The role of beachrocks on the evolution of the Holocene barrier systems in Rio de Janeiro, southeastern Brazil

Abstract

Silva, A.L.C., Silva, M.A.M., Souza, R.S., Pinto, M.L.V., 2014. The role of beachrocks on the evolution of the Holocene barrier systems in Rio de Janeiro, southeastern Brazil. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 170–175, ISSN 0749-0208.Beachrocks at different locations along Rio de Janeiro coast (southeastern Brazil) play an important role in the understanding of coastal evolution and sea-level fluctuations in the Holocene. The studied area presents a long and wide stretch of coastal plain characterized by two sandy barriers, which confine a series of small isolated chain-like lagoons in addition to large lagoons on the reverse side of the inner barrier. The beachrocks occur at different positions in relation to present-day mean sea level. These can be observed as submarine outcrops and along the beaches. The 8.100 years old B.P. beachrocks reveal a phase of retrogradation of the barrier system, and the drowning and partial preservation of this barrier during the sea transgression through the Holocene. The beachrocks are composed of medium-to coarse sand grained bioclastic quartz-rich sandstones which are strongly cemented by calcite. Three successive events of calcite cementation have drastically reduced the porosity of the rock. Calcite cement occurs as three basic forms: isophacous fringe of very fine crystalline calcite, aphanocrystalline calcite and coarsely crystalline calcite. Each textural type indicates different geochemical composition of the pore waters during the very early diagenetic evolution within the intertidal zone of the coastal barrier system.