Stratigraphy, correlation, and age estimates for fossils from Area 123, Koobi Fora

Abstract

Geological data from the Bura Hasuma region at Koobi Fora provide important constraints for estimating the ages of hominin fossils recovered there, including the cranium KNM-ER 1813. Strata of the upper Burgi, KBS, and Okote members in this part of Koobi Fora reflect three depositional regimes driven by changing paleogeography through time. The upper Burgi and lowermost KBS sequence in the southern Bura Hasuma region accumulated in a lacustrine to delta front setting, with highly localized depositional patterns, limiting the lateral extent of lithostratigraphic markers. Farther north, uppermost upper Burgi through KBS member strata document a fluctuating lake margin, with complex facies patterns. This interval is marked by laterally extensive lithostratigraphic markers, including molluscan packstones, beach sandstones, and stromatolite beds. The uppermost KBS and Okote members show a transition to dominantly fluvial character, with localized and discontinuous accumulation. An age model for the richly fossiliferous Area 123 sequence demonstrates the complexity of terrestrial accumulation patterns. Early lacustrine and delta front accumulation is marked by fairly continuous sedimentation, and high accumulation rates (up to ca. 91 cm/k.yr.). The fluctuating lake margin interval reflects lower sedimentation rates coupled with intervals of exposure, decreasing accumulation significantly (to ca. 13 cm/k.yr.). The capping fluvial interval is marked by significant erosion surfaces, breaks which may drop the overall accumulation rate even lower (ca. 0.3 cm/k.yr.). The data provided here establish a geological framework at odds with a recent proposal of ages considerably younger (by ca. 250 k.yr.) for many of the fossils from Area 123 and elsewhere. Tests of age models demonstrate that the younger ages are not possible. While minor refinements to age estimates for fossils are indicated by improved chronostratigraphic control, in the case of KNM-ER 1813, an age of younger than 1.78 Ma is precluded on magnetostratigraphic grounds.