INSIGHTS INTO THE MORPHOLOGY, GEOMETRY, AND POST-IMPACT EROSION OF THE ARAGUAINHA PEAK-RING STRUCTURE, CENTRAL BRAZIL

Abstract

The 40-km-wide Araguainha structure is the largest and best-exposed complex impact crater in South America. It was excavated in flat-lying sediments of the intracratonic Paraná Basin, where target rocks are well exposed and have remained undeformed since the impact event ca. 245 Ma. Despite the excellent state of preservation and exposure, information available on the target rock stratigraphy, post-impact erosion, and morphology of the structure is limited. Our combined field observations and remote sensing analysis demonstrate that Araguainha preserves all features of a shallowly eroded peak-ring crater. The interior of the structure exposes a central peak surrounded by a 5-km-wide annular basin and two main ring features 10-12 and 14-18 km from the center. Analysis of the pre-impact stratigraphy, present morphology, and crater dimensions indicates that excavation related to the transient cavity formation was extensive in the annular basin, but minimal to nonexistent beyond the inner ring feature. The formation of the inner ring feature can be reconciled with compressional stresses during outward collapse of the central uplift and inward slumping of the crater walls. Regional stratigraphic data combined with field observations indicate two periods of post-impact erosion associated with exhumation of the Paraná Basin. We estimate that 250-350 m of fallback deposits and target rocks have been removed by erosion. Erosion also accounted for removal of the proximal ejecta immediately outside the rim of the structure. In contrast to previous suggestions that the impact took place in a shallow-marine environment, our observations are more consistent with an extremely shallow, brackish to freshwater lacustrine environment, with little effect on the developing impact crater.