The Colônia structure, São Paulo, Brazil

Abstract

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.

Why deep drilling in the Colônia Basin (Brazil)?

“…The basin stratigraphy essentially comprises organic-rich and fine-grained sediments (silt and clay), with intercalations of sandy mud. The presence of Precambrian rock fragments in a matrixsupported conglomerate found at ∼ 350 m depth points towards a fanglomeratic deposit derived from the highest parts of the outer rim and is most probably related to the initial infilling of the depression (Riccomini et al, 2011). Riccomini et al (2011) discussed various alternative processes to explain the origin of the Colônia structure.…”

“…The presence of Precambrian rock fragments in a matrixsupported conglomerate found at ∼ 350 m depth points towards a fanglomeratic deposit derived from the highest parts of the outer rim and is most probably related to the initial infilling of the depression (Riccomini et al, 2011). Riccomini et al (2011) discussed various alternative processes to explain the origin of the Colônia structure. Impact cratering has remained the preferred agent (e.g., Velazquez et al, 2014) despite a lack of unambiguous evidence.…”

“…This impact hypothesis is supported by the overall geometry of the structure because the geology does not provide any other plausible explanation for the formation of such a distinct circular shape by endogenic or exogenic processes. Riccomini et al (2011) also noted that the current depth-to-diameter ratio is different from similarly sized and comparatively deeper known impact structures (e.g., Brent Crater, Ontario, attains a depth in the inner part of the basin of 1000 m). This was interpreted as indicating that the crater could have been substantially eroded since its formation.…”

“…1a), as it has remained a closed basin system ever since its formation. Based on geophysical data by Riccomini et al (2011), the continuous sedimentary infill could provide an extended and unique paleoenvironmental record for the Southern Hemisphere tropics and, in particular, for Atlantic tropical rainforest evolution. Much of what is currently known about the Colônia infill comes from sedimentological and palynological studies of the upper 8 m, which provided a record of 130 ka ( Fig.…”

Petrographic Investigation of Target Rock Transformation under High Shock Pressures from the Colônia Impact Crater, Brazil

“…The crater rim rises about 120 m with respect to the central lowland and is almost completely filled by younger siliciclastic sediments. Riccomini et al, 2011);2, 197 m, (Velázquez et al, 2013); 3, 304 m (this study)…”