Pre-Moenkopi karst topography formed on the nonresistant gypsum beds of the Harrisburg Member in the Beaver Dam Mountains of southwestern Utah. Local relief on the erosional surface may be more than 140 m (450 ft), forming potential unconformity traps with substantial closure.

Upper Permian and Lower Triassic rocks in the Beaver Dam Mountains accumulated on a gently westward-sloping continental shelf as both carbonate and clastic tidal-flat environments. Facies analysis shows seven lithofacies, including gypsum, dolomitic and calcareous mudstone, dolomitic and calcareous siltstone, dolomite, silty wackestone, fossiliferous packstone, and pelloidal-ooidal grainstone to packstone. Gypsum and dolomite facies formed mainly on supratidal flats. Packstone and grainstone rocks formed mainly on shoals, bars, and banks in the intertidal zone of a carbonate tidal flat. Mudstone and siltstone units formed mainly on muddy tidal flats.

High-displacement Basin and Range normal faults have uplifted the rocks, forming good exposures of the Permian and Triassic strata in the Beaver Dam Mountains and perhaps forming structural traps to the east. Stratigraphic traps may also occur throughout the Harrisburg and Shnabkaib members. Thick anhydrite beds form the cap. Grainstone, packstone, and dolomite units may be effective reservoir rocks. Source rocks include algal-rich wackestone and dolomite beds, fossiliferous units, including the underlying Fossil Mountain Member, and organic-rich mudstone units. Though trapping mechanisms are abundant in the Beaver Dam Mountains, oil exploration has not been very successful to date; this may be due, in part, to the difficulty of locating suitable traps.

End_of_Article - Last_Page 939------------