SEDIMENT DEPOSITION, ACCUMULATION, AND SEABED DYNAMICS IN ARE ENERGETIC FINE-GRAINED COASTAL ENVIRONMENT

Sedimentary processes on the continental shelf and shoreline northwest of the Amazon River mouth were investigated as part of A Multidiscipl inary Amazon Shelf SEDiment Study (AmasSeds) during four field expedit ions between 1989 and 1991. Periodic deposition and resuspension of se abed layers as much as a meter thick dominate sedimentary processes fo r most of the inner shelf and for the shoreface and foreshore north of Cabo Cassipore. Strata forming as a result of this process consist of decimeter-thick mud beds separated by hiatal (scour) surfaces. The vo lume of sediment resuspended seasonally from the inner shelf surface l ayer (SL) is of the same order of magnitude as the annual input from t he river, indicating that resuspension is an important control on susp ended-sediment distributions in shelf waters. Most resuspension from t he SL occurs during February-May (the period of maximum wind stress), which is also the time of rapid deposition on the mudflats, suggesting that sediment resuspended from the SL could contribute to shoreface a nd foreshore accretion for the northern portion of the study area. In addition, some of the sediment resuspended from the SL is transported seaward periodically in the form of near-bottom fluid-mud flows. This results in non-steady-state input of certain particle-reactive trace m etals, which is reflected in the occurrence of quasi-cyclic Pb-210 pro files in the foreset region of the subaqueous delta. As determined usi ng Ra-228/Ra-226 geochronology, sediment accumulation rates in this re gion are 10-60 cm y(-1). Farther seaward, in the bottomset region, acc umulation rates decrease and there is increased evidence of biological activity preserved in sedimentary structures. However, episodic (but reduced) sediment input from fluid-mud flows also extends to this regi on, affecting the fauna and fine-scale stratigraphy.