BIOSTRATIGRAPHIC FRAMEWORK OF BED II, OLDUVAI GORGE, TANZANIA

Olduvai Gorge, Tanzania is a prominent paleoanthropological site within an off-axis rift platform basin separated from main East African Rift System by the Ngorongoro Volcanic Highlands (NVH). Of the ~2 Ma exposed sedimentary record, Bed II is of significance because of the appearance of Homo erectus and disappearance of Paranthropus boisei and Homo habilis. Bed II ranges from ~1.8–1.14 Ma and records five major lake transgression-regression cycles coupled with climatic variability, both of which have been a major focus in hominin paleoecological studies. Synsedimentary faulting and NVH volcanism had major impacts on drainage patterns in the basin, including reversal of main drainage direction. These processes also impacted lake size, but their role in hominin paleoecology is less understood.

We digitized and georeferenced published stratigraphic cross sections of Bed II localities across the gorge and identified the major disconformities that separate the five lake sequences. Facies within each sequence were correlated to assign paleoenvironments and depositional changes across the basin. Vertebrate fossil sites were placed within the spatially expanded stratigraphic framework to identify if faunal material was primarily or secondarily deposited, to assess correlations between faunal diversity and depositional environments, and investigate paleohabitat distribution changes.

The cross sections were regionally flattened on primary marker tuff of Bed II in order to reconstruct paleotopography at time of tuff deposition. This series of paleotopographic reconstructions with geological facies data highlight faulting and intervening geomorphological processes that changed the landscape throughout Bed II times. By connecting biostratigraphy with paleotopography we identify the spatial and temporal patterns of vertebrate faunal diversity and distribution during an integral period of hominin evolution.