Tertiary Formations of Rim Rock Country, Presidio County, Trans-Pecos Texas

RI0036. Tertiary Formations of Rim Rock Country, Presidio County, Trans-Pecos Texas, by R. K. DeFord. 37 p., 4 figs. Reprinted from The Texas Journal of Science,v. 20, no. 1, 1958.

ABSTRACT
The rim rock of the Vieja Rim, the quartz pantellerite of Lord, is named the Bracks Rhyolite. Beneath the Bracks, the “Vieja series” of Vaughn (1900), which is newly subdivided into five formations named, in descending order, the Chambers Tuff, Buckshot Ignimbrite, Colmena Tuff, Gill Breccia, and Jeff Conglomerate, rests unconformably on Upper Cretaceous formations. The Vieja Group is expanded to include also the Bracks and three overlying formations, named, in ascending order, the Capote Mountain Tuff, the Brite Ignimbrite, the Petan Basalt. An ancient post-volcanic gravel above the Petan antedates the bolson fill. The minimum hiatus at the base of the Vieja may include part of the Upper Cretaceous, all the Paleocene, and most of the Eocene epochs. The age of major faulting that created the bolsons is pre-Pleistocene, and probably most of the bolson fill was deposited before the end of the Tertiary Period.

Keywords: bolsons, Vieja Rim, Bracks Rhyolite, Petan Basalt, Paleocene, Eocene, rim rock, Trans-Pecos Texas, Presidio County, Texas, Tertiary


Citation
DeFord, R. K., 1958, Tertiary Formations of Rim Rock Country, Presidio County, Trans-Pecos Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 36, 37 p.