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

TmMarkagunt megabreccia, undivided – The Markagunt megabreccia (restrictedusage following Sable and Maldonado, 1997a) is undivided where exposures areinsufficient to delineate bedrock units, and in the remote northwest part of theFlanigan Arch quadrangle; most areas mapped as Tm consist predominantly ofthe Isom Formation (which is typically pervasively and finely fractured so thatit weathers to grussy soils and rounded hills), but locally includes Wah WahSprings and Brian Head strata, and, north of Panguitch Lake, large amounts ofmafic block and ash-flow tuff, volcaniclastic sandstone, and pebblyconglomerate; on top of the Markagunt Plateau, north of the latitude ofPanguitch Lake, Tm is unconformable on the resistant, planar, gently eastdippingsurface of the Isom Formation, but between Brian Head peak and ClearCreek, it unconformably overlies the Leach Canyon Formation; at the west edgeof the plateau, south of Iron Peak, it unconformably overlies Brian Head strata;maximum thickness exceeds 500 feet (150 m).Tm(Ta) Markagunt megabreccia – middle Tertiary alluvium component –Volcaniclastic conglomerate and pebbly sandstone on the north flank ofHaycock Mountain that may be a coarse alluvial facies of the Mount DuttonFormation; contains quartzite cobbles and small boulders in the basal part of thedeposits; typically forms cobble-covered hillsides, but is locally wellconsolidatedin exposures on the southwest side of Haycock Mountain; thismiddle Tertiary alluvium that locally caps Haycock Mountain is likely olderthan similar alluvium under the Haycock Mountain Tuff described by Anderson(1993); as mapped here, the alluvium (Tm[Ta]) at Haycock Mountain overliesand is in turn locally overlain by Isom Formation (Tm[Ti]), all interpreted to bepart of the upper plate of the Markagunt megabreccia; additionally, thisalluvium is as much as 800 feet (250 m) above Panguitch Creek and the typesection of Haycock Mountain Tuff (thus to have postdated emplacement of themegabreccia, the alluvium we map as [Tm(Ta)]would have had to completelyfill the ancestral Panguitch Creek drainage and then been exhumed, but we seeno compelling evidence for this interpretation); maximum thickness in this areais probably about 100 feet (30 m).Tm(Td) Markagunt megabreccia – Mount Dutton Formation component – Massiveblock and ash-flow tuff of mafic and intermediate composition, volcaniclasticpebble to boulder conglomerate, and minor tuffaceous sandstone; characterizedby brown or locally reddish-brown hues; mapped northwest of HaycockMountain; tentatively interpreted to be part of the Markagunt megabreccia, butmay contain strata that postdate emplacement of megabreccia (ongoing mappingin areas to the north may resolve this uncertainty); maximum thickness in thisarea is probably about 300 feet (100 m).Tm(Thm) Markagunt megabreccia – Haycock Mountain Tuff component – Consistsof two cooling units: lower unit is white to very light pink, unwelded, crystalpoorrhyolite tuff that is overlain by light-pink, poorly welded, moderately33