-grained bottomset deposits; and bottomset strata ilhat form a thin, acoustically Geologists, 1976). Notably missing from these gently clipping topset deposits accrete upward transparent layer of , which is burying detailed presentations is the world's largest river, upon the foreset deposits. Gilbert (1885, 1890) sandy on the surface of a basal the Amazon River. The Amazon River mouth is studied deltas, and some signifi- (transgressive) layer. The large-scale discussed in the literature both as a delta cant differences exist between these delUis and structure which the deposits of these regions (Wright, 1978) and as an estuary (Friedman modern deltas, which generally contain create is a sigmoidal clinoform. Fine-scale and Sanders, 1978); however, its sedimentary finer sediment and more gentle foreset slopes. have been examined nature has not been documented sufficiently for The has become the type ex- from the inner continental shelf using X- accurate appraisal. ample of modern deltas, and use of Gilbert's radiographs of about 140 cores (box cores The early Greeks used the word "delta" to topset-foreset-bottomset terminology has. been and large gravity cores). These X-radio- describe the shape of subaerial deposits at the replaced by a new classification of sedimentary graphs reveal: (1) physically stratified sand mouth of the River. The first scientific study environments which is more compatible with near the river-, (2) interbedded of delta stratification included subaerial deposits observations of the Mississippi delta (Coleman, mud and sand (in the shelf adjacent to the (topset) as a requisite to deltaic sequences (Gil- 1976; Wright, 1978). crcss the river mouth, and (3) faintly laminated and bert, 188S). The first geological definition of a subaerial deltaic plain, and at their mouths is mottled mud surrounding the interbedded "delta" (Barrell, 1912, p. 381) described it as "a found the delta front, consisting of distributary- deposit partly subaerial built by a river into or mouth bar deposits and distal bar deposits. The against a body of permanent water." These asso- delta fiont has the greatest relief of the delta, but •Present address: Department of Geology, Univer- even so, the slopes for the delta front of the sity of , Columbia, South Carolina ciations of subaerial features within a delta help 29208. to explain the confusion about the Amazon river Mississippi are less than 1°. The deposits of the

Geological Society of America Bulletin, v. 97, p. 444-458,18 figs., April 1986.

444

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Figure 1. Bathymétrie chart for the mouth of the Amazon River (modified from chart 200, Brazilian Directorate of Hydrography and Navigation). Hachured area represents broad transverse bar across the river mouth. Locations are shown for cores presented in this paper.

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distributary-mouth bar consist largely of the iment, resulting in faulting, slumping, and mud also should affect the subaqueous structure of bedload of the river and form physically strati- flowage (Coleman and others, 1974). Decay of deltas. fied . The accumulates in organic matter in the seabed can cause produc- the deeper and more quiescent region of the dis- tion of methane gas. The presence of such gas Sedimentation near the Amazon River Mouth tal bar, forming deposits of mud interlaminated usually is recognized by disruption of the seismic with sand. Beyond the delta front, there are pro- record and can lead to instability in the seabed The annual discharge of Amazon River water delta deposits, where the remaining and (Coleman, 1976). represents one-fifth of the world total for river accumulate as horizontally laminated deposits. Where the potential for delta formation exists, discharge, and it is more than an order of magni- Because of reduced accumulation rate, these the large-scale morphology of a delta is con- tude larger than that of the deposits can become mottled and homogenized trolled by several fluvial and oceanographic fac- (Gibbs, 1967). The difference between seasonal by bioturbation (Moore and Scruton, 1957). The tors (Coleman, 1976; Wright, 1978). The high flow and low flow of the Amazon is only a above sedimentary environments, with associ- amount and of sediment discharged by factor of about two (Oltman, 1968). Because of ated sedimentary structures, commonly occur in a river affect estuarine infilling and delta pro- the large, continuous discharge, ocean water other deltas (for example, ; Allen, gradation. The nature of the fluvial jet entering never enters the Amazon River mouth ( Gibbs, 1965,1970). The seaward transition of environ- the ocean influences the dispersal of deltaic sed- 1970). On the adjacent continental shelf, the ments exhibits a progressive fining (Scruton, iments (Bates, 1953; Wright, 1977). The pri- fluvial discharge becomes a hypopycnal jet, and 1960). Delta projjradation causes the deposits to mary oceanographic processes affecting deltaic low- water is transported hundreds of be superimpose^ which leads to upward configuration are surface waves and tidal cur- kilometres as a surface plume (Gibbs, 1970; coarsening. For many deltas, the basal (pro- rents (Wright and Coleman, 1973; Galloway, Curtin, 1983). The annual sediment discharge of delta) are not competent to bear the 1975). In some situations, coastal (shelf) cur- the Amazon is about 1.2 x 109 tons (Meade and weight of the overlying deposits, and the mud is rents also have a significant effect on dispersal of others, 1985), ranking it second in the world forced upward us diapiric intrusions (Nota, deltaic (Murray and others, 1980). (Milliman and Meade, 1983). About 85% to 1958; Morgan and others, 1968; Shepard and The studies examining the impact of the above 95% of sediment discharged by the Amiizon is others, 1968). The rapid of deltaic factors have focused on effects to the large-scale mud (silt and clay) (Gibbs, 1967; Meade, 1985), deposits can lead also to failures of surficial sed- subaerial morphology of deltas, but these factors and most sediment is injected to the ocean in suspension (Gibbs, 1976). 52°w Oceanographic processes affecting the disper- sal of Amazon sediment include strong ticlal and coastal currents as well as moderate surface waves. The near the river mouth commonly exceeds 6 m (NOAA, 1982), and tidal currents exceeding 100 cm/s have been measured on the shelf (near the river mouth) a couple of metres above the seabed (Curtin, 1983). The North Brazilian Coastal flows northwestward along the shelf with sur- face speeds generally exceeding 50 cm/s (Gibbs, 1982). In addition, the discharge of Amazon water as a surface plume entrains water from below and causes a landward bottom current on the shelf (Gibbs, 1976). The surface waves are primarily generated by prevailing trade winds, and most wave energy is dissipated by inner- shelf muds (as described for the Guiana by Wells and Coleman, 1981). The immense river flow and the strong tidal and coastal currents effectively disperse Amazon sediment from the river mouth. This region con- sists of two channels, north and south, separated by a shallow region (<5-m water depth) con- taining several islands (Fig. 1). Each has a depth of 20-30 m and widens near its mouth into a field of longitudinal banks. These give

Figure 2. Bathymétrie chart for the continental shelf ad- jacent to the Amazon River mouth. Locations are slliown for cores (numbered) and seismic profiles (lettered) pre- sented in this paper.

