Revised magnetostratigraphy of the Plio-Pleistocene sedimentary sequence of the Olduvai Formation (Tanzania)

The magnetic stratigraphy of the Plio-Pleistocene sedimentary formation of the Olduvai Gorge (Tanzania), which contains a well known series of prehistoric pebble cultures and hominid remains, has been reinvestigated. Sixty one sites were sampled along 90 metres of stratigraphic section extending from the base of Bed I into the Masek Beds. Palaeo- and rock-magnetic analyses reveal that magnetizations are stable, carried by low coercivity magnetic minerals (titano-magnetites) and are likely to be of depositional origin. A revised magnetostratigraphic scale is proposed. The base of the major normal magnetozone, N1, interpreted as the Olduvai sub-chron, lies ca 6 m below Tuff IA, i.e. significantly below the previously defined position. This revision, associated to 40Ar/ 39Ar ages recently determined on the Tuffs of Bed I, supports the validity of the astronomical calibration of the geomagnetic polarity time scale for the Olduvai sub-chron. The correlation of the normal magnetozone N2 with Jaramillo (or Jaramillo + Cobb Mountain), is suggested by the discovery of a reverse magnetozone (R3) in the uppermost part of the Pleistocene sequence (Masek Beds), and implies that the fossiliferous sequence of the Olduvai Formation was entirely deposited before the Brunhes/Matuyama boundary.