Interim Geologic Map of the West Part of - Utah Geological Survey

ends of the Asay Knoll (Qbak), Bowers Knoll (Qbbk), and Coopers Knoll (Qbck) lavaflows.Several lava flows cross and are cut by range-bounding normal faults, includingthe Water Canyon (Qbw), Summit (Qbs), and Red Hills (Qbrh) flows. For example, the0.44 Ma Water Canyon flow crosses a relay ramp between two en echelon sections of theParowan-Paragonah fault zone; at the mouth of Water Canyon, the flow reveals about250 feet (75 m) of displacement, yielding a long-term slip rate of about 0.17 mm/yr(about 0.007 in/yr or 550 feet/Ma) for the eastern fault strand.Basaltic magmas are partial melts derived from the compositionallyheterogeneous lithospheric mantle, and this, coupled with fractional crystallization, mayaccount for most of the geochemical variability between individual lava flows (Lowder,1973; Best and Brimhall, 1974; Leeman, 1974; Nealey and others, 1995, 1997; Nelsonand Tingey, 1997; Nusbaum and others, 1997; Smith and others, 1999; Downing, 2000;Johnson and others, 2010). Nb/La ratios for virtually all samples of basaltic and andesiticlava flows from the map area are less than 1.0, thus suggesting a lithospheric mantlesource (Fritton and others, 1991). Rock names are from LeBas and others (1986).QTb Basaltic lava flow, undivided (Pleistocene? to Miocene?) – Medium- to darkgraybasalt lava flow that caps a ridge north of Wilson Creek, a southern tributaryof Mammoth Creek, in the Asay Bench quadrangle; correlation is uncertain, butmajor- and trace-element geochemistry shows affinities to the 5.3 Ma HoustonMountain lava flow, although its degree of topographic inversion suggests that itis not that old; about 20 to 30 feet (6-9 m) thick.Qbpl 1, Qbpl 2, Qbpl 3Panguitch Lake lava flows (middle Holocene to upper Pleistocene) – Mapped asthree separate lava flows, with Qbpl 1 being the youngest; all three flows aremostly unvegetated, blocky, and exhibit steep flow fronts 100 to 200 feet (30-60m) high: Qbpl 1 is dark-gray to black latite (potassium-rich trachyandesite)containing small (1 mm), stubby plagioclase phenocrysts in a glassy to aphanaticgroundmass; Qbpl 2 and Qbpl 3 are dark-gray latite containing small stubbyplagioclase and abundant acicular hornblende phenocrysts in a fine-grainedgroundmass; the Qbpl 1 lava flow lacks collapsed lava tubes and exhibits blockyflow lines similar to those of the Dry Valley lava flow (Qbdv); the smaller Qbpl 2lava flow has collapsed lava tubes and partly buries the Qbpl 3 lava flow; the Qbpl 3flow, which has abundant collapsed lava tubes and branching distributary lobes,erupted from a vent apparently now concealed by the younger vents of the Qbpl 1and Qbpl 2 lava flows (immediately northeast of Miller Knoll, the large cindercone about 3 miles [5 km] south of Panguitch Lake) and flowed northward about3 miles (5 km) nearly to Panguitch Lake; this is the “northern Panguitch flow” ofStowell (2006); age uncertain, but may be as young as middle Holocene;individual lava flows are typically about 200 feet (60 m) thick.Qbdv Dry Valley lava flow (middle Holocene to upper Pleistocene) – Dark-gray latite(potassium-rich trachyandesite) that contains olivine and abundant hornblendephenocrysts in an aphanatic to fine-grained groundmass; forms a thick, blocky,laterally restricted flow west of Black Rock Valley that exhibits high, steep flow13