Despite the annual discharge of more than a billion tons of sediment by the Amazon River, the sedimentary environment near the river mouth has little subaerial expression and thus does not meet the classic definition of a delta. The river mouth, however, is not an estuary, either. These observations raise a major question as to what type of sedimentary environment the Amazon river mouth represents.
Seismic stratigraphy has been examined on the continental shelf at the mouth of the Amazon River using high-frequency (3.5-kHz) seismic records from about 6,000 km of ship track. These records demonstrate three regions. (1) <40-m water depth—topset strata composed primarily of muddy sediment with sandy interbeds that dip gently and diverge seaward; stratification (sandy interbeds) that becomes less predominant northwestward along the dispersal system. (2) ∼40- to ∼60-m water depth—foreset strata that dip relatively steeply (but <1°) and converge seaward. (3) ∼60- to ∼100-m water depth—bottomset strata that form a thin, acoustically transparent layer of mud, which is burying sandy bedforms on the surface of a basal (transgressive) sand layer. The large-scale structure which the deposits of these regions create is a sigmoidal clinoform. Fine-scale sedimentary structures have been examined from the inner continental shelf using X-radiographs of about 140 cores (box cores and large gravity cores). These X-radiographs reveal: (1) physically stratified sand near the river-mouth bar, (2) interbedded mud and sand (in the shelf adjacent to the river mouth, and (3) faintly laminated and mottled mud surrounding the interbedded deposits. This fine-scale stratification resembles deposits of (respectively) the distributary-mouth bar, distal bar, and prodelta observed for the Mississippi and other deltas.
The Amazon River has built a subaqueous feature which stretches for hundreds of kilometres offshore and alongshore from its mouth. The feature is prograding seaward and accreting upward, and it contains fine-scale stratification typical of classic deltas. The feature forming at the mouth of the Amazon is a subaqueous delta; it differs from classic deltas primarily in its lack of subaerial expression. Subaqueous deltas, such as the Amazon, represent the general case of a major river entering an energetic oceanic regime.