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Figure 3. A. Cruise tracks from which 3.5-kHz seismic profiles were collected. B. Locations of cores from which sedimentary structure was examined by X- radiography. Box cores were collected at locations shown for E6A79. Both box cores and large gravity cores were collected at most locations shown for CI8306.

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way to a broad transverse bar (<10 m deep) 12 cm wide and 2.5 cm thick, from which X- the river mouth (Fig. 5), a sandy layer is expjsed which stretches across the mouth of the entire radiographs were obtained. A total of about 90 at the seabed surface and reduces penetration of river. Although the river mouth is predominant- box cores and 50 gravity cores from the inner- the seismic signal. Evidence of diapiric intru- ly subaqueous, water depths increase offshore shelf deposit have been examined by X- sions on the shelf (Fig. 7) is confined to a small only gradually (Fig, 2); the 40-m isobath is radiography (Fig. 3B). The locations for cores area just north of the river mouth (see Fig. 5). located 150 km seaward of the river-mouth bar. described in this paper are shown in Figures 1 For small segments of the seismic records, topset These shallow depths probably result from high and 2. About 20 piston cores (400 to 1,200 cm reflectors appear parallel (Fig. 6). Viewed in rates of sediment accumulation (generally length) have been obtained also. These are larger scale, however, they reveal a gradual sea- >1 cm/yr; Kuehl and others, 1982), which cylindrical cores, with a diameter of only about ward divergence (that is, spacing between reflec- increase seaward across this region (Kuehl and 5 cm, and they are not as useful as box cores and tors increases). Approaching the 40-m isobath, others, 1985b). The seabed abruptly steepens be- gravity cores for examining the detailed nature the gently dipping topset strata can be observed tween the 40-m and 60-m isobaths, where the of sedimentary structures. They provide infor- to overlie more steeply dipping foreset strata inner-shelf mud tliins (Milliman and others, mation about longer time scales, however, and (Fig. 8). 1975; Milliman, 1S»79) and accumulation rates have been used primarily to interpret seismic Foreset. Both the sea floor and the underly- decrease (Kuehl and others, 1982). Farther sea- profiles. ing seismic reflectors have their greatest slopes ward, accumulation rates are negligible, and the between about 40-m and 60-m water depth transgressive sand layer is exposed, showing var- RESULTS (Fig. 9). As described in Figure 5, the gradients ious bedforms on its surface (Barreto and others, are relatively gentle (1:1,000) along the south- 1975). Figueiredo and others (1972) have de- Seismic Stratigraphy ernmost portion of the region. Northward, the scribed the Amazon inner shelf as a subaqueous gradients increase to 1:200, decrease to 1:500, delta. Support for this idea and an understanding Figures 4 and 5 summarize the stratigraphic and then increase again to 1:100 (note: these are of its meaning require detailed examination of setting of the Amazon continental shelf. Three all < 1°). A more flat-lyingbasa l reflector am be Amazon shelf stratification, over a range of distinct regions can be recognized: (a) topset observed beneath the foreset strata in this region vertical scales. (<40-m water depth), (b) foreset (-40 m to (Fig. 9). Disruption of seismic reflectors by gas is -60 m), and (c) outer shelf (bottomset and uncommon on the Amazon shelf but is observed METHODS transgressive sand; ~60 m to-100 m). The suit- in some areas (for example, landward portions ability of deltaic terminology is considered in the of the area shown in Fig. 9). No evidence of The samples and data presented in this paper section entitled "Discussion." mass movement (slumps, mud flows) is ob- were collected during two cruises: E6A79 on the Topset. This region is characterized by a gen- served on the shelf, even in the relatively ¡¡teep R/V Eastward (October 1979; low flow of tle offshore gradient (< 1:3,000) for the sea floor foreset region; this is supported by studies with river) and CI8306 on the R/V Iselin (May-July and for sub-bottom seismic reflectors. Textural side-scan sonar (Adams and others, 1983). 1983; high flow of river) (Fig. 3). High- analyses of piston cores reveal sand interbedded Outer Shelf (bottomset and transgressive frequency (3.5-kH:!) seismic records were col- within silt/clay (Nittrouer and others, 1983). sand). On the outer shelf, the general gradic nt of lected along 6,0(X) km of ship track using The number of sandy beds at a site generally the surface is again gentle (<1:2,000). At the Raytheon systems (hull mounted on E6A79 and corresponds to the number of seismic reflectors foot of the steep middle shelf, a thin laysr of towed on CI8306) Ship speed during profiling observed. Other factors besides sediment texture acoustically transparent mud overlies the basal ranged from 9 to 18 km/hr. Satisfactory records (for example, degree of consolidation, seismic reflector (Fig. 10). This mud layer thins and were obtained from most of the shelf, although interference patterns) can affect an acoustic sig- disappears in the seaward direction. The basal profiles were of red uced quality in water depths nal and can cause real or apparent reflectors on reflector is exposed as a sand layer on the out- less than about lCt-20 m. Satellite navigation seismic records. Although every reflector may ermost portion of the shelf. The surface cf the generally was used for positioning, but the equa- not be a sandy bed, the empirical observations in sand contains bedforms of different scales (Fig. torial location of the study area caused the the piston cores and on the 3.5-kHz records in- 11). South of about 2°N, the surface is defo rmed number of satellite passes to be few (especially dicate a general correlation between beds and on E6A79). Dead reckoning was inaccurate be- seismic reflectors. Reflectors become less nu- cause of strong and variable currents. Sub- merous in a northwestward direction away from bottom stratigraphy on the Amazon shelf, the river mouth (Fig. 6). In one area offshore of however, is closely associated with regional bathymetric features; thus, the effect of un- certain positioning on the interpretation of seis- mic records was griiatly reduced. A box corer and a large gravity (kasten) corer were used to collect samples for examination of sedimentary structures. Box cores (cross section 20 cm x 30 cm) were about 50 cm long, and gravity cores (cross section 12.7 cm x 12.7 cm; Kuehl and others, 1985a) were about 300 cm long. The cores were subsampled to obtain slabs

Figure 4. Cartoon depicting the stratigraphie setting of the Amazon continental shelf. A subaqueous delta is prograding seaward over a basal (transgressive) sand layer.

