Estratigrafia, morfodinâmica e evolução de um esporão arenoso em ambiente macromaré: Ponta da Areia, São Luís-MA

Abstract

The Ponta da Areia is an oceanic beach located on the eastern shore of São Marcos Bay, in the Coastal Zone of São Luís city, being dominated by a macrotidal with variation of up to 6 meters. The Ponta da Area beach like so many other beaches in Brazil and around the world is subject to coastal erosive processes. As a mitigation measure of the erosive processes in this border, a coastal barrier was built along the coast line, totaling 560 m of rockfill. This construction resulted in an artificial beach/antepractic development. The Holocene origin and evolution of this coastal subject is linked to the formation of a headland spit barrier anchored in the cliffs of the Alcântara Formation. This sandspit develops towards WSW as a result of sediment longshore drift of sediments. Its extension is interrupted by the hydraulic barrier effect caused by the tide prism at the mouth of the Anil River. This work aims to establish an evolutionary model for the holocene coastal barrier system as well as the stratigraphic recognition of anthropic and natural changes in the sandyspit of Ponta da Areia. In order to reach its objective, monthly monitoring was carried out through topographic profiles in sectors exposed to erosion (P1, P4 and P5) and accretion (P2, P3 and P6) and wind transport and longshore currents. Three geological drills (percussion and vibrocore) were also performed, two in the progradational beach compartment and one in the retrograded beach compartment (erosive). The SP1 record obtained 4.20 m of sedimentary data intercepting at its base sandy facies with litoclastic gravel analogous to a transgressive lag, bounded at the top by a wave raving surface (SR). This surface represents the erosive stages of this coast when the engineering construction (groin/coastal barrier) was not yet present and the headland spit barrier system retrograded towards the continent over a lagoon complex and its backbarrier. Above these facies, the progradational phase of the coastal barrier was triggered by the construction of the groin and overlapping SR within thin shore face sands (foreshore / shoreface). The SP2 drill represents the continuity of the transect towards the ocean and the offshore end of the coastal barrier. Its sedimentary data is 4.90 m and includes the most recent progradational phase of the coast. It intercepts two sedimentary facies one from the lower shoreface in the basal position with intercalations of silt lenses and upper facies related to beach/shoreface. The SV1 drill recorded 3 m of registration including the evolution of a retrogradation barrier lagoon system about 7,240 ± 30 cal. years A.P. caused by the migration of the headland spit barrier from Ponta da Areia towards the mainland. This coastal barrier morphotype has always been found anchored on a system of cliffs, but it evolved retrograding to a lagoon complex of backbarrier, the old Jansen lagoon. In an evolutionary context, the old paleolagoon was filled by the retrogradation process, originating a small river, called “igarapé” (Igarapé da Jansen), which in the 70s was artificially flooded giving rise to the current Jansen Lagoon. The two testimonies in the progradational phase of the Ponta da Areia beach record the moment when the beach system was in a place previously characterized by retrograde/erosion, while the SV1 drill records the original behavior of this coastal subject, which is, retrogradational/erosive. The anthropic intervention caused by the construction of the groin has prevented coastal erosion in a certain sector of the beach until then, while there are still sectors that are still exposed to erosion and destruction of the longshore.