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

of the Claron Formation (Tcwt), so that only their middle volcaniclastic unit is present inthe map area; in the map area, Brian Head strata are unconformably overlain by the 30Ma Wah Wah Springs Formation (Tnw), or locally by the 26 to 27 Ma Isom Formation(Ti), and so is early Oligocene to latest Eocene (Sable and Maldonado, 1997b).Maldonado and Moore (1995) reported 40 Ar/ 39 Ar ages of 33.00 ± 0.13 Ma (plagioclase)and 33.70 ± 0.14 Ma (biotite) on an ash-flow tuff in the northern Red Hills that lies in theupper part of the formation. We obtained a U-Pb age on zircon from an airfall tuff at thebase of the formation at Cedar Breaks National Monument of 35.77 ± 0.28 Ma.Tbhv Middle volcaniclastic unit – White to light-gray volcaniclastic mudstone,siltstone, silty sandstone, sandstone, conglomerate, volcanic ash, micriticlimestone, and multi-hued chalcedony; near Mineral Canyon and northwest ofLittle Salt Lake (hill 7292), conglomerate consists of pebble- to boulder-size,rounded clasts of intermediate volcanic rocks of unknown affinity and quartzitepebbles and cobbles; Maldonado and Williams (1993a) reported clasts of ash-flowtuff that resemble some overlying rocks (for example the Bauers Tuff), andadditional work is underway to investigate this possibility; sandstone iscommonly bioturbated with pencil-size root or burrow casts that weather out inrelief; soft-sediment slump features are locally common; chalcedony is variousshades of white, gray, yellow, red, black, and brown, typically has a whiteweathering rind, is commonly highly brecciated and resilicified, typically occursin beds 1 to 3 feet (0.3-1 m) thick but locally as much as 8 feet (2.5 m) thick, islocally stained by manganese oxides, and may have resulted from silicification oflimestone beds (Maldonado, 1995; Sable and Maldonado, 1997b) or possiblyvolcanic ash beds (Bakewell, 2001); chalcedony is almost always highlyfractured, but some is useful for lapidary purposes (Strong, 1984); the formationis typically nonresistant, poorly exposed, and extensively covered by colluvium,but locally well exposed near Panguitch Lake and on the southwest side of BrianHead peak; because of abundant bentonitic clay derived from weathered volcanicash, this unit weathers to strongly swelling soils (unlike underlying ClaronFormation) and forms large landslide complexes; deposited in low-relief fluvial,floodplain, and lacustrine environments in which large amounts of volcanic ashaccumulated; thickness uncertain, but maximum exposed thickness, at Brian Headpeak, is about 500 feet (150 m).TbhtRhyolitic tuff of middle volcaniclastic unit – Pinkish-brown, unwelded rhyolitetuff in the upper part of the formation in the northern Red Hills, on the west flankof Jackrabbit Mountain; yielded K-Ar ages of 34.2 ± 2.1 Ma (plagioclase) and36.3 ± 1.3 Ma (biotite), and 40 Ar/ 39 Ar ages of 33.00 ± 0.13 Ma (plagioclase) and33.70 ± 0.14 Ma (biotite) (Maldonado and Williams, 1993b); as much as about200 feet (60 m) thick.unconformityClaron Formation (Eocene to Paleocene) – Mapped as five informal lithostratigraphicunits described below: an upper white member (which is itself divided into an uppermostmudstone interval, an upper limestone interval, a middle mudstone and sandstone45