Geophysical investigation of the Colônia structure, Brazil

Abstract

The origin of the nearly circular Colônia structure, located at the southwestern edge of the city of São Paulo, Brazil, has been the subject of a long‐standing debate, ever since the 1960s when the structure was first investigated by geophysical methods. The structure still raises interest for geological research, as its sedimentary infill holds important paleoclimatic information about the evolution of the tropical rainforest, as well as the interplay between the South American summer monsoon, the Intertropical Convergence Zone, and the southern Westerly wind belt—for possibly as long as several million years. In addition, the search for evidence to conclusively establish the origin of this structure continues, and the answer most likely lies in the lower portions of the basin's sedimentary infill, which also holds a significant potential for underground water resources. Here, we present the results from recent seismic (reflection and HVSR), gravimetric, and geoelectrical surveys. They have provided a reliable image of the sedimentary infill, and the maximum depth to basement within the structure has been constrained consistently by the different methods to approximately −400 m. The geophysical data have also allowed to map the lateral contact between the crystalline basement rocks and the sedimentary infill, which indicates a diameter of approximately 2.8 km for the sedimentary basin, with 3.6 km being the diameter of the outermost limit of the structure. A total of six seismic stratigraphic boundaries were identified within the sedimentary infill, providing a framework for the planning of a deep drilling campaign and subsequent sampling program aimed at geological and paleoclimatology studies.

Chronostratigraphy of a 1.5±0.1 Ma composite sedimentary record from Colônia basin (SE Brazil): Bayesian modeling based on paleomagnetic, authigenic 10Be/9Be, radiocarbon and luminescence dating

“…Indeed, a conservative extrapolation using derived sedimentation rates gives an age comprised between 5.3 and 11.2 Ma (median age: 7.2 Ma) for the base of the Colônia basin sedimentary infill at ~280 m (excluding the ~120 m interval of the putative breccia layer; Prado et al, 2019), which is fully coherent with previous estimation between 5 and 36 Ma (Riccomini et al, 1991(Riccomini et al, , 2011. The Colônia basin sediments have therefore a strong potential for documenting long-term behavior of the SASM and regional paleoenvironmental responses of the Atlantic forest to major global paleoclimatic variations and reorganizations that occurred within the course of the Mio-Pleistocene time period.…”

“…1). It is located on the Paulistano Plateau (South of São Paulo city) at an altitude of about 800 m. The most accepted hypothesis of its origin is that of an impact crater, although not yet demonstrated (Riccomini et al, 1991;Velazquez et al, 2013;Prado et al, 2019), dated between 5 and 36 Ma. This coarse estimate is based on the apparent deformation of imbrication contact from a regional outcrop, alongside structure diameter, filling and preservation state (Riccomini et al, 1991(Riccomini et al, , 2011.…”

“…Seismic investigations within the Colônia structure indicate a sedimentary basin of 2.8 km diameter with a maximum sedimentary filling comprised between 250 and 370 m (Neves 1998;Riccomini et al, 2011;Prado et al, 2019). The seismic stratigraphic model proposed recently by Prado et al (2019) comprises three main sedimentary units below the bog (Riccomini et al, 1991;Ledru et al, 2005Ledru et al, , 2009. A first lacustrine unit of ~180 m thickness composed of organic-rich clay with few sandy mud layers deposited in humid conditions (Riccomini et al, 2005(Riccomini et al, , 2011.…”

“…A first lacustrine unit of ~180 m thickness composed of organic-rich clay with few sandy mud layers deposited in humid conditions (Riccomini et al, 2005(Riccomini et al, , 2011. A second ~70 m lacustrine unit composed of sandy muds with grains and pebbles of quartz deposited under drier conditions during the initial sedimentary filling of the depression (Prado et al, 2019). A lowermost unit approximately 120 m thick possibly includes an impact breccia layer.…”

“…A lowermost unit approximately 120 m thick possibly includes an impact breccia layer. Altogether, Prado et al (2019) proposed a total depth of ~400 m (considering uncertainties on data calibration) from seismic and gravimetric studies. Cores previously drilled in the Colônia basin for groundwater exploration and palynological studies allow first insights into sediments composition (Riccomini et al, 1991 and provide a tenuous age estimate of ∼2.6 Ma for the formation of the basin.…”