Observations of sediment transport on the Amazon subaqueous delta

Observations of sediment transport on the Amazon subaqueous delta

 

 

Sediment transport on the Amazon subaqueous delta. A 19-day time series of fluid, flow, and suspended-sediment characteristics in the  benthic boundary layer is analyzed to identify major sedimentary processes active over the prodelta  region of the Amazon subaqueous delta. Measurements were made by the benthic tripod  GEOPROBE placed on the seabed in 65 m depth near the base of the deltaic foreset beds from 11  February to 3 March 1990, during the time of rising water and maximum sediment discharge of the  Amazon River; and the observations included: hourly measurements of velocity and suspendedsediment  concentration at four levels above the seabed; waves and tides; and seabed elevation.  Results of the first 14-day period of the time series record indicate that sediment resuspension  occurred as a result of tidal currents (91% of the time) and surface gravity waves (46% of the time).  Observations of suspended sediment indicated that particle flux in this region is 0.4-2% of the flux  measured on the adjacent topset deposits and is directed to the north and landward relative to the  Brazilian coast (268”T). Fortnightly variability is strong, with particle fluxes during spring tides five  times greater than during neap tides. On the 15th day of the data record, a rapid sedimentation  event was documented in which 44 cm of sediment was deposited at the study site over a 14-h  period. Evaluation of various mechanisms of mass sediment movement suggests that this event  represents downslope migration of fluid muds from the upper foreset beds that were set in motion  by boundary shear stresses generated by waves and currents. This transport mechanism appears to  occur episodically and may represent a major source of sediment to the lower foreset-bottomset  region of the subaqueous delta.