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52°W 51° 50° 49° 48° 47° 46°

Figure 5. Distribution of characteristic types of seismic stratigraphy. Topset deposits are well stratified near the river mouth, and they reveal diapirs and a sandy surface reflector in the approximate locations shown. Topset deposits become less stratified northwestward along the dispersal system. Foreset deposits reveal along-shelf variations in their seaward gradient. The transgressive sand layer, which is exposed on the outer shelf, contains bedforms of different scales (sand , sand waves) and is being buried by bottomset deposits.

into sand waves about 3-6 m in height and tions of the 80-m isobath in Fig. 2) and are being Sedimentary Structures 100-200 m in wavelength. Farther north, the buried by mud (Fig. 12). Generally, the 3.5-kHz dominant features are very large sand "shoals" signal cannot penetrate the sand. Where pene- Four distinct types of sedimentary structures (Palma, 1979), 20-30 m high and 6-8 km in tration is possible, however, the lower surface is are present within the inner-shelf muds and are wavelength. These features are oriented perpen- irregular. Infilled channels can be observed ex- distributed as shown in Figure 14 (modified dicularly with (as can be seen by deflec- tending into the underlying surface (Fig. 13). from Kuehl and others, 1982,1986).

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-5

•10

•15m Figure 6. Seismic records from the topset deposits, demonstrating the well-stratified siouthwest northeast deposits near the river mouth (upper ri;cord) I -H 1 1 km and the less stratified deposits farther north- vertical exaggeration 18x westward along the inner shelf (lower record). Textural observations in piston cores indicate that the seismic reflectors are sandy layers (Nittrouer and others, 1983). (Upper record is profile A and lower record -25m is profile B shown in Fig. 2.) -0

-5

-10

-15m northeast southwest I H 1 km B vertical exaggeration 18x

25 m

25 m

Figure 7. Seismic records from the small area of the topset deposits where diapirs are observed. Some seismic reflectors appear to be bent upward by diapiric intrusions from below. The diapirs probably result from mud deposits not competent to bear the weight of ovei lying deposits. These records suggest the presence of some gas associated with the diapirs. (Upper record is profile C and lower record is prolile D shown in Fig. 2.)

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L40m southwest northeast I 1 2 km vertical exaggeration 18 x

Figure 8. Seismic record from near the outer topset deposits, which overlie more steeply dipping foreset deposits. Careful inspection of topset reflectors reveals seaward divergence, which is typical of topset deposits and results from a seaward increase in accumulation rates. (This record is profile E shown in Fig. 2).

southwest northeast I 1 1 2 km vertical exaggeration 18X

f•i* ' • •.Mv- i ^y«mu ' ¡2¡ f x* b30 f 40 m

southwest northeast I 1 2 km vertical exaggeration 18x

Figure 9. Seismic records from foreset deposits, revealing relatively steeply dipping sea floor and seismic reflectors overlying the basal (transgressive) sand surface. The reflectors converge seaward because of seaward-decreasing accumulation rates. The sea floor in the upper record has a maximum gradient of about 1:200 and in the lower record has a maximum gradient of 1:400. Both records suggest the presence of gas in landward portions. (Upper record is profile F and lower record is profile G shown in Fig. 2.)

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Figure 10. Seismic record of bottomset deposit overlying the basal (transgressive) sand surface. The bottomset deposit thins seaward, and the sand is exposed on the outer shelf. The bottomset deposits typically are thin and acoustically transparent because of low accumulation rates and absence of sand. (This record is profile H shown in Fig. 2.)

m ! !

northeast southwest I 1 1 - 0 1 2km O vertical exaggeration 18 x

Figure 11. Seismic records of sandy bedforms on the outer shelf. The upper record shows sand shoals, thought to be relict estuarine features from lower stand of sea level. The lower record shows sand waves, which probably are responding to intense tidal currents. (Upper record is profile I and loweir record is profile J shown in Fig. 2.)

Physically Stratified Sand. This is very fine grain sizes, see Nittrouer and others, 1983). The Mottled Mud. Located on the rest of the to medium sand, revealing predominantly ripple sand layers reveal both horizontal and ripple inner shelf, surrounding the other types of sedi- laminations, with some convolute bedding (Fig. laminations. According to the classification of mentary structures, mottled mud is found The 15a). The sand is observed throughout some Reineck and Singh (1980), the relationship of mottling ranges from heterogeneous to nearly box cores and as a surface layer upon over- sand and mud would be described as wavy bed- homogeneous structure (Fig. 17c). consolidated mud in other box cores. In the lat- ding in some box cores and as lenticular bedding ter cores, the surface of the mud is erosional and in other box cores. In gravity cores, the interlam- DISCUSSION indicates production of rip-up clasts (Fig. 16). inated mud and sand is observed as beds ~50 The clasts are observed as rounded mud balls cm thick, separated by thicker beds of faintly Stratigraphic Relationships near within the sand (Filg. 15b). Physically stratified laminated mud. This interbedding of mud and the Amazon River Mouth sand was collected from the bar region at the sand is present seaward of the river-mouth bar mouth of North Channel (Fig. 1). Unfortu- and extends northwestward along the inner shelf According to terminology of Mitchum and nately, ship operations were difficult and limited (Fig. 14). others (1977), the large-scale structure of the in this region. Shallow areas could not be Faintly Laminated Mud. Farther northwest- Amazon inner shelf is a sigmoidal clinotbrm. sampled, and thus spatial characteristics of the ward, the beds of interlaminated mud and sand The term "clinoform" refers to a gently sloping sand could not be resolved. disappear, and only faintly laminated mud is accretionary feature, and "sigmoidal" refers to Interbedded Mud and Sand. This sediment present (Fig. 14). This sediment is entirely silt the configuration of the generally concordant contains layers, several millimetres to -5 cm and clay, and stratification is derived from subtle seismic reflectors. Rich (1951) first introc uced thick (Fig. 17a), of silt/clay and fine sand (for changes in the amount of each (Fig. 17b). the concept of accretionary topography, ¡ilong

