Historical Geology Field Trip

Fall 2005

Rio Grande city, Roma and San Ygnacio

Stop 1: Rio Grande City - The Catahoula Tuff, Frio Fm. (Oligocene)

1.         What is a tuff? How does a tuff form?

How has this deposit been formed?

2. At the top of the outcrop is the base of a well-cemented sandstone. Considering the difference in resistance to erosion of this sandstone compared to the Catahoula, why do bluffs form in this area?

3. Examine the paleoenvironmental map of the Frio Fm. What environment these sediments have formed in? What does this imply about sea level change from the Eocene to the Oligocene?

4. Given the lithologies and the inferred paleo-environment, what is the probable reason for the sharp-base of the sandstone member above the Catahoula?

Stop 2: Roma - The Roma Sand, Jackson Group (Eocene)

1.         Describe the grain size, sorting, roundness, mineralogy, degree of cementation, and sedimentary structures (including bedding thickness) observed. If the characteristics vary depending on where you are, describe how they vary. Is there any cyclicity (repetition of a pattern of variation)? Does the rock change in any progressive way upward? Classify the sedimentary rock(s) seen here.

2. What depositional environment is likely represented here? Why?

STOP 3: San Ygnacio - The Laredo Formation (Eocene)

l.          Our elevation here is significantly higher than in Edinburg. According to the law of superposition, younger rocks should overly older rocks, but the material here is much older (Eocene) than what is seen at Edinburg (Pleistocene). Why?

2.         Examine the rocks at the lower part of the section.  Describe the grain size, sorting, roundness, mineralogy, degree of cementation and type of cement, and sedimentary structures (including bedding thickness) observed. If the characteristics vary depending on where you are, describe how they vary.  Is there any cyclicity (repetition of a pattern of variation)? Does the rock change in any progressive way upward? Classify the sedimentary rock(s) seen here.

3.         Note the layers of white crystalline material in the outcrop. What mineral is it composed of? What does that mineral imply about the conditions in which the rock formed?

5.         What depositional environment do you think might be represented by this outcrop? Why?

6.         Given your answer to 5, what has happened to sea level since the Eocene. Given what you know about the pattern of global change during the Cenozoic, what has caused this change in sea level?

7.         Note how the river is deeply incised (i.e. It has down cut into the underlying material). What may have caused the river to downcut its valley?