Named as the upper member of the Phosphoria formation (new) for Rex Peak in the Crawford Mountains of Rich Co, UT on the Wasatch uplift. No type locality designated, but a section 240 ft thick measured in sec 12, T10S, R44E, Bear Lake Co, ID on the Wasatch uplift described. Rex at that locality is composed of gray limestone banded by gray to black chert at base, an iron-stained chert in the middle, and a black, cherty, red-brown to purple-weathering shale at top. Overlies an unnamed phosphatic part of the Phosphoria. Is older than Woodside shale. Is locally difficult to distinguish from Woodside wherever the shaly facies forms the top. Conspicuous unit because of its resistancy. Has sponge spicules, crinoid stems, and brachiopods, though it is usually unfossiliferous. Recognized in parts of southeast ID, northeast UT, and western WY [Greater Green River basin?]. Assigned a Permian?, Carboniferous age.

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

Named by Richards and Mansfield (1912) for Rex Peak, Crawford Mountains, Rich Co, UT. No description of a section in Crawford Mountains ever published but a section west of Phosphoria Gulch was described where chert is dominant constituent. Rex beds in Crawford Mountains are carbonate rock. Hypothesized by McKelvey (p. 25-28) that Richards and Mansfield intended Phosphoria Gulch section as type. Term Rex retained because of wide use. Includes a variety of black to white cherty rocks (chert, mudstone, phosphorite and carbonate with masses of chert). Bedding may be structureless, nodular, wavy, or lenticular. Contains coarse, light-gray, bioclastic limestone near middle at many localities. Occurs in northern UT (Wasatch uplift), eastern ID (Snake River basin), southwest MT (Montana folded belt province) and western WY (Greater Green River basin). Overlies Meade Peak Member of Phosphoria; underlies Franson tongue of Park City Formation at most localities. Theories of depositional environment discussed. Fossils described (crinoids, brachiopods, bryozoans, etc.). Is of Permian age.

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

Is member of Phosphoria Formation in Caribou Co (Wasatch uplift) and Bingham Co (Snake River basin), southeastern ID. Overlies Meade Peak Phosphatic Shale Member of Phosphoria. Biostratigraphic dating on basis of conodonts. NEOGONDOLELLA PHOSPHORIENSIS Zone (earliest Wordian) found in uppermost Meade Peak and most of Rex. Wordian Stage, Guadalupian Series of Early Permian age.

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