The purpose of our paper (Fildani et al., 2009) was to report a suite of 205 reliable single-grain U-Pb zircon ages from ash horizons in the southwestern Karoo Basin, thus demonstrating the potential of this technique to determine ages of ash beds elsewhere in the basin. The zircons yielded a late Permian to earliest Triassic population of ages that young upward with the stratigraphy. These results challenge the oversimplified tabular lithostratigraphic model for this part of the Karoo basin, suggesting that a reevaluation of the Ecca and lower Beaufort groups is necessary.
We have no reason to question the peer-reviewed interpretations of the Skoorsteenberg Formation and correlative Laingsburg Formation as having been deposited in marine environments (Bouma and Wickens, 1994; Fildani et al., 2007; Flint et al., 2007; Hodgson et al., 2006; Johnson et al., 2001; Scott et al., 2000; Wickens and Bouma, 2000; Wild et al., 2009). Higgs (2010) would prefer that we accept his lacustrine interpretation for the Skoorsteenberg Formation, which is bewildering since he admits that the ichnofauna Chondrites, Lophoctenium, and Lorenzinia are “invariably marine.” He emphasizes the two ichnogenera, Gordia and Helminthopsis, which apparently support his non-marine interpretation, except that they are also common to marine deposits, including those of the deep sea (e.g., Wetzel et al., 2007; Gingras et al., 2008; Heard and Pickering, 2008). In fact, Helminthopsis is often recognized in association with the Cruziana, Zoophycos, and Nereites ichnofacies (Pemberton et al., 2001; MacEachern et al., 2007). The trace fossil assemblage reported by Johnson et al. (2001) is overwhelmingly consistent with a marine setting. Of the eleven ichnogenera recognized, six (Chondrites, Cosmorhaphe, Helminthoida, Helminthopsis, Lorenzinia, and Paleodictyon) are common components of the Nereites ichnofacies (MacEachern et al., 2007); the other five ichnogenera are also widely reported from marine environments. We do acknowledge salinity fluctuations during early Karoo basin evolution (lower Ecca Group formations), but this in no way contradicts a deep-water setting.
Interestingly, Higgs disregards the recent publications of Tankard et al. (2009) and Scheffler et al. (2006) that address the basin's dynamics and paleogeography. Scheffler et al. (2006) clearly identified proxies for a fully marine Karoo Basin through their “turbiditic phase” (their figure 9). Wild et al. (2009) also supported a marine setting on the basis of tidal facies. Despite this corroborating evidence, Higgs claims that a marine interpretation is “doubtful” but then admits that he would not be surprised if there were marine bands in the succession. Higgs’ insistence on a non-marine or lacustrine setting for this part of the Karoo Basin is weakly supported only by ambiguous, isolated evidence. Together, however, the sedimentary facies, chemical proxies, and ichnofacies associated with the ash beds of Fildani et al. (2009) strongly support a marine interpretation for the Permian-Triassic (P-T) boundary in the Karoo basin.