Spherule Layers Near the Archean-Proterozoic Boundary

Abstract

As noted in Chapter 7, strata deposited during a relatively short period of time straddling the Archean-Proterozoic boundary contain an unusually high concentration of spherule layers. At least 4 bodies roughly the size of the end-Cretaceous impactor or larger hit the Earth in a span of some 140 million years (Table 8.1). To date, spherule layers from these impacts have only been found in two limited geographic areas where sedimentary strata are unusually well preserved: the Hamersley Basin of Western Australia and the Griqualand West Basin of South Africa. These two successions are thought to be largely contemporaneous and host some of the largest banded iron formations (BIFs) and iron mines in the world (Fig. 8.1; Beukes and Gutzmer 2008; de Kock et al. 2009). Spherule layers are hosted by both BIFs and associated strata and it appears that most can be correlated between the two basins (Simonson et al. 2009a; Hassler et al. 2011; see Sect. 8.4). The Hamersley layers are described first, then those in the Griqualand West Basin, and finally their correlations are discussed.