RECORD OF SPRINGS, WETLANDS AND LAKE LEVELS IN OLDUVAI BASIN: THE RELATIVE ROLE OF TECTONICS AND CLIMATE CHANGE
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.