Sedimentary facies and Holocene depositional evolution of the Maricá lagoon, Rio de Janeiro, Brazil

Abstract

Abstract This study aims to improve the knowledge on the geological evolution of the Marica Lagoon (Rio de Janeiro) during the Holocene. One hundred and three samples from four boreholes and GPR data were acquired. Laboratory analyses included grain size and morphology, color, organic matter contents, fossils, mineral composition, and 14C dating. The results indicate that the 12.5\xa0m thick sedimentary section studied here represents the last 8111 years of lagoonal deposition. Five sedimentary facies were identified: (1) subrounded sand facies (containing a gravel subfacies) which occur along the southern margin; (2) silty sand facies occurring at the transition from the margin to the lagoon center; (3) sandy silt facies and (4) silt/clayey silt facies which cover the calmer, central areas of the lagoon; and (5) angular sand facies which occur only at the surface of the northern margin. The organic matter content in the mud varied from 18% to 81%. Sedimentation rates up to 4277\xa0cal\xa0years BP in the western margin were 0.27\xa0cm/y and 0.15\xa0cm/y in the deeper, central lagoon areas, and 0.20\xa0cm/y since then. The sedimentary facies and organisms (molluscs and sponges) indicated that the Marica Lagoon has changed little during the Holocene. At about 8111\xa0cal\xa0years BP, the lagoon was an important freshwater basin with occasional saltwater influence. At about 5600–5000 years ago, during the maximum Holocene transgression, the lagoon attained its maximum dimensions and prevailing marine influences. Overwash processes and inlets enabled the formation of washover fans and flood-tidal delta deposits that have been recognized since the early formation stages of the Pleistocene barrier. From 4645\xa0cal\xa0years BP to the present day, the connection between the lagoon and the sea has occurred sporadically at the southeast extreme of the lagoon (Barra Lagoon). Today s sedimentary dynamics are influenced by fluvial input (which is building up a lagoonal delta) and by winds, which transport sands from the barrier into the lagoon, in addition to generating waves capable of remobilizing the marginal and bottom lagoon sediments.