Correlation of volcanic ash layers between the Early Pleistocene Acheulean sites of Isinya, Kariandusi, and Olorgesailie, Kenya

Olorgesailie, Kariandusi, and Isinya are archeological sites with Acheulean artifacts in or on the flanks of the southern Rift Valley of Kenya. 40Ar/39Ar ages on feldspar from tuffs in the Olorgesailie Formation of 992 ± 39 ka (Member 1), 974 ± 7 ka (Member 5), and 747 ± 6 ka (Member 9), 662 ± 4 ka (Member 10), and 601 ± 3 ka (Member 11) bracket most of the Acheulean sites there. A 40Ar/39 age on a tuff in the Kariandusi sequence is 977 ± 10 ka, and both sites are associated with thick diatomite deposits. No age control has been available for the site of Isinya. Electron microprobe analyses of glass from volcanic ash layers from these sites allow tephrostratigraphic correlations between them. Here we report that samples from Kariandusi below the archeological levels correlate with samples in Member 4 and Member 2 at Olorgesailie (992 ka-974 ka). Samples from a volcanic ash at Isinya above the archeological site correlate with an ash layer in Olorgesailie Member 4, showing that these artifacts are >974 ka, providing the first age control for the artifacts at Isinya. The correlation from Olorgesailie to Kariandusi shows that the diatomites at those sites were deposited at the same time ∼120 km apart but now differ in elevation by ∼700 m. Chemical similarity with obsidians from localities in the Naivasha region suggest that some of the ash layers may originate from that area.