Radar de penetração no solo aplicado à caracterização de estruturas tectônicas miocênicas e quaternárias no leste da ilha do Marajó (PA)

Abstract

There are several inferences about the occurrence of neotectonic reactivations in the Marajo Island, but studies aiming to demonstrate the unambiguous presence of tectonic structures are still lacking. The goal of this work is to record these structures in the shallow subsurface in the Eastern portion of this island applying an electromagnetic geophysical method using ground penetrating radar (GPR). Analysis of radar sections led to the recognition of nine reflection categories, designated as: chaotic, divergent, parallel to subparallel, undulating, hyperbolic, channel, mound, lack of reflection, and stratigraphic surface. The latter reflectors are of high frequency and amplitude, and are correlated along the entire study area, which led to its usage as a stratigraphic marker between two radar units. The combination of radar data with geological information allowed us to conclude that the lower unit corresponds to Miocene deposits of the Barreiras Formation and the upper unit corresponds to the post-barreiras sediments of Late Pleistocene-Holocene age. The stratigraphic surface reflects an unconformity at the top of the Barreiras Formation, which is highlighted in the radar sections by ferruginous concretions associated with a lateritic paleosol of regional extension. Analysis of radar sections showed that the strata from the Barreiras Formation are strongly displaced by vertical to subvertical faults, including normal faults and flower structures, the latter related to transcurrent deformation. These structures continue upward, affecting the post-barreiras sediments, though at a lower degree. The reflectors related to the unconformity at the top of the Barreiras Formation disappear rapidly to the west and north of the study area. The replacement of this unconformity by Quaternary strata led to the conclusion that those localities experienced subsidence. The subsiding areas became sites for renewed sedimentation during the Quaternary. In addition to faults, the base of the Barreiras Formation contains widely undulatory reflectors, which were related to synclines and anticlines. The occurrence of these features only at the base of the Barreiras Formation allows to formulate the hypothesis that deformation by compression has occurred still during the sedimentation of this unit, i.e., in the Miocene. The association of faults and folds in the Miocene strata, as well as of faults in the Quaternary sediments, probably reflects reactivations of transcurrent faults linked to the latest evolutionary stages of the rift systems developed along of the Brazilian Equatorial passive margin.