Big Bend and the Geology of west texas

〉 Seminole Canyon State Park – 

To take a quick tour through the Nature Center. We don’t have time to hike and see the petroglyphs in the canyon, but you are encouraged to return to see these interesting historic features during a later vacation. Look at the fossiliferous limestone along the sidewalk exposures. Caprinid bivalves or Globigerina are visible in this Salmon Peak limestone of the Edwards formation.

〉 Pecos River Overview – 

We will stop momentarily to look at the steep vertical cliffs carved by the Pecos River as the Edwards Plateau was being uplifted, beginning in early Tertiary time. Look at the exposures of Cretaceous massive lower Devil’s River Limestone overlain by patches of the Buda Limestone and Del Rio Clay Formations. To the left, downstream, you can see the confluence with the Rio Grande. We will see the Pecos River again on Thursday further upstream and we can compare its character to our observations here. As we cross the bridge, we will enter the Trans Pecos Region, true West Texas!

〉 Judge Roy Bean Visitor’s Center and Garden – 

We will make a quick stop here to use the restrooms, tour the desert gardens, and to pick up a valuable packet of information of all the facilities, including a map of Texas. 

〉 Big Bend’s Dog Canyon – 

Provides an excellent opportunity to look close up, at a cross-section of a folded ridge, exposing a variety of fold structures, and a thrust sheet of Laramide origins. In Mexico, they extend into the Sierra Madre Oriental and Occidental ranges.

〉 Big Bend’s Fossil Bone Exhibit – 

Here we will stop momentarily to visit this exhibit of see bones of terrestrial dinosaurs from the Late Cretaceous and mid-Tertiary Mammal bones deposited in an environment which was changing from a grassland to a windswept desert. Fossils of a huge pterosaur, the Quetzalcoatlus, with a 36 foot wingspan and a 40 foot crocodile, Phobosuchus with a 6 foot skull were unearth near here, both of which are the largest ever found.

A variety of grazing mammals is on display at the Visitor Center. Look at the exceptional cross-bedded layers of sand. Hurry back to the van. In this part of the Park, you can see the volcanic features sitting on top of the underlying limestone in the eroded low basins.

〉 Big Bend’s Visitor’s Center at Panther junction – 

There are excellent books and maps available as well as general information about the park area. There is a small botanical garden outside to help you learn to recognize the typical flora of the desert.

〉 Big Bend’s Boquillas Canyon – 

We will walk into a very different canyon in the southeast area of the park, with its own unique scenery. Exposures of older Paleozoic rocks of the Sierra del Carmen Mts can be seen here. A major slump block has created monocline at the entrance to the canyon. From the high overlook, we can see the village of Boquillas, and deep into Mexico where the fluorite mines are operating. Recent events in world politics restrict any travel to Mexico from any of the locations to this region.

〉 Big Bend’s Hot Springs – 

At this stop we will relax at the old spa where early settlers came to treat their aches and pains from their harsh life by sitting in soothing fresh hot springs. The community was abandoned after WW II, the bath house building was destroyed in the ‘70s, but the warm waters still flow up deep fissures with a constant temperature of 95°F, filling the foundation like a tub. Hope you brought your bathing suit.

〉 Big Bend’s Santa Elena Canyon (A Super Photographic Site) – 

After our adventurous journey over the old Maverick trail, we will come upon a view of the canyon, and marvel at its majesty. We will venture as far as time and conditions permit. This is one of the most photographed sites in the Southwest. Canyons surround much of the south side of the Big Bend Area. The steep walls along the front face of the canyon are part of a fault escarpment that was slowly uplifted, producing the Terlingua Monocline to the west and south; it dips down to the north. The big Bend block is also a very gently dipping monocline; it dips to the south. The super imposed Rio Grande has down cut through the crystalline Cretaceous limestone as the block was uplifted, carving the canyon. The river has not meandered much as its energy has been directed to the tracks of down cutting. Because of the lack of stream, meandering, the boundary between the United States and Mexico has been clearly recognized by both nations here, unlike in the El Paso area, where boundary disputes were settled by cementing in the channel of the meandering river.

We will follow along the vertical cliffs where peregrine falcons, eagles, and other endangered species nest in the rocky crags. The rocks are composed of the Santa Elena Formation, a thin-bedded fossiliferous limestone which can be easily eroded by the rushing waters of Rio Grande. This rock is of similar age to those we saw in Austin and will have familiar fossils. The Zuni Sea flooded this area also and deposited 1000s of feet of limestone, 50-25 million years before any of these faults and surrounding volcanic activity began to form.

〉 Beautiful Lava Flow Structures along us 90 – 

Here we can visualize the violent explosion of ignimbrites as they flow down the slopes of the caldera type volcanoes. Large boulders included in the streaming incandescent flows reflect the dynamics of the events. Imagine a fiery cloud of ash boiling down the volcanic slopes at speeds of as much as 150 mph, with temperatures over 1000° C. Many dikes are exposed as walls flanking the hills as we travel west of Alpine.

Pissano Peak and Twin Sisters Peaks are vents of these local calderas. If we have the time and opportunity to travel south from Alpine we will visit the Woodward ranch where they have a rock shop and collection of agate that has been exhibited around the world because of its beauty and unique character.

〉 McDonald Observatory – 

After dinner we will climb to Mt. Locke, the second highest peak in the region, and one of the best spots to study astronomy in the country. Our approach to the observatory will allow us to observe many of the features of the volcanic flows and development of several calderas. From the observatory we will have a beautiful scenic overlook of the Davis Mountains and the West Texas sunset. The research facilities of McDonald Observatory, part of The University of Texas system, are some of the finest in the world. The clear air, lack of clouds, and dark ground provide optimal conditions for studies. Most of the work done does not require people to look through the eyepiece anymore; data is collected by sensors and recorded by computers so that it can be analyzed by a variety of techniques.