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I 1 1 0 1 2km vertical exaggeration 18x

HTjimtrfVZZ* 50 m

north south h— 1 1 2 3 4 km vertical exaggeration 28x

Figure 12. Seismic records showing partial and complete burial of sand shoals by progradation of the subaqueous delta. The upper record shows transparent bottomset mud in the trough between two sand shoals; the sharp contact between sand and mud suggests immobility of the sand and an ancient origin for the shoals. (Upper record is profile K and lower record is profile L shown in Fig. 2.)

Figure 13. Seismic record showing infilled channels at the base of the transgressive sand layer. These channels were cut into the old subaerial surface and were filled as sea level rose. Today the infilled channels are being buried by bottomset deposits and prograding foreset deposits. (This record is profile M shown in Fig. 2.)

with its three contiguous stratigraphie regions: Strictly interpreted, "topset, foreset, and bot- lar: elinoform refers to the over-all accretionary undaform, elinoform, and fondaform. Deposits tomset" refer only to deltaic sequences (see sec- feature; topset, foreset, and bottomset are the of these correspond to topset, foreset, and bot- tion entitled "Background"). These confusing subordinate regions. tomset, which are the terms most commonly semantics will be addressed in following sec- On the Amazon shelf, topset strata reveal a used today. The word "elinoform" survives, but tions. The nomenclature used in this paper is decrease of internal reflectors northwestward represents the composite of all three regions. that of Mitchum and others (1977), in particu- along the dispersal system. This indicates a pro-

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52°w 51° 50° 49° 48° 47° 46°

Figure 14. Distribution of characteristic types of sedimentary structures within deposits on the inner shelf (modified from Kuehl and others, 1982, 1986). Physically stratified sand is present near the river-mouth bar. Inter- bedded mud and sand are present adjacent to the river mouth and extending northwestward along the shelf. Sand is not found northwestward, and the sediment is faintly laminated mud. Where sediment accumulation is slow, benthic organisms mix the seabed causing mottled sediment.

gressive depletion of sand, which is a common observation along dispersal systems and results from preferential accumulation of sand relative to finer sediment (Nittrouer and others, 1986). Figure 15. X-radiographs (positives) of box Topset strata also reveal a seaward divergence. cores from the physically stratified sand near Divergent reflectors should indicate increasing the river-mouth bar. a. Station 30 (length, 18 accumulation rate. Radiochemical studies (Pb- cm); reveals ripple laminations and convolute 210, Cs-137) on the Amazon shelf demonstrate bedding, indicating bedload transport and that nearshore accumulation rates are 1 cm/yr liquefaction (respectively), b. Station 32 or less and that rates increase to as much as 10 (length, 21 cm); reveals about 13 cm of ripple- cm/yr on the outer topset region (Kuehl and laminated sand separated by an erosional others, 1985b). The relatively low rates near- contact from laminated mud below. The sand shore result from resuspension of sediment by contains rip-up clasts (mud balls) formed by shoaling surface waves and by strong tidal cur- of the mud. rents. As with the Amazon shelf, maximum ac- cumulation rates an: commonly observed on the middle portions of other modern shelves (Nit- trouer and others, 1985).

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Comparison with Sedimentation at Other River Mouths

