RECENT DISCOVERIES HELP CLARIFY ORIGINS OF LATE ARCHEAN IMPACT SPHERULE LAYERS IN THE HAMERSLEY BASIN OF WESTERN AUSTRALIA

Three formations in the Hamersley basin of Western Australia each contain a single late Archean layer rich in distinctive sand-size spherules of former silicate melt produced by major bolide impacts. (A Paleoproterozoic spherule layer in the Dales Gorge iron formation is not considered here). Recent discoveries shed new light on the questions of how many impacts were involved and what the target materials were. The spherule layers near the top of the Jeerinah Formation and in the upper Wittenoom Formation were formed by two different impacts around 2.63 and 2.54 Ga respectively. The Wittenoom spherule layer, exposed throughout an area >18,000 km2, is typically a few decimeters thick and contains an aggregate thickness of one to several centimeters of spherules. In contrast, the Jeerinah spherule layer was discovered in one drill core in 1997 where it is only 6 mm thick. In 2000, a surface outcrop was discovered 50 km away in which the Jeerinah layer is 1.5 m thick with an aggregate spherule thickness of several decimeters. The third late Archean spherule-bearing layer occurs near the base of the Carawine Dolomite in the Ripon Hills area, where it is ca. 20 m thick and has the characteristics of a carbonate debris flow. Although we originally correlated it with the Wittenoom layer, this interpretation is now suspect. In July 2001, we discovered a surface exposure of a spherule-rich layer 60-70 km southeast of Ripon Hills which is about 10 m thick and similar in character to the Carawine layer, but it is in the upper part of the Jeerinah Formation, i.e., close beneath the Carawine Dolomite stratigraphically. This strongly suggests the spherule layers in the Jeerinah and Carawine represent a single ejecta layer formed by the same impact. Both of these layers also contain larger, more irregular particles with close petrographic similarities, suggesting the ejecta was deposited closer to the site of impact than any other Archean spherule layer. In contrast, the apparent absence of irregular melt particles as well as teardrop and dumbbell-shaped droplets suggest the Wittenoom layer is more distal. The Carawine spherule layer also contains quartz crystals which may have planar deformation features, suggesting a continental impact, whereas the composition of the target rocks responsible for the Wittenoom layer is still uncertain.