The changing shoreline of Suriname (South America)

Abstract

The Surinam coast forms part of the extensive tropical mud coast between the Amazon River (Brazil) and the Orinoco River (Venezuela). It is classified as a low to medium energy coast. A substantially longshore supply of mud originates from the Amazon River. The fine grained sediment (pelite) is transported in suspension and by steadily westward migrating shoreface-attached mudflats. These mud shoals are separated by intermudflat troughs. The mudflats migrate in a westward direction owing to erosion of their east side and accretion of the west side. The pelite contains a small fine sandy component. There is mineralogical evidence that the Amazon River is the source of this sand. It is winnowed from the mud and particularly along the western part of the Surinam coast it forms cheniers in a few cases. Along the eastern part of the Surinam coast, the cheniers are built up of medium to coarse sand which is supplied by the Marowijne River and the coast of French Guiana. Shell clastics originate from the shelf. Cheniers are perched, shallow based beach ridges, resting on clay. Those which are built up from alongshore supplied medium to coarse sediment develop at approximately mean high-water level. The formation of fine sandy cheniers in west Surinam depends on winnowing of the small sandy component form the pelite. It therefore begins at a much lower level (around mean low water). Sedimentary structures of both types of cheniers show much similarity. The underlying clayey deposits appear to be laminated. Locally the horizontal lamination is strongly bioturbated. The mudflats are overgrown with mangroves at the higher levels. Avicennia germinans is the principal representative. This type of coast is called a mud accretionary coast. Although the volume of the cheniers is reduced when compared with that of the mud deposits, they have a dominant effect on the coastal landscape. When a chenier accretes seaward a broad chenier complex with important protective properties develops: a sand accretionary coast. When this chenier is eroded the protective property gradually disappears. First a straight erosion coast with a landward receding chenier is formed (due to wash-over processes), while the exposed clay surface is impacted by the waves. Finally the volume of the chenier may decrease to such an extent that protective effects no longer exist. The erosion protrudes through existing and newly formed straight erosion channels and an indented erosion coast, characterized by bights and capes, develops. Cheniers may also impede the drainage of the landward mangrove swamps. This may cause the death of the mangrove vegetation by asphyxiation or hypersalinity of soil and/or water. As a result an Avicennia forest may change into an Avicennia “churchyard”. Afterwards a new pioneer vegetation may settle and a saltmarsh develops. This may gradually become a mangrove swamp again. There appears to be a close relationship between the various landscapes and the processes of sedimentation and erosion. This can be organized into a system which can be applied to the whole coast. It shows a cyclic alternation of deposition and erosion and the related landscapes, which gradually moves westward along the coast.