Does the Amazon form an estuary or a delta? The estuarine nature of the Amazon is more simply evaluated than its deltaic nature. Despite the estuarine-like physiographic form near its river mouth, the Amazon is not an estuary— neither according to physical nor geological oceanographic definitions. Physical oceanog- raphers view as semi-enclosed bodies of water where and salt water meet and are mixed (Pritchard, 1967). Salt water never enters the Amazon River mouth (Gibbs, 1970), and typical frontal-zone processes occur on the continental shelf (which is not semi- enclosed). Geological oceanographers view es- tuaries as river valleys, drowned by sea-level rise, which still efficiently trap sedi- ment. In less than 100 yr, the sediment discharge Figure 16. Photograph of the top section of a box core (station 34). Truncated laminae in of the Amazon could fill to sea level the region mud demonstrate erosional nature of contact between sand and underlying mud. extending out across the river-mouth bar (that is, to the outer 10-m isobath, Fig. 1). Instead, the sediment is transported to the adjacent continen- tal shelf. Obviously the mouth of the Amazon is On the Amazon shelf, foreset strata dominate Additional information about the mechanisms not trapping sediment efficiently. In addition, seaward progradation (offlap). The foreset strata of sediment accumulation can be gained from the prevalence of erosion associated with the show seaward convergence and a downlap rela- observations of fine-scale stratification. The North Channel and bar suggests that accumula- tionship with underlying strata. The seaward por- sands just landward of the river-mouth bar tion is inhibited by shear stresses resulting from tions of the foreset strata meld into bottomset indicate the following: bedload transport and the immense discharge of the river and other strata. The convergence results from accumula- migration, which create ripple lamina- intense physical processes (tidal currents, the tion rates which decrease relatively abruptly tions; intense flow and rapid deposition, which North Brazilian Coastal Current, surface gravity (-10 cm/yr to <1 cm/yr; Kuehl and others, cause liquefaction and convolute bedding; and waves). The Amazon River mouth is thus open 1985b), probably because of an offshore reduc- continually intense flow, which prevents deposi- not because of passive drowning by sea-level tion of sediment supply. Other contributory tion of mud, even during low flow of the river. rise, but more probably because it is in equi- causes might include (1) an increase in bottom Net accumulation is not great, as evidenced by librium with a very energetic physical regime. resulting from the North Brazilian the thin nature of the sand and the active erosion The Amazon River mouth nearly meets the Coastal Current and (2) the presence of a land- of older sediment below (production of mud definition of a delta. Certainly the discharge of ward bottom current. The acoustic transparency balls). Most modern sediment is transported Amazon sediment has built a deposit into a of bottomset strata demonstrates the absence of beyond the river-mouth bar, where some (see Barrell, 1912) and certainly sand, which apparently is not transported accumulates (at centimetres per year) in a more that deposit contains topset strata accreting up- beyond the foreset region. variable environment, creating interlamination ward and foreset strata prograding seaward (see The basal reflector beneath the inner-shelf and interbedding of mud and sand. The primary Gilbert, 1885). The major difference between clinoform is the surface of the transgressive sand fluctuation in flow near the river mouth occurs the Amazon and classic deltas is the absence of layer, which formed during Holocene sea-level with tidal frequency (Curtin, 1983), and the in- subaerial deposits. The Mississippi delta has be- rise. As the sand layer formed, it unconformably terlamination is probably tied to this fluctuation. come the type example of modern deltas, and covered the old land surface. The surface of the The presence of sandy interbeds might represent the distinction of the Amazon region can be rec- sand layer is still exposed on the outer shelf, and unusually large of the river (when disper- ognized by comparison with the Mississippi bedforms indicate that some portions are being sal of coarse sediment was extensive) or might delta. The Mississippi delta possesses a range of reworked more than other portions. Sand waves represent winnowing and removal of fine sedi- subaerial/shallow deposits (, cravasse-splay, probably are forming in response to strong tidal ment by physical processes. Far from the river interdistributary-) associated with numerous currents, whereas sand shoals probably are relict mouth, accumulation rates decrease sufficiently distributary channels (Coleman, 1976; Wright, features that formed as longitudinal ridges in the that benthic organisms are able to destroy physi- 1978). The region just landward of the Amazon mouth of the ancestral Amazon, during the last cal stratification and homogenize the seabed River-mouth bar contains a few with low stand of sea level (Palma, 1979). The sharp (Kuehl and others, 1982). Closer to the river . The largest portion of this contact between the sand shoals and the mouth, benthic habitation is inhibited by insta- region is subaqueous and has not been well doc- acoustically transparent mud, which is accumu- bility of the seabed (frequent deposition and ero- umented, but it probably does not contain the lating in their troughs (Fig. 12), suggests sion) and by little food supply from the water complex array of distributary channels and asso- immobility of the sand and an ancient origin for column (primary is prevented by ciated environments found with the Mississippi. the shoals. turbid water; DeMaster and others, 1983). For the areas near the bar deposits, however,

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Figure 17. X-radiographs (positives) of cores from the Amazon shelf, a. Station 28 (length, 34 cm); interbedded mud and sand. Mud shows horizontal laminations, and sand shows cross laminations, suggesting deposition from suspended load and bedload, respectively. The upper portion of the core shows ball-and-pillow structure, b. Station 170 (length, 28 cm); faintly laminated mud. Thin silt laminae cause stratifica- tion. c. Station 127 (length, 33 cm); mottled mud. Mixing by benthic organisms destroys physical stratification and causes mottled sediment.

fine-scale stratigraphie relationships (that is, more extensive. The prodelta muds (faintly lam- On a larger scale, some notable differences physically stratified sands) associated with the inated, mottled) are the same in both areas; even exist between the Mississippi and Amazon re- Amazon and Mississippi are similar. The inter- the relationship of the accumulation rate to the gions. The Mississippi has prograded neaily to laminated mud and sand of the Amazon shelf transition from laminated to bioturbated sedi- the shelf break. Much sediment is being supplied are very similar to the distal-bar deposits of the ment is similar (Moore and Scruton, 1957; to the continental slope, and only a fraction is Mississippi, although the Amazon deposits are Kuehl and others, 1982,1986). being transported westward by along-sheli cur-

