GY480 Field Geology Virtual Tour 

 

Llano Uplift of South-central Texas 
 
 
Barrel cactus flowers surrounded by Town Mt. Granite float. The first two weeks of the field course are spent in the Llano Uplift of south-central Texas, just west of Austin. The field course group home base is Inks Lake State Park, located on the banks of the Colorado River. Scenes like this are typical near the camp sites. Most of the mapping projects are within the State Park. The pink rock exposed here is the Town Mountain Granite, the famous "Texas Pink" polished dimension stone that is used throughout North America. The K-feldspar is so pervasive in the granite that large exposures literally turn the landscape a pink color.
 

 
Total station mapping project..
Total station mapping area at Inks Lake State Park, Texas.


Exposure of the Lion Mt. member of the Upper Cambrian Riley formation. Topographically high areas of the Llano uplift are held up by the more resistant Paleozoic cover sequence. The exposure below overlooks Lake LBJ near Marble Falls, Texas, and is used as a type locality for several members of the Cambrian Riley and Wilberns Formations. In the photograph below the distinctive glauconitic Lion Mt. member of the Upper Cambrian Riley Formation is exposed along the roadside, and is overlain by the Welge Sandstone and basal portion of the Morgan Creek members, both being stratigraphic components of the Wilberns Formation. Mapping of these stratigraphic units is a major component of the projects conducted in Texas. 
 
Normal fault in Lion Mt. Member of the Riley Formation. The rocks exposed in the Llano uplift have been deformed by late Paleozoic extentional tectonics that was primarily accommodated by horst and graben structures that trend southwest to northeast. Fault-line escarpments mark the large-displacement faults that juxtapose Precambrian crystalline rocks against early Paleozoic carbonate rocks. In the semi-arid climate of the Llano, carbonate rocks are more resistant to erosion, therefore, the topographic ridges are held up by the grabens whereas the valleys are underlain by Precambrian crystalline rocks. In the photograph below, an outcrop-scale horst and graben structure is exposed within the Lion Mt. member of the Riley Formation.
 
 
 
Typical exposure of the Valley Springs Gneiss (Precambrian). Much of the Inks Lake State Park property is underlain by Precambrian Valley Springs Gneiss. Below is a typical exposure of the gneiss near the lake. The protolith of the gneiss was probably a felsic volcanic tuff. Xenoliths of the gneiss are typically found within Town Mt. Granite intrusives. The high quartzofeldspathic component of both the gneiss and granite result in similar appearance and weathering characteristics.
 
 
View looking southwest over the lower summit of the Enchanted Rock exfoliation dome, Enchanted Rock State Park.
The Enchanted Rock exfoliation dome is located approximately 40 miles from Inks Lake State Park. A morning field trip is taken to this state park to observe the well-exposed characteristics of this stock. A variety of textures and fractional crystallization trends may be determined by careful observation of the exposure from the intrusive margin at lower elevations, to the center of the intrusive at the summit.
 
Vernal pool with unique flora near the summit of Enchanted Rock S.P. The flora and fauna of the Texas hill country are unique. The region around Inks Lake State Park is similar in climate to the African savanah, and the interaction between climate and geology have produced an environment that is as scenic as it is unique. Below is one of the vernal pools that is an interpretive stop on the trail to the summit of Enchanted Rock. 

Northern New Mexico
 
 

Taos Pueblo, the oldest continuously inhabited pueblo in North America (courtesy of the City of Taos Tourism Office). The middle 2 weeks of the field course are spent in the vicinity of Taos, New Mexico, while several mapping projects are completed. These mapping projects expose students to a diverse assemblage of geological formations and structures. Mapping projects include rocks that range from Proterozoic to Cenozoic, structures ranging from megascopic isoclinal flow folds to Tertiary Rio Grande Rift normal faults that have produced the horst and graben structures of the present terrain. Accommodations are located in the Taos Ski Valley, at approximately 9,000 feet elevation. The town of Taos lies in the rift valley at an elevation of just under 7,000 above sea level. Towering over Taos and the ski valley is Wheeler Peak, at 13,160 the highest elevation point in the state. The area in and around the town of Taos offers visitors many historic and cultural landmarks. Below is a photograph of the Taos Pueblo with Wheeler Peak and the Sangre De Cristo range in the background.
 
 
View of the west facing side of the Sangre De Cristo Range near Taos, NM.
The Sangre De Cristo range of northern New Mexico are horst structures associated with the Rio Grande Rift system. The range exposes Precambrian high-grade metamorphic rocks unconformably overlain by Paleozoic clastic and carbonate strata. Along the topographic break at the fault scarp, it is common to find Proterozoic crystalline rocks in fault contact with Quaternary volcanics- structural vertical relief in excess of 25,000 feet.
 
 
Dr. Allison and wife Sherry enjoying the Sangre De Cristo Mountains
The climate zones of the Sangre De Cristo range are spectacular. Flora range from sage in the valley (7,000 feet elevation) to Douglas fir and Ponderosa pine at the base of the range to Aspen and Blue Spruce above 9,000 feet. Above 11,000 feet the trees give way to emerald green pastures above the tree line. In the background of this photograph are the ski runs of the Taos Ski valley.
 
Before Wheeler peak..
Just before the climb to Wheeler Peak, NM, highest peak (13,161 ft) in New Mexico.
 
Cross stratification in the Dakota sandstone, northern New Mexico
New Mexico mapping projects include the classic Mesozoic strata of western North America, in this example the Dakota sandstone, well-known for its characteristic high-angle crossbeds. 

Cross stratification in the Dakota sandstone. northern New Mexico
Mapping the Pillar metaconglomerates at the Copper Hill project area near Dixon, NM.

Crossing the cable bridge.
Crossing the dreaded "cable bridge" at Box Canyon.


Harding Mine
The Harding Pegmatite mine tour.

North Kaibab trail
View from the north rim of the Grand Canyon down the north Kaibab trail.


Shiprock, NM..
Shiprock, NM near the 4 corners area.

View from Bright Angel point..
View from Bright Angel point at the Grand Canyon north rim lodge.



dallison@jaguar1.usouthal.edu

Department of Geology and Geography
University of South Alabama