2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 12
Presentation Time: 11:10 AM


MCDOUGALL, Ian, Australian National Univ, Research School Earth Sciences, Canberra, ACT 0200, Australia, BROWN, Francis H., Geology and Geophysics, University of Utah, 135 South 1460 East, Salt Lake City, UT 84112 and FLEAGLE, John G., Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794, Ian.McDougall@anu.edu.au

The Kibish Formation of southern Ethiopia comprises four members separated by disconformities with a cumulative thickness of about 100 m. Each member represents an interval of mainly deltaic deposition associated with the Omo River where it entered a much expanded Lake Turkana, about 100 km north of the present lake. Geological evidence indicates rapid deposition of each member. Tephrostratigraphic studies enable correlation of the members throughout the area, and 40Ar/39Ar ages on alkali feldspars from small pumice clasts in Member I give ages of 198 ± 14 ka (weighted mean age 196 ± 2 ka) and 104 ± 7 ka for a tuff in Member III. Radiocarbon ages on Member IV show deposition occurred between about 10 and 3 ka ago. These ages are remarkably similar to ages derived via the astronomical time scale for several sapropels in the eastern Mediterranean Sea, 195 ka for sapropel S7, 102 ka for S4 and 8 ka for S1. The link between the two widely separated regions is paleoclimatic. The Omo River and the Nile River system share a drainage divide and both the formation of the sapropels and the deposition of the members of the Kibish Formation reflect greatly increased precipitation in the Ethiopian highlands and high runoff during periods of intensification of the African monsoon when precession was at a minimum and solar insolation was at a maximum. The linkage between sapropel formation in the Mediterranean and very high lake levels of Lake Turkana was a serendipitous discovery arising from investigations at Kibish relating to the stratigraphic position and numerical age of early anatomically modern hominid fossils assigned to Homo sapiens, confirming that the fossils are from comparable levels in Kibish Formation Member I, and showing that their age is 195 ± 5 ka.