Reply to ‘‘Comment on: ‘K–Ar evidence from illitic clays of a Late Devonian age for the 120 km diameter Woodleigh impact structure, Southern Carnarvon Basin, Western Australia’’

Abstract

The isotopic and/or stratigraphic age and morphometric analysis of major asteroid/comet impacts are directly relevant to identification of their potential environmental consequences. Renne et al.’s criticisms hinge on the premise that the diameter of Woodleigh is ∼60 km and the impact is of probable pre-Devonian age. Here we clarify the evidence for a Late Devonian age and a diameter of 120 km. We elucidate (A) the methodology and implications of K–Ar analyses of illitic clay minerals; (B) stratigraphic age constraints on the impact event; (C) seismic reflection evidence for a composite fault-bounded central uplift dome, consisting of a 20–25 km wide granitoid plug mantled by a 31–37 km wide updomed sedimentary collar, which widens with depth; (D) morphometric evidence relevant to definition of the outer diameter; and (E) correlations with extinction events. The impact origin of the 120 km diameter multi-ring geophysical anomaly identified by Iasky and Mory and Mory et al. was confirmed by drilling (GSWA Woodleigh 1), recovering shock metamorphosed granitoid at 171–333 m depth. Stratigraphic constraints indicate a pre-early Jurassic post mid-Devonian age. Rb–Sr biotite, K–Ar biotite, K–Ar feldspar, UV-laser Ar–Ar biotite, feldspar and devitrified glass, apatite fission track, and paleomagnetic pole analyses yielded a range of relic and reset isotopic ages inconsistent with these stratigraphic constraints. However, K–Ar studies of coarser grained illite and mixed layered illite/smectite, separated from the basement core and derived clasts, yielded a concentration of isotopic ages around 359±4 Ma, as well as younger ages, interpreted in terms of intermediate-temperature (200–250°C) hydrothermal activity triggered by the impact event. The impact interpretation of Mory et al. was questioned by Reimold and Koeberl, who claimed that, should Woodleigh prove to be of impact origin, it is not larger than about 40 km across. However, shock metamorphic features were confirmed by later studies. Bevan and Hough and Hough et al. reiterated the criticism of a 120 km diameter, instead suggesting a 60 km diameter. © 2002 Elsevier Science B.V. All rights reserved.