Magnetostratigraphy of the lower member of the hadar formation (Ethiopia): Evidence for a short normal event in the mammoth subchron |
| |
| Authors: | Endale Tamrat Nicolas Thouveny Maurice Taieb |
| |
| Institution: | (1) Laboratoire de Géologie du Quaternaire CEREGE, BP 80, 13545 Aix-en-Provence, Cedex 4, France |
| |
| Abstract: | Summary Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis
of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial
and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis
measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex
magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in
composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most
sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about
72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α95=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history
of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of
the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing40Ar/39Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed
Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of
the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr. |
| |
| Keywords: | hominid site rock magnetism polarity Kada-Hadar event |
| 本文献已被 SpringerLink 等数据库收录！ |
|
