CONTROLS ON PLIO-PLEISTOCENE GEOGRAPHY AND HOMININ HABITATS IN THE TURKANA BASIN (ETHIOPIA AND KENYA): INFERENCES FROM SEQUENCE STRATIGRAPHY

Dick Hay's study of the paleo-Olduvai Basin, Tanzania represents a landmark contribution to the paleogeographic analysis of early hominin habitats. In memoriam, we use this as a starting point to review early concepts and suggest how our paleoenvironmental understanding of East African Plio-Pleistocene hominin evolution can be furthered through sequence stratigraphy theory and methods. In particular, we interpret possible tectonic, climatic, and accommodation controls on basin-wide paleogeographic changes for the Turkana Basin, and the hominin deposits of upper Koobi Fora Formation (northeast basin, Kenya) for the period of about 2.5-1.5 Ma.

We recognize that two previously documented unconformities, with origins relatable to regional uplift and down-warping, define a third-order tectono-volcanic sequence stratigraphic cycle in the upper Koobi Fora Formation. Constituent lacustrine facies successions are divisible into lowstand, possibly transgressive, and highstand systems tracts. Fifth-order lake-margin parasequences are apparent within the early-middle highstand systems tract. Parasequence sets are packaged by fourth-/fifth-order sequence boundaries that are defined by well-developed channel scours and paleosols.

Geochronological correlations between global paleoclimate changes, fourth- to fifth-order sequence units, and the inferred base-level cycle are used to argue that the sensitivity of the system to climate forcing is dependent on the accommodation potential, as predicted by other lacustrine basin-fill models. Infilling of the lake and the eventual transition to a fluvial basin is a likely result of decreasing subsidence. A refined paleogeographic model is offered, which synthesizes the sequence stratigraphic interpretations with new and old data from facies, chronostratigraphy, sedimentation rates, and stable isotopes. Implications for East African paleoclimate issues, hominin habitat structure, and Homo erectus evolution are briefly discussed.