Cerro do Jarau, Rio Grande do Sul: A possible new impact structure in southern Brazil

Cerro do Jarau is a prominent, ∼13.5-km-wide, circular landform rising >200 m above the plains of the "pampas" in southern Brazil. The name (meaning Jarau hills) comes from the prominent crests of silicified sandstones, which form a semiring of elevated hills in the northern part of the structure. The origin of this structure has been debated for decades, and varied suggestions of its formation include either endogenous tectonic processes or large meteorite impact. However, no conclusive evidence to support either hypothesis has been presented to date. This structure was formed in Mesozoic volcano-sedimentary rocks of the Paraná Basin and consists of the Jurassic-Cretaceous Guará (sandstones), Botucatu (sandstones), and Serra Geral (basalts) formations. The Botucatu Formation sandstones are intensely silicified and deformed, and were subject to radial and annular faulting. Our investigations at Cerro do Jarau identified the occurrence of parautochthonous monomict lithic breccia and polymict breccias resembling suevite and striated joint surfaces resembling crude shatter cones in sandstones and basalts. In addition, our first mineral deformation studies show the presence of rare planar features in quartz clasts in polymict breccias. The identification of these features at Cerro do Jarau, for the first time, is suggestive of an impact origin for the structure. If confirmed by further investigation of possible shock features, Cerro do Jarau would become the sixth known impact structure in Brazil, as well as the fifth basalt-hosted impact structure on Earth. © 2010 The Geological Society of America. All rights reserved.