REPLY: doi: 10.1130/G23941Y.1 2005). The ages presented in the fi eld guide transitions. The thicker volcanic units in Upper were provided without supporting data, and Guandao section are tuffaceous siliciclastic Daniel J. Lehrmann were not intended to be used for boundary age mudstone. Thicker volcaniclastic units at Upper Department of Geology, University of assignments or cited, as specifi ed in the fi eld Guandao, and correspondingly thinner carbon- Wisconsin–Oshkosh, Oshkosh, Wisconsin guide (Lehrmann et al., 2005). ate units, resulted from lower rates of carbonate 54901, USA Bucher et al. suggest that the fi rst appearance accumulation and more rapid siliciclastic ac- Jahandar Ramezani of the conodont Chiosella timorensis, which we cumulation at the basin margin farther from the Samuel A. Bowring used as a proxy for the O-A boundary, is diachro- platform source of carbonate. Mark W. Martin nous based on comparison of the preliminary The argument of Bucher et al. that the fi rst Department of Earth, Atmospheric, and geochronology from Upper Guandao section appearance of Cs. timorensis was diachronous Planetary Sciences, Massachusetts Institute (Lehrmann et al., 2005) and the high-precision in our sections is fl awed because it rests on of Technology, Cambridge, Massachusetts dates presented in Lehrmann et al. (2006). Be- the preliminary age of sample GDGB-O from 02139, USA low we demonstrate that the conodont occur- Upper Guandao section. Correlation between Paul Montgomery rences are isochronous within the constraints the two sections (Fig. 1) is corroborated by Paul Enos of paleomagnetic-reversal and carbon-isotope paleo magnetic reversals and a large positive iso- Department of Geology, University of Kansas, stratigraphy between the sections. tope excursion, showing that the fi rst occurrence Lawrence, Kansas 66045, USA Figure 1 illustrates the correlation between of Cs. timorensis, and the occurrences of asso- Jonathan L. Payne Lower and Upper Guandao sections. The de- ciated conodonts, are isochronous within the Department of Geological and Environmental piction of the Lower Guandao section is the constraints of the data. In both sections, the O-A Sciences, Stanford University, Stanford, same as our Figure 2 (Lehrmann et al., 2006) boundary is delineated by the fi rst occurrence of California 94305, USA with the addition of high-resolution conodont Cs. timorensis (and faunal turnover of several Michael J. Orchard data that resulted in a 2.6 m downward shift of associated species; Fig. 1) that occurs below the Geological Survey of Canada, Vancouver, the O-A boundary. The O-A boundary remains peak of the positive carbon isotope excursion British Columbia V6B 5J3, Canada bracketed by volcanic-ash horizons PGD-2 near the base of the Aegean, and the shift from Wang Hongmei and PGD-3. Adjustment in the boundary posi- predominantly reversed to normal polarity near Wei Jiayong tion yields a new interpolated boundary age of the base of the Bithynian. We agree with Bucher Guizhou Bureau of Geology and Mineral 247.24 Ma. The Upper Guandao section has et al.’s suggestion that a boundary age should Resources, Guiyang, Guizhou, People’s been updated from the very preliminary form not be constrained solely by the fi rst occurrence Republic of China given in our fi eld guide (Lehrmann et al., 2005) of one species, and we have used several cono- by integrating stratigraphic thicknesses of sev- dont species in delineating the boundary. Bucher et al.’s Comment (2007) questioned eral measurements, adding high-resolution We chose not to use the Upper Guandao the validity of our 247.2 Ma age estimate for conodont data, paleomagnetic-reversals, and section for delineation of the O-A boundary in the Early-Middle Triassic (Olenekian-Anisian, carbon-isotope data. Lehrmann et al. (2006) because the geochrono- O-A) boundary based on a comparison of recent Lower and Upper Guandao sections occur in logical data for GDGB-O indicated a great data from Lower Guandao section presented the deep-marine slope (Lower Guandao) to toe deal of complexity. However, as discussed by us (Lehrmann et al., 2006) with preliminary of slope (Upper Guandao) facies adjacent to a in Ramezani et al.’s Reply (2007) to Buchur data reported from the adjacent Upper Guandao carbonate platform (Lehrmann et al., 2005). et al.’s Comment, we can now confi dently as- section from a fi eld-trip guide (Lehrmann et al., Rapid facies changes are the norm in such sign a depositional age to this ash. Ovtcharova et al. (2006) interpreted the O-A boundary to lie between 248.1 Ma and 247.8 Ma on the basis of a single new age of 248.1 ± 0.4 Ma they obtained from the Upper Spathian, and the citation of a preliminary age date (GDGB-O) from our fi eld trip guide (Lehr- mann et al., 2005). The extensive and integrated chronostratigraphic