This field trip begins at the Twisted Shoe campsite along the usually dry Juniper Creek. A clear evening makes for a fabulous view of the Sierra del Carmen, Lower Cretaceous limestone cliffs to the southeast of the campsite, beyond the Rio Grande in Mexico. The two most prominent cliffs are composed of the Santa Elena Limestone (upper) and the Del Carmen Limestone (lower) (or whatever these units are called in Mexico). The slopes between these cliffs are due to more easily eroded units. The Sue Peaks Formation of shale, marl, and limestone lies above the Del Carmen and below the Santa Elena. The limestone and marl Telephone Canyon Formation lies below the Del Carmen. These are the same units exposed at a lower elevation in Santa Elena Canyon at the western side of the park. Below these units, forming a sort of stair-step of alternating limestone and marl, is apparently the Glen Rose Formation at the base of the Cretaceous, or whatever its equivalent is in Mexico. The high point in the Santa Elena is Pico Cerda, "bristle peak" in Spanish.
Limestone largely consists of calcite, a mineral (calcium carbonate) that fizzes when in contact with acid, much like Tums. (In fact, that's what Tums is.) Marl is a sort of dirty limestone, and shale is basically mud layers turned into stone. The Sierra del Carmen are relatively high due to a major fault that lies along the eastern side of the park. They were uplifted along the east side of the fault with respect to the rocks to the west (where we are). All these rocks were originally deposited in shallow seas, where the limestones formed in the absence of sediment (from shells and tests of organisms as well as direct precipitation of calcite), and the shales and marls formed when sediment was available. These are clues as to how deep the sea was and how far it was from land when these units were deposited.
Retraction (!) From our perspective at Twisted Shoe, I wrote the following:
Note what appears to be a small normal fault to the right of Pico Cerda. A normal fault is one where the "hanging wall" (the one on the right in this case) is down-faulted with respect to the "foot wall" (here, on the left). A normal fault indicates the rocks were pulled apart (tensional stress). The detail below indicates the fault. The fault seems to die out in the Glen Rose Formation - particularly in the thick, soft bed above the lowest limestone cliff - where the rocks appear to be merely "bent" (folded) by the tensional stress. Softer beds often deform rather than break under stress.
"Upon further review", as they say in football, I'm going to have to change my mind. When I saw the Sierra del Carmen escarpment from another direction, I had to conclude the evidence for a fault was not as strong as it appeared from Twisted Shoe. The limestone beds that form the cliffs on the escarpment have been folded into a monocline (see below) and apparently dip away (in an easterly direction) from an observer there. Therefore cliffs that are farther away are lower in elevation than cliffs made of the same limestone beds that are closer. Since distances are extremely difficult to judge in the desert, I now think the beds to the right of Pico Cerda are really just farther away than those below Pico Cerda, and therefore appear to have been down-faulted. The drawing below illustrates this, on the left with the escarpment viewed from the side in perspective and on the right from the front. The red "bed" plays the role of one of the limestone beds in the escarpment.
The real "fault" was probably mine, it turns out. However, there is a good lesson in this for the aspiring geologist. When you have erosion of tilted beds, an apparent fault may just be due to the angle of observation. Furthermore, if you do have faults in tilted beds, they can look like one type of fault when further "review" (field mapping) shows they are really a different kind of fault. This is why field mapping has to be done carefully and the results examined meticulously.
Now Back to the Trip.
The major faulting episode that uplifted the Sierra del Carmen had to occur after these rocks were deposited (a geological principle called "the principle of cross-cutting relations"). Since these are Lower Cretaceous rocks (140 to 100 million years old), the faulting had to occur later than that. Other evidence indicates the faulting was due to the extension of the crust in western North America that formed the Basin and Range Province. This was relatively recent, within the past 25 million years. The faulting associated with this activity reactivated the Sierra del Carmen monoclinal fold formed during the Laramide orogeny, a mountain-building episode of the Late Cretaceous to Early Tertiary (around 70 to 50 million years ago), which resulted from the subduction of the Farallon Plate beneath western North America.
My brother, Randy, got up early and took this picture of the South Rim of the High Chisos Mountains in the morning light. The South Rim is over 3000 feet above the Twisted Shoe campsite. Toll Mountain, to the right, is just catching the sun, rising in the northeast over the elevated terrain of Lost Mine Peak, Crown Mountain, and Hayes Ridge off the right side of the image. The rocks of both Toll Mountain and the South Rim are volcanic in origin.
Being the lazier of the two of us, I got up somewhat later and snapped this picture looking north up Juniper Canyon. You see the South Rim and Toll Mountain again, and, at the head of the canyon, Casa Grande Peak, which looks quite different from this perspective than it does in the Basin.
The Dodson trail heads west from Twisted Shoe, winding between two igneous intrusions, originally masses of magma that moved up into rock already there ("country rock" - not a music genre in this case) and cooled. At that time this area was deep underground and has subsequently been uplifted by subterranean forces and exposed by erosion. The intrusions, being more resistant to erosion, now form topographic highs.
1We were a little apprehensive upon heading out. We had heard that the trail was poorly marked and maintained. Also it was August and extremely hot. The showers of the West Texas "monsoon" season had vanished the past few days, and, even though you don't expect to get much rain on the desert floor, the cloud cover would have been very welcome. (Days later, we were drenched on the South Rim while watching thunderstorm after thunderstorm cross the desert toward the Chisos Mountains from Mexico - right across where we had sweltered days before. Now, however, we were wet and cold. Murphy is everywhere.)
