Devonian conodont biochronology in geologic time calibration

Abstract

The conodont-based biochronology of the Devonian Period is calibrated herein on the basis of: (1) a recent, reliable radiometric dating (CLAQUE??-LONG et al. 1992) of a stratigraphic position just above the Devonian-Carboniferous boundary; and (2) new calculations of stage durations based on our estimates and those of a large number of other Devonian workers, which pertain to many stratigraphie sequences in several different paleotectonic settings globally. Using these tools we maintain the 46-m.y. duration for the Devonian Period given by the recent global timescale (GTS 89) of HARLAND et al. (1990). The main changes to GTS 89 are accomplished by raising the dating of the end of the Devonian by 9 m.y. from 363 Ma to 354 Ma and considerably shortening the bizarre estimate given by GTS 89 for the length of the oldest two Early Devonian stages. We reiterate our earlier expose?? (ZIEGLER & SANDBERG 1994) of the inaccuracies and misestimates of the ages of stage boundaries inherent in GTS 89 because of its misuse of conodont zones, which it employed as chrons to subdivide the Devonian Period. Our revisions provide the following new ages: 354 Ma, end of Devonian Period: 364 Ma. start of Famennian Stage; 369 Ma, start of Frasnian Stage; 376 Ma, start of Givetian Stage; 381 Ma, start of Eifelian Stage; 389 Ma start of Emsian Stage; 393 Ma, start of Pragian Stage; and 400 Ma, start of Lochkovian Stage and Devonian Period. Durations of stages are thus: Famennian, 10 m.y.; Frasnian, 5 m.y.; Givetian, 7 m.y.; Eifelian, 5 m.y.; Emsian, 8 m.y.; Pragian, 4 m.y.; and Lochkovian, 7 m.y.