Using impact spherule layers to correlate sedimentary successions: a case study of the Neoarchean Jeerinah layer (Western Australia)
Spherule layers constitute the only trace of impacts by large extraterrestrial bodies in the early Precambrian. Strata in the Hamersley Basin (Western Australia) contain layers from at least three impacts; the older two date to ∼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, is restricted to an area of the basin where the Wittenoom Formation is absent. Upon discovery, the Carawine and Wittenoom layers were attributed to the same impact, but the subsequent discovery of the Jeerinah layer raised questions about this interpretation. To test it, petrographic characteristics of spherules and irregular particles in the Jeerinah layer at Hesta were quantified by point counting. Minor amounts of quartzo-feldspathic sand are present in both the Jeerinah and Carawine layers and their textural characteristics were also quantified for comparative purposes. The textures of the impact melt particles in the Jeerinah and Carawine layers are indeed very similar to one another and differ from those in the Wittenoom layer. The quartz in the Jeerinah and Carawine layers is broadly similar, but its textural characteristics are more variable. Moreover, the feldspar grains in the Jeerinah layer appear to have a more volcanic provenance than those in the Carawine layer. All of the quartz shows textures typical of plutonic to regionally metamorphosed basement rocks instead of shock metamorphism, affirming its earlier interpretation as detrital sand rather than impact ejecta. Correlations have also been proposed between Hamersley layers and an impact spherule layer in the roughly coeval Monteville Formation (Griqualand West Basin, South Africa) that is locally rich in quartzose sand. Petrographically, the melt population in the Monteville layer is remarkably similar to those of the Jeerinah and Carawine layers, including similar diagenetic histories. The quartzose detritus is again broadly similar but not identical to that of the Hamersley layers. In summary, our petrographic data are consistent with the Jeerinah, Carawine and Monteville layers all being products of a single large impact. However, the quartzo-feldspathic detritus appears to have multiple sources, consistent with its being locally derived detritus. These results suggest petrographic analysis can be used to test correlations in well-dated Precambrian successions that contain impact spherule layers.
Jones-Zimberlin, Sarah, Bruce M. Simonson, David Kreiss-Tomkins, Daniel Garson. 2006. "Using impact spherule layers to correlate sedimentary successions: a case study of the Neoarchean Jeerinah layer (Western Australia)." South African Journal of Geology 109(1/2): 245-261.
Geological Society of South Africa
South African Journal of Geology
Spherules (Geology), Sedimentary structures, Geology, Stratigraphic--Precambrian, Basins (Geology)