MAGNETOSTRATIGRAPHY OF THE EARLY MIOCENE HIWEGI FORMATION (RUSINGA ISLAND, LAKE VICTORIA, KENYA)

The early Miocene fossils from Rusinga Island (Lake Victoria, Kenya) provide some of the best evidence for understanding early hominoid evolution and Miocene faunal succession in East Africa. The majority of the fossils from Rusinga have been recovered from several localities within the Hiwegi Formation. Determining accurate ages for these fossil sites is crucial for developing an understanding of East African paleobiology. Previous researchers have suggested that the major fossil localities within the Hiwegi Fm. are correlative and were deposited synchronously. However, correlating between the fossil sites is difficult and the age relationships of the fossil localities are uncertain. To address these problems, we conducted paleomagnetism analyses at four of the major Hiwegi Formation fossil localities (R1, R3, R4, and R5). Our results document a series of reversals through the formation that can be used to correlate the fossils sites. Analyzed paleomagnetic samples indicate that the base of the formation was deposited in normal polarity, the middle of the formation in reversed polarity, and the top of the formation in normal polarity. Importantly, fossil sites at the different localities are found within strata of different polarities. This indicates that (1) there are at least three different, major fossil-bearing horizons within the Hiwegi Formation and (2) the fossils within the Hiwegi Formation are not all contemporaneous. These results indicate that previous paleontological and paleoenvironmental studies that have lumped all of the fossils collected from the Hiwegi into a single unit have time-averaged fossils from multiple intervals. Thus, models of Rusinga’s paleoenvironments and faunal communities based on analyses of combining fossils from the entire Hiwegi Formation may be oversimplified.