|Name||Upper part of Shely Group, including Tm4 or Morita Ranch Formation|
Igneous > Volcanic > Felsic-volcanic > Rhyolite (Pyroclastic-ash flow)
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
|Comments||Upper part of Shely Group is a sequence of densely welded, commonly silicified, rhyolitic ash-flow units that crop out north and east of the Chinati Mountains caldera (units 6, 7, and 8 of the Shely Group of Amsbury (1958)). Tm4 of Morita Ranch Fm. is aphyric basalt lava flow and is one of the more voluminous units in the formation. It extends westward to near Shafter amd continues as far south as Sierra Blanca dome. The upper part of unit 4 of Morita Ranch Fm. is sparsely porphyritic flows of mafic trachyte (Henry and others, 1992, p. 34). The unit erupted about 34 Ma ago about the same time as the Mitchell Mesa Rhyolite (unit that erupted to form the Chinati Mountains and led to collapse of Chinati Mt. caldera). T4 unit of Morita Ranch Fm may correlate to unit 6 of the Shely Group (Henry and others, 1992). All the units discussed here, except the Mitchell Mesa Rhyolite, were designated the Shely Tuff by Henry and others (1992).|
Henry, C.D., Price, J.C., Duex, T.W., and James, E.W., 1992, Geology of the Infiernito caldera and magmatic evolution of the Chinati Mountains, Trans-Pecos Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations no. 206, 56 p.
Amsbury, D.L., 1958, Geology of the Pinto Canyon area, Presidio County, Texas: University of Texas at Austin, Bureau of Economic Geology Geologic Quadangle Map 22, scale 1:63,360.
Bureau of Economic Geology, 1992, Geologic Map of Texas: University of Texas at Austin, Virgil E. Barnes, project supervisor, Hartmann, B.M. and Scranton, D.F., cartography, scale 1:500,000.