Imageamento com GPR das geometrias internas de depósitos fluviais em rochas da Formação Barreiras, Praia de Cacimbinhas/RN

Abstract

This study comprises a geophysical survey utilizing the Ground Penetrating Radar (GPR) method, conducted along siliciclastic rocks outcropping at Cacimbinhas Beach, in the municipality of Tibau do Sul, on the southeastern coast of Rio Grande do Norte state, Brazil. These rocks are often found in the form of live cliffs, arranged along the coastline (CL) throughout the region. The stratigraphic framework of the area is synthesized by units of the Barreiras Formation (Miocenic age) and the "Supra-Barreiras" covers, represented by the Potengi Formation and the "Dunes Field". However, the focus of these research is the Barreiras Formation, as regional studies point to the occurrence of fluvial deposits associated with this lithostratigraphic unit, but it still lacks a better characterization in terms of its geophysical signatures, the internal geometry of the fluvial deposits, architectural elements, and geometric parameters (width and thickness) of the studied deposits. Thus, the main objective of this work was to apply the GPR method to image, identify, and obtain geometric parameters of the architectural elements of depositional geometries of fluvial siliciclastic rocks that occur in the southern coastal region of Natal and neighboring cities. To this end, 2 GPR lines were processed and interpreted, with the assistance of the ReflexW version 8.1 and CorelDRAW version 22.0.0.412 software, respectively. They were acquired with a SIR-3000 model, manufactured by Geophysical Survey Systems (GSSI), with a 200 MHz antenna, one approximately N-S oriented (parallel to the CL) and the other E-W oriented (perpendicular to the CL), in addition to using complementary geological data (columnar section) for a more accurate identification of the subsurface sedimentary facies. Based on the terminations and internal pattern of the reflections, 4 radarfacies were defined, with 3 associated with the rocks of the Barreiras Formation (RF2, RF3, and RF4) and 1 with the aeolian deposits that cover such lithotypes (RF1). The identified architectural element was the Channel (CH) and downstream accretion deposits (DA), and the bounding surfaces (SL) separating lithosomes of different physical and temporal scales are represented by SL of at least 4 distinct orders. Concerning the geometric parameters, it was noted that the imaged fluvial channels have an average width (W) of 52.69 m, thickness (T) of 2.11 m, and a W/T ratio of 24.40. The results obtained are consistent with the model of a braided fluvial system for the study area, composed of multi-episodic and multilateral sandy complexes.