The park rangers couldn't give us any information about the availability of water along the route, although we had hopes of finding water at Fresno Creek, our first day destination. But, to be on the safe side, we loaded ourselves down like beasts of burden with enough water to get us to Blue Creek Ranch, the other end of the trail. Now, neither my brother nor I are what you would call young bucks, and this hike became quite an ordeal with the heavy packs, 100-degree heat, rocky trail, and up-and-down terrain. However, the good thing was that, in spite of the cautions, the trail was easy to follow, and I never had to use my compass and maps to navigate cross-country. Since people have been lost, and even died, on this trail, we suspect the park service has recently done some work on it.
Let's take a closer look at the South Rim. Recently, there has been a revision of how these rocks are interpreted, and the new USGS geological map (Scientific Investigations Map 3142) has drastically changed the geological landscape from what I originally learned. Gone from the South Rim are the Pine Canyon (formerly Brown) Rhyolite, the Wasp Spring Flow Breccia, and the Lost Mine Rhyolite. The rocks that make up the South Rim cliffs are now assigned to the Boot Rock member of the South Rim Formation (age 32.3 million years). The Boot Rock member consists of lava flows, ash flows, and breccia erupted from the Pine Canyon Caldera, the remnants of which are in the Pine Canyon area (where else) in the park (White et al, Petrogenesis and Tectonic Setting of the Peralkaline Pine Canyon Caldera, Trans-Pecos Texas, USA, 2006). The composition of the Boot Rock member volcanic rocks is trachyte to rhyolite. Both trachyte and rhyolite have a very high silica content and is often erupted violently; however, trachyte has a low quartz content compared to rhyolite. The material below the cliffs consists of the Chisos Formation (volcanic and sedimentary rocks, mostly covered in this picture with colluvial debris). Colluvium is just the accumulation of debris below a hill or cliff due to various erosional processes.
The Dodson trail tracks mostly through the Chisos Formation, riddled here and there with igneous intrusions. The Chisos Formation has not been mapped in detail and is not well understood, although the recent effort to remap the park and study its geology has already improved our knowledge of these rocks. Prior to this effort, a few units of this group had been named and mapped, but most of the sedimentary and volcanic rocks that make up the Chisos Formation have, until recently, at least, been lumped together as unknowns. This is not hard to understand, as much of the Chisos Formation is buried under soil and debris, and good outcrops are not common due to the soft nature of most of this rock. Below you see a rather "typical" outcrop (in that it is small and isolated) along the trail, which appears to be a thin bed of welded volcanic tuff within beds of unconsolidated tuff or ash, colored red by iron oxide.
There are some hard-rock outcrops, however. Check out the image below of what appears to be a mesa topped by several dark lava flows (possibly basalt) north of Tortuga Mountain. The "basalt" may to rest upon volcanic ash, indicated by the light color where not covered with vegetation or debris eroded from the dark beds, but it could also be sediment. Basalt is a volcanic rock that is relatively high in iron and low in silica compared to other volcanic rocks. Also in the picture is an outcrop of what is likely to be or welded tuff similar to that found at other points along the trail and at the lower end of Blue Creek Canyon. The "basalt" is such a thick deposit, I initially wondered if it could be the Bee Mountain Basalt, one of the known and mapped units of the Chisos Formation. However, the USGS map only indicates what it labels the "Tcstr" member of the Chisos Formation in this area, and the description of this member does not mention basalt beds. Possibly the apparent lava flows are rhyolitic rather than basaltic, despite the dark color.
In the following image, beyond Randy, toiling under his heavy pack, you see a prominent sill in the Sierra Quemada ("burned mountains"). This is apparently the same sill that can be seen from the South Rim. A sill is an igneous intrusion that typically follows a weakness between two layers of rock, assuming a tabular form. A sill is termed a "concordant" intrusion, because it follows the geologic structure already in place rather than cutting across it (which, not surprisingly, is called "discordant"). According to the USGS map, this is a basaltic sill, like so many others found in the park.
Once you get past the mesa in the second figure above, you have come across a window to the south in the mountainous terrain. What greets you is a fantastic view of two fabled intrusive features of the park, Elephant Tusk and Backbone Ridge. To the left is Tortuga Mountain. Both Elephant Tusk and Backbone Ridge are Tertiary granitic intrusions. Tortuga Mountain is topped with the upper Cretaceous Aguja Formation, in whose sediments you see the transition from sea to land in Big Bend.
And here is a great picture Randy got of "the Tusk".
Well, there was water in Fresno Creek after all - plenty of water. We felt silly for having lugged so much water across the desert. "Logic would dictate" that with all the monsoon showers occurring over previous weeks, Fresno Creek would have water flowing in its bed. On the other hand, you don't want to be caught in the desert without water.
From our camp at Fresno Creek, the South Rim was pretty awesome in the evening light. (The image below is a telephoto.) According to recent work in this general area (Sedimentation, Volcanism, and Intrusion in the Lower Chisos Group, Big Bend National Park, Texas, by Barker et al), the prominent tannish-orange outcrop below the rim could be a bed of trachyandesite, rhyolite, or welded tuff. I originally took it to be the 30-million-year-old trachyandesite that was previously thought to be the Tule Mountain Trachyandesite. The new USGS map (Scientific Investigations Map 3142) of this area indicates no trachyandesite, just the youngest undivided rocks of the Chisos Formation of Oligocene and Eocene age. If this rock correlates with that along Blue Creek, it consists of welded tuff. There is a lot of whitish material exposed on the slope where the vegetation is thinner, indicating quite a bit of volcanic ash was deposited before the South Rim rocks, which form the high cliffs, were erupted.
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