Variation of coastal dynamics during the last 7000 years recorded in beach-ridge plains associated with river mouths: example from the central Brazilian coast

The reconstruction of relative sea-level changes, during the last 7000 years, has shown that the central Brazilian coast was been subjected to submergence until approximately 5100 yr. B.P. However, the emergence after 5100 yr. B.P. was interrupted by two important fluctuations with an amplitude of approximately 2–3 m and a duration of 200–300 yr. This evolutionary history, which is quite different from that of several other regions in the world, played an essential role in the development of the central Brazilian coastal plains, whether or not they are associated with important river mouths. The periods of submergence, characterized by erosional phases, introduced noticeable changes in the geometry of the coastal deposits. The periods of emergence gave rise to sandy terraces covered by beach-ridges whose orientation is determined by the longshore drift. Elsewhere on a low sandy coast, the direction of longshore sand transport depends on the orientation of the swell. In a coast subjected to various swell patterns the efficient swells are defined as those which determine the resulting longshore transport direction. Such swells are not necessarily the most prevalent. For instance, along the central Brazilian coast the southern sector waves, in spite of their infrequent occurrences, are much more powerful than the northern sector waves and therefore generate a longshore sand transport from south to north. When fossil beach-ridges are present, as is the case on the central Brazilian coast, their geometry reflects the past directions of longshore sand transport. This makes it possible to determine the provenance of past efficient swells and to establish the past wind patterns. A detailed study of the beach-ridge geometry of some sections of the central Brazilian coast, showed a sequence of reversals of the longshore drift during the last 5100 yr, with a duration ranging from 10 to 100 yr. These reversals represent changes in the direction of effective waves, which determine the longshore transportation of sediments, and, consequently, indicate changes in the wind pattern.