The U-Bar Formation (Aptian-Albian) consists of five members: they are (in ascending order) brown limestone, oyster limestone, limestone-shale, reef limestone, and suprareef limestone. These members crop out in the area southwest of the Big Hatchet Mountains, Hidalgo County, New Mexico. The U-Bar Formation is overlain by the Mojado Formation (Albian) and underlain by the Hell-to-Finish Formation (pre-Aptian). A composite stratigraphic section (3,115 ft; 950 m) southwest of the Big Hatchet Mountains has been compiled from measured sections at Pierce Tank and Hell-to-Finish Tank.

The brown limestone member contains thin-bedded, fine-grained, ledge-forming, silty, gypsiferous, dolomitic to oolitic limestones, arkosic sandstones and siltstones which weather red-brown to dusky red. The sparse fauna contains fragmented diminutive bivalves, Ostrea-type bivalves, and turritelloid-type gastropods.

The yellow-gray to red-brown oyster limestone member primarily contains thin-bedded, fine to medium-grained, clastic, silty limestones and covered shale intervals. Massive limestones, thin-bedded arkosic sandstones, and siltstones occur near the base and middle of the member. This richly fossiliferous member contains some limestones consisting almost entirely of small Exogyra and Ostrea-type bivalves. Also common are Pecten, turritelloid-type gastropods, and serpulid worm tubes which commonly encrust Ostrea-type bivalves.

The limestone-shale member contains thin to medium-bedded ledge-forming, fine-grained limestone. These pitted blue-gray to medium-gray weathering limestones contain a sparse fauna of turritelloid-type gastropods, small, thin-shelled bivalves, fragmented echinoid spines, and abundant Orbitolina, and large Lunatia-type gastropod steinkerns in the upper part of the member.

The reef limestone member is a massive, fine to medium-grained limestone. The light-gray weathering limestone contains fragmented Ostrea-type bivalves, rudists, and abundant orbitolinids at the base.

The suprareef limestone member (locally thin) is a medium-grained, clastic limestone, weathering light gray and containing abundant orbitolinids in the base and fragmented Ostrea-type bivalves above. The fossils commonly are iron-stained and weather in relief.

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