A CRYPTIC VOLCANIC SOURCE: IMPROVING THE TEPHROSTRATIGRAPHIC RESOLUTION OF THE “ZINJANTHROPUS” ARCHEOLOGICAL SURFACE AT OLDUVAI GORGE, TANZANIA

Tuffs and volcaniclastic sandstones make up much of the stratigraphic record of Bed I at Olduvai Gorge, Tanzania, and their individual compositions discriminate them. Geochemical “fingerprints” of juvenile feldspar, augite, hornblende, Ti-magnetite, and rare glass separates correlate major Bed I marker tuffs between world-renowned archeological sites throughout the 20km Olduvai basin. Our contribution improves further the resolution of this framework within a paleoanthropologically rich area, linking the classic Zinjanthropus site at FLK to nearby sites.

47 samples of tuffs and volcaniclastic sandstones were collected from ten small archaeological trenches mapped in detail at the FLK, FLKN, FLKNN, FLK-Maiko Gully, and VEK sites over a <1km² area. Previously identified Marker Tuffs (IB, IC, ID) were confirmed using phenocryst compositions measured by electron microprobe (EPMA). Grains from volcaniclastic sandstones between these layers were also analyzed by EPMA, as was a newly discovered thin black fall-out tuff between Tuffs IB and IC.

A previously unknown volcaniclastic composition characterizes sandstones between Tuffs IB and IC, with higher K anorthoclase and higher Ti, Al, and Mg augite than other Bed I tuffs. The black fall-out tuff also has a distinct composition, containing augite with even higher Mg and Ti. This suggests that additional tephra not yet identified as distinct ash-fallout units were deposited within the Olduvai catchment. The unique fingerprint of volcaniclastics between Tuffs IB and ID improves tracing of archeologically important levels between sites, such as the classic Zinjanthropus hominin exploitation surface at FLK and FLKN, which lies between the black tuff and Tuff IC.

In contrast, a volcaniclastic sandstone close above Tuff IC is compositionally indistinguishable from overlying Tuff ID. This early appearance of Tuff ID compositional grains in sandstones below ID limits the usefulness of Tuff ID composition as a geochemical fingerprint. Definition of compositional trends, integrated with high-resolution sequence stratigraphic mapping, helps refine the reconstruction of landscapes on which hominin activities occurred during the early Pleistocene.