RATES AND MECHANISMS OF SHOREFACE PROGRADATION AND RETREAT DOWNDRIFT OF THE AMAZON RIVER MOUTH

Field surveys of the 350-km shoreline adjacent to the Amazon river mou th reveal three distinct types: erosional mud, accretionary sand, and accretionary mud. Formation of these zones is controlled by the delive ry of Amazon suspended sediment, mediated by the hydrodynamic regime. Erosional mud shorelines extend from Rio Araguari (near the Amazon riv er mouth) northwestward 280 km to 3.5 degrees N (near Cabo Cassipore). Shoreface (<5 m water depth) retreat annually yields 1-4 x 10(6) tons of fine-grained sediment deposited during an earlier phase of shorefa ce progradation. Sandbodies up to 5 m thick overlie erosional mud shor efaces for 10-30 km downdrift of small rivers. The sand is supplied fr om these rivers and released by shoreline retreat of the river-mouth a reas. Amazon River sand is restricted in the coastal zone from its mou th north to the Cabo Norte shoal area, where it mixes with sand carrie d by Rio Araguari from the Guiana Highlands. North of 3.5 degrees N, m ud aggradation and progradation is taking place on underconsolidated, low-gradient tidal flats backed by mangrove swamps. Pb-210 and C-14 ge ochronology of vibracores from the mudflats indicate that sediment acc umulation is rapid (0.24-2.0 cm/yr) landward of the 2-m isobath, suppl ied from a thick (50-150 cm) seasonal surface layer. Shoreface prograd ation is episodic, separated by decadal hiatuses. Fine-grained suspend ed-sediment flux from the Amazon and minor amounts of sand and mud fro m the local rivers supply sediment to the mudflats. Shore-normal tidal currents and solitary waves rework the surface mud layer, preferentia lly transporting available sand landward into the mangrove fringe, and producing very fine-grained accumulation on the tidal flat (10-12 phi mean grain size). Lateral accretion of features 10-100 km long, terme d mudcapes, produces tens of kilometers of seaward coastal-plain addit ion along the northern coast. Similar features are identified downdrif t along the Guianas coast as far as the Orinoco River (1600 km). The n orthern Amapa shoreface deposits are a locus of modern sediment accumu lation, which progrades over subaqueous deltaic strata.