Two normal paleomagnetic polarity intervals in the lower Matuyama Chron recorded in the Shungura Formation (Omo valley, southwest Ethiopia)

摘要

Two hundred fifty meters of fluvio-lacustrine deposits were sampled from the 760-meter thick Shungura Formation to study magnetostratigraphy within the lower Matuyama Chron. Two to three block samples (at different positions) were collected from 46 units in the section. Characteristic remanent magnetization (ChRM) is isolated at temperatures between 390 degrees C and 620 degrees C or alternating fields between 25 and 100 mT. The ChRM is carried by detrital magnetite and maghemite. Directional analyses revealed three reversed and three normal polarity zones: R3-N3-R2-N2-R1-N1 in the sequence. The overall mean direction D,=359.1 degrees, I-s=3.4 degrees; alpha(95)=3.9 degrees, N=44 differs marginally from the expected mean dipole field direction (D=1.0 degrees, I=8.8 degrees, alpha(95)=2.3 degrees, N=26) [Besse, J., Courtillot, V, 2003. Apparent and True Polar wander and the geometry of the geomagnetic field in the last 200 million years. J. Geophys. Res. 108 (B10), 2469. doi 10.1029/2003JB002684]. Using stratigraphic scaling between Ar-40/Ar-39 and K/Ar isotopic age data [Brown, F.H., McDougall, I., Davies, I., Maier, R., 1985. An integrated Plio-Pleistocene chronology for the Turkana basin. In: Delson, E. (Ed.), Ancestors: The Hard Evidence. Alan R. Liss, Inc., New York, pp. 82-90; McDougall, I., Brown, F.H., 2006. Precise Ar-40/Ar-39 geochronology for the upper Koobi Fora Foramtions, Turkana Basin, northern Kenya. J. Geo. Soc. (Lond.) 163, 205-220], the normal zones N3, N2, and N1 are dated between 2.06 +/- 0.01-2.08 +/- 0.01 Ma, 2.15 +/- 0.01-2.20 +/- 0.01 Ma, and at 1.93 +/- 0.02 Ma respectively. These age estimates imply that NI represents the bottom of the Olduvai Subchron while N2 and N3 might represent Reunion Subchrons. If the sedimentation rates are assumed to be constant, the duration for the older Reunion (I) Subchron is about four to seven times (57 +/- 15 kyr) that currently adopted by the recent GPTS [Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093-6095] and the younger Reunion (II) Subchron has a shorter duration of 18 +/- 25 kyr. Published by Elsevier B.V.