Download Itinerary - Cin

Itinerary for ESCI 301 Field Trip to the Llano Uplift
March 24-27
Cin-Ty Lee 281 250 3606
Additional Drivers
Wenrong Cao
Lexi Malouta
Hehe Jiang
Depart: March 24, 1 PM
Return: March 27, by 7 PM
What to bring: hat, good walking shoes, hand lens, note book, water bottle
Day 1. March 24
7 AM – breakfast in lobby
7:45 AM - depart
Depart Houston. Take I-10 west towards San Antonio. Before Luling, get off on 183
(there is a BUCEES there). This will take you through Luling. In Luling, take 80 west to
the city of San Marcos. Continue to HWY 35 and turn right on the frontage road. Turn
left on Aquarena Springs Drive and under the freeway to our hotel.
Rodeway Inn
512-353-8011
6 rooms
1635 Aquarena Springs Rd, San Marcos, TX 78666
Day 2. March 25
Depart San Marcos. Head south on 35, then exit north on 12. Continue on 12 to the
town of Dripping Springs. Turn left on Hwy 290, then north on CR 3232 to Pedernales
Falls.
Stop 1. Pedernales Falls State Park
30.334398, -98.252623, 287 m.
Ordovician, Pennsylvanian, and Cretaceous rocks, mostly limestones. Fossiliferous.
 Describe the make-up of the limestones. How do you think these sediments were
formed? What sort of paleo-environment?
 Describe the large-scale structure of the limestones. Are they tilted? Are they
fractured/jointed?
 Describe any features associated with erosion associated with the current river
system.
From Pedernales Falls State Park, take CR 2766 west to the town of Johnson City.
Turn right (north) 281 to the town of Marble Falls. Just before town, turn right on Max
Starke Dam Road. Proceed to the end and park.
Stop 2. Ordovician Ellenberger group with stromatolites
30.55404, -98.26261, 247 m.
Stromatolitic limestones in profile view. Stromatolites are limestones formed by algal
(cyanobacteria) mounds. They form by the precipitation of carbonate around the
cyanobacteria, which are photosynthetic. During photosynthesis, CO2 is consumed,
causing the waters around the bacteria to increase in pH, promoting carbonate
precipitation. In order to photosynthesize, the algal mounds must be shallow.
 Describe and sketch these algal mounds. What are the scales of these mounds?
Scale of the laminations?
Return to 281. Turn right into the town of Marble Falls. Turn left on 10 th street to the
Town Mountain Granite Quarry.
Stop 3. Town Mountain Granite
30.59236, -98.29926, 263 m.
Town Mt Granite, age ~1115 Ma. ‘Post-orogenic’ granite. Used for building stone.
 Describe the texture and mineralogy of the granite
 What characteristics tell you this is a plutonic rock?
 What depth do you think these granites were formed at?
From the Town Mtn Granite, turn north on Slaughter Mountain Road. Continue for about
2 miles.
Stop 4. Cambrian non-conformity
30.61995, -98.29328, 277 m
Cambrian nonconformity over the Town Mountain Granite. There is a 500 my time gap.
Cambrian marine sediments were deposited directly on top of Proterozoic basement.
 Speculate on how a non-conformity is formed
 Why are the ridges capped by these Cambrian limestones?
Continue north on Slaughter Mountain Road to Fairland Road (121). Turn left on
Fairland all the way to 1431. Turn right on 1431 and continue for 2.4 miles to river
lookout on the left.
Stop 5. 30.64518, -98.41587, 294 m.
Backbone Ridge/ Hoover Point Cambrian glauconitic sandstone, trilobite coquina.
 Describe the overall sedimentary structures
 Describe any post-sedimentary structures, such as jointing and faulting
 How do these sediments differ from limestones?
Continue west on 1431 to Kingsland. Turn N on SR 2342 towards Inks Lake, merge
onto Park Rd. 4. Proceed to north side Spring creek and park in pull-out.
Stop 6: 30.749394, -98.35851, 287 m.
Valley Springs Gneiss. Here, we will see multiple episodes of ductile deformation. You
will see different colored rocks here, the colors reflecting different proportions and types
of minerals, which are ultimately controlled by the protolith composition. Some of these
include pelitic protoliths (clay-rich sediments) and mafic protoliths (metavolcanic rocks).
 Sketch examples of ductile deformation?
 Sketch examples of a fold
 Sketch examples of foliation. What minerals help define foliation?
 Describe the mineralogy of the rock
 Is this rock metamorphic, igneous or sedimentary?
Take Park Road. 4 N to Hwy 29. Turn left and proceed to Llano. In Llano, turn south on
Rte 16 and once in town, turn right onto Legion Drive just before bridge, into city park.
Pull down to near dam in Llano River.
Stop 7: 30.754211, -98.67678, 304 m
Folded granitic dikes and Valley Spring Gneiss metasediments.
 Describe the relationship between the dikes and the gneiss?
 How do you think the dikes formed?
Hotel
Llano Best Western
325 247 4101
901 W young St, Llano TX 78643
Day 2
7 AM – breakfast in lobby
7:30 AM – depart
Take 16 south to 965. Turn right on 965 to Enchanted Rock State Park.
Stop 8. Enchanted Rock State Park
Mid-Proterozoic alkali feldspar granite.
 Describe mineralogy and textures of the granite
 Describe jointing and fracturing
Return to 965 and go north back to 16. Turn south on 16. After ~4.1 miles, turn left on
Willow City Loop Road. Continue to second major creek crossing.
Stop 9. Migmatites
Mid-Proterozoic migmatites. This is the birth place of some granites. What you are
seeing is partial melting (anataxis) of the lower crust. You will see evidence of ductile
deformation in the solid state, but also the generation and migration of melts.
 What temperatures do rocks melt?


How deep do you think these rocks were when they formed?
How did they get to the surface?
Continue on Willow City Loop road to serpentinite
Stop 10: 30.4795, -98.6311, 351 m
Coal Creek Serpentinite. These rocks represented the hydrated equivalents of
ultramafic protoliths, olivine-bearing rocks called peridotites. These rocks were once
part of the uppermost mantle, but tectonic processes brought these mantle rocks
towards the surface, subjecting these rocks to hydration that led to serpentinization. The
protoliths could have been represented by sub-arc mantle that was exhumed during
collision of an island arc with the continent. Serpentine is a hydrous mineral, specifically
a phyllosilicate. It is very soft and hence quite ductile. Do you see evidence for
deformation?
Return Home. Continue south on Willow City Loop to Willow City. Turn left on 1323,
then right on 281 (south) through Johnson City to 290. Take 290 east to Austin and
continue on 71 to I-10 back to Houston.
Other questions to answer.
 Describe the overall geologic history of the Llano, from continental collision to the
deposition of marine sediments in the Phanerozoic. Place each of our stops in
the context of this geologic history.