Chisos Formation of Schiebout et al (1987) and the Big Yellow Sandstone Member of their Tornillo Formation, undivided (TXOGEOc;0)

State	Texas
Name	Chisos Formation of Schiebout et al (1987) and the Big Yellow Sandstone Member of their Tornillo Formation, undivided
Geologic age	Phanerozoic \| Cenozoic \| Tertiary \| Eocene Oligocene
Original map label	OEc
Comments	Big Yellow Sandst. Mbr 30-50 ft thick overlies the Hannold Hill Mbr. of Tornillo Fm)(revised from Maxwell and others (1967). Above the Big Yellow Mbr is as much as 1130 ft of mainly tuffaceous beds--(Chisos Formation) clay, tuffaceous clay, mudstone, and sandst., sandst. and conglomeratic sandst., calcareous tuff of tuffaceous limestone and silicified tuff. Includes tuff and lava flows (basalt, trachyte, trachybasalt, and massive, dense, porphyritic trachyandesite). Unit varies greatly in thickness and lithology from place to place.
Primary rock type	sandstone
Secondary rock type	mudstone
Other rock types	tuff; conglomerate; mixed clastic/volcanic; basalt; trachyandesite
Lithologic constituents	Major Sedimentary > Clastic > Mudstone > Claystone > Bentonite (Bed) Minor Igneous > Volcanic > Felsic-volcanic > Trachyte (Flow)more than 1,000 ft (as much as 3,400 ft) of interbedded tuffaceous fine and coarse-grained sedimentary rocks, tuff, lava flows Sedimentary > Clastic > Conglomerate (Bed) Sedimentary > Clastic > Sandstone (Bed) Incidental Sedimentary > Clastic > Mixed-clastic > Sandstone-Mudstone (Bed)more than 1,000 ft (as much as 3,400 ft) of interbedded tuffaceous fine and coarse-grained sedimentary rocks, tuff, lava flows, some volcanic flow breccia Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Map references	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
Unit references	Maxwell, R.A., Lonsdale, J.T., Hazzard, R.T., and Wilson, J.A., 1967, Geology of Big Bend National Park, Brewster County, Texas: The University of Texas Bureau of Economic Geology Publication 6711, 320 p. Schiebout, J.A., Rigsby, C.A., Rapp, S.D., Hartnell, J.A., Standhardt, B.R., 1987, Stratigraphy of the Cretaceous-Tertiary and Paleocene-Eocene transition rocks of Big Bend National Park, Texas: Journal of Geology, v. 95, p. 359-371.
Geographic coverage	Brewster - Presidio