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Abstract
Sterkfontein Caves is the single richest early hominin site in the world with deposits
yielding one or more species of Australopithecus and possible early Homo, as well
as an extensive faunal collection. The inability to date the southern African cave
sites accurately or precisely has hindered attempts to integrate the hominin fossil
evidence into pan-African scenarios about human evolutionary history, and especially
hominin biogeography. We have used U-Pb and U-Th techniques to date sheets of calcium
carbonate flowstone inter-bedded between the fossiliferous sediments. For the first
time, absolute age ranges can be assigned to the fossil-bearing deposits: Member 2
is between 2.8 +/- 0.28 and 2.6 +/- 0.30 Ma and Member 4 between 2.65 +/- 0.30 and
2.01 +/- 0.05 Ma. The age of 2.01 +/- 0.05 Ma for the top of Member 4 constrains the
last appearance of Australopithecus africanus to 2 Ma. In the Silberberg Grotto we
have reproduced the U-Pb age of approximately 2.2 Ma of for the flowstones associated
with StW573. We believe that these deposits, including the fossil and the flowstones,
accumulated rapidly around 2.2 Ma. The stratigraphy of the site is complex as sediments
are exposed both in the underground chambers and at surface. We present a new interpretation
of the stratigraphy based on surface mapping, boreholes logs and U-Pb ages. Every
effort was made to retain the Member system, however, only Members 2 and 4 are recognized
in the boreholes. We propose that the deposits formally known as Member 3 are in fact
the distal equivalents of Member 4. The sediments of Members 2 and 4 consisted of
cone-like deposits and probably never filled up the cave. The U-Th ages show that
there are substantial deposits younger than 400 ka in the underground cave, underlying
the older deposits, highlighting again that these cave fills are not simple layer-cakes.