Named for Opex mine, [T10S R3W, Eureka 7.5' quad], Juab Co, UT, Great Basin province. No type locality designated. Poorly exposed. Can be identified only by recognition of Cole Canyon dolomite (first used) beneath and Emerald dolomite member (named) above it. Mapped in central west part of area. Geologic map. Measured section described west of Eureka Peak where Opex is a 388 to 393 ft thick unit of dark-gray dolomite and dark shaly limestone. Some beds are mottled; some have short white spangles; some beds are thinly banded; some beds are cross-bedded. Conformably overlies the Cole Canyon dolomite; unconformably underlies the Ajax limestone (new). Fossils found up in the Opex were assigned to the Late Cambrian. These fossils were 300 ft above fossiliferous beds of the Cole Canyon. Boundary between Middle and Late Cambrian cannot be precisely located. Age is Late(?) Cambrian based on fossils.

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

Restricted to sandstone, limestone, dolomite, and shale sequence above the Middle Cambrian Cole Canyon dolomite (revised). The dusky blue dolomite may be oolitic, cross-bedded, mottled or studded with twig-shaped bodies (about 200 ft thick) removed from Opex and reassigned to underlying Cole Canyon dolomite. Two measured stratigraphic sections measured: 1) SE1/4 sec 24, T10S, R3W, Juab Co, Great Basin province, near Opex shaft; 2) W1/2 sec 34, T9S, R2W, Utah Co, Wasatch uplift, on east spur of Pinyon Peak. Underlies Ajax dolomite. Fossiliferous. Assigned to Late Cambrian. Geologic map. Cross sections.

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

Known in the East Tintic Mountains, Juab Co, UT in the Great Basin province. Correlation with newly named members of Orr Formation in the House Range and Wah Wah Mountains, Millard and Beaver Cos., UT and with other formations in the Great Basin province shown on fig. 2. Is of Late Cambrian age.

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