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

unwelded, white, rhyolite tuff that is overlain by a 10-foot-thick (3 m) moderateorange-pinkrhyolite tuff that has sparse reddish-brown lithic clasts, whichbecomes slightly more indurated in the upper part of the unit. This is overlain bya massive, 12-foot-thick (4 m) black vitrophyre, which is in turn overlain by a 25-foot-thick (8 m) resistant, pale-red, moderately welded rhyolite tuff that containspale-lavender flattened pumice lenticules and as much as 1% distinctive, small,reddish-brown lithic clasts of flow rock torn from the vent walls; this resistantupper unit forms the cap rock of Black Ledge northward to beyond the SidneyPeaks area.To the east, west of Panguitch Lake, the Leach Canyon Formationunconformably overlies the Brian Head Formation or locally stream gravelcontaining clasts of Isom Formation welded tuff (for example, on the southeastside of Prince Mountain at sample location PL061708-3); pumice makes up about10% of the tuff and is typically less than 0.5 inch (1 cm) in length, but somewhatlarger near the top of the cooling unit; a nonresistant, moderate-orange-pink ashfalltuff identical to that at Brian Head peak is present at the base of the unit; themain part of the cooling unit contains only rare, small, reddish-brown lithicfragments.Previously, there was considerable confusion over the distribution of thepetrographically and chemically similar Leach Canyon Formation and theHaycock Mountain Tuff in the map area (the two units are not reliablydistinguishable based on their major- and trace-element chemistry, but theHaycock Mountain Tuff is typically less welded than the Leach Canyon andcontains conspicuous black lithic fragments, unlike the reddish-brown lithicfragments of the Leach Canyon). Detailed mapping of the Panguitch Lakequadrangle (Biek and Sable, in preparation) has resolved this problem. The LeachCanyon Formation can be traced in continuous outcrop from Brian Head peaknorthward to the head of Bunker Creek and then east to the east end of PrinceMountain just west of Panguitch Lake; it is unconformably overlain by theMarkagunt megabreccia, which consists mostly of the Isom Formation. Samplesfrom the south side of Prince Mountain yielded K-Ar ages of 22.8 ± 1.1 Ma(biotite) and 24.8 ± 1.0 Ma (sanidine) (Rowley and others, 1994a, sample 89USa-1a) and a duplicate K-Ar age of 24.3 ± 1.0 Ma (sanidine) as well as an 40 Ar/ 39 Arage of 23.86 ± 0.26 Ma (biotite) (Sable and Maldonado, 1997a, on the samesample 89USa-1a). The Leach Canyon Formation is widely agreed to be about23.8 Ma (Best and others, 1993; Rowley and others, 1995). However, bothRowley and others (1994a) and Sable and Maldonado (1997a) interpreted this tuffto be the Haycock Mountain Tuff, which yielded a slightly younger 40 Ar/ 39 Ar ageof 22.75 ± 0.12 Ma (sanidine) at its type section one mile (1.6 km) northeast ofPanguitch Lake (Sable, unpublished data, 1996). The facts that the tuff at PrinceMountain yielded an age analytically indistinguishable from the Leach CanyonFormation, that it can be traced continuously to outcrops at Brian Head peak, andthat it is unconformably overlain by the Markagunt megabreccia, are irrefutableevidence that it is the Leach Canyon Formation and not the Haycock MountainTuff.41