MAGNETOSTRATIGRAPHY OF THE EARLY MIOCENE KULU AND HIWEGI FORMATIONS ON RUSINGA ISLAND (LAKE VICTORIA, KENYA)

Early Miocene fossils from Rusinga Island (Lake Victoria, Kenya) provide some of the most extensive evidence for understanding early hominoid evolution and East African faunal succession. The majority of the fossils on Rusinga have been recovered from the Rusinga Group. Understanding the age relationship of the stratigraphic formations in the Rusinga Group is particularly important to help provide a geochronological context within which the abundant fossil evidence can be interpreted. While there is general consensus over the stratigraphic position of most of the formations in the sequence, the placement of the lacustrine Kulu Formation and the volcaniclastic Hiwegi Formation has been debated since the 1940s. Recently researchers suggested that the Kulu and Hiwegi Formations were simultaneously deposited. However, detailed geological analyses indicate that the Hiwegi and older formations were locally deformed and deeply eroded prior to deposition of the Kulu Formation. New paleomagnetic data, presented here, support this interpretation. Paleomagnetic samples from the basal Hiwegi Formation demonstrate normal polarity while samples throughout the entire Kulu Formation indicate reversed polarity. Therefore, the formations could not have been deposited contemporaneously. A new ⁴⁰Ar/³⁹Ar age determination from the upper Hiwegi constrains the age of the top of the formation to ca. 18 Ma. Based on this date, the series of geomagnetic reversals in the Hiwegi Formation most likely correlate to C5En – C5Dr.2r of the Geomagnetic Polarity Time Scale. The stratigraphic relationship of the two formations indicates that the Kulu Formation strata cannot be any older than ca. 18 Ma, which is the base of C5Dr.2r. As the basal contact of the Kulu is a major erosional unconformity, it is plausible that it is considerably younger than 18 Ma.