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Abstract-The near-circular Coloˆnia structure, located in the southern suburbs of the megacity of Sa˜o Paulo, Brazil, has attracted the attention of geoscientists for several decades due to its anomalous character and the complete absence of any plausible endogenous geologic explanation for its formation. Origin by impact cratering has been suggested repeatedly since the 1960s, but no direct evidence for this has been presented to date. New seismic data have been recently acquired at Coloˆnia, providing new insights into the characteristics and possible layering of infill of the structure, as well as into the depth to the underlying basement. We review the current knowledge about the Coloˆnia structure, present the new seismic data, and discuss the existing-as yet still indirect-evidence for a possible origin by an impact. The new data suggest the existence of a sedimentary fill of approximately 275 m thickness and also the presence of two intermediate zones between sediment and basement: an upper zone that is approximately 65 m thick and can be interpreted as a possible crater-fill breccia, whereas the other zone possibly represents fractured ⁄ brecciated basement, with a thickness of approximately 50 m. Although this depth to basement seems to be inconsistent with the expected geometry of a simple, bowl-shape impact structure of such diameter, there are a number of still unconstrained parameters that could explain this, such as projectile nature, size and velocity, impact angle, and particularly the current erosion depth.

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Study of the clay mineral content of Oligocene lacustrine sediments of the Tremembé Formation and laterally correlative deposits of a marginal system of distal alluvial fans of the Resende Formation has furnished important indicators of paleoenvironmental and paleoclimatic conditions during the génesis of these minerais within the context of the Paleogene evolution of the Continental Rift of Southeastern Brazil. X-ray diffraction and scanning electron microscopy revealed clay minerais of the smectite and kaolinite groups and, to a much lesser extent, members of the mica and chlorite groups. Detrital clay minerais in the Tremembé and Resende Formations are mainly represented by smectites, generated under generally arid conditions probably during the Eocene. Ancient soils developed upon Precambrian basement rocks acted as remarkable source-areas for the smectites, although the conditions for their genesis are still unresolved. Eodiagenetic halloysite in deltaic sandstones of the Tremembé Formation was derived by decomposition of feldspar and is indicative of pluvial seasonality during sedimentation. Worm-like kaolinite was formed during a later diagenetic stage, without the influence of the depositional environment. The presence of kaolinite in the meandering fluvial system of the São Paulo Formation suggests a significam climatic change to humid conditions during the Late Oligocene. It appears, therefore, that the paleoclimatic and paleoenvironmental changes inferred from the clay mineralogy of the sediments studied here are in general accord with the pattern of global changes for the Paleogene as deduced from other geological and paleoceanographic evidence.

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