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

esistant crystal-poor rhyolite tuff; both contain common pumice fragments andlithic fragments of black, aphanatic, mafic volcanic rock; forms gently eastdippingledge that at its type section overlies apparently undeformedvolcaniclastic conglomerate and elsewhere overlies deformed Bear ValleyFormation sandstone; petrographically and chemically similar to the LeachCanyon Formation (see the Leach Canyon Formation unit description fordetails); yielded 40 Ar/ 39 Ar age of 22.75 ± 0.12 Ma (Ed Sable, U.S. GeologicalSurvey, unpublished data, 1996); Anderson (1993) interpreted the HaycockMountain Tuff to postdate emplacement of the Markagunt megabreccia, butlead-author Biek tentatively interprets the tuff as riding largely undisturbed inthe upper plate of the megabreccia; mapped north of Haycock Mountain; about35 feet (11 m) thick.Tm(Tbvs) Markagunt megabreccia – Bear Valley volcaniclastic strata component –White to light-gray, moderately to well-sorted, fine- to medium-grainedvolcaniclastic sandstone having high-angle cross-beds, and similarly coloredtuffaceous mudstone and siltstone; typically pervasively deformed by faults andfolds indicative of deformation as part of the Markagunt megabreccia; mappednorth of Haycock Mountain; exposed thickness as much as 120 feet (35 m).Tm(Tbvt) Markagunt megabreccia – Bear Valley ash-flow tuff component – White,unwelded, massive, likely rhyolitic ash-flow tuff that contains common pebblesizelithic fragments of intermediate volcanic rocks and rounded quartzitepebbles; mapped north of Haycock Mountain; exposed thickness as much as100 feet (30 m).Tm(Tl) Markagunt megabreccia – Limerock Canyon Formation component –White and light-gray, locally tuffaceous, volcaniclastic sandstone, pebblysandstone, mudstone, minor tuffaceous limestone, and local multi-huedchalcedony; mapped on the southeast side of Haycock Mountain, where it isfaulted and folded, indicative of deformation as part of the Markaguntmegabreccia; we are uncertain of the identification of the Limerock CanyonFormation at Haycock Mountain (it is possible that these beds are in the pre-30Ma Brian Head Formation, or are part of the 24 Ma Bear Valley Formation, orthat the Limerock Canyon has a wider age range than the 20 to 21 Ma age wecan now demonstrate), and this awaits further geochemical and radiometric agecontrol; exposed thickness as much as 150 feet (45 m).Tm(Tdm) Markagunt megabreccia – Mafic block and ash-flow tuff of the MountDutton Formation component – Dark-gray, vesicular, basaltic andesite andbasalt present as angular cobble- to boulder-size blocks floating in a light-graysandy matrix of the same composition; monolithic; minor basaltic scoria, likelya rafted block, is present north of Bunker Creek in the NE1/4NE1/4 section 12,T. 36 S., R. 8 W.; mapped between Panguitch Lake and Sidney Peaks where itunconformably overlies 24 Ma Leach Canyon Formation (Tql); in the BunkerCreek drainage west of Panguitch Lake, the map unit is overlain by Isom34