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rents. The offshore progradation is the result of a sediment begin to escape from estuaries, mud persed from the region near their river mouths. low-energy regime: small surface waves, little accumulates in mid-shelf regions (Curray, 1965; Where large rivers (for example, Amazon tidal range, and weak coastal currents (Wright Swift, 1970). Depending on the relative rates of River) discharge their sediment loads into open- and Coleman, 1973). The sediment transported sediment supply and dispersal, mud can accum- marine environments, subaqueous deltas can westward from the Mississippi has formed a thin ulate also on the inner shelf (McCave, 1972). In form. These deltas represent the general case of layer on the shelf (Curray, 1960). In contrast, the extreme case, deltas can form subaerial de- major rivers entering an energetic oceanic re- the Amazon has built thick clinoforms, by ac- posits which extend onto the shelf. The sedi- gime. The supply of sediment is sufficient to cumulating much of its sediment on the adjacent mentary environments, ranging from complete create a major topographic deposit on the shelf, shelf. High accumulation rates are largely re- exposure of the transgressive sand layer to but the shear stresses associated with the am- sponsible for the accretionary clinoforms; al- deltaic complexes, represent a spectrum which bient physical processes inhibit the deposit from though some of the associated relief (and includes the shelf near the mouth of the accreting to sea level. If coastal currents are variation in relief) may result from antecedent Amazon. An arbitrary decision has been made present, the sediment is dispersed along-shelf, topography, as observed on other parts of the by the community of geological scientists to de- and it can produce thick clinoform deposits Brazilian continental shelf (Kowsmann and fine deltas as "that portion of the spectrum stretching for hundreds of kilometres along the Costa, 1979). Despite high accumulation rates which contains extensive subaerial deposits." coast. Similar deposits are represented in the and relief, diapirism and mass movement are not The unique characteristic of deltas, however, is geological record, in particular as and silt- common near the Amazon. This differs from that they form distinct accretionary deposits stones formed in the Cretaceous seaway of observations of the Mississippi region and is which stand well above the regional topography. North America (see Fig. 18; Asquith, 1970). probably the result of the following factors. (1) Whether these deposits reach an arbitrary datum The Amazon subaqueous delta has both modern Gas production is not as prevalent in Amazon plane (sea level) is of much less importance. In and ancient counterparts and represents an im- sediments. This may be a consequence of the this more general view, the Amazon River portant subclass of deltaic sequences. high content of iron oxides (-8% by weight), mouth is a delta, a subaqueous delta. As a which inhibits chemical reduction from reaching corollary, the terms "topset, foreset, and bottom- CONCLUSIONS the stage of methanogenesis ( and Mackin, set" can be used in connection with subaqueous 1984). (2) The Mississippi deposits are forming deltas. For example, although the topset strata of Sedimentation on the continental shelf near in proximity to the shelf break, and sediment is the Amazon delta are not subaerial, they are the mouth of the Amazon River is producing a slumping down the continental slope (Coleman gently dipping deposits which are accreting sigmoidal clinoform. Topset strata dip gently and others, 1974). This process removes mate- upward upon more steeply dipping, seaward- seaward and reveal (1) an offshore divergence rial from the toe of shelf deposits and allows prograding strata (foreset). In this more general associated with increasing accumulation rates slumping to progress across the shelf. Such a usage, Amazon topset strata meet the essence of (~ 1 cm/yr to ~ 10 cm/yr) and (2) a northwest- mechanism is not active on the Amazon shelf, the original definition. ward decrease in seismic stratification associated because progradation has not reached the shelf The fundamental difference of subaqueous with depletion of sand along the dispersal sys- break. (3) The higher energy regime of the deltas can be recognized by contrasting their lo- tem. Foreset strata dip relatively steeply seaward Amazon region intensely reworks the seabed, cations with those of classic deltas. Most classic but reveal no evidence of mass movement. Bot- and may impart a more stable fabric on the deltas (for example, in Coleman and Wright, tomset strata are thin because of slow accumula- sediment which remains as net accumulation. 1975) are located in quiescent (for example, tion (< 1 cm/yr) and are acoustically transparent This sort of relationship has been observed for , of Mexico, Arctic because of an absence of sand. The modern sandy sediment (Allen, 1982, p. 173ff.). Ocean) or within locally quiescent settings (for clinoforms are prograding over a transgressive Although the Amazon River mouth possesses example, embayments, ). Where moderate sand layer, which includes modern sand waves some deltaic attributes, it also lacks many char- or small rivers (for example, ) and relict sand shoals (old estuary-mouth acteristics of the Mississippi and other deltas. discharge their sediment loads into open-marine features). Classification of the Amazon sedimentary de- environments, the sediment is effectively dis- The accretionary feature at the mouth of the posit is difficult; however, the Amazon is not a unique case. For example, the mouth of the River is dominated by an extensive subaqueous structure with many stratigraphic re- lationships which resemble those of the Amazon (Butenko and others, 1985; DeMaster and oth- ers, 1985; Rhoads and others, 1985). Because the Amazon and Yangtze regions accumulate much sediment and form extensive strata, un- derstanding and classification of their type of sedimentation is important.

The Generality of Deltaic Sedimentation

Holocene sea-level rise formed estuarine traps for fluvial sediment, causing many continental shelves to be characterized by complete expo- Figure 18. Cross section showing pattern of clinoforms observed within Cretaceous shales sure of the transgressive sand layer (Emery, and siltstones of Wyoming (from Asquith, 1970). Note the similarity of large-scale structure to 1968). River discharge is dominated by fine- that demonstrated in Figure 4 for the Amazon shelf (the authors thank The American Associa- grained sediment, and, as significant amounts of tion of Petroleum Geologists for permission to include this figure).

