The orogeny produced belts of deformation forming topographic highs, between which coarse volcanoclastic and extrusive Tertiary rocks were unconformably deposited on eroded Cretaceous beds." A central folded belt oriented NNW-SSE through the center of the Chisos Mountains appears to have acted as a barrier to eastward deposition of the TACB and westward deposition of Paleocene and Lower and Middle Eocene clastic rocks that occur east of the Chisos Mountains.
Those interested in the detailed location of the TACB are referred to the Geologic Map of Big Bend National Park, provided as plate II in Geology of Big Bend National Park, Texas, Bureau of Economic Geology Publication 6711 (Maxwell et al 1967) and also supplied with the Bureau of Economic Geology Guidebook 7, The Big Bend of the Rio Grande (Maxwell 1968).
The TACB is exposed in Big Bend National Park around the western base of the Chisos Mountains from Dogie Mountain in the north to the Gauging Station in the south, and Lajitas Mesa, just outside the Park, on the west (Fig.
At least one of the eight units of the TACB consists of three separate flows, as distinguished by mapping and petrography and illustrated in their chemistry.
In all exposures within the National Park the TACB rests unconformably on the uppermost Cretaceous Javelina Formation, but west of the Park, at Lajitas Mesa (Fig.
Either Unit A or B is at the base of the TACB where well exposed elsewhere, and each is overlain by Unit C or D.
Unit A is at the base of the TACB, resting unconformably on the Javelina clay from the north-central to the southernmost part of the area at the Gauging Station, where it is overlain by Unit C (Fig.
Un it G comprises the entire outcrop of the TACB at Sierra Aguja (Figs.
The eight units of the TACB include six of olivine basalt, one basaltic trachyandesite and one trachyandesite.
Pale green to brown smectite is the dominant alteration product in the TACB lavas, commonly pseudomorphing olivine and as interstitial patches and veinlets throughout the rocks.
Table 3 shows the overall chemical nature of the TACB units, and serves to demonstrate the general makeup and similarity of these rocks, as well as their characterization as alkaline olivine basalts.
11a) the TACB suite of rocks shows a successive increase in LREE (light rare earth elements), suggesting that, over all, it might be some sort of evolutionary sequence.