New Mexico Geological Society Annual Spring Meeting — Abstracts

Global time scales, beginning with Harland, et al. (1989), followed by Gradstein et al. (2004) and Gradstein et al. (2012) have been based primarily on a combination of radiometric ages as related to paleomagnetic reversals (and more recently by orbitally tuned rock sequences) in rock strata throughout the world. These time scales have grown more precise as more radiometric ages have become available to fill in gaps present in earlier time scales. Unfortunately, rocks that can be dated using radiometric methods have not always been present in the same stratigraphic sequences where good paleomagnetic data have been obtained and vice versa. In an effort to improve on the precision of the age of one paleomagnetic reversal Fassett (2000) presented paleomagnetic data for a rock sequence in the southern San Juan Basin that included the uppermost Cretaceous Fruitland and Kirtland Formations. The paleomagnetic reversal from C33n to C32r was found in the upper part of this sequence. Within these same strata, eight altered volcanic ash beds were discovered and sanidine crystals from those ashes yielded relatively precise ⁴⁰Ar/³⁹Ar ages – six of these ash beds were below the C33n/C32r reversal and two were above it. Based on these ages, it was determined that this paleomagnetic reversal had an age of 73.50±0.19 Ma. The uncertainty for these eight ash-bed ages ranged from 0.13- 0.41 Ma. In 2008, some of the ash beds dated by Fassett (2000) were recollected in order to use modern mass spectrometry to more precisely determine the age of the C33n/C32r paleomagnetic reversal. Two of the recollected ash beds below the reversal yielded much more precise results compared to previous data yielding ages of 76.42±0.04 Ma and 75.24±0.04 Ma; an ash bed above the boundary had an age of 73.36±0.025 Ma. Based on these bracketing ages, and assuming a linear deposition rate, the age of the C33n/C32r reversal was found to be 73.782 Ma. The age for this reversal in Gradstein et al. (2012) is reported to be74.309 Ma – about 0.5 m.y. older than our newer and more precise age. We recommend that our new age of 73.782 for the C33n/C32r paleomagnetic reversal become the established age for this reversal used in future global time scales.

References:

1. Fassett, J.E., 2000, Geology and coal resources of the Upper Cretaceous Fruitland Formation, San Juan Basin, New Mexico and Colorado, 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, USGS Professional Paper 1625-B, Chapter Q (published in digital form on CD-ROM).
2. Gradstein, F.M., Ogg, J.G., Schmitz, M.D., and Ogg, G.M., 2012, The geologic time scale 2012: Elsevier, 2 volumes, 1144 p.
3. Gradstein, F.M., Ogg, J.G., and Smith, A.G., 2004, A geologic time scale 2004: Cambridge University Press, Cambridge, UK., 589 p.
4. Harland,W.B., Armstrong, R.L., Cox, A.V., Craig L.A., Smith, A.G., Smith, D.G., 1990, A geologic time scale, 1989, Cambridge University Press, Cambridge, 263 p.