CONTENTS – R through S The Colônia Crater , a Probable Impact Structure in Southeastern Brazil

About 35 km from São Paulo, near Colônia, there is a conspicuous ring feature, 3.64 km in diameter, centered at 23 o 52'S, 46 o 42'20"W, within Precambrian crystalline basement rocks, and defined by a hilly circular outer rim up to 125m higher than an inner alluvial plain presently occupied by a swamp. The crater is filled with organic-rich clayey sediments of Quaternary age. Seismic survey indicated a maximum depth of 450m to the Pre-cambrian crystalline substratum within the crater [1], in accordance with magneto-telluric data [2] and inferences based on morphological parameters [3,4]. A zone with low seismic velocity at the top of the basement may suggest the presence of brecci-ated and/or fractured rocks [1]. Paleogene sediments are tectoni-cally imbricated within Precambrian gneisses of the basement in a low-angle fault zone one kilometer southward from the structure and this situation could represent a part of the overturned rim. The presence of higher altitudes in the southwestern part of the rim suggests a body trajectory from northeast before the pre-sumable shock. The hypothesis that this feature is a probable impact structure has long been postulated, ever since the first studies carried out in the area [5]. Further studies based on morphological parameters [3,4] and geophysical data [1, 2] have reinforced this idea. Alternative hypotheses to the origin of the Colônia crater, such as sinkhole, structural interference pattern, intrusion and phreato-magmatic structure related to a kimberlite, among others, may be rejected, respectively, by the absence of carbonate rocks in the region, the persistence of the ENE structural trend of the basement , the lack of structures and/or minor intrusive bodies (dikes, sills) that should be associated with such intrusions, and the excessively unusually large dimension for a kimberlite pipe. Despite the above arguments, no direct evidence of an impact, such as shock metamorphism, has yet been observed in the area, most likely due to the intense weathering. The peculiar circular shape and the typical depth/diameter ratio [3], as well as the semi-circular outcrop patterns of the Paleogene sedimentary rocks along the southern and southeastern inner part of the rim, are the main indicators of a impact structure. The lack of convincing proof of endogenous process also reinforces this hypothesis. Nowadays, the Colônia crater is a probable impact structure with ejecta removed, rim partly preserved, although deeply eroded, and crater-fill products preserved. Based on palynologi-cal data [6,7], the impact age is probably …