Paleomagnetism of a polarity transition in the Lower (?) Triassic Chug Water Formation, Wyoming

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Abstract
A total of 156 oriented specimens, taken at stratigraphic intervals of 3 to 20 cm, have been collected from the Red Peak Member of the Chugwater Formation at three localities approximately 25 km southeast of Dubois, Wyoming. The stratigraphic interval containing the polarity transition in the remanent magnetic field is about 1.0 m. All specimens were thermally demagnetized at 500°, 550°, 620°, and 660°C and show directions of magnetization of normal, reversed, and transitional character as well as a remarkable decrease of the remanent intensity in the transition zone. The section sampled is divided into five intensity periods: two periods recording the dipolar field, one recording a dipolar field of decreasing intensity, one recording a dipolar field of increasing intensity, and a period of low intensity recording the field present during the 180° directional swing. A virtual geomagnetic pole (VGP) path obtained for a N → R polarity transition makes a fairly smooth transit, with the path confined to a sector of longitude between 85°W and 110°E. The reversal's onset is first indicated by a slow movement of the VGP across northern latitudes. The VGP then lingers to two positions 60°N and 40°N (35°E to 70°E) and 20°N and 40°N (335°E to 5°E), suggesting brief periods of field stability (or nondipolar field dominance). The onset of the transition appears to be more rapid than the recovery. The VGP path observed at a northern hemisphere site is located about 90° from the site. The pole position derived from stable normal and reversed portions of this study (115.5°E, 46.4°N, α95 = 4.1, dp = 2.2, dm = 4.2) lies approximately in the center of the main group of poles reported for other Triassic units.