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Lucy Gomes SantAnna, University of Sao Paulo - School of Arts, Sciences and Humanities (Brazil)
M. Dolores Carvalho, Petroleo Brasileiro S.A. (Brazil)
Alcides Nobrega Sial, Federal University of Pernambuco - Institute of Geosciences (Brazil)
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In southeastern Brazil, the Paleogene continental sedimentary record is nestled in the Continental Rift of southeastern Brazil (CRSB), whose sedimentary filling is mainly represented by Eocene-Oligocene mudstones (smectitic clayey matrix containing variable proportions of quartz and feldspar) of alluvial fans of the Rio Frio (in the Itaboraí basin), Resende (Taubaté, Resende and São Paulo basins) and Guabirotuba (Curitiba basin) formations. Only in the Rio Frio Formation occur Paleocene basal travertines formed in hot springs and derived from Precambrian marbles of the basement; the other stratigraphic units have mainly Precambrian gneisses in their basement.
The Eocene-Oligocene mudstones are closely associated with pedogenic calcretes, whose dominant morphological type is nodular calcrete. The nodules (2 -10 cm in diameter) consist of dense, red luminescent micritic to microsparitic calcitic matrix, containing floating, etched grains, as well as yellow luminescent sparry calcite-filled cristallarias and microfractures. The matrix of nodules from Rio Frio Formation is more enriched in strontium and depleted with respect to Fe, Mn, Zn, when compared with that ones of nodules from Resende and Guabirotuba formations. The stable isotope data obtained for the matrix also showed heavier dC13 (∼ -3.5‰ PDB) values for nodules from Rio Frio Formation than that ones of nodules from Resende and Guabirotuba formations (dC13PDB values between -10.73 and -7.49‰). Despite that these nodular calcretes have similar petrographic characteristics, host materials (smectitic mudstones) and origin (pedogenic process in the vadose zone), they show geochemical differences which are attributed to the influence of geological setting. In the case of Rio Frio Formation, the nodular calcretes inherited their geochemical characteristics, in terms of trace elements contents and isotope signature, in part from the Paleocene travertines. (Financial support by FAPESP).
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