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

Panguitch quadrangle) of several voluminous calc-alkaline, subduction-relatedvolcanic centers and underlying source plutons that characterized the West fromOligocene to Miocene time at this latitude (Lipman and others, 1972; Rowley andDixon, 2001); Fleck and others (1975) and Rowley and others (1994a) reportedseveral K-Ar ages of 21 to 30 Ma on rocks of the coeval vent facies; at least 1000feet (300 m) thick in the map area in the northern Markagunt Plateau (Andersonand Rowley, 1987) (and at least 6000 feet [2000 m] thick farther north; Rowleyand others, 2005), but pinches out radially from an east-trending string ofstratovolcanoes along the southern part of the Marysvale volcanic pile.TbvBear Valley Formation, undivided (lower Miocene to upper Oligocene) – Whiteto light-gray, yellowish-gray, and olive-gray, moderately to well-sorted, fine- tomedium-grained volcaniclastic sandstone; sand is about 60% quartz, and theremainder is feldspar, biotite, hornblende, augite, and relict pumice replaced byzeolite; cement is mostly zeolite (clinoptilolite) (Anderson, 1971); sandstone ischaracterized by high-angle cross-beds indicative of eolian deposition; thickeolian sand was derived from the south and west and accumulated in a low-reliefbasin bounded on the north by an east-trending fault scarp possibly associatedwith the 25 Ma Spry intrusion (Anderson, 1971; Anderson and others, 1990a);also contains lesser interbedded lava flows, volcanic mudflow breccia,conglomerate, and ash-fall and ash-flow tuff beds, especially in the upper part ofthe formation; where mapped as part of the Markagunt megabreccia, sandstonefacies (Tbvs) and ash-flow tuff facies (Tbvt) are mapped separately; Fleck andothers (1975) reported two K-Ar ages of 24.0 ± 0.4 Ma and 23.9 ± 0.5 Ma fromthe upper part of the formation; an incomplete section of Bear Valley strata isonly 100 feet (30 m) thick in the Panguitch quadrangle (Moore and Straub, 1995)and about 400 feet (120 m) thick in the Cottonwood Mountain quadrangle, but theformation is in excess of 1000 feet (300 m) thick on the northern MarkaguntPlateau (Anderson, 1971); the unit is as much as about 260 feet (80 m) thick inthe Red Hills (Maldonado and Williams, 1993b).Tbvb Bear Valley Formation mudflow breccia (lower Miocene to upper Oligocene) –Pale-yellowish-brown breccia composed of pebble- to cobble-size clasts, mostlyof intermediate composition volcanic rocks and lesser amounts of tuff andtuffaceous sandstone; as much as about 800 feet (245 m) thick in the CottonwoodMountain quadrangle.Quichapa Group (lower Miocene to upper Oligocene) – Regionally consists of threedistinctive ash-flow tuffs: in ascending order, the Leach Canyon Formation, CondorCanyon Formation, and Harmony Hills Tuff (Mackin, 1960; Williams, 1967; Andersonand Rowley, 1975; Rowley and others, 1995); the lower two formations erupted from theCaliente caldera complex, but the Harmony Hills Tuff likely erupted from the easternBull Valley Mountains (Rowley and others, 1995).Tqh Harmony Hills Tuff (lower Miocene) – Resistant, pale-pink to grayish-orangepink,crystal-rich, moderately welded, dacitic ash-flow tuff; contains about 50%phenocrysts of plagioclase (63%), biotite (16%), horneblende (9%), quartz (7%),39