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

unconformityThe Leach Canyon Formation unconformably overlies the Isom Formationat Brian Head peak and the southern part of Black Ledge. North of Castle Valleyand at Prince Mountain, however, the Leach Canyon unconformably overliesBrian Head strata. This distribution suggests that the Prince Mountain-CastleValley area was a paleohigh of Brian Head strata during Isom time, and that, oncethe resistant Isom was in place, this paleohigh was preferentially eroded to form abroad, east-trending stream valley in which the Leach Canyon accumulated; theLeach Canyon is not present north of Clear Creek in the map area. We speculatethat northwest-trending Clear Creek may conceal a pre-Isom down-to-the-northnormal fault that helped control distribution of the Isom Formation.TiIsom Formation (upper Oligocene) – Medium-gray, crystal-poor, denselywelded, trachydacitic ash-flow tuff, locally having distinctive rheomorphicfeatures including flow folds, elongated vesicles, and flow breccia; small (1-3mm) euhedral crystals constitute 10 to 15% or less of the rock and are mostlyplagioclase (90%) and minor pyroxene and Fe-Ti oxides set in a devitrified-glassgroundmass; exhibits pronounced platy outcrop habit and is thus accompanied byextensive talus deposits; rarely, a black basal vitrophyre is exposed, and locallyfracture surfaces and elongated vesicles (lenticules, described below) are darkreddish brown to dusky red; query indicates uncertain correlation in the upperreaches of the Clear Creek drainageThe best and most extensive exposures of the Isom Formation are at BrianHead peak and to the northeast along Black Ledge where at least three coolingunits are locally present; at Brian Head peak, the lower part of the formation isclassic tufflava about 80 feet (24 m) thick, whereas the upper part is a flowbreccia 60 to 90 feet (18-27 m) thick; along Black Ledge, about 7 miles (11 km)northeast of Brian Head peak, the flow breccia is absent and the Isom thereappears to consist of a single cooling unit about 350 feet (100 m) thick; the Isomalso forms prominent cliffs north of Clear Creek and Panguitch Lake.Regionally, many outcrops of all cooling units in the Isom Formationreveal secondary flow characteristics, including flow breccias, contorted flowlayering, and linear vesicles such that the unit was considered a lava flow untilMackin (1960) mapped its widespread distribution (300 cubic miles [1300 km 3 ]today spread over an area of 9500 square miles [25,000 km 2 ]; Best and others,1989a) and found evidence of glass shards, thus showing its true ash-flow tuffnature; for that reason it is commonly referred to as a tufflava, and is also called arheomorphic ignimbrite, an ash-flow tuff that was sufficiently hot to move withlaminar flow as a coherent ductile mass – see, for example, Anderson and Rowley(1975) and Andrews and Branney (2005); exhibits pronounced subhorizontallamination or platiness, which Mackin (1960) called “lenticules”; Fryman (1986,1987), Anderson and others (1990b), and Anderson (2002) described the lightgray,pancake-shaped lenticules, which are typically spaced 4 to 8 inches (10-20cm) apart and that may extend for 30 feet (10 m) or more, and which are locallycontorted, suggesting turbulence in the flow as it moved over uneven topography;42