New magnetic Polarity Stratigraphy of the Mae Moh Basin in northern Thailand, and Implications for the Age of the First Miocene Hominoids

HR: 10:50h
AN: GP52A-03 [Abstracts]
TI: New magnetic Polarity Stratigraphy of the Mae Moh Basin in northern Thailand, and Implications for the Age of the First Miocene Hominoids
AU: * Benammi, M
EM: mouloud@igeofcu.unam.mx
AF: Instituto de Geofisica, Depto de Paleomagnetismo y Geofisica Nuclear, Universidad Nacional Autonoma de Mexico, CU, Mexico, D.F 04510, Mexico
AU: Coster, P
EM: paulinecoster@yahoo.fr
AF: Lab. Géobiologie, Biochronologie et Paléontologie humaine, UMR CNRS 6046, Bāt Sci. Naturelle, 40 av. Recteur Pineau - Poitiers, Poitiers, F86022, France
AU: Jaeger, J
EM: jean-jacques.jaeger@univ-poitiers.fr
AF: Lab. Géobiologie, Biochronologie et Paléontologie humaine, UMR CNRS 6046, Bāt Sci. Naturelle, 40 av. Recteur Pineau - Poitiers, Poitiers, F86022, France
AU: Chaimanee, Y
EM: jean-jacques.jaeger@univ-poitiers.fr
AF: Geological Survey Division, Department of Mineral Resources, Rama VI Road, Bangkok, 10400, Thailand
AB: This magnetostratigraphic study has been conducted on the Miocene Mae Moh basin, in the Lampang province, Northern Thailand. 194 paleomagnetic samples were collected from 65 stratigraphics levels from Na Khaem and Huai Luang formations. The studied sections are mainly composed of clay, sandstone and lignite. Rock magnetic experiments revealed that titanomagnetite, magnetite and hematite are the mains carriers of magnetisation. Samples subjected to progressive thermal or alternating field demagnetization procedures exhibit a low stability and a high stability component, with either normal and reversed polarities. The reversal test is positive and indicates that the characteristic remnant magnetization directions correspond to the primary magnetization directions (McFadden and Mc Elhinny, 1990). The mean direction calculated for section 1 are: incl : 23.24 and decl. : 5.01 and incl. : 31.22 et decl. : 7.01 for section 2. These results don't document any rotation with respect to previous study (Benammi et al., 2002). In total, nine polarity zones (four normal and five reverse), that can be reliably be correlated the geomagnetic polarity time scale developed by Gradstein et al, 2004, are recognized from the studied sections. Based on the biochronological constraints, the magnetostartigraphic column of the Mae Moh formations correlates best with chron C5ACr-C5r.3r, between 14.1and 12 Ma. This correlaton revealed a mean sedimentation rate of approximately 21 cm/ky, and a age of 12.7 et 13.2 for for the fossilferous levels (J5, K1, K2) where the mammals remains were found. The analysis of the elements traces spectrum of two ash levels coming from the basins of Mae Moh and Chiang Muan made it possible to establish a new correlation of the Chiang Muan sequence with the GPTS. This correlation prove that the age of the Chiang Muan sequence would be between 13.1 and 12 Ma, and the fossiliferous levels with hominoid (Khoratpithecus Chiangmuanensis) would be dated between 12.2 and 12.4 Ma for the upper lignite and between 13 and 12.8 My for the lower lignite.
DE: 0444 Evolutionary geobiology
DE: 1135 Correlative geochronology
DE: 1520 Magnetostratigraphy
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly