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HOW MANY LATE ARCHEAN IMPACTS ARE RECORDED IN THE HAMERSLEY BASIN OF WESTERN AUSTRALIA? B. M. Simonson 1 , S. W. Hassler 2 , J. Smit 3 , and D. Sumner 4 1 Geology Department, Oberlin College, Oberlin, OH 44074 (bruce.simonson@oberlin.edu), 2 John F. Kennedy University, Orinda, CA 94563 USA (shassler@juno.com); 3 Faculty of Earth Sciences, Vrije Universiteit Amsterdam, the Netherlands (smit@geo.vu.nl); 4 Geology Dept., University of California at Davis, Davis, CA 95616 (sumner@geology.ucdavis.edu) Previous work: Various layers rich in sand-size spherules of former silicate melt and interpreted as impact ejecta have been identified in Precambrian successions [1,2]. About half of the layers occur in the Hamersley basin of Western Australia, but there has been some uncertainty as to how many impacts they represent. One of the Hamersley spherule layers was described as early as 1966 [3], but they were not connected with impacts until much later. Shocked minerals have yet to be reported from any of the spherule layers, but an impact origin is indicated by numerous lines of geologic [4,5] and geochemical [6-8] evidence. Spherule layers were initially discovered in three units in the Hamersley basin: the Wittenoom Formation, the Carawine Dolomite, and the Dales Gorge Member of the Brockman Iron Formation [4]. The Dales Gorge layer is the youngest and probably early Paleoproterozoic in age, whereas the Wittenoom and Carawine layers both formed in the late Archean [9]. The Wittenoom only occurs in the main part of the Hamersley basin whereas the Carawine is restricted to a separate area to the east known as the Oakover River area [10,11]. The Wittenoom and Carawine are thought to be contemporaneous because they each represent the only thick succession of carbonates in their respective parts of the Hamersley basin [10]. Given the rarity of impact ejecta layers and the petrographic similarity of the spherules, we interpreted the Wittenoom and Carawine spherule layers as products of a single impact which constituted a time plane across the Hamersley basin [4,11]. This correlation was problematic because one layer is stratigraphically high in the Wittenoom whereas the other layer is stratigraphically low in the Carawine. Further searching yielded an additional spherule layer near the top of the stratigraphically lower Jeerinah Formation. The Jeerinah Formation extends throughout the entire Hamersley basin. We discovered the spherule layer in drill core FVG-1 in the eastern part of the main Hamersley basin [12] and subsequently located it in a surface exposure at Hesta ~60 km to the northwest [13]. The layer is very different at these two sites (Table 1), and in July 2001 we discovered another surface exposure of a spherule layer
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