Pg. 12-18. Name proposed for member of the Haystack Mountains Formation (new) of Mesaverde Group (rank raised). At type section, composed of thin-bedded, pale-yellowish-gray, brown-weathering, fine-grained sandstone with sandstone concretions and abundant fossil burrows (OPHIOMORPHA), 220 feet (67 m) thick; is cliff-forming. Gradationally overlies lower unnamed shale member and conformably underlies middle unnamed shale member of the Haystack Mountains Formation. Is a shallow-water marine deposit. Age is Late Cretaceous (early Campanian). Table 1 shows correlation with other units of the same age in south-central Wyoming.
Type section: measured 9.6 km north of town of Sinclair, southern Haystack Mountains, in NE/4 NE/4 sec. 22, NE/4 sec. 23, and SE/4 SE/4 sec. 14, T. 22 N., R. 86 W., Carbon Co., south-central WY (Greater Green River basin). Named from exposures at O'Brien Spring, in SW/4 sec. 9, T. 24 N., R. 86 W., Seminole Dam SW 7.5-min quadrangle.

Source: US geologic names lexicon (USGS Bull. 1520, p. 225); GNU records (USGS DDS-6; Denver GNULEX).