Cabo São Braz Lagoon (Angola): an example, of oceanic coastal evolution

Abstract

The São Braz Lagoon occupies a triangular bay located immediately to the north of the São Braz promontory, located itself about 200 km south of Luanda (Angola) along the African eastern coast of the Southern Atlantic. Our investigation demonstrated that the system of spit-bars separating the brackish lagoonal water from the open ocean is a very recent and rapidly evolving feature that first appeared in the seventies. A previous phase of coastline progradation (also ascribed to recent historical times) is shown by traces of an older lagoon, nowadays turned into a small sabka. The Presence, shape and current morphodynamics of the São Braz Lagoon appear clearly controlled by the profile of the coastline (i.e. the sharp bend occurring at the promontory) as well as by the SW winds that dominate the region (St. Helen Winds) and the swell phenomena they generate. The resulting northward re-distribution of sediments resulting from sea cliff erosion and fluvial inputs feed the construction of spit bars downdrift of the promontory’s head. Comparison of maps and aerial photos of different ages demonstrated that the present configuration of the lagoonal area is mostly the result of phenomena occurring during the last 20 years, while our repeated surveys indicate that it still suffers appreciable changes from one year to another. The depositional progradation and progressive closure of the bay that have dominated the recent decades does not represent the continuation of a longer term monotonous morphodynamic tendency. In fact, the morphological evidence we have collected are sufficient to state (notwithstanding the present lack of dates for the older stages of evolution) that the São Braz morphostructural bay has alternatively experienced periods of opening (with destruction of bars and spits and re-activation of the inner sea cliff) and periods of partial or total closure (with creation of lagoons, marshes and sabkas, accompanied by continental re-shaping of the abandoned sea cliffs). Such alternations were probably the consequence of the minor sea level fluctuations that occurred during the second half of the Holocene, but changes of the mean oceanographic conditions induced by climate occurring during the last millennia could also have played a major role.