USING IMPACT SPHERULE LAYERS FOR STRATIGRAPHIC CORRELATION: A CASE STUDY OF THE NEOARCHEAN JEERINAH LAYER (HAMERSLEY BASIN, WESTERN AUSTRALIA)

In the early Precambrian, spherule layers constitute our only evidence of impacts by large extraterrestrial bodies. Strata in the Hamersley Basin (Western Australia) contain layers of at least three impacts; the older two date to ca. 2.63 and 2.54 Ga and occur in the Jeerinah and Wittenoom Formations respectively. A third formation containing one spherule layer, the Carawine Dolomite, only occurs in a geographically isolated part of the Hamersley Basin. The Carawine spherule layer was originally correlated with the Wittenoom spherule layer, but recorrelated with the Jeerinah spherule layer based partly on the presence of irregular melt particles found only in those two layers. To further test this recorrelation, the proportions of 8 different textural categories spherules and irregular particles were quantified to compare the melt populations of each. Minor amounts of detrital quartz are also present in all three layers and their petrographic characteristics were quantified using the method of Basu et al. (1975, JSP). Our data indicate the textures in the impact melt particles are very similar in the Jeerinah and Carawine layers, whereas those in the Wittenoom layer are significantly different. Moreover, the quartzose sand in both the Jeerinah and Carawine layers shows textures typical of a low-rank metamorphic provenance instead of evidence of shock metamorphism, indicating there is very little solid impact ejecta (at least preserved) in these layers. A spherule layer in the Monteville Formation (Griqualand West Basin, South Africa) is similar in age and character to the older Hamersley layers. More specifically, the Jeerinah, Carawine, and Monteville layers all contain remarkably similar former melt populations and share a similar diagenetic history. Quartz is very abundant in the Monteville and likewise has textures typical of epiclastic detritus rather than impact ejecta. These data support the interpretation of the Jeerinah, Carawine, and Monteville layers as products of a single large impact around 2.63 Ga. It also reaffirms the usefulness of straightforward petrographic analysis as an inexpensive tool to help correlate impact spherule layers in well-dated Precambrian successions, even those on different cratons.