Structural control of HM placer deposits at RGS (Brazil) Coastal Plain using ground penetrating radar

This paper presents and discusses the structural control on heavy mineral (HM) placer deposits in the Coastal Plain of Rio Grande do Sul State (Brazil). The investigation was mainly based on GPR surveys and drill-holes sections. High resolution images were also used to evaluate the sedimentation process, and the placer control and development. The RGS Coastal Plain is the immersed part of the Pelotas Basin, which was developed from Barremian/Aptian on, after Gondwana rupture. The actual geomorphic and stratigraphic configuration of the RGS Coastal Plain is due to a deltaic alluvial plains and four lagoons/barrier systems developed as consequence of sea level changes from Middle Pleistocene to Holocene. The HM placer deposits are present in areas between the Upper Pleistocene (121.5 ka) and Holocene (5.6 ka) barriers. GPR surveys were acquired perpendicular to shoreline and parallel to drill-hole sections in the Bujuru District area. GPR survey sections revealed that a holocenic peat layer equivalent radar-facie underwent normal fault displacement (synthetic and antithetic geometry) and arcuate close to faults (drag folds). The vertical fault displacement in the Bujuru District area amount 1-7 m. The rotational faulting developed a sequence of domino-like blocks, stablishing elevated crest lines seaward and depressions inward. The fault depressions controls aeolian sedimentation, which hosts actual HM placer deposits.