Rates and mechanisms of shoreface progradation and retreat downdrift of the Amazon river mouth

Abstract

Field surveys of the 350-km shoreline adjacent to the Amazon river mouth reveal three distinct types: erosional mud, accretionary sand, and accretionary mud. Formation of these zones is controlled by the delivery of Amazon suspended sediment, mediated by the hydrodynamic regime. Erosional mud shorelines extend from Rio Araguari (near the Amazon river mouth) northwestward 280 km to 3.5 °N (near Cabo Cassipore). Shoreface (<5 m water depth) retreat annually yields 1–4 × 10 6 tons of fine-grained sediment deposited during an earlier phase of shoreface progradation. Sandbodies up to 5 m thick overlie erosional mud shorefaces for 10–30 km downdrift of small rivers. The sand is supplied from these rivers and released by shoreline retreat of the river-mouth areas. Amazon River sand is restricted in the coastal zone from its mouth north to the Cabo Norte shoal area, where it mixes with sand carried by Rio Araguari from the Guiana Highlands. North of 3.5 °N, mud aggradation and progradation is taking place on underconsolidated, low-gradient tidal flats backed by mangrove swamps. 210Pb and 14C geochronology of vibracores from the mudflats indicate that sediment accumulation is rapid (0.24–2.0 cm/yr) landward of the 2-m isobath, supplied from a thick (50–150 cm) seasonal surface layer. Shoreface progradation is episodic, separated by decadal hiatuses. Fine-grained suspended-sediment flux from the Amazon and minor amounts of sand and mud from the local rivers supply sediment to the mudflats. Shore-normal tidal currents and solitary waves rework the surface mud layer, preferentially transporting available sand landward into the mangrove fringe, and producing very fine-grained accumulation on the tidal flat (10–12 φ mean grain size). Lateral accretion of features 10–100 km long, termed mudcapes, produces tens of kilometers of seaward coastal-plain addition along the northern coast. Similar features are identified downdrift along the Guianas coast as far as the Orinoco River (1600 km). The northern Amapa shoreface deposits are a locus of modern sediment accumulation, which progrades over subaqueous deltaic strata.