Chronostratigraphy and age modeling of Pleistocene drill cores from the Olduvai Basin, Tanzania (Olduvai Gorge Coring Project)

The Olduvai Gorge Coring Project drilled a total of 611.72 m of core (575.48 m recovered) of mostly fluviolacustrine and fan-delta volcaniclastic Pleistocene strata at three sites in the Olduvai Basin, Tanzania, in 2014. We have developed a chronostratigraphic framework for three of the cores based on 40Ar/39Ar dating of core and outcrop volcanic and volcaniclastic units, core paleomagnetic stratigraphy, and tephrochemical correlation between cores and from core to outcrop. This framework is then used to constrain Bayesian stratigraphic age models which permit age estimates for desired core levels with realistic confidence intervals. The age models reveal that the deepest core level reached at 245 mbs is similar to 2.24 Ma, similar to 210 kyr older than the oldest strata exposed at Olduvai Gorge. Strata net accretion rates in this early phase of basin history were relatively rapid (57-69 cm/kyr), but decreased within similar to 250 kyr to similar to 15 cm/kyr in Lower Bed I. Rates rebounded partially in Upper Bed I, but subsequently declined to <10 cm/kyr by Middle to Upper Pleistocene. The age models also provide new estimates for the basal contacts of upper Olduvai Gorge stratigraphic units that have been previously difficult to calibrate: Bed III at 1.14 +/- 0.05 (95% confidence interval), Bed IV at 0.93 +/- 0.08, Masek at 0.82 +/- 0.06, and Ndutu at 0.50 +/- 0.04 Ma. Finally, based on recently acquired seismic imaging identifying basement another 135 m beneath the bottom of the deepest core, extrapolation of net accretion rates suggests that sedimentation began at this site in the Olduvai Basin at similar to 2.5 Ma.