Pennsylvanian and Lower Permian carbonates of Clark County, Nevada, were studied at Arrow Canyon (AC), Frenchman Mountain (FM), and Azure Ridge (AR). Distinct facies changes are observed from the AC section (2,193 ft, Bird Spring Group, restricted, miogeosynclinal) where limestones are 65.5% micrite to FM section (1,055 ft, Callville Formation, outer shelf) where limestones are 57.2% pelsparites, biosparites, or oosparites to AR (779 ft, Callville Formation, inner shelf) where limestones are 50.9% pelsparites, biosparites, and oosparites. The marked change in facies provides evidence for support of a dual terminology, Bird Spring Group for the basin facies and Callville Limestone for the shelf.

The Bird Spring-Callville interval is subdivisible into a lower eastward thinning member (1,634 ft, AC; 634 ft, FM; 322 ft, AR) characterized by abrupt vertical fluctuations in facies (AC 14.3, FM 8.0, AR 15.2 fluctuations per 100 ft) causing the steplike outcrop appearance characteristic of the interval. Gray to buff weathering and lack of quartz silt distinguish the lower member from the arenaceous buff to brown upper member. The latter contains several quartz sandstone units (zero AC, 41.7% FM, 51.1% AR) in which the size of the fine (< 0.1 mm maximum apparent diameter in thin section) angular grains increases eastward. The upper member ranges from 559 ft at AC to 421 ft minimum at FM (491 ft maximum depending on contact position in a covered interval) to 457 ft at AR. A relativ ly small number of fluctuations in facies (8.4 AC, 5.2 FM, 3.9 AR) produces a uniform outcrop appearance.

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