Part 1: Observations for the Edwards Formation

Transcription

1 Name: Spring 2018 Professor name: Class Time: The Virtual Field trip is to be completed by your self. You should not work with a partner. This is not group work. Nor should you search for answers on the internet. The answers that have been posted in previous years are not correct as the students who posted these did a poor job with the answers. If you look at the videos with closed captioning, the spelling for some geologic terms is incorrect. If you are unfamiliar with a word or geologic concept, look it up and get the correct spelling. By submitting this work, I attest that I have not violated the UH Academic Honesty code. I did this work by myself and did not copy any portion of my answers from a website, another student or any other source (your initials here) When a geologist goes on a field trip or does their field research, they take notes and make sketches of what they see and also try to interpret these rocks (outcrops). So, to do this field trip you will be asked to take notes on what you see using both the videos and gigapan images. The first step at every outcrop is to identify if you are looking at igneous, metamorphic or sedimentary rocks. Then, describe features of the rocks and possibly sketch them. After describing the rocks, you need to describe any deformation that has occurred. The general process for writing rock descriptions is to start off with a general description (color, grain size, texture, etc.), followed by identification of the minerals within the rock (the mineral assemblage), the name of the rock (deduced from the assemblage) and then finally your interpretation on how and where it was formed. If you need help describing rocks, your text has many figures that are helpful. We ve noted many of the figures for each stop. All figures referred to are from Earth, 12 th Edition by Tarbuck, Lutgens, and Tasa. If you do not have a copy of the text book, go look at in the Geoscience Learning Center (GLC is in Fleming 136) or ask TAs in the GLC for assistance. All answers should be written as full sentences and not one or two words. Part 1: Observations for the Edwards Formation Watch the video at Bee Cave road, view the gigapan image (you may want to zoom in closely to see rock features), describe the rocks, and interpret what you ve seen. This is a chemical sedimentary rock. In sedimentary rocks, you also need to observe whether or not the beds are tilted or if faults cross-cut the sedimentary beds (strata). Use Figure 7.22 (p. 228) to classify the rock type.

2 1) Describe the Edwards limestone. Use full sentences and include information such as minerals present, grain size, types of fossils, color, weathering, size of bedding, etc. finally you can interpret the depositional environment (see Figure 7.25, p ). Include evidence to support your interpretation. 2) There is a small fault at this outcrop. What kind of fault is seen here? Circle one: reverse/thrust/normal/strike slip 3) Is Austin, TX in the hanging wall or footwall of the fault system? 4) San Antonio and Austin both depend on water from the Edwards aquifer. How does the Edwards aquifer get recharged with water? Is this outcrop in the recharge or discharge region? Explain your answer. Gigapan Questions Answer the following questions using the gigapan image for the Edwards formation/bee cave road cut out For each of the following questions take a partial screenshot of the relevant portion of the gigapan image, creating a separate, smaller image for each. For direction on how to do this refer to the link Directions for Interpreting Geologic structures in gigapan images. 1) Trace three contacts between beds. 2) Circle an area where chemical weathering has taken place. 3) Using the contacts you drew, how thick are the individual limestone beds? (Hint: trees/shrubs are 5-10 ft. tall.) 4) Are all the beds in the Edwards formation the same thickness? 5) Does the thickness of individual beds remain constant across the outcrop? You may have to relook at the entire gigapan image to answer this question.

3 Part 2: Observations for Slaughter Gap Same as for the first stop: watch the video at Slaughter Gap, look at the two gigapan images, write two rock descriptions, and describe the contact between the two rock types. To see both rock types, you need to zoom into both the top and bottom of the first gigapan images. Then scroll up from the bottom to the top, to find the contact between these. As before, you must describe the rocks. In this outcrop, there are two rock types (one behind the students and one they are standing on). One of these is sedimentary and the other is igneous. For igneous rocks, look at Figure 4.7 (p. 113) for texture and use Figure 4.13 (p. 117) to classify the rock. The other rock type is sedimentary. You may want to refer to Figures 7.3 (p. 215), 7.7 (p. 217) and 7.22 (p. 228). 1) Use full sentences and describe the rock type are the students standing on (igneous, metamorphic, or sedimentary)? Include its color(s), list of minerals, texture, grain size and its rock name. 2) Use full sentences and describe the rock type behind the students (igneous, metamorphic, or sedimentary)? Include grain size, grain shape, grain sorting, color, bedding thickness, and other features. 3) What type of contact is between the rock on which the students are standing and the rock that is behind the students? What is your evidence for this interpretation? 4) For the igneous rock, interpret whether this is intrusive or extrusive and its cooling rate.

4 Gigapan Questions Answer the following questions using the gigapan image Slaughter gap 1 for the first question and Slaughter gap 2 for the second question. For each of the following questions take a partial screenshot of the relevant portion of the gigapan image, creating a separate, smaller image for each. For direction on how to do this refer to the link Directions for Interpreting Geologic structures in gigapan images. The first image was taken at a location you can easily walk up to as shown by the red arrow on the topographic map of Slaughter Gap shown below. The second image is the view across a small stream and railroad track from first location as shown by the red line on the topographic map. The base of the gigapan image is parallel to the red line shown below. So, your contacts should be able to be traced across the valley with a similar geometry. Topographic Map of Slaughter Gap region provided by Dr. Leon Long, University of Texas at Austin 1) Draw a line approximately where the contact between the Precambrian basement and Hickory sandstone is. Hint: look for layered rock in contact with non-layered rock. You may have to zoom in and move around the image to look at the rock textures to identify this. Look down at the level where the students would have been standing. 2) Draw a line where the contact between the Precambrian basement and Hickory sandstone is and label the two rock formations with text. Hint: look for layered rock above the break in slope. Also look carefully at all rocks within the trees. Be careful as some of these have fallen down the cliff and others are in place. You may have to zoom in to identify the rock type. Since this gigapan image was taken looking across a small creek to the west of the first image, the dip (angle) of the contact should be the same on both sides of the creek.

