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
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.
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
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
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
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.
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.
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!
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.
Bud Johnson (1-9) August 2003