Paleoweathering features in the Sergi Formation (Jurassic-Cretaceous), northeastern Brazil,and implications for hydrocarbon exploration |
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| Authors: | Cristina Pierini Ana M. Mizusaki Nuno Pimentel Ubiratan F. Faccini Claiton M.S. Scherer |
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| Affiliation: | 1. Museo Paleontológico Egidio Feruglio, Ave. Fontana 140, 9100 Trelew, Argentina;2. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA;3. Departamento de Ciencias Geológicas, Universidad de Buenos Aires, Ciudad Universitaria Pab. II, 1428 Buenos Aires, Argentina;4. Laboratorio de Paleopalinología, Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), Casilla de Correo 131, 5500 Mendoza, Argentina;5. Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina;1. Dipartimento di Biologia, Ecologia e Scienze della Terra, Università degli Studi della Calabria, Via P. Bucci, 87036 Arcavacata di Rende, CS, Italy;2. Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences, Waseda University, 1-6-1, Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan;1. Univ Lyon, Université Lyon 1, ENTPE, CNRS, UMR 5023 LEHNA, F-69622 Villeurbanne, France;2. Univ Lyon, Université Lyon 1, ENS-Lyon, CNRS, UMR 5276 LGL-TPE, F-69622, Villeurbanne, France;3. Mathematics and BioSciences Group, Faculty of Mathematics, Oskar-Morgenstern-Platz 1, University of Vienna, A-1090 Vienna, Austria;1. University of Tübingen, Department of Geosciences, Hölderlinstr. 12, 72074 Tübingen, Germany;2. Georg-August-Universität Göttingen, Geoscience Centre, Goldschmidtstr. 3, 37077 Göttingen, Germany;3. Carleton University, Department of Earth Sciences, 1125 Colonel By Drive, K1S 5B6 Ottawa, ON, Canada;4. University of Waterloo, Department of Earth & Environmental Sciences, 200 University Ave, N2L 3G1 Waterloo, ON, Canada;1. UMR 7329 GEOAZUR Dynamique Orogénique Bat 1 Ave A. Einstein 06560 Valbonne, France;2. Dept de Géologie, Université Jean Monnet UMR 6524, 23, rue du Dr P. Michelon, 42023 St. Etienne, France |
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| Abstract: | Paleoweathering in the Sergi Formation has been classified and analyzed to ascertain its origin and relationship with stratigraphic evolution. The Sergi Formation belongs to the pre-rift sequence of the Recôncavo Basin (northeastern Brazil) and comprises a complex association of eolian and fluvial sandstones and lacustrine mudstones. This formation can be subdivided into three depositional sequences bounded by regional unconformities. Four paleoweathering types, each one related to a distinct origin, have been described in the Sergi Formation: (1) textural mottling, which is distinguished by alternating rock colors as a result of the iron oxide mobilization within mineral phases that evolved under alternating oxidation (yellowish, brownish and reddish shades) and reduction (grayish or greenish hues) conditions; (2) non-textural mottling, which displays a discoloration pattern that is independent of the original rock texture; (3) carbonate concentrations, usually related to carbonate nodule formation, which display a massive internal structure that reveals their origin through continuous growth or crystallization; and (4) banded carbonates (silicified), associated with the beginning of regular surface formation due to the chemical precipitation of carbonates within lacustrine environments. Both mottling color motifs and carbonate accumulation usually represent groundwater oscillation rather than pedogenesis. Only carbonate intraclasts and banded carbonate (silicified) have their origin ascribed to pedogenesis sensu stricto, although the carbonate intraclasts do not represent soil deposits in situ, but calcretes eroded from areas close to channels, and the banded carbonates (silicified) have strong diagenetic modifications. Therefore, it is reasonable to assume that fluvial and meteoric water have controlled paleoweathering evolution as well as deposition, yet both aspects are ruled by the same mechanisms (relief, sedimentation rate and, above all, climate). |
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