Rio Grande Relay Zone, Rio Grande do Sul Coastal Plain, Brazil: I—Retiro-Estreito Main Fault and Sedimentation on Subsidence

Abstract

This paper shows the results of geophysical and geological surveys carried out in the southern tip of the Retiro-Estreito Main Fault, and important fault scarp extending more than 60 km NE in the Rio Grande do Sul Coastal Plain (RGSCP). The investigated areas include mainly the Ilha dos Marinheiros (Rio Grande County) and Retiro (São José do Norte County). Ground Penetrating Radar (GPR, 50 MHz RTA antenna), drillholes for heavy minerals (HM) exploration and fieldwork were applied to investigate deformational structures controlling the faulted Pleistocene sedimentary units (basement) and the Holocene sedimentary radarfacies geometry and their inner reflectors architecture. The Retiro-Estreito Main Fault is part of a group of recently recognized gravity sliding tectonic structures. The radargrams enabled to distinguish that the Retiro-Estreito Main Fault shows strain partitioning into a sequence of normal splaying faults, giving rise to a stepped structure in the hangingwall block toward the east. The Retiro-Estreito Main Fault footwall block was later faulted by propagating splay fault from the southern Quinta Main Listric Fault and gave rise to the Retiro Horst. Diachronous fault displacements, each one showing different displacement magnitude and chronology, gave rise to complex fill pattern radarfacies, mainly in the first sedimentary stages (beach ridges) in the hangingwall fault block. After the Retiro Horst was built up, significant beach ridges radarfacies structural pattern is developed, followed by Retiro Horst and footwall block erosional degradation, building up backshore and washover radarfacies in the footwall block, and foreshore radarfacies in the hangingwall block of the Retiro-Estreito Main Fault. These sedimentary units were deposited still under a high mechanical subsidence rate. The vanishing period of the normal fault’s displacements (final sedimentary stage) is characterized by widespread thin transgressive dune sheet (TDS) cover and by transgressive dunes (TD) prograding southwestward to isolate the Retiro-Estreito lagoon, and to build up the main Holocene sedimentary barrier close to the Atlantic Ocean. The Retiro-Estreito lagoon shows evidence of advanced clogging processes, such as erosional degradation of fault scarp and TD + TDS southwestward progradation.