Fluvial sediment supply, mud banks, cheniers and the morphodynamics of the coast of South America between the Amazon and Orinoco river mouths

Abstract The overarching morphosedimentary control on the coast of South America between the Amazon and Orinoco rivers is the massive muddy discharge of the Amazon (from c. 754×106 to 1000×106 t a−1). This mud supply by the world’s largest river reflects sediment sourcing from the Andes, weathering in a tropical–equatorial climate and the extremely high water discharge (ca 173 000 m3 s−1). Amazon mud forms an estuarine mega-turbidity maximum on the shelf that feeds the growth of a subaqueous delta. About 15–20% of this mud forms coastal banks that migrate towards the mouth of the Orinoco, which has constructed a large subaerial delta. This muddy coast exhibits interspersed beaches and cheniers constructed from sand supplied by the smaller rivers and by the Orinoco, and is characterized by extremely large spatio-temporal geomorphic variability resulting from intense wave-reworking of the migrating banks, mangrove colonization and destruction, and erosion in interbank areas. These dynamic processes also drive important biogeochemical recycling, enhancing coastal productivity while efficiently remineralizing organic matter and promoting authigenic mineral formation. This muddy coast is being impacted by human-induced changes, in Guyana in particular. The Amazon–Orinoco coast provides an analogue for muddy, wave-dominated shorefaces in the geological record.