Double Reunion Event From Revisited Shungura Formation, Omo Valley, Southwest Ethiopia.
Abstract
Two hundred fifty meters of fluvio-lacustrine deposit were sampled from the 760 meter thick fossil rich Shungura Formation to study magnetostratigraphy within the Matuyama Chron. This formation is exposed in the lower Omo valley in southwestern Ethiopia and is divided into 12 members (Basal, followed by A to L (but excluding I)) on the basis of volcanic ash layers (de Heinzelin, 1983). This study focuses on strata between Tuff F and the upper part of Member G. Two to three block samples were collected from 46 levels, from which four to five standard cube specimens were produced resulting in 240 laboratory specimens. Two sets of specimens were first subjected to a pilot step-wise Alternating Field (Af) and Thermal demagnetization (Th) with 17 to 21 steps for Af and 19 to 25 steps for Th. The remaining samples were treated with Th techniques. On average samples have strong Natural Remanent Magnetization Intensity (NRM) with a mean of μ= 0.21 A/m. In most cases, up to three components of magnetization were revealed by both demagnetization techniques. The Characteristic Remanent Magnetization (ChRM) is represented by the highest temperature- coercivity component and appears by demagnetization at temperatures between 390°C-620°C or alternating fields between 25-100mT. As shown by study of the magnetic mineralogy, ChRM is carried by detrital magnetite and maghemite. Directional analyses revealed three reversed and three normal polarity zones: R3-N3-R2-N2-R1-N1. Reversed polarity (R3) is observed in the bottom 60 m of section, the next 44 meters are normal (N3), followed by 36 meters of reversed (R2) sstrata, then 10 meters of normal (N2) strata, 84 meters of reversed (R1) section, and 16 meters (N1) of normal strata at the top. The overall mean direction Ds=359.5°, Is= -0.3°, α95=3.9°, N = 46 is not different from the expected direction D=1°, I=8.8°, α95= 2.3°, N = 26 obtained from the Apparent Polar Wander Path (APWP) curve for Africa at 2 Ma (Besse and Courtillot, 2003). Using stratigraphic scaling between Ar{40}/Ar{39} and K/Ar ages of 1.85±0.02 Ma, 1.87±0.02 Ma, and 2.34 Ma (McDougall and Brown, 2006; Feibel et al., 1989) for Tuff H4 and Tuff H2 and Tuff F, respectively the normal zones N2 and N3 are dated between 2.04-2.07 Ma and 2.12-2.19 Ma respectively while the bottom of N1 is dated at 1.93 Ma. These ages imply that N1 represents the bottom of the Olduvai event while N2 and N3 represent the Reunion events that have been identified in Gamarri volcanic section (Kidane et al. 1999) in Afar and elsewhere.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMGP11A0055B
- Keywords:
-
- 1513 Geomagnetic excursions;
- 1519 Magnetic mineralogy and petrology;
- 1520 Magnetostratigraphy;
- 1535 Reversals: process;
- timescale;
- magnetostratigraphy;
- 1540 Rock and mineral magnetism