High-resolution seismic stratigraphy and its sedimentological interpretation on the Amazon continental shelf

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Seismic profiles (3.5 kHz) from over 6000 km of shiptrack and piston cores from 26 stations were examined to evaluate the seismic-stratigraphic framework of the Amazon subaqueous delta. Acoustic reflectors in the seismic profiles were related to sedimentological and acoustic properties in piston-core sediment. Grain size in cores from the Amazon shelf correlates well with seismic velocity and saturated bulk density, and thus with changes in acoustic impedance. Acoustic reflectors on the Amazon shelf generally correspond to coarser layers in the seabed. The observed acoustic reflectors can be attributed to both depositional and erosional processes. Depositional reflectors, which correspond to stratal surfaces, reveal typical stratification of the subaqueous delta. In a seaward direction, topset strata (<30 m water depth) dip gently (<1:1500) and diverge (∼1.5m/50km); foreset strata (30–70 m water depth) dip steeply (>1:1500) and converge abruptly (∼1.5m/5km); and bottomset strata (>70 m water depth) dip gently (<1:1500) and downlap onto the transgressive sand layer. Erosional reflectors correspond to hiatuses or to unconformities which truncate tops of seismic sequences composed of depositional reflectors. A stratum corresponding to an erosional reflector is actively forming over an area of 6 × 10 3km 2 seaward of the river mouth, and is thick (>2 m thickness) and coarse (5.1 phi, mean grain size) as compared to depositional reflectors. This erosional reflector overlies two seismic sequences of top-truncated foreset reflectors. North of the river mouth, an acoustically transparent layer, covering 1.5 × 10 4km 2 and representing 100–200 years of sediment accumulation, overlies truncated relict mud <1000 years old. The presence of truncated strata and erosional reflectors illustrates the spatial and temporal variability of sedimentary conditions on the Amazon shelf, and the difficulty of extrapolating modern (i.e. surficial) sedimentation patterns into the past.