Physical Geology Field Trip

Fall 2005

Rio Grande city, Roma and San Ygnacio

Stop 1: Sullivan City – Goliad Fm. (Pliocene)

1.                  Describe these unconsolidated deposits and draw a x-section

2.                  How these sediments have been deposited? By which agency?

Stop 2: 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?

Stop 3: 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?

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?