2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 1
Presentation Time: 8:15 AM


ASHLEY, Gail M., Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854, gmashley@rci.rutgers.edu

Olduvai basin has 2 million year long paleontological and archaelogical records including at least three hominin species. Nearly 75% of the recovery sites are in the junction area which represents only 1% of the 250 km2 of sedimentary outcrop of fluvio-lacustrine deposits. This concentration of life appears to be due to a fortuitous interplay of rift-related tectonics and astronomically controlled climate fluctuations that created localized lush freshwater environments (~1.85-1.75 Ma) in this arid setting. Olduvai was an apparent attraction for hominins, as well as a wide range of animals from the nearby Ngorongoro Volcanic Highland and adjacent Serengeti grasslands.

The East African (Gregory) Rift lies east of Olduvai. Volcanism during early basin history produced thick volcaniclastic deposits; drainage was westward from the highlands into the basin. Periodic deformation under extensional stresses (NW-SE to W-E) created a regional tilt to the east shifting the depo center eastward through time. An eastward shift occurred at ~ 1.8 Ma (Bed I to Bed II time). The basin evolved to an entirely fluvial environment (post 1.7 Ma). A fault system comprised of steep-dipping normal faults and low amplitude drag folding disrupted the layer cake stratigraphy into fault blocks. This study recognizes 4 closely spaced N-S faults in the junction area that created adjacent horsts and grabens.

Superimposed on this background of active tectonics are long term astronomically controlled wet-dry cycles, i.e. Milankovitch precession cycles (19-23 kyrs). Changes in solar insolation are thought to drive stronger summer monsoon maxima increasing net annual rainfall. The level of Lake Olduvai fluctuated in sympathy with the wet/dry cycles; the groundwater system also responded. Aquifers swelled as meteoric water trapped on nearby 2000 m high Ngorongoro Highland moved into the basin. Water surfaced along fault planes and probably at the intersection of groundwater and impermeable beds. A 1 km2 lush wetland developed within the faulted junction area sourced by seeps and a number of point sources (springs). Analysis of lithic artifact assemblages showed a preferentially high concentration of artifacts at the springs sites compared to non-spring sites and also suggest high use of these freshwater resources during both wet and dry periods.