New 40Ar/39Ar Chronostratigraphy for Olduvai Gorge, Tanzania, and Orbital Forcing of Lacustrine Sedimentation

The Pleistocene sedimentary and volcanic deposits of Olduvai Gorge, north-central Tanzania, have been a focus of geological, paleontological, paleoanthropological and archaeological investigation since Louis Leakey initiated fossil and artifact excavations there in 1931. Although more than half a century of effort has been applied to radiometric dating of the volcaniclastic strata within this sequence, vexing questions persist regarding details of the chronostratigraphy. Renewed dating efforts have enabled revision of the chronostratigraphy of Bed I, providing a more accurate evaluation of the absolute ages of volcanic events, and the rates of geological processes. The new ⁴⁰Ar/³⁹Ar geochronology, based primarily on phenocrystic K-feldspar from interbedded marker tuffs, resolves some of the anomalies apparent in earlier Olduvai basin Bed I chronostratigraphies. A new age of 1.88 ± 0.05 for Tuff IA near the base of Bed I, while relatively imprecise, agrees with a projected age 1.91 Ma based on magnetostratigraphy and sedimentation rates toward the base of Bed I. The key marker tuff IB in the upper part of Bed I has a well-determined age of 1.848 ± 0.003 Ma. Comparative dating of early and late erupted phases of this unit yields identical ages, resolving the conundrum of an apparent hiatus of ~80 ka between the crystal-rich fallout base and the body of the Tuff IB ignimbrite in the eastern exposures proposed by Walter et al. (1991, 1992). New ages for tuffs above Tuff IB to the top of Bed I (i.e., Tuff IE 'vitric,' Tuff IE, Ng'eju tuff, and Tuff IF), fall in stratigraphic order and are consistent with the paleomagnetic age constraint of 1.778 Ma imposed by the top of the Olduvai Subchron in overlying Bed II. The new high-precision chronostratigraphy provides age control for a series of wet/dry paleoclimate indicators previously documented in upper Bed I (Hay, 1976; Hay and Kyser, 2001; Sikes and Ashley, 2007). The most prominent event is a major lake expansion encompassing the emplacement of Tuff IB. The age of this tuff is coincident within 2s error with the highest amplitude insolation peak of the entire calculated insolation time-series (30° N latitude, July) of the Bed I/lower Bed II interval at 1.852 Ma. A second, relatively minor lake expansion event (Sikes and Ashley, 2007), now recognized to underlie the Ng'eju tuff, apparently coincides with the next calculated precessional peak, the second most intense of the Bed I/lower Bed II time interval. Age control of the Ng'eju tuff should be improved to better constrain this younger lake expansion event.