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Bates, C. C„ 1953, Rational theory of delta formation: American Association of ment to the : Journal of Geology, v. 91, p. 1-21. Amazon can be classified as a subaqueous delta. Petroleum Geologists, v. 37, p. 2119-2162. Milliman, J. D., Summerhayes, C. P., and Barretto, H. T., 1975, Quaternary Despite the absence of extensive subaerial de- Broussard, M. L., ed, 1975, Deltas: Models for exploration: Houston, Texas, sedimentation on the Amazon : A model: Geological Houston Geological Society, 555 p. Society of America Bulletin, v. 86, p. 610-614. posits, the topset-foreset-bottomset stratigraphy Butenko, J., Milliman, J. D„ and Ye, Y. C„ 1985, , shallow Mitchum, R. M., Vail, P. R., and Sangree, J. B., 1977, Seismic stratigrap ly and structure, and geological hazards in the East Sea: Continental global changes of sea level, Part 6: Stratigraphic interpretation of t eismic has functional similarities to classic deltas. Shelf Research, v. 4, p. 121-141. reflection patterns in depositional sequences, in Payton, C. E, ed,, Seis- Fine-scale stratigraphic relationships reveal Coleman, J. M., 1976, Deltas: Processes of deposition and models for explora- mic stratigraphy—Application to hydrocarbon exploration: Tulsa, Ok- tion: Champaign, Illinois, Continuing Education Publication Co., 102 p. lahoma, American Association of Petroleum Geologists Menvnr 26, more direct similarities. Physically stratified Coleman, J. M., and Wright, L. D., 1975, Modem river deltas: Variability of p. 117-133. processes and sand bodies, in Broussard, M. L., ed., Deltas: Models for Moore, D. G., and Scruton, P. C., 1957, Minor internal structures o' some sands are found near the Amazon River-mouth exploration: Houston, Texas, Houston Geological Society, p. 99-149. recent unconsolidated sediments: American Association of Pet oleum bar and resemble de posits of distributary mouth Coleman, J. M„ Suhayda, N. N., Whelan, T., and Wright, L. D„ 1974, Mass Geologists Bulletin, v. 41, p. 2723-2751. movement of Mississippi sediments: Gulf Coast Association Morgan, J. P., ed., 1970, Deltaic sedimentation: Modem and ancient: Tulsa, bars. Distal-bar depDsits of the Amazon are rep- of Geological Societies Transactions, v. 24, p. 49-68. society of Economic Paleontologists and Mineralogists Special Publica- Curray, J. R., 1960, Sediments and history of Holocene transgression, continen- tion no. 15,312 p. resented by interlaminated mud and sand, and tal shelf, northwest Gulf of Mexico, in Shepard, F. P., Phleger, F. B„ and Morgan, J. P., Coleman, J. M., and Gagliano, S. M., 1968, Mudlumps: Eiapiric prodelta muds show a trend from laminated (in van Andel, Tj. H., eds., Recent sediments, northwest Gulf of Mexico: structures in Mississippi delta sediments, in Braunstein, J., and O'Brien, Tulsa, Oklahoma, American Association of Petroleum Geologists, G. D., eds., Diapirism and diapirs: Tulsa, Oklahoma, American -Associ- areas of rapid accumulation) to homogeneous p. 221-266. ation of Petroleum Geologists Memoir 8, p. 145-161. 1965, Late Quaternary history, continental shelves of the United States, Murray, S. P., Coleman, J. M., Roberts, H. H , and Salama, M., 1980, (in areas of slow accumulation) sedimentary in Wright, H. E, and Frey, D. G., eds., The Quaternary of the United currents and off the Damietta Nile: Intern itional structures. States: Princeton, , Princeton University Press, p. 723-735. Conference, 17th, Proceedings, p. 1680-16! >9. Curtin, T. B„ 1983, The Amazon plume region: Physical observations and National Oceanic and Atmospheric Administration, 1982, tables 1983, The Amazon subaqueous delta and its clino- dynamics: EOS (American Geophysical Union Transactions), v. 64, east coast of North and South America: , D.C., U.S. De- p. 1026. partment of Commerce, 285 p. form structure represent the general case of a DeMaster, D. J., Knapp, G. B., and Nittrouer, C. A., 1983, Biological uptake Nittrouer, C. A., Sharara, M. T., and DeMaster, D. J., 1983, Variations of and accumulation of silica on the Amazon continental shelf: Geochim- sediment texture on the Amazon continental shelf: Journal of Se< limen- major river entering an energetic oceanic re- ica et Cosmochimica Acta, v. 47, p. 1713-1723. tary Petrology, v. 53, p. 179-191. gime. Other modern, and ancient examples with DeMaster, D. J., McKee, B. A., Nittrouer, C. A., Qian, J. C., and Chen, G. D., Nittrouer, C. A., DeMaster, D. J., Kuehl, S. A., McKee, B. A., and 1985, Rates of sediment accumulation and reworking based on Thorbjarnarson, K. W., 1985, Some questions and answers about the similar stratification exist. The Amazon and radiochemical measurements from continental shelf deposits in the East accumulation of fine-grained sediment in continental margin environ- China Sea: Continental Shelf Research, v. 4, p. 143-158. ments: -Marine Letters, v. 4, p. 211-213. these other examples are distinguished from Emery, K. O., 1968, Relict sediments on continental shelves of the world: Nittrouer, C. A., DeMaster, D. J., Kuehl, S. A., and McKee, B. A., 1986, classic deltas by the prevalence of subaqueous American Association of Petroleum Geologists Bulletin, v. 52, Association of sand with mud deposits accumulating on continental p 445-464. shelves, in Knight, R. J., ed., Sedimentology of shelf sands anc sand- deposits. The Amazon River mouth and conti- Figueiredo, A. G„ Gamboa, L.A.P., Gorini, M„ and Costa Alves, E, 1972, stones: Calgary, Canadian Society of Petroleum Geologists (in piess). Natureza da sedimentacao actual do Rio Amazonas—Testemunhos e Nota, DJ.G., 1958, Sediments of the western Guiana shelf, in Reports of the nental shelf demonstrate an important type of geomorfologia submarina, () Amazonas testemunhos sub- Shelf Expedition, Volume 2: Utrecht, Mededelingen van de deltaic sedimentation, which deserves more marinos: Congresso Brasfleiro de Geologia, 26th, v. 2, p. 51-56. Landbouwhoogeschool te Wageningen, v. 58, p. 1-98. Friedman, G. M., and Sanders, J. E, 1978, Principles of sedimentology: New Oltman, R. E., 1968, Reconnaissance investigations of the discharge anc water attention. York, Wiley, 792 p. quality of the Amazon River U.S. Geological Survey Circular 552, Galloway, W. E, 1975, Process framework for describing the morphologic and 16 p. stratigraphic evolution of deltaic depositional systems, in Broussard, Pal ma, J J.C., 1979, Geomorfologia da plataforma continental norta Brasfleira, M. L., ed., Deltas: Models for exploration: Houston, Texas, Houston in Chaves, H.A.F., ed, Geomorfologia da margem continental Brasi- ACKNOWLEDGMENTS Geological Society, p. 87-98. leira e das areas oceanicas adjacentes: Rio de Janeiro, Petrobras, l*rqjeto