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

others, 2007); lava flow is typically several tens of feet thick, but is as much asabout 300 feet (90 m) thick where it fills the ancestral Coal Creek channel.Qbub, QbubcUpper Bear Springs lava flows and cinder cones (middle to lower Pleistocene)– Medium- to dark-gray, fine-grained olivine basalt; lava flows (Qbub) eruptedfrom vents at cinder cones (Qbubc) about 2 miles (3 km) southwest of NavajoLake and flowed mostly south onto the Kolob Terrace; probably about 750,000years old because they appear to be the same lava flows as those at Horse Knoll(Sable and Hereford, 2004; Doelling, 2008), which yielded a K-Ar age of 0.81 ±0.05 Ma and an 40 Ar/ 39 Ar age of 0.73 ± 0.02 Ma (Biek and Hylland, 2007; UGSand NMGRL, 2008); lava flows are several tens of feet thick.Qbbm Black Mountain lava flow (middle to lower Pleistocene) – Medium-gray, finegrainedolivine basalt with small plagioclase and pyroxene phenocrysts; lava flowcaps the northwest sloping surface of Black Mountain in the Webster Flatquadrangle; vent unknown but may be concealed by nearby younger lava flows orsurficial deposits to the southeast; yielded K-Ar ages of 0.80 ± 0.24 and 0.87 ±0.24 Ma (Anderson and Mehnert, 1979; Best and others, 1980); lava flow istypically several tens of feet thick.Qbhp 1 , Qbhp 2 , QbhpcHancock Peak lava flows and cinder cone (middle to lower Pleistocene) –Medium-gray basalt that contains clusters of olivine and clinopyroxenephenocrysts in a fine-grained groundmass; based on chemistry and morphologythe map unit is divided into two flows, both of which are well vegetated; eruptedfrom Hancock Peak, a large, well-preserved cinder cone (Qbhpc) southeast ofBrian Head peak; Qbhp 1 appears to overlie Qbhp 2 and extends fartherdownstream where it caps an inverted valley about 600 feet (180 m) aboveMammoth Creek just north of the community of Mammoth Creek; age unknown,but estimated to be middle to early Pleistocene based on comparison with the600,000-year-old Long Flat lava flow (Qblf) and the 2.8 Ma Blue SpringMountain lava flow (Tbbm); lava flows are typically several tens of feet thick, butlikely exceed 100 feet (30 m) thick where they fill paleotopography.QbcFirst Left Hand Canyon vent area (middle to lower Pleistocene?) – Lower partcontains abundant angular blocks of Claron Formation and mafic volcanic rocksand minor rounded quartzite pebbles and cobbles; the whole is cut by severalbasaltic dikes; some blocks are as large as 12 feet (4 m) in size, but most arepebble to small cobble size; unbedded; appears to be a volcanic mudflow deposit.Upper part is mostly basaltic blocks and lesser Claron blocks, welded intoscoriaceous matrix. Unconformably overlies the lower conglomerate and middlesandstone members of the Grand Castle Formation on the northwest side ofHenderson Hill in First Left Hand Canyon; forms deeply eroded vent area about600 feet (180 m) above modern drainage, and may be associated with adjacentbasaltic dikes; about 400 feet (120 m) thick.20