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

Qbtp The Pass lava flow (Pleistocene?) – Medium- to dark-gray basalt that containsclusters of olivine and clinopyroxene phenocrysts in a fine-grained groundmass;caps small knob just south of The Pass east of Panguitch Lake that Wagner (1984)interpreted as a small gabbroic intrusion, but that appears to be a flow remnantpartly involved in a landslide; chemically similar to the 5.3 Ma Houston Mountainlava flow (Tbhm), but source is uncertain; lava flow may be about 50 feet (15 m)thick.Qbs, QbscSummit lava flow and cinder cone (lower Pleistocene) – Medium- to dark-gray,fine-grained olivine basalt that Maldonado and others (1997) referred to as theCinder Hill cone and flow; lava flow (Qbs) erupted from vent at cinder cone(Qbsc) at the base of the Hurricane Cliffs, about 2 miles (3 km) southwest ofSummit; lava flow also crops out at the southeast margin of the Red Hills and ispresumed to underlie the southern part of the Parowan Valley graben, where it isdisplaced by graben-bounding faults; yielded K-Ar ages of 1.00 ± 0.16 Ma and0.94 ± 0.14 Ma (Anderson and Mehnert, 1979); lava flow is typically several tensof feet thick.QbeElliker Basin lava flow (lower Pleistocene) – Medium- to dark-gray, fine-grainedolivine basaltic trachyandesite; vent area unknown, but minor scoria and blockyflow breccia is present on the north rim of Elliker Basin, suggesting that the ventcould underlie the basin, which is southwest of Summit; yielded K-Ar ages of1.00 ± 0.16 Ma and 1.11 ± 0.11 Ma (Anderson and Mehnert, 1979); lava flow istypically several tens of feet thick.Qbrh, QbrhcRed Hills lava flows and cinder cones (lower Pleistocene) – Medium- to darkgray,fine-grained basaltic andesite with small olivine and plagioclasephenocrysts; lava flows (Qbrh) erupted from vents at three cinder cones (Qbrhc)in the northwest corner of the Summit quadrangle and adjacent Enoch quadrangle(Rowley and Threet, 1976); lava flows are mostly covered by eolian sand and silt,and locally by small areas of fan alluvium, but only larger such areas are mappeddue to map scale; lava flow is cut by faults associated with the Red Hills horst andgraben; yielded K-Ar ages of 1.28 ± 0.4 Ma (Anderson and Mehnert, 1979) and1.30 ± 0.3 Ma (Best and others, 1980); lava flow is typically several tens of feetthick.Tbbm, TbbmcBlue Spring Mountain lava flow and cinder cone (Pliocene) – Medium-grayhawaiite and mugearite (sodium-rich trachybasalt and basaltic trachyandesite,respectively) lava flow (Tbbm) that contains clusters of olivine and clinopyroxenephenocrysts in an aphanitic to fine-grained groundmass; erupted from vents at acinder cone (Tbbmc) on Blue Spring Mountain and flowed east and south, mostlytoward the ancestral Mammoth Creek drainage; an erosional outlier caps21