New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic time scales such as those by Gradstein et al. (2004) and Gradstein et al. (2012) are valuable reference tools for all geologists involved in geochronologic studies. Unfortunately the Gradstein et al. 2012 Time Scale has failed to include important published geochronologic data for latest Cretaceous strata generated in the San Juan Basin, New Mexico. Fassett and Steiner (1997) published 40Ar/39Ar sanidine ages for multiple altered volcanic ash beds from uppermost Cretaceous strata in the San Juan Basin, New Mexico. Five of these ash beds were in a 21-km long area of bedrock exposure in the southwest part of the San Juan Basin. Two independent paleomagnetic traverses had been obtained in the same area and both contained the paleomagnetic reversal from magnetochron C33n to C32r. With three precise 40Ar/39Ar ages below this reversal and two above, it was thus possible to precisely date this reversal as 73.50 ± 0.18 Ma. Fassett (2000) added an additional 40Ar/39Ar age and recalculated the reversal at exactly the same age. The Gradstein et al. (2004) paper ignored these two publications and gave the C33n-C32r paleomagnetic reversal an age of 73.577 Ma. Fassett (2009) addressed the incorrect age for the C33n-C32r reversal in Gradstein et al. (2004) and published a corrected time scale for uppermost Cretaceous magnetochrons from C33n to C28n. In this paper Fassett also recalibrated Western Interior Ammonite zone (WIAZ) boundaries that were also shown incorrectly in the Gradstein et al (2004) report. Gradstein et al. (2012) ignored all of these publications and assigned an age of 74.309 Ma to this reversal determined by projections and extrapolations vs. the actual, precise recalibrated age of 74.22 Ma and also showed outdated and incorrect WIAZ boundary ages. The correct ages for these important boundaries are in Fassett (2009) but must be recalibrated to current standards.

References:

1. Fassett, J.E., 2000, Geology and coal resources of the Upper Cretaceous Fruitland Formation, San Juan Basin, New Mexico and Colorado, Chapter Q, in Kirschbaum, M. A., Roberts, L.N.R., and Biewick, L.R.H., eds., Geologic Assessment of Coal in the Colorado Plateau: Arizona, Colorado, New Mexico, and Utah, compiled by Colorado Plateau Coal Assessment Group: U.S. Geological Survey Professional Paper 1625-B, p. Q1-Q131. [CD-ROM]. http://pubs.usgs.gov/pp/p1625b/Reports/Chapters/Chapter_Q.pdf
2. Fassett, J.E., 2009, New geochronologic and stratigraphic evidence confirms the Paleocene age of the dinosaur-bearing Ojo Alamo Sandstone and Animas Formation in the San Juan Basin, New Mexico and Colorado: Palaeontologia Electronica, v. 12, no. 1, 146 p. (on-line pub. at http://palaeo-electronica.org/splash/index12_1.html).
3. Fassett, J.E., and Steiner, M.B., 1997, Precise age of C33n-C32r magnetic-polarity reversal; San Juan Basin, New Mexico and Colorado, in Anderson, O. J., Kues, B.S., and Lucas, S.G., eds., Mesozoic geology and paleontology of the Four Corners Region: New Mexico Geological Society 48th Annual Field-Conference Guidebook, p. 239-247.
4. Gradstein, F.M., Ogg, J.G., Schmitz, M.D., and Ogg, G.M., The geologic time scale 2012: Elsevier, 2 volumes, 1144 p.
5. Gradstein, F.M., Ogg, J.G., and Smith, A.G., 2004, A geologic time scale 2004: Cambridge University Press, Cambridge, UK., 589 p.