Although my experience as a geologist lies in the Eastern Shelf of the Midland Basin, following is a brief overview of the geology of the Big Bend area of Texas as I understand it, beginning with the Cambrian:

1. Deposition of the Cambro-Ordovician sedimentary rocks is consistent with other Texas deposition; that is, sandstones grading upward into carbonates formed by the transgressive sequence of the Sauk Transgression. Outcrops of these rocks can be found at the Solitario only a few miles from Terlingua House or through Big Bend State Park. A guided tour can be arranged to visit these outcrops.

2. Silurian rocks of the upper Tippecanoe Transgression seem to be missing in the area; this could either be the result of non-deposition or erosion. The same sequence is missing along the Ft. Chadbourne Fault Trend in the San Angelo to Sweetwater, Texas area.

3. Devonian/Mississippian sedimentary rocks lie unconformably on the Ordovician. This rock group is primarily the Caballos Novaculite. Novaculite is a cryptocrystalline siliceous sedimentary rock associated with the Ouachita Front (see 5 below).

4. Next are the Pennsylvanian/Permian aged sedimentary rocks. Locally the principal group of this age is the Tesnus Sandstone as found in the Solitario. Other Pennsylvanian and Permian rocks are found in the far West Texas area. These aged rocks are typically formed by cyclothemic deposition; that is, worldwide sea level changes caused by glaciation (glaciers forming and melting). As glaciers form over a large region, they rob water from ocean basins resulting in oceans regressing or lowering. When the glaciers melt the oceans recover their waters and transgress back over the granitic continents. Glaciers have a way of melting faster than they form. From an oceanic perspective this results in a slow regression followed by a rapid transgression.

5. During the Pennsylvanian and Early Permian the Ouachita Front, a now extinct mountain range wound its way through Texas. Entering Texas northeast of Dallas, it came south-southeast approximately along Interstate 35, wrapped itself around the Llano Uplift northwest of Austin, then headed west (approximately along US 90) to the Big Bend country. This mountain range probably turned south-southwest from Persimmon Gap (north gate of Big Bend Nat’l Park) and continued into Mexico.

6. Next, the Zuni Transgression resulted in the Cretaceous sedimentary rocks. At Terlingua House, Reed Plateau on the south property line is a massive shallow water carbonate consistent with the transgression (Santa Elena Formation). A large fault lies along the south property line of Terlingua House; Terlingua House sits on a younger (down faulted) Cretaceous outcrop of alternating dirty marine carbonates and terrestrial to highly clastic influx shallow marine shales (Boquillas Formation). The north property line of Terlingua House (TX 170) is also a fault with the north side being only the clastic shales (Penn Formation – I believe?). This gives the visitor a good look at the Cretaceous regressive sequence. It is important to note that during the Cretaceous this area was situated in a shallow coastal plain between mountains to the west and a shallow ocean basin to the east. By virtue of this location, this area would have been an ideal location for both dinosaur life and dinosaur fossilization – and in fact, numerous fossilized dinosaurs have been found in the Big Bend area

7. The next major event is the volcanoes of the Cenozoic. To the non-hard rock geologist this area is both fascinating and somewhat overwhelming. Two things stand out: First is the sheer volume of volcanic material suggesting a time of huge and frequent volcanoes; second is the fact that the volcanics are both granitic (rhyolitic) and basaltic. The granitics tend to predate the basalts but in certain cases they seem to be almost interbedded. Granitic igneous rocks tend to come from the granite continents (the high ground we live on now). Basalts, on the other hand tend to come from oceanic plates. Basalts are heavier (more dense) than granites. As a result the granite continents tend to float and the basalts tend to sink. The theory of plate tectonics rests generally on this theory. The youngest of these volcanics seem to be about 22 million years old and are basaltic.

West of Ruidoso, New Mexico, the Malpais, or badlands on the north end of the Tularosa Basin, are made of a relatively young basaltic lava flow (2 million years old?). The fact that the younger lava flows of the Big Bend area are basaltic and that the basaltic flow of New Mexico is as young as 2 million years and are essentially “on trend” with Big Bend basalts, strongly suggests that this area was (or is) headed for a future oceanic basin.

It is interesting to note that the previously mentioned Llano Uplift of Central Texas is probably a thick section of granitic material, “floating” in the earth’s crust. In as much as a US Naval Carrier sits deeper in the ocean and floats higher above the sea surface than a two man johnboat – the Llano uplift sits deeper and floats higher than the surrounding granitic rocks of the continent.

8. Development of the Rocky Mountains. Numerous horst and graben (basin and range or mountain and bolson) structural features are found in this area. Terlingua House is located in a graben. This graben is referred to as either Long Draw Graben or Terlingua Graben. Following is a schematic of this graben as I believe it exists.

9. Last is the “current conditions.” The thing that stands out to me is what you don’t see. In most basin and range desert environments you get the basins with the alluvial fans and overlapping alluvial fans forming bajadas. I had made several trips to the area before I noticed this was missing. This isn’t to say there are none, but by and large what you see are the older Paleozoic, Mesozoic or Cenozoic (volcanic) rocks without the layers of recent detrital covering the sections. Of course, there are plenty of examples of mass wasting and desert pavement but it is nice seeing the rocks in their original depositional state, even in grabens.

So if you don’t see the alluvial fans and bajada’s of a desert environment, what do you see? Once I looked past the agaves, prickly pear and ocotillos what I noticed was signs of a very wet, highly fluvial environment. Reed Plateau, right beside Terlingua House, has dissolution cavities (caves with a limited degree of karsting). I have come across numerous fracture/faults that are completely filled with calcite. And moreover the original major industry of the area was mercury (known as quicksilver). I know very little about the geology of quicksilver as of yet, but it is associated with the mineral cinnabar, and the veins of cinnabar I have seen are lined with calcite.

In short, when you come to Terlingua you will predominantly see what I believe to be regressive, intermediate to shallow marine and coastal environments grading to terrestrial environments which were later blanketed with volcanic lavas and ash, then faulted through compressional and tensional forces, and then subjected to a cool, high rainfall glacial climate, which was abandoned for the dry, warm desert conditions which now exist - As Terlingua Twirls!

Thank you for taking the time to read this very brief geological overview of the Big Bend Area – I hope it was enlightening for you. For those of you who might have read this and are more experienced in the area and would like to edit any of the above sections, please copy any section onto an email and edit as you see fit and forward it on to info@terlinguahouse. If it is used you will be listed on the following contributors list.

Contributors:
Bud Johnson (1-9) August 2003