The Amazon-influenced mud-bank coast of South America: short- to long-term morphodynamics of ?inter-bank? areas and chenier development
Résumé
Part of the huge suspended-sediment discharge of the Amazon River is transported as mud banks along the 1500 km-long coast of South America between the Amazon and the Orinoco river mouths. As the banks migrate alongshore, their interaction with waves results in markedly fluctuating shorelines that are associated with space- and time-varying depositional 'bank' phases and erosional 'inter-bank' phases. Bank zones are protected from wave attack as a result of wave-energy dampening by mud, and undergo significant accretion accompanied by mangrove colonization. Erosion by waves propagating across relatively mud-deficient shoreface zones in interbank areas can lead to retreat rates of tens of metres to several kilometres over a few months to a few years, accompanied by massive removal of mangroves. Notwithstanding the higher incident wave energy on inter-bank shores, inter-bank shorefaces are permanently muddy due to the pervasive influence of the Amazon muddy discharge. Inter-bank areas are associated, in places, with periodic sandy cheniers formed from sand reworked by waves from the nearshore zone, but also occasionally from reworking of older cheniers exposed along the coast by erosion. Erosion during each inter-bank phase results in the partial removal of the Holocene coastal stratigraphic package, thus enabling the progressive incorporation of cheniers into the prograding muddy coastal plain. Inter-bank areas sometimes exhibit km-scale megacuspate horns and bays the origin of which is unknown, although it is surmised that they may be the muddy-coast equivalents of similar forms associated on sandy coasts with infragravity wave energy cascades or with self-organized behaviour. ADDITIONAL INDEX WORDS: Amazon mud supply, mud banks, wave energy gradien