A New Calibration Point for the Late Cretaceous Time Scale: The 40Ar/39Ar Isotopic Age of the C33r/C33n Geomagnetic Reversal from the Judith River Formation (Upper Creataceous), Elk Basin, Wyoming, USA

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Abstract
We present a new calibration point for the latest Cretaceous time scale, an interval that at present contains no well-dated polarity reversals. The magnetostratigraphy of Campanian-aged sediments in the northern Bighorn basin of Wyoming documents a geomagnetic reversal in the upper part of the Judith River Formation that occurs in direct association with ash fall layers dated by the 40Ar/39Ar method. Extrapolation based on three new 40Ar/39Ar laser fusion dates from sanidines extracted from the ash fall layers dates the reversal at 79.34 Ma; thus the geomagnetic reversal in the Judith River formation can be confidently correlated to the C33r/C33n boundary of the Geomagnetic Reversal Time Scale. This age estimate is in good agreement with recent time scales that date this polarity interval by interpolation between few and widely spaced calibration points. The Judith River Formation is nonmarine and cannot be correlated regionally by traditional ammonite biostratigraphic methods. This study shows that in northern Wyoming the formation was deposited from 80 to 79 Ma, and can be correlated both magnetostratigraphically and by isotopic age to the Campanian-aged ammonite zones of the marine record that range from the top of Baculites obtusus to the lower part of B. perplexus (early form). The Judith River is underlain by the Claggett Shale, which contains one of the ash fall levels of the Ardmore bentonite. This ash fall is an important datum level found throughout the Western Interior U.S. in the range zone of B. obtusus, one we have dated at 80.71 +/- 0.55 Ma.

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