DEPOSITIONAL PROCESSES OF BED III AND BED IV, OLDUVAI GORGE, TANZANIA: EXPANDING ON THE WORK OF RICHARD HAY, 1976

Richard Hay documented the stratigraphy of Olduvai Gorge, Tanzania in 1976, with primary focus on Bed I and Bed II (Hay, 1976). Little has been done to expand upon the depositional environmental interpretations of Bed III and Bed IV beyond Hay’s original differentiation of eastern and western fluvial sources. Bed III and Bed IV were deposited between 1.15 and 0.6 mya, and archaeological evidence shows Homo erectus inhabited the area and utilized the Acheulean stone tool industry during this time interval. The lack of detailed paleoenvironmental data make it difficult for interpretations on the interaction between H. erectus and its environment to emerge. Recent stratigraphic fieldwork has shed new light on the processes involved in the deposition of these beds.

Field investigations allowed for volcanic, fluvial, and gravity-induced processes to be recognized from Bed III and Bed IV rocks. The only geographically widespread tephra deposits were identified as Tuff IIIA and Tuff IVA, and these proved essential for correlation across several stratigraphic sections. Most volcanic deposits, however, consisted of reworked tephra, from which stratigraphic positions of unique mineral associations are sought for secondary correlations. Channel to overbank floodplain deposits were interpreted from sedimentary structures and grain characteristics preserved in rocks across the Main and Side Gorge. Gravity-induced processes produced diamictites with mud matrices and clast sizes and compositions reflecting their proximity to the Ngorongoro Volcanic Highlands in the eastern extreme of the region. Paleosols, identified by the presence of root casts, result from both the floodplain deposits and finer-grained diamictites. As such, they are often stacked one on top of another, especially in the western reaches of the gorge.

Diagenesis makes Bed III distinctly red in the Side Gorge and east of the FLK Fault in the Main Gorge. This is likely from iron from the volcanic rocks derived from the NVH to the east, explaining why the red color in not present in the western-most portions of the gorge.

Hay, R.L. (1976). “Geology of the Olduvai Gorge: A Study of Sedimentation in a Semiarid Basin.” University of California Press, Berkeley (203 pp.).