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

thickness uncertain but outcrop patterns suggest that displaced Brian Head stratalikely exceed 100 feet (30 m) thick.TlTipLimerock Canyon Formation (lower Miocene) – White, light-gray, and pale- toolive-green, locally tuffaceous, volcaniclastic sandstone, pebbly sandstone,gritstone, pebbly conglomerate, mudstone, and minor tuffaceous limestone; maycontain minor multi-hued chalcedony in areas north and west of the type sectionnear Hatch, but as described below, we are as yet uncertain in differentiatingBrian Head strata (which contain abundant chalcedony), Bear Valley Formation,and Limerock Canyon Formation; includes at least 10 thin beds of ash-fall tuff;commonly bioturbated; clasts are about 90% volcanic but include as much as 10%quartzite and sandstone; Kurlich and Anderson (1997) stated that the formationlacks Needles Range, Isom, Bear Valley, and Mount Dutton clasts, but most clastsappear to lead-author Biek to be Isom; mapped on the east part of the MarkaguntPlateau north and west of Hatch, where it appears to cut out much of the BrianHead Formation, although this interpretation is uncertain as described below;unconformably overlain by upper Tertiary unconsolidated fan alluvium (Taf);query indicates uncertain identification; deposited in fluvial and minor lacustrineenvironments; two ash-fall tuff beds, about 100 feet (30 m) and 200 feet (60 m)above the base of the formation at the type section west of Hatch, respectively,yielded K-Ar ages of 21.5 ± 0.6 Ma (biotite) and 21.0 ± 1.0 Ma (sanidine), and20.2 ± 1.4 Ma (biotite) and 19.8 ± 0.8 Ma (sanidine) (Sable and Maldonado,1997b); Sable and Maldonado (1997b) also reported an 40 Ar/ 39 Ar age of 20.48 ±0.8 Ma (biotite) and 21.0 ± 1.0 Ma (sanidine); as much as 290 feet (88 m) thick ina composite type section west of Hatch (Kurlich, 1990; Kurlich and Anderson,1997).Sable and Maldonado (1997b) described the difficulty of differentiatingsimilar volcaniclastic strata of the Limerock Canyon, Bear Valley, and BrianHead Formations. The type section of the Limerock Canyon Formation (west ofHatch) contains a few tens of feet of strata that we tentatively reassign to theBrian Head Formation, and we interpret that the limestone that Kurlich andAnderson (1997) assigned to the Brian Head Formation at the base of this typesection is in fact the upper white member of the Claron Formation as originallydescribed by Kurlich (1990). Relationships of strata that we assign to the BrianHead and Limerock Canyon Formations are much clearer in the southeast cornerof the Haycock Mountain quadrangle, 1 to 2 miles (1.5-3 km) west of the typesection. There, and along the flanks of Hatch Mountain and Haycock Mountain,we place the contact between the two formations at the top of a massivemultihued chalcedony bed that is typically 3 to 6 feet (1-2 m) thick. Strataunderlying this chalcedony are typical of Brian Head strata as they are exposed atBrian Head peak; overlying strata are typically sandier and locally, as on the southflank of Hatch Mountain, contain significant pebble conglomerate.Iron Peak laccolith (early Miocene) – Medium-gray gabbro-diorite porphyrycomposed almost entirely of augite and plagioclase (calcic labradorite) and about8% opaque oxide minerals, mostly magnetite, with diorite the dominant phase36