Named as the lower member of the Arapien shale (new) for outcrops in the valley of Twelvemile Creek west of Arapien Valley, Sanpete Co, UT on the Wasatch uplift. No type locality designated. Divisible into four units (ascending): 1) gray limestone, generally thin-bedded, 2) light-gray, very thin bedded shale and siltstone with occasional thin beds of finely rippled sandstone, 3) gray argillaceous and gypsiferous gray shale with irregular red blotches which are locally prominent, and 4) compact red-salt-bearing shale. Is about 7,000 ft thick [inferred]. Compared with the Carmel formation. Underlying unit not discussed. Overlain by the newly named Twist Gulch member of the Arapien. Fossils (cephalopods, crinoids, mollusks) in unit 2 are like those found in the Carmel and are of Late Jurassic age. Stratigraphic table.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Term Arapien shale restricted stratigraphically to rocks formerly assigned only to the Twelvemile Canyon member of the Arapien. Twelvemile Canyon member not used.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Arapien Shale of central UT in the Nephi-Levan area, west side of Gunnison Plateau, Juab Co (Great Basin province) is 5,464 ft thick, and is divisible into Twelvemile Canyon Member (3590 ft thick, at base) of Bajocian, Bathonian, and early Callovian age, and Twist Gulch Member (1874 ft thick, at top) of early and middle Callovian age. In the Gunnison-Salina-Richfield area, Sevier and Sanpete Cos (Wasatch uplift), Arapien represents less of Middle Jurassic time; the Twelvemile Canyon is 1800-2700 ft thick and of late Bathonian and early Callovian age; the overlying Twist Gulch is 1730 ft thick and of early and middle Callovian age. Twelvemile Canyon thought to resemble members of Twin Creek Limestone from base of Gypsum Spring to basal Leeds Creek Member; Twist Gulch thought to be lithologically similar to Preuss Sandstone. Stratigraphic chart. Arapien unconformably overlies Navajo Sandstone. Arapien unconformably underlies Curtis Formation in Gunnison area, and younger rocks elsewhere.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Abandoned as a member of Arapien Shale of the Wasatch uplift. Rocks formerly assigned to it now assigned to the Arapien Shale which is not divided into members. Abandonment of Twelvemile Canyon and stratigraphic restriction of term Arapien to rocks formerly called Twelvemile Canyon is a recognition by U.S. Geological Survey of a stratigraphic change made by Hardy in 1952 that has been followed by most workers.

Source: GNU records (USGS DDS-6; Denver GNULEX).