The Santo Antonio Bank: a high-resolution seismic study of a deflected ebb-tidal delta located at the entrance of a large tropical bay, eastern Brazil

摘要

The Santo Antonio Bank (SAB) is a large sand accumulation on the inner continental shelf of Salvador City, next to Todos os Santos Bay (TSB) entrance, eastern Brazilian coast. Since the sixteenth century, chroniclers report this feature as a navigational hazard. Previous studies have tried to determine the origin and volume of sands stored within this feature without success. In the present study, high-resolution shallow seismic surveys were used to investigate the architecture, origin, and evolution of the SAB. One hundred seventy kilometers of high resolution shallow seismic lines was acquired during December 2014 and February 2015 using a Meridata (R) sparker system operating on frequencies between 0.3 and 1.5 kHz, with a maximum 200 J of energy. Three major stratigraphic units were identified: the bedrock, a transgressive unit, and a highstand unit. The bedrock comprises sedimentary rocks of the Northern Camamu Sedimentary Basin and high-grade metamorphic rocks of the Precambrian basement. The transgressive unit is interpreted as the result of deposition in an estuarine environment infilling incised valleys. The highstand unit represents the SAB itself. A well-preserved proto-bank exists at the core of the SAB marking the early stages of bank development. This proto-bank drowned in place possibly during the 8.2-ka event. Present-day SAB and tidal prism volumes show that the bank reached a maturity phase of development. The volume of siliciclastic sands stored in the SAB is 5.98 x 10(8) m(3). The SAB is potentially a major source of sands for beach nourishment for the urban beaches of the city of Salvador and metropolitan area, thus increasing the resilience of the city to future sea-level rises.