and geochronological data presented by us (Lehrmann et al., 2006) provide GDGB a far more robust constraint on the boundary age GDGB-0 at 247.2 (± 0.4). REFERENCES CITED Bucher, H., Hochuli, P.A., Schaltegger, U., PGD-3 Ovtcharova, M., Galfetti, T., Brayard, A., Goudemand, N., and Guex, J., 2007, Timing PGD-2 of recovery from the end-Permian extinction: PGD-1 Geochronologic and biostratigraphic constraints from south China: Comment: Geology, doi: 10.1130/G23609C.1. Lehrmann, D.J., Ramezani, J., Bowring, S.A., Martin, M.W., and Montgomery, P., Enos, Paul, Payne, J. L., Orchard, M. J., Wang, H., GDGB-0 Wei, J., 2006, Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China: Geology, v. 34, p. 1053–1056, doi: 10.1130/ Figure 1. Olenekian-Anisian boundary at Lower and Upper Guandao section showing G22827A.1. correlation of biostratigraphy, magnetostratigraphy, and carbon isotope data. mab—meters Lehrmann, D.J., Payne, J.L., Enos, P., Montgomery, above base. P., Wei, J., Yu, Y., Xiao, J., and Orchard, M.J., e136 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/35/1/e136/3534326/i0091-7613-35-1-e136.pdf by guest on 26 September 2021 2005, Field Excursion 2: Permian-Triassic in detail by presenting new U-Pb zircon data boundary and a Lower-Middle Triassic for the vol canic layer (sample GDGB-0) and boundary sequence on the Great Bank of Guizhou, Nanpanjiang basin, southern Guizhou to demonstrate the unequivocal correlation be- Province: Albertiana, v. 33, p. 169–186. tween the Upper and Lower Guandao sections. Ovtcharova, M., Bucher, H., Schaltegger, U., Forty U-Pb analyses of single zircons are Galfetti, T., Brayard, A., and Guex, J., 2006, presented for sample GDGB-0 (Table 1). The New Early to Middle Triassic U-Pb ages from sample was collected from a ~7-m-thick layer of south China: Calibration with ammonoid biochronozones and implications for the dominantly volcaniclastic tuff that occurs a short timing of Triassic biotic recovery: Earth and distance above the Olenekian-Anisian bound- Planetary Science Letters, v. 243, p. 463–475, ary in Guandao (Lehrmann et al., 2006, 2007). doi: 10.1016/j.epsl.2006.01.042. Figure 1 shows 26 out of 40 analyses, including Ramezani, J., Bowring, S.A., Martin, M.W., Figure 1. U-Pb geochronologic results for Lehrmann, D.J., Montgomery, P., Enos, P., new CA-TIMS analyses, which yield concor- 206 238 sample GDGB-0, Upper Guandao section, Payne, J.L., Orchard, M.J., Wang, H., and dant Pb/ U dates between 244 and 248 Ma. south China. Dashed boxes delineate data Wei, J., 2007, Timing of recovery from the A coherent population of 17 analyses that over- used for age calculation. Not all analyses are end-Permian extinction: Geochronologic and lap within uncertainty yields a weighted mean shown; see Table 1 for complete data. biostratigraphic contstraints from south China: 206Pb/238U date of 246.301 ± 0.073(0.11)[0.38] Reply: Geology, doi: 10.1130/G23942Y.1. Ma with a MSWD of 1.17. A subset of 12 most precise analyses from this group (including REPLY: doi:10.1130/G23942Y.1 8 CA-TIMS analyses), produce an identical tween the two Guandao sections. We further date of 246.302 ± 0.064(0.10)[0.37] Ma with a believe that our estimate for the age of the Jahandar Ramezani* MSWD of 0.56. Thus, we interpret 246.30 Ma Olenekian-Anisian boundary (and duration of Samuel A. Bowring as the best estimate for the age of the dominant the Early Triassic) supersedes all previous esti- Mark W. Martin† volcanic component in sample GDGB-0, and mates because it is based on a set of internally Department of Earth, Atmospheric, and inferentially its (maximum) depositional age. consistent geochronologic data from a single Planetary Sciences, Massachusetts Institute The 246.30 ± 0.07 Ma date for sample stratigraphic section with excellent biostrati- of Technology, Cambridge, Massachusetts GDGB-0 is consistent with its position above graphic, chemostratigraphic, and magneto- 02139, USA the Olenekian-Anisian boundary and with stratigraphic controls. Daniel J. Lehrmann our estimate of 247.2 Ma for the boundary REFERENCES CITED Department of Geology, University of itself determined in the Lower Guandao sec- Wisconsin–Oshkosh, Oshkosh, Wisconsin tion (Lehrmann et al., 2006). It unequivocally Bucher, H., Hochuli, P.A., Schaltegger, U., 54901, USA substantiates the correlation between the two Ovtcharova, M., Galfetti, T., Brayard, A., # Goudemand, N., and Guex, J., 2007, Timing Paul Montgomery Guandao sections as constrained by conodont of recovery from the end-Permian extinction: Paul Enos biostratigraphy and carbon isotopes (Lehrmann Geochronologic and biostratigraphic con- Department of Geology, University of Kansas, et al., 2007). straints from south China: Comment: Geol- Lawrence, Kansas 66045, USA In their ammonoid and ash bed U-Pb study ogy, doi: 10.1130/G23609C.1.