Paleontology and magnetostratigraphy of the Eocene Krabi basin (Southern Thailand)

Abstract

Excavations undertaken since several years in the Eocene sites of the Krabi basin (southern Thailand) has yielded six genera of reptiles and 31 different species of mammals, among which are several forms new to Southeast Asia. Nearly all vertebrate remains have been collected from lignite levels, and their study indicates a rather closed area submitted to a tropical climate. In addition to considerably increase our knowledge ofthe early Tertiary terrestrial vertebrate faunas of southern Asia, these Thai remains allow to propose an accurate age for this mammal fauna. They also can be related to known late Eocene and/or early Oligocene forms from Europe, North Mrica, Asia, or even North America. The study of the evolutionary stages ofthe different taxa of the Krabi fauna, and comparisons with other well dated mammalian faunas from Eurasia and North Mrica, led us to propose a late Eocene age for the mammal fauna (about ±37 to ±34 Ma). A magnetostratigraphic study has been carried out on the late Eocene fluviatilell acustrine Krabi deposit (southern Thailand). 87 samples in 43 different stratigraphic levels were collecte d from a thickness of 105 m of the section. These paleomagnetic samples were analysed, and they yielded a paleomagnetic direction. Rock magnetic experiments show that the NRM generally results from the presence of magnetite. The Krabi section that has been sampled displays only reverse polarity. The mean direction for reversed polarity samples was D = 192.34, I = -15.5 (N = 71, <X95 = 4.7, k = 14). The correlation to the Geomagnetic Polarity Time Scale (GPTS) relies on the biostratigraphic data previously proposed for the fauna found in the same section. These correlations have been achieved by comparing variations in the sedimentary rate derived from the alternative correlation. The proposed correlation puts Krabi section in the chron C13r (34.1 Ma) or C15r (35.14 Ma). This suggests sedimentation rates of 9.4 and 26 cm/ky respectively, but the second sedimentation rate is too high for this type of sediments (lignite). Hence, the most probable correlation for the Krabi section is with chron C13r (33.54-34.65 Ma).