Gibbs, R. J., 1967, The geochemistry of the Amazon River system: Part I. The Remac7, p. 25-51. factors that control the salinity and the composition and concentration Pritchard, D. W., 1967, What is an estuary: Physical viewpoint, in Lauff, G. D., This research was sponsored by the National of the suspended solids: Geological society of America Bulletin, v. 78, ed., Estauries: Washington, D.C., American Association for the Ad- p. 1203-1232. vancement of Science, p. 3-5. Science Foundation (Grants OCE-7908496 and 1970, Circulation in the Amazon River estuary and adjacent Atlantic Reineck, H. E., and Singh, I. B., 1980, Depositional sedimentary structures: OCE-8117709). The authors thank the crews Ocean: Journal of Marine Research, v. 28, p. 113-123. , Springer-Vedag, 549 p. 1976, Amazon River sediment transport in the Atlantic Ocean: Geol- Rhoads, D. C., Boesch, D. F., Tang, Z. C., Xu, F. S„ Huang, L. Q., and Wilsen, and scientists on E6A79 and CI8306, who are ogy, v. 4, p. 45-48. K. J., 1985, and sedimentary fades on the Chaiigjiang 1982, Currents on the shelf of north-eastern South America: Estuarine, delta platform and adjacent continental shelf, East China Sea: Continen- responsible for making this paper possible. The Coastal and Shelf Science, v. 14, p. 283-299. tal Shelf Research, v. 4, p. 189-213. authors appreciate helpful reviews of this paper Gilbert, G. K., 1885, The topographic features of lake : U.S. Geological Rich, J. L., 1951, Three critical environments of deposition and criteria for Survey 5th Annual Report, p. 104-108. recognition of rocks deposited in each of them: Geological Society of by J. E. Damuth and R. H. Meade. 1890, : U.S. Geological Survey Monograph 1,438 p. America Bulletin, v. 62, p. 1-20. Kowsmann, R. O., and Costa, M. P., 1979, Evidence of Late Quaternary Scruton, P. C., 1960, Delta building and the deltaic sequence, in Shepard, F. P., sealevel stillstands on the upper Brazilian continental margin: A synthe- Phleger, F. B., and van Andel, Tj. H., eds., Recent sediments, Northwest REFERENCES CITED sis: International Symposium on Coastal Evolution in the Quaternary, Gulf of Mexico: Tulsa, Oklahoma, American Association of Peti oleum Proceedings: Sao Paulo, IGCP Project 61, p. 170-192. Geologists, p. 82-102. American Association of Petroleum Geologists, 1976, Modern deltas: Tulsa, Kuehl, S. A., Nittrouer, C. A., and DeMaster, D. J., 1982, Modem sediment Shepard, F. P., Dill, R. F., and Heezen, B. C., 1968, Diapiric intrusions in American Association of Petroleum Geologists, Reprint Series no. 18, accumulation and strata formation on the Amazon continental shelf: forcset slope sediments off Magdalena delta, Colombia: Americin As- 205 p. Marine Geology, v. 49, p. 279-300. sociation of Petroleum Geologists Bulletin, v. 52, p. 2197-2207. Adams, C. E., Wells, J. T., ani Coleman, J. M., 1983, Bedforms on the Kuehl, S. A., Nittrouer, C. A., DeMaster, D. J., and Curtin, T. B., 1985a, A Shirley, M. L, and Ragsdale, J. A., eds., 1966, Deltas: In their geologic .Tame- Amazon shelf: EOS (Aaerican Geophysical Union Transactions), long, square-barrel gravity corer for sedimentologica! and geochemical work: Houston, Tesas, Houston Geological Soaety, 251 p. v. 64, p. 1025. investigation of fine-grained sediments: Marine Geology, v. 62, Swift, DJ.P., 1970, Quaternary shelves and the return to grade: Marine Geol- Allen, J.R.L., 1965, Late Quaterauy Niger Delta, and adjacent areas: Sedimen- 365-370. ogy, v. 8, p. 5-30. tary environments and lithofaries: American Association of Petroleum 1985b, An overview of sedimentation on the Amazon continental shelf: van Straaten, LMJ.U., ed, 1964, Deltaic and shallow marine deposits: New Geologists Bulletin, v. 49, p. 547-600. Geo-Marine Letters, v. 4, p. 207-210. York, Elsevier, 464 p. 1970, Sediments of the modern Niger Delta: A summary and review, in Kuehl, S. A., Nittrouer, C. A., and DeMaster, D. J., 1986, Distribution of Wells, J. T., and Coleman, J. M., 1981, Physical processes and fine-grained Morgan, J. P., ed, Deltai: sedimentation: Modern and ancient: Tulsa, sedimentary structures in the Amazon subaqueous delta. Continental sediment dynamics, coast of Surinam, South America: Journal of Sedi- Society of Economic Paleontologists and Mineralogists, Special Publica- Shelf Research (in press). mentary Petrology, v. 51, p. 1053-1068. tion no. 15, p. 138-151. McCave, I. N., 1972, Transport and escape of fine-grained sediment from shelf Wright, L. D., 1977, Sediment transport and deposition at river mouths: A 1982, Sedimentary structures: Their character and physical basis, areas, in Swift, DJ.P., Duane, D. B., and Pilkey, O. H., eds., Shelf synthesis: Geological Society of America Bulletin, v. 88, p. 857-868. Volume 1: New York, Elsevier, 593 p. sediment transport: Process and pattern: Stroudsburg, Pennsylvania, 1978, River deltas, in Davis, R A., ed., Coastal sedimentary environ- Aller, R. C., and Mackin, J. E, 1984, Apparent dominance of Fe-reduction Dowden, Hutchinson, and Ross, p. 225-248. ments: New York, Springer-Verlag, p. 5-68. during early diagenesis oi* organic matter in Amazon shelf sediments: Meade, R. H., 1985, Suspended sediment in the Amazon River and its tributar- Wright, L. D., and Coleman, J. M., 1973, Variations in morphology of major EOS (American Geophyscal Union Transactions), v. 65, p. 905. ies in Brazil during 1982-84: U.S. Geological Survey Open-File Report river deltas as functions of ocean wave and river discharge regimes: Asquith, D. O,, 1970, Depositicnal topography and major marine environ- 85-492. American Association of Petroleum Geologists Bulletin, 57, ments, Late Cretaceous, Wyoming: American Association of Petroleum Meade, R. H„ Dunne, T., Richey, J. E., Santos, U. de M., and Salati, E., 1985, p. 370-398. Geologists Bulletin, v. 54, p. 1184-1224. Storage and remobilization of suspended sediment in the lower Amazon Barrel!, J., 1912, Criteria for the lecognition of ancient delta deposits: Geologi- River of Brazil Science, v. 228, p. 488-490. cal Society of America Bulletin, v. 23, p. 377-446. Milliman, J. D., 1979, Morphology and structure of Amazon upper continental Barreto, L. A., Milliman, J. D., Amaral, C.A.B., and Francisconi, O., 1975, margin: American Association of Petroleum Geologists Bulletin, v. 63, MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 4,1985 Upper continental margin sedimentation off Brazil: Northern Brazil: p. 934-950. REVISED MANUSCRIPT RECEIVED NOVEMBER 1,1985 Contributions to Sedimeniology, v. 4, p. 11-43. Milliman, J. D., and Meade, R H., 1983, World-wide delivery of river sedi- MANUSCRIPT ACCEPTED NOVEMBER 14, 1985

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