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

Formation or locally by Wah Wah Springs Formation, each as part of theMarkagunt megabreccia; exposures north of Bunker Creek (in the north-centralpart of section 12, T. 36 S., R. 8 W.) show 30 feet (9 m) of paleotopography cutinto the upper surface of the Leach Canyon, with blocks of the rhyolite tuffincorporated into the base of the overlying mafic block and ash flow; similarmafic block and ash-flow tuff is present north of Panguitch Lake, where it isclearly part of the Markagunt megabreccia and so mapped as Tm(Tdm) andlumped as Tm in hill 8980 to the north; it is unclear if the deposits betweenPanguitch Lake and Sidney Peaks partly underlie or are everywhere part of themegabreccia; maximum thickness is about 80 feet (24 m).Tm(Ti) Markagunt megabreccia – Isom Formation component – Medium-gray,crystal-poor, densely welded, trachydacitic ash-flow tuff; small (1-3 mm)euhedral crystals constitute 15 to 20 % of the rock and are mostly plagioclase(90%) and minor pyroxene, magnetite, and quartz set in a devitrified glassygroundmass; most outcrops and blocks weather to grussy soils and roundedhills; except at Haycock Mountain, rarely forms cliffs as is typical of theautochthonous Isom Formation; although generally poorly exposed, constitutesthe great bulk of the megabreccia mapped between Panguitch Lake and BrianHead; may locally include areas of Brian Head Formation, Wah Wah SpringsFormation, and mafic block and ash-flow tuff that are difficult to delineategiven extensive forest cover and inconspicuous outcrop habit; generallyunconformably overlies the Leach Canyon Formation west of Panguitch Lake(but also locally overlies Wah Wah Springs Formation and Brian HeadFormation in this area), whereas east of Panguitch Lake, unconformablyoverlies what we tentatively map as Limerock Canyon Formation; maximumthickness as much as about 400 feet (120 m).Tm(Tnw) Markagunt megabreccia – Wah Wah Springs Formation component –Pale-red, grayish-orange-pink, and pale-red-purple, crystal-rich, moderatelywelded, dacitic ash-flow tuff; phenocrysts of plagioclase, hornblende, biotite,and quartz (with minor Fe-Ti oxides and sanidine) comprise about 40% of therock; forms a single cooling unit; mapped northeast of Castle Valley, where itrests on displaced Brian Head strata (Tm[Tbhv]) and in the upper reaches ofBunker Creek, where it rests on displaced Isom Formation (Tm[Ti]) ordisplaced mafic block and ash-flow tuff (Tm[Tdm]); about 40 feet (12 m) thick.Tm(Tbhv)Markagunt megabreccia – Brian Head Formation component – Poorlyexposed, but distinctive white to light-gray volcaniclastic mudstone, pebblysandstone, micritic limestone, and chalcedony are present as colluvium, thusbetraying the formation’s presence northeast of Castle Valley where it rests outof-sequenceon the Leach Canyon Formation; on the ridge at the commonborder of sections 9 and 16, T. 36 S., R. 8 W., pebbly volcaniclastic sandstoneof the Brian Head Formation, well exposed at the head of a landslide, dips about25° northeast and is overlain by similarly dipping Wah Wah Springs Formation;on the hill to the south, however, Brian Head strata appear to be subhorizontal;35