Designated as upper of two major oil producing zones in Paradox Salt basin and assigned to lower part of upper member of Hermosa formation. Probably named for Shell Bluff No. 3, discovery well for Bluff field, sec 4, T40S, R23E, San Juan Co, UT, Paradox basin. No type section designated, though occurs in 5470-5495 ft pay interval in discovery well. Zone is also productive at Boundary Butte, North Boundary Butte, Recapture Creek, and South Hovenweep fields, San Juan Co, UT, Paradox basin; East Boundary Butte and Bita Peak field, Apache Co, AZ, Black Mesa basin; and Hunav field, San Juan Co, NM, San Juan basin. Zone consists of predominately shelf limestones interbedded with some dark gray and black shale beds and laminae. Limestones are light gray fossiliferous, vuggy calcilutites, grading in part into fine-grained rarely oolitic calcarenites; chert is present in varying amounts as nodules and limestone replacements. Overlies black shale marking top of Desert Creek zone (new) of Paradox member of Hermosa. Grades northeastward into Paradox member. Stratigraphic chart; cross sections. Assigned Des Moines (Pennsylvanian) age.

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