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

Fryman (1986, 1987) also described fumaroles in the Isom of the northernMarkagunt Plateau, a result of degassing of the flow as it came to a rest.The source is unknown, but isopach maps and pumice distribution suggestthat the Isom Formation was derived from late-stage eruptions of the 27-32 MaIndian Peak caldera complex that straddles the Utah-Nevada border, possibly inan area now concealed by the western Escalante Desert (Rowley and others, 1979;Best and others, 1989a, 1989b); estimated crystallization temperature andpressure of phenocrysts of the Isom is 950°C and < 2 kbar (Best and others,1993), and this relatively high temperature is supported by its degree of weldingand secondary flow features; at its type area in the Iron Springs district southwestof the map area, Mackin (1960) defined three members, a lower unnamedmember, the Baldhills Tuff Member, and the upper Hole-in-the-Wall TuffMember; Rowley and others (1975) redefined the Baldhills Tuff Member toinclude Mackin’s lower unnamed member, and noted that the Baldhills consists ofat least six cooling units; Maldonado and Williams (1993a, b) described nineapparent cooling units in the northern Red Hills at the west edge of the map area;in the northern Markagunt Plateau, Anderson and Rowley (1975) defined the BlueMeadows Tuff Member, which underlies the Baldhills Tuff Member, but it ispossible that the Blue Meadows Tuff is part of the Mount Dutton Formation, andthus a local tuff of the Marysvale volcanic field (Rowley and others, 1994a); insome places in the Panguitch 30’ x 60’ quadrangle, autochthonous IsomFormation may include the Baldhills Tuff Member, but this member was notrecognized in the southern part of its outcrop belt in the map area; the IsomFormation is about 26 to 27 Ma on the basis of many 40 Ar/ 39 Ar and K-Ar ages(Best and others, 1989b; Rowley and others, 1994 a); maximum exposedthickness is about 350 feet (110 m) at Black Ledge and about 250 feet (75 m)along Ipson Creek.Mapped as the Blue Meadows Tuff Member of the Isom Formation at theeast edge of the Cottonwood Mountain quadrangle (Maldonado and others, inpreparation), but refered to simply as Isom Formation undivided here pendingongoing mapping in areas to the east.TnTnlNeedles Range Group, undivided (lower Oligocene) – Lund Formation andWah Wah Springs Formation undivided in the Red Hills due to map scale.Lund Formation (lower Oligocene) – Grayish-orange-pink, moderately welded,crystal-rich, dacitic ash-flow tuff exposed in the Red Hills; similar to underlyingWah Wah Springs Formation, but with generally smaller mafic phenocrysts and alighter-colored matrix; locally contains spheroidal masses of tuff as large as 1 foot(0.3 m) in diameter near the top of the unit; base of the formation includes about12 feet (4 m) of pale-greenish-yellow tuffaceous sandstone and lesser pebblyvolcaniclastic conglomerate; exhibits normal magnetic polarity (Best and Grant,1987); derived from the White Rock caldera, the southwest part of the olderIndian Peak caldera, and is of similar volume to the underlying Wah Wah SpringsFormation (Best and Grant, 1987; Best and others, 1989a, b); preferred age is 27.943