5 3. Use your interpretation of the two gigapan images to draw the unconformity on the topographic profile provided below. You should be able to locate this by looking at the breaks in the slopes that indicated the different types of rocks (either cliff former or valley former). Also, from your interpretations, you know if the contact is relatively flat or dipping. If you need help to locate this, go to the GLC for assistance. You can also look at Figure 9.12 (p. 280) for an example of the topographic profile with various types of unconformities. You should draw this a solid line through the pink colored earth and as a dashed line through the air. Also, label the two rock types with their formation names. Part 3: Observations for Inks Lake Watch the video of Inks Lake, look at the gigapan image, and describe the two rock types as well as deformation seen in this location. The students are standing on one rock type and the other type is seen across the stream in the gigapan image. 1) What is the rock type the students are standing on (igneous, metamorphic, or sedimentary)? Use full sentences and describe the rock. Include observations on its color, grain size, minerals present and texture (use terms such as granoblastic, foliated or layered). Look at Figure 8.14 (p. 251) to classify the rock type. Interpret the metamorphic grade from the minerals present (use Figures 8.26 and 8.27, p. 261 and 262). 2) What geologic deformation structures are present in the rocks on the other side of the river from where the students are standing? 3) Describe the cross-cutting relationships between the igneous and metamorphic rocks. What does this tell you about which came first?

6 Gigapan Questions Answer the following questions using the gigapan image Inks Lake 1 for the first two questions and Inks Lake 2 for the third question. For each of the following questions take a partial screenshot of the relevant portion of the gigapan image, creating a separate, smaller image for each. For direction on how to do this refer to the link Directions for Interpreting Geologic structures in gigapan images. 1) Draw a polygon around a dike. 2) Draw a polygon around a sill. 3) In these two images, use a pen tool to highlight the foliation. 4) Draw a box around a fold. (Hint: look for the yellow notebook.) When describing metamorphic rocks, in which geologists cannot tell which layer is older or younger, they use the terms antiform and synform instead of anticline and syncline (look at figure 10.8 in your text). Is this an antiform or synform? Is it symmetrical, assymetric or overturned? Geologic Map of granitic intrusives of the Llano Uplift adapted from Mosher (1996)

7 Part 4: Observations for Enchanted Rock Enchanted Rock State Natural Area has two exfoliation domes formed by erosion of Grenville age (1,082 million years old) Town Mountain granite similar to what you heard about at Slaughter Gap. For an explanation of exfoliation domes, look at Smart Figure 6.5 in your text. Also, you may want to watch a movie of exfoliation at The Town Mountain Granite intruded into the Packsaddle Schist (schist is not visible in the Gigapan image). The two granite domes are part of the second largest batholith in the Llano Uplift. The tallest dome is 425 above ground level. There are two gigapan images. One is taken from the parking lot at the entrance to the Enchanted Rock State Natural Area showing two granite domes. The second gigapan image was taken from the south dome looking north at the main dome that everyone hikes up. For this stop, there are only gigapan images to use for your observations Answer the following questions using these gigapan image For each of the following questions take a partial screenshot of the relevant portion of the gigapan images, creating a separate, smaller image for each. For direction on how to do this refer to the link Directions for Interpreting Geologic structures in gigapan images. 1) For Enchanted Rock Gigapan 1 - draw a polygon around granite blocks on the dome to show the exfoliation process. Are the edges of the blocks straight or curved? How do these blocks form? 2) For Enchanted Rock Gigapan 1 - do you think the granite blocks will undergo either (1) further mechanical weathering, break into smaller pieces, slide down the side of the dome and be added to the granite rubble pile at the bottom of the dome or (2) be chemically eroded by rain? Explain your answer. 3) Enchanted Rock Gigapan 2 shows black stripes running down the slope of the dome. Using a pen tool (and not a line tool), trace one of these stripes down the dome from top to bottom. These stripes are caused by what type of weathering? 4) Sketch a cross section for the tectonic setting for Inks Lake and Enchanted Rock stops. You can use Figures 2.12 or 2.16 from your text (p. p. 47 or 50). Your sketch must by hand drawn and labeled showing were the Enchanted Rock batholith and Valley Springs gneiss would have been located. Also label geologic features such as oceanic and continental crust, lithosphere/asthenosphere boundary, volcanoes, mountains, and trench.

8 Part 5: Interpretation of all stops On the geologic time scale provided to indicate the age for all the geologic events for each stop on this field trip. The ages for each rock type are given in the video. How much time has occurred since the oldest rocks seen in this field trip to the youngest fault activity seen in the first outcrop? How much time elapsed between the two rock types seen at Slaughter Gap? How much time elapsed between the two rock types at Inks Lake?

9 Turn in two pdf files via 1) The questions, (This Document) with answers, saved as yourlastname_firstname_qa.pdf 2) Your interpretations from the gigapans, cross-section at Slaughter Gap, and geologic time scale for the trip should be done in powerpoint and saved as yourlasname_firstname_vft.pdf 3) Both files should be ed to