Sismoestratigrafia de alta resolução da plataforma continental adjacente a cidade de Natal - Rio Grande do Norte (NE do Brasil)

Abstract

The mapping of subsurface features contribute to the understanding of chronostratigraphic boundaries associated with different depositional phases, as well as to record hydrodynamic conditions in the system. Thus, sediment availability, relative sea level changes and local circulation are controllers of the depositional environment and they are analyzed from seismic stratigraphy. Therefore, high resolution seismic data collected in a plataformal coastal system, specifically the Natal shelf (Rio Grande do Norte - Brazil), allowed to investigate the interaction between these factors during the last post glacial transgression until present day. The aim of this study is obtain seismic sections with satisfactory visual definition from processing routines applied to high resolution seismic data so that it is possible to map the chronostratigraphic limits and sedimentary deposits on the shelf during the Quaternary in order to understand evolution. Thus, 31 lines (transverse and longitudinal) distributed at 62 km along the inner shelf, collected with a boomer system that operates with energy of 300 J and frequency 1-2 kHz were used. The acquired data were processed and a processing flow was established that improved the signal-to-noise ratio of the seismic sections and the visualization of the reflectors. Through the analysis of the profiles three main horizons and their respective Seismic Units were identified, both related to the Holocene. An older regional discordance registers a Pleistocene/Holocene reactivation surface about average depth of 35 meters with deeper regions near the coast - a semi-confined environment. The next surface shows a great influence of the hydrodynamics in the shallower regions and begins the final stage of closing of the semi-confined regions. The subsequent, newer surface at approximately 18 m below modern sea level is related to a maximum flooding surface occurring at highstand after transgression. The units between the surfaces showed: a) high rate of deposition with high infilling of sediments in the semi-confined portions with medium-high energies at the beginning of the transgression; b) low sedimentation and low energy in a shallow marine environment with small excavations in the middle of the transgression; c) and lower energy environment with low rate of aggradation and low sedimentation after highstand. The morphology and depths of the discordances allow identify the events occurred. Sediment losses occur from the shelf to the continent in a regressive event and later, with the drown shelf in transgression, the sediments deposit as on the shelf as transported by longshore drift. In addition, the coastal zone has a greater influence on the sedimentation occurring on the shelf in ending transgression until present day. In this way, this research contributes to the understanding of the Holocene stratigraphic evolution of a shallow marine environment.