Bucher et al.'s Comment (2007) on our recently reported age for the Olenekian-Anisian boundary in south China (Lehrmann et al., 2006) concerns a discrepancy between our results and a preliminary age from a nearby section reported in a field excursion guide (Lehrmann et al., 2005). Specifically, the controversy concerns the age of a volcanic layer from the Lower Anisian of the Upper Guandao section (Nanpanjiang Basin, south China). We welcome this opportunity to address the geochronology in detail by presenting new U-Pb zircon data for the vol canic layer (sample GDGB-0) and to demonstrate the unequivocal correlation between the Upper and Lower Guandao sections.
Forty U-Pb analyses of single zircons are presented for sample GDGB-0 (Table 1). The sample was collected from a ~7-m-thick layer of dominantly volcaniclastic tuff that occurs a short distance above the Olenekian-Anisian boundary in Guandao (Lehrmann et al., 2006, 2007). Figure 1 shows 26 out of 40 analyses, including new CA-TIMS analyses, which yield concordant 206Pb/238U dates between 244 and 248 Ma. A coherent population of 17 analyses that overlap within uncertainty yields a weighted mean 206Pb/238U date of 246.301 ± 0.073(0.11)[0.38] Ma with a MSWD of 1.17. A subset of 12 most precise analyses from this group (including 8 CA-TIMS analyses), produce an identical date of 246.302 ± 0.064(0.10)[0.37] Ma with a MSWD of 0.56. Thus, we interpret 246.30 Ma as the best estimate for the age of the dominant volcanic component in sample GDGB-0, and inferentially its (maximum) depositional age.
The 246.30 ± 0.07 Ma date for sample GDGB-0 is consistent with its position above the Olenekian-Anisian boundary and with our estimate of 247.2 Ma for the boundary itself determined in the Lower Guandao section (Lehrmann et al., 2006). It unequivocally substantiates the correlation between the two Guandao sections as constrained by conodont biostratigraphy and carbon isotopes (Lehrmann et al., 2007).
In their ammonoid and ash bed U-Pb study of the nearby Jinya section (Nanpanjiang Basin, south China), Ovtcharova et al. (2006) placed the Spathian-Anisian boundary between 248.1 Ma and 247. 8 Ma, using our preliminary date for sample GDGB-0 reported in a non–peer reviewed field excursion guide (Lehrmann et al., 2005). This was despite a clear assertion in the guide that the date was “preliminary” and that it “should not be cited” (Lehrmann et al., 2005, p. 179 and Fig. 17). The cited date, however, provided Ovtcharova et al. (2006) with an age constraint on the boundary and thereby substantiated their estimate of 4.5 ± 0.6 m.y. for the minimal duration of Early Triassic. Our combined age and stratigraphic results from Guandao provides a more refined estimate of 5.4 ± 0.6 m.y. for the duration of the Early Triassic, given a similar age of 252.6 Ma for the base of the Triassic. Furthermore, considering the occurrence of ~10 m of “Transition Beds” of unknown biozonal affinity and evidence of drastic changes in depositional environment at the Spathian–Anisian boundary interval in the Jinya section (Ovtcharova et al., 2006), the Guandao uniform pelagic carbonate sections provide a more reliable constraint on the boundary age.
In summary, our new U-Pb data and refined stratigraphic results from the Upper Guandao section invalidate the argument of Bucher et al. regarding correlation inconsistencies between the two Guandao sections. We further believe that our estimate for the age of the Olenekian-Anisian boundary (and duration of the Early Triassic) supersedes all previous estimates because it is based on a set of internally consistent geochronologic data from a single stratigraphic section with excellent biostratigraphic, chemostratigraphic, and magnetostratigraphic controls.
