The São Francisco strandplain: a paradigm for wave-dominated deltas?

Abstract

Listen

Translate article

Abstract The São Francisco ‘delta’ (total area 800 km 2 ) has been considered for more than two decades as a paradigm for wave-dominated deltas. Although it is frequently cited in the literature, there is a paucity of published data about this sedimentary feature. At the time the modern models of deltaic sedimentation were proposed, many of those deltas receiving modifiers such as ‘wave-dominated’ and ‘tide-dominated’ were very poorly understood. Circularity of reasoning did the rest. Inevitably phenomena described for well-studied areas such as the Mississippi River (i.e. dip-feeding and autocyclicity) were uncritically extrapolated to other areas and assumed to be valid for all other members of the socalled delta category. There are three major aspects that make sedimentation in the São Francisco strandplain very different from the classical deltas. (i) Sea-level history for the east coast of Brazil is characterized by a maximum around 5100 years ago when sea level reached 5 m above the present level. This sea-level history has exerted a major control on the evolution of the São Francisco Quaternary plain. During the 5.1 ka highstand the coastal plain was drowned and a barrier island/lagoonal system formed. When sea level dropped afterwards the shoreline prograded and Holocene beach ridges were deposited. As a result, sand body distribution is intimately controlled by sea-level history. (ii) In wave-dominated environments, the fluvial effluent behaves effectively as a groyne, retaining in the updrift side sediments transported shorewise by the longshore currents. This phenomenon creates a clear asymmetry in facies distribution at the Quaternary plain. Whereas the updrift side is characterized by a continuous sand sheet, progradation on the downdrift side occurs by incorporation of sandy islands (reworked river-mouth bar sediments) that extend coastwise, protecting small lagoons rapidly filled by suspended sediments. This manner of sediment dispersal is also reflected along the entire shoreline and inner shelf in the vicinity of the river mouth. Therefore, the São Francisco ‘delta’ is not simply a dip-fed system. In the updrift side progradation of the shoreline is accomplished with sediment contribution from outside sources. (iii) Autocyclic processes such as changes in the lower river course (the delta cycle concept) have not been observed for the São Francisco strandplain.