ISOTOPIC DATING OF AN ARCHEAN BOLIDE IMPACT HORIZON, HAMERSLEY BASIN,WESTERN-AUSTRALIA

Detailed geologic field work in the Hamersley basin of Western Austral ia has identified a single horizon that contains predominantly sand-si zed spherules similar to those found in impact ejecta such as at the C retaceous-Tertiary boundary. Evidence suggests that these spherules re present a reworked distal strewn held formed by a large bolide impact in late Archean time. This so-called ''spherule marker bed'' occurs th roughout the main body of the Hamersley basin and in stratigraphically equivalent, but shallower-water lithologies in the northeastern corne r. Carbonate constitutes both a matrix component of the spherule marke r bed and minor interbeds in the local stratigraphic section, We have utilized carbonate Pb-Pb dating methods to provide, for the first time , an age estimate (2541 +18/-15 Ma) for this important marker bed and therefore of the bolide impact event. Samples from both facies define, within error, an identical isochron age that is also consistent with the few zircon-based age estimates for the Hamersley succession. These findings highlight the great potential of carbonate Pb-Pb geochronolo gy in the dating of Archean and Proterozoic sedimentary rocks.