Maybe we will be able to see the Laser telescope sending out its beams of green light. Hopefully we will get to see the new Hobby-Eberly telescope which has a new structural design and segmented mirror lens totaling 362”, instead of one single large mirror lens of 108”, the largest at this facility. We will have an evening of star gazing. The visitor’s center has movies and other attractions to see. This stop has been a heavenly experience for many groups.

〉 El Capitan Reef, Guadalupe Mountains, Texas and New Mexico – 

Standing at the south end of the reef, we study evaporate deposits of the Castile formation. These rocks represent the last minerals to crystallize from the ocean waters as thtey evaporated in the hot arid climate that developed in these region at the close of the Permian time when huge Ouachita Mountains to the south cut off and stranded this basin from the circulating ocean waters. These evaporate salts are part of the Salado and Castillo Formations and include halite NaCl (table salt), sylvite: KCl (salt substitute), potash: K2CO3 (used as a fertilizer), and other less abundant minerals, some of which are used for medications and vitamins.

Along the southwest corner of the range is Salt Flats, an area where the halite and some Bittern salts have leached out of the mountains and collected into a playa. For decades, settlers, Indians and the Calvary have fought for control of these deposits. Eventually, the need diminished and the populations looked for other sources.

〉 Guadalupe Mts. National Park Headquarters – 

We will be traveling north along the eastern flank of the Guadalupe Mts. to Guadalupe Mountains National Park at Pine Spring Canyon, where we will pick up information about McKittrick Canyon. After a brief stop for you to use the facilities quickly, we will drive up to the park entrance a few kilometers to the north; se the Guadalupe Mountains National Park map. At this site we are looking at the entrance to a beautiful canyon carved out of the Permian age (280 – 230 mya) El Capitan Limestone which is primarily a fossil reef which is exposed in the Guadalupe Mountains here. This barrier reef extends for over 600 km in a horseshoe shape around what was the shelf edge of the Delware Basin as far south as Marathon, where it is exposed in the Glass Mountains. The bottom of this paleo geographic map of the Permian Basin, outlines the edge of the Laurentian Continent, and defines its location near the Equator, accounting for the tropical conditions of the ocean and the influence on the growing such an extensive reef. The reef would be similar to the Great Barrier Reef off the coast of Australia today. It is rich in marine fossils including sponges, brachiopods, crinoids, algae, fusulinid forams, mollusks, and bryozoans.

Surprisingly, compared to most reefs today, El Capitan had very few corals associated with its growth. All of these organisms lived in dense populations and were encrusted into this massive biohermal mound. Recent uplift and subsequent weathering and erosion in this arid environment produces the prominent and resistant cliffs of the limestone.

〉 Carlsbad Caverns National Park –

Virtually all caves from in limestone. Lava tubes in Hawaii and fractures in rocks are like caves, but will not develop the interesting features we are about to see. Caves form in three stages. First, limestone is deposited below sea level sublittoral environments over time. Later the limestone is uplifted into a terrestrial environment, where it develops solution cavities when flooded and chemically attacked by flowing groundwater. The long they are submerged and exposed to the process, the more extensive the caverns develop. If the same process occurs at the surface, karst topography will develop with sinkholes, disappearing streams, and other solution features. These can be dangerous as the surface rock loses it integrity and can collapse, and with it comes our community’s roads, houses, schools, etc. At some later point in time when the conditions change, the ground water level can drop below the caverns, leaving them dry. As surface acidic rain waters seep through limestone, calcite dissolves and drips in the cavern. Changes in the cave’s air pressure, oxygen content, or evaporation cause the calcite to precipitate out of solution and form the exotic features seen in the caves. Since every cave is different, every feature is also. Living caves are actively forming, while dry caves are not developing, due to the lack of water.

All the caves of Texas and New Mexico have developed in just the past few million years. Most of the caves in this region including Caverns of Sonora (we’ll see tomorrow) are found in Cretaceous limestone. Carlsbad Caverns are distinctive as they have formed in Permian limestone, while Long Horn Cavern, which we will visit next month in Burnet, is rare because it formed in Ordovician limestone.

〉 San Angelo State Park Synapsid Tracks –

We will stop at San Angelo State Park to see synapsid (possibly Dimetrodon) tracks. During the Early Permian Period about 280 million years ago San Angelo was a coastal environment with an ocean to the west. When people here the Permian basin and associate it with oil and gas, it is the this ocean that generates that petrolium. 

〉 Caverns of Sonora –

We have reservations for an educated tour of the caves. There are two sections, and we will get a partial tour of each. You will want to return here to see the whole system at another time. The features and formations of this cavern are very different from Carlsbad. While the magnitude of Carlsbad is breathtaking, so here are the delicacy of the unique formations of this caves. Helectites, the soda straws, lions tails and the Butterfly are exquisite features that make this cavern one of the most beautiful in North America.

Before and after the tour, look in the gift shop and at the exposures around the park as many echinoids and other fossils have been found in the loose alluvium. Then hop into the van as we have a long but interesting ride back home.

Trip Details:

• To book a trip or inquire more details, email me at: jurassicjames65@gmail.com

• Note: It is illegal to collect vertebrate fossils in Texas, we will only keep marine invertebrate shells.

• More information coming soon!