HR: 0800h
AN: GP21A-06    [Abstracts]
TI: Magnetostratigraphy of the Neogene sediments of SW Uruguay
AU: * Sanchez Bettucci, L
EM: leda@fcien.edu.uy
AF: Dra. Leda Sanchez Bettucci, Dpto. de Geología, Facultad de Ciencias, UDELAR,Igua 4225, Malvin Norte, Montevideo, CP 11400, Uruguay
AU: * Sanchez Bettucci, L
EM: leda@fcien.edu.uy
AF: Cristina Bertoni, UFRGS, CNPq/CAPES, Av. Bento Gonçalves, 9500 CEP 91509-900., Porto Alegre, Brazil
AU: Orgeira, M J
EM: orgeira@fcen.uba.ar
AF: Dra. M. Julia Orgeira, Laboratorio de Paleomagnetismo D.A. Valencio, Ciudad Universitaria, Buenos Aires, 1428, Argentina
AU: Sanchez, G
EM: geogonza@gmail.com
AF: Dra. Leda Sanchez Bettucci, Dpto. de Geología, Facultad de Ciencias, UDELAR,Igua 4225, Malvin Norte, Montevideo, CP 11400, Uruguay
AU: Bertoni-Machado, C
EM: cristina.bertoni@bol.com.br
AF: Cristina Bertoni, UFRGS, CNPq/CAPES, Av. Bento Gonçalves, 9500 CEP 91509-900., Porto Alegre, Brazil
AU: Farina, R
EM: fari~a@fcien.edu.uy
AF: Dra. Leda Sanchez Bettucci, Dpto. de Geología, Facultad de Ciencias, UDELAR,Igua 4225, Malvin Norte, Montevideo, CP 11400, Uruguay
AB: Preliminary results on the magnetostratigraphy of three Neogene formations from SW Uruguay are presented: Camacho, Raigón and Libertad, and a relative age from the record of reversions in the Earth's magnetic field polarity is proposed. The sediments outcrop in the SW region of Uruguay,and have been received attention due to their fossil contents. The oldest is the late Miocene-Pliocene Camacho Fm, of Huayquerian to Montehermosan affinities. A Pliocene-early Pleistocene age has been assigned to Raigón Fm, of Chapadmalalan affinities. The overlying Libertad Fm has been considered early-middle Pleistocene in age, and to have Marplatan-Ensenadan affinities. The lithology of this facies of Camacho Fm is identified by the presence of fine to very fine sandstones and is composed of silty-sandy and bioturbed silty deposits. The marine facies of this formation is rich in fossil content, as several invertebrate and vertebrate taxa are found. The upper Raigón Fm is formed by sandstones of varied grain size and includes lenses and levels of claystones and conglomerates. It shows the sedimentological features of a deep, pebble-rich braided fluvial system. The 383 vertebrate specimens found in the sandy facies and studied in a taphonomically-oriented study belong to 19 genera and 13 higher taxa. The bones are disarticulated, with smooth fractures and little weathered or abraded, which is congruent with a short time of contact, as observed in strong and sudden flows. The elements belong to the three Voorhies groups and fractured specimens are found along with well preserved materials, implying different taphonomical histories and reworking. The consequent inferred time averaging urges caution at using these remains for defining precise ages. The Libertad Fm, top of the sequence, is conformed by greenish clays, clayey fine sandstones, medium sized sandstones and conglomeratic levels, corresponding to deposits of continental origin under a semiarid climate, which allowed the accumulation of fine material through aeolic transport. Several genera of mammals have been found in that formation. Three localities were sampled; Mauricio, Arazatí and Kiyú. A reverse polarity was found for the specimens of the Camacho Fm sampled in the locality of Mauricio, assignable to the Gilbert magnetic zone. The sediments of Raigón Fm have normal polarity, both in those specimens from Mauricio and in those from Arazatí, interpreted as belonging to Gauss magnetic zone. The samples from Kiyú of Libertad Fm showed a reverse magnetic polarity, which we refer to Matuyama magnetic zone. However, in the samples of this Formation taken in the locality of Medina, a normal magnetic polarity was found, which suggests they correspond to Brunhes. The chronology is roughly compatible with that based on the fossil contents of those formations, although several details must be worked out.
DE: 1520 Magnetostratigraphy
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly