|Name||Chisos Formation of Schiebout et al (1987) and the Big Yellow Sandstone Member of their Tornillo Formation, undivided|
|Geologic age||Eocene to Oligocene|
Sedimentary > Clastic > Mudstone > Claystone > Bentonite (Bed)
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Conglomerate (Bed)
Igneous > Volcanic > Felsic-volcanic > Trachyte (Flow, Pyroclastic-tuff)more than 1,000 ft (as much as 3,400 ft) of interbedded tuffaceous fine and coarse-grained sedimentary rocks, tuff, lava flows
Igneous > Volcanic > Mafic-volcanic > Basalt (Flow)
Sedimentary > Clastic > Mixed-clastic > Sandstone-mudstone (Bed, Tuffaceous)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
|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.|
|Stratigraphic units||Big Bend Park Group(Chisos Formation), Chilicotal Group(Big Yellow Sandst. Member of Tornillo Fm);Alamo Creek Basalt, Bee Mountain Basalt, Mule Ear Spring Tuff|
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.
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.
|Counties||Brewster - Presidio|