SHATTER CONES CONFIRM THE VISTA ALEGRE METEORITE IMPACT STRUCTURE IN THE PARANA FLOOD BASALT PROVINCE, SOUTHERN BRAZIL

The Vista Alegre structure, centered at 25^o 57’S and 52^o 42’W, has been recently proposed as a meteorite impact structure due to findings of small clasts of shatter cones and possible microscopic evidence of shock metamorphism in breccias within the 9.5-km-diameter feature. The structure is located in the Paraná State of southern Brazil, within the Paraná Basin, which contains one of the largest and most extensive flood basalt provinces on Earth. The Paraná flood basalts belong to the Serra Geral formation and are temporally related to the opening of the Atlantic Ocean, having been dated at about 133-132 Ma. Tholeiitic basalts dominate, with some minor rhyodacites. The Vista Alegre structure has a circular outline (in the form of an incomplete ring of hills) and a central depression. The presence of a central uplift is not obvious, but is indicated by the limited occurrence of sandstones, possibly related to the Botucatu formation, that are normally at a stratigraphic depth of about 800 m. The structure must be deeply eroded, and is heavily vegetated and agriculturally used. There are limited outcrops of polymict breccias (some of them possibly melt-bearing). We report on the recent finding of extensive shatter cone occurrences in the form of large (and small) clasts of a fine-grained sedimentary rock within the polymict breccias. The shatter cone-bearing breccias are found at different locations within the structure, separated by several kilometers. The nested shatter cones range in size from about 0.5 to 20 cm for individual cones, and up to half a meter for complete assemblages. This finding clearly confirms the impact origin of Vista Alegre, which may thus be an important analog for impact craters in basalts on Mars and other planets.