Magnetostratigraphy of the Fossil-Rich Shungura Formation, southwest Ethiopia

Abstract

Three hundred eighty-six oriented block samples were collected from the lower 334m of Pliocene and Pleistocene fluvial strata of the Shungura Formation in two field seasons. Paleomagnetic polarity of these was determined on samples from these blocks, using a routine of 8–13 steps for AF demagnetization, and 15–20 steps for thermal demagnetization. Rock magnetic properties revealed titanomagnetite with subordinate maghemite as the dominant magnetic minerals with pseudo-single domain magnetic grain sizes. Directional analyses indicate one or two components of magnetization both in AF and thermal techniques. The first component is mostly removed by 5–10mT AF fields or heating to 300°C. The magnetization component after these steps generally defined straight-line segments directed towards the origin which are interpreted as the Characteristic Remanent Magnetization (ChRM). ChRM directions so determined and averaged at the site level revealed both normal and reversed polarities. The directions of the normal and reversed polarities are respectively Dec=357°, Inc=−0.1 (N=36, α95=3.7°) and Dec=176°, Inc=−3.4 (N=55, α95=3.1°) and are antipodal. A sequence of R4-N3-R3-N2-(R2)-N1-R1 polarities was identified with R2 being an anomalous direction within N2/N1. Published weighted mean 40Ar/39Ar ages on volcanic ash layers in this part of the section with height from the base and ages in parentheses are: Tuff A (34m; 3.60±0.02Ma), Tuff B-α (65m; 3.43±0.01Ma), Tuff B-δ (77m; 3.41±0.01Ma), Tuff in B-10-1 (150m; 2.97±0.01Ma), Tuff C (170m; 2.99±0.09Ma), Tuff D (252m; 2.53±0.01Ma), Tuff D-3-2 (272m; 2.47±0.01Ma), and Tuff F (228m; 2.32±0.01Ma). These direct age results imply that the identified polarity zones correspond to the Gilbert Chron, lower Gauss Chron, Mammoth Subchron, upper Gauss Chron, and lower Matuyama Chron, respectively. No new paleomagnetic data are available for the Usno Formation, but tephrostratigraphic work results in reinterpretation of the stratigraphy suggesting that only the Mammoth Subchron is preserved in the upper part of this formation. In the Shungura, Usno and Koobi Fora formations, the thickness represented by the lower Gauss and Mammoth Subchron is very similar, but the Kaena Subchron is either not recorded in the type area of the Shungura Formation, or only incompletely revealed.