ABSTRACT

Correlations of continental Triassic and marine Jurassic rocks in Wyoming, northwestern Colorado, and northeastern Utah are based on 150 measured sections. In central Wyoming, these rocks are about 1,000 ft thick and consist, from bottom to top, of the following formations: Crow Mountain and Popo Agie (Triassic), Nugget (Triassic? and Jurassic?), and Gypsum Spring and Sundance (Jurassic). The lower part of the Nugget has been named. South and east of the Wind River basin, rocks equivalent to the Crow Mountain are called the Jelm Formation. In northwestern Colorado and northeastern Utah, Jurassic and Triassic rocks discussed here are about 1,500 ft thick and consist, from bottom to top, of the following formations: Chinle (Triassic), Glen Canyon (Triassic and Jurassic), and Carmel and Curtis (Jurassic). These formations thin southeast toward central Colorado. Near Boulder they are represented by only a 35-ft-thick equivalent of the Sundance. Particularly useful in correlation are purple and ocher analcime-rich rocks of the Popo Agie; the various units of the Redwater Shale Member and underlying member of the Sundance; and unconformities at the top of the Popo Agie, and at the base, in the middle, and near the top of the Sundance. The unconformity at the base of the Sundance is of Middle Jurassic age and is characterized by chert pebbles which occur in an area extending from the northeastern corner of Wyoming to Zuni, New Mexico (700 mi), and from the San Rafael Swell, Utah, to Boulder, Colorado (300 mi).

The Jelm extends from Wyoming about 50 mi into north-central Colorado. The Popo Agie and the lower and upper parts of the Nugget of central Wyoming correlate with the lower and upper parts of the Chinle, and with the Glen Canyon of the Uintas, respectively. The Gypsum Spring as defined at the type section is represented by the unconformity beneath the chert-pebble zone at Manila, Utah, on the north flank of the Uintas. The Sundance correlates with the Curtis and Entrada, and possibly with part of the Carmel of Manila, Utah.

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