We conducted natural‐source audiomagnetotelluric (AMT) soundings at 31 stations along a 215 km profile in the Paraná Basin (South America) across the previously postulated Araxá‐Rio Grande alignment. Our objective was to study the resistive structures of the upper layers of the sedimentary sequence; our results made it possible to map the surficial layer of sediments of the Bauru group, a layer that reaches thicknesses of up to 250 m. We concluded that AMT soundings have reasonably good potential for groundwater exploration in the region. The varying thicknesses of Bauru group sediments and the irregularities of their contact surface with resistive volcanic rocks of the Serra Geral formation along the profile were not known previously. Our AMT profile crossed an important positive gravity anomaly located near the Rio Grande (a natural boundary between the states of São Paulo and Minas Gerais, in Brazil) which has not been fully interpreted. Five sites for deeper MT soundings, which may throw light on possible causes of the gravity anomaly, were selected based on the conclusions from the AMT survey.