The deltaic nature of Amazon shelf sedimentation C. A. NITTROUER \ S. A. KUEHL* > Department of Marine, Earth and Atmospheric Sciences, North Carolina 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 distributary- tons of sediment each year, prograding subaerial Despite the annual discharge of more than mouth bar, distal bar, and prodelta observed deposits near the Amazon River mouth are a billion tons of sediment by the Amazon for the Mississippi and other deltas. much subdued relative to many other rivers. In River, the sedimentary environment near the The Amazon River has built a subaqueous fact, the physiographic expression at the Amazon river mouth has little subaerial expression feature which stretches for hundreds of River mouth closely resembles that of an estuary. and thus does not meet the classic definition kilometres offshore and alongshore from its This paper describes recent stratigraphic ob- of a delta. The river mouth, however, is not mouth. The feature is prograding seaward servations of the continental shelf 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 water depth—foreset strata that deltas (see, for example, van Straaten, 1964; delta progradation: relatively steeply dipping dip relatively steeply (but <1°) and converge Shirley and Ragsdale, 1966; Morgan, 1970; foreset deposits prograde outward over thi i, flat- 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 mud, which is burying detailed presentations is the world's largest river, upon the foreset deposits. Gilbert (1885, 1890) sandy bedforms on the surface of a basal the Amazon River. The Amazon River mouth is studied Pleistocene lake deltas, and some signifi- (transgressive) sand 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 ocean deltas, which generally contain create is a sigmoidal clinoform. Fine-scale and Sanders, 1978); however, its sedimentary finer sediment and more gentle foreset slopes. sedimentary structures have been examined nature has not been documented sufficiently for The Mississippi delta 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 Nile River. The first scientific study environments which is more compatible with near the river-mouth bar, (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). Distributaries 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 South Carolina, 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/4/444/3445127/i0016-7606-97-4-444.pdf by guest on 25 September 2021 30' 50° 30' 49° 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/4/444/3445127/i0016-7606-97-4-444.pdf by guest on 25 September 2021 446 NITTROUER AND OTHERS 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 sands. The suspended load 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 silt and clay (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 Mississippi River 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 grain size 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, Niger delta; 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-salinity 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) muds 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 sediments (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 deposition 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 tidal range 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).