We welcome a discussion of the Jeerinah Impact Layer and its likely stratigraphic correlation. Glikson accepts the interpretation of the Jeerinah Impact Layer as a distal ejecta layer as first proposed by Simonson et al. (2000), but questions our correlation of the Jeerinah Impact Layer with the Carawine layer, as well as our suggestion that the layer contains debris from a continental impact.
Glikson argues for a correlation between the Wittenoom (rather than Jeerinah) and Carawine spherule beds based on two lines of evidence, the first of which is the different location of spherules relative to interpreted impact-related tsunami deposits in the Jeerinah Impact Layer and the Carawine layer. Specifically, spherules in the Jeerinah Impact Layer from the Hesta Site (see Figure 1 in Rasmussen and Koeberl, 2004) underlie a debris flow breccia, whereas in the Ripon Hills area, spherules occur above a thick dolomixtite unit (Simonson, 1992), suggesting differences in the timing of ejecta fallout and tsunami-induced debris flow in the two layers.
Secondly, Glikson cites Pb/Pb carbonate isochron dates by Woodhead et al. (1998) as further supportive evidence of a correlation between the Wittenoom layer (2541 +18/–15 Ma) and the Carawine layer (2548 +26/–29 Ma). While the Pb/Pb date for the Wittenoom layer is consistent with a SHRIMP U-Pb zircon date (2561 ± 8 Ma) for an underlying tuff in the Wittenoom Formation (Trendall et al., 1998), there is no supportive age data for the Carawine layer. Simonson et al. (2002) recently proposed a correlation between the Jeerinah Impact Layer and the Carawine layer based on palaeogeographic considerations, the presence of “larger, more irregular particles of impact-generated, flow-banded melt” in the Jeerinah Impact Layer (at the Hesta site) and the Carawine layer (at Ripon Hills and the Tarra Tarra turnoff site), and similar 53Cr/52Cr compositions (Shukolyukov et al., 2002) in the Jeerinah Impact Layer and the Carawine layers, pointing to an ordinary chondrite as the impactor for both layers.
This correlation was recently confirmed by SHRIMP zircon geochronology of a tuff bed ~30 m below the spherule-bearing dolomixtite unit in the Ripon Hills area. The tuff yielded a U-Pb date (Rasmussen et al., 2005) that is indistinguishable from a SHRIMP U-Pb zircon date (2629 ± 5 Ma) from a tuff in the uppermost Jeerinah Formation in the Hamersley Province (Trendall et al., 2004). This result suggests that the Jeerinah Impact Layer and the Carawine spherule bed formed during a single impact ~2.63 Ga. This result has implications for the veracity of Pb/Pb carbonate dating and raises questions about the relationship between the spherule layers and debris flows used as evidence for a Wittenoom-Carawine correlation.
Glikson also questions our suggestion that “The presence of shocked quartz in the Jeerinah spherule bed indicates that the impact site contained quartz, favoring a continental target rather than an oceanic site” (Rasmussen and Koeberl, 2004, p. 1031). Glikson (2005, p. 125) states that “the absence of shocked quartz grains in recorded early Precambrian ejecta and the largely ferromagnesian compositions of the microkrystite spherules … are consistent with impacts into mafic to ultramafic crust.” Now that we document shocked quartz in one of the layers, Glikson argues that the presence of shocked quartz is still consistent with his conclusions. We note that the only demonstrable target component in the Jeerinah Impact Layer is the shocked quartz. All other minerals (including chlorite) could be diagenetic, metamorphic, or detrital (and unrelated to the impactor or target rock). Therefore, we maintain that the presence of shocked quartz favors a continental, over an oceanic, impact site.
Finally, Glikson's comment that “A study of orientation of the planar deformation features in the shocked quartz fragment (Rasmussen and Koeberl, 2004) may yield further clues to the shock pressures involved in the ~2.63 Ga impact,” is an impractical suggestion. Rasmussen and Koeberl (2004) already noted that the orientations of the planar deformation features in the quartz grain were measured by universal stage, but it is obvious that such measurements are only meaningful if a statistically significant number of grains are present, which is clearly not the case here.