Geology 2 Exercise 4 Relative Time In the following diagrams, place the age of formation of the units or features in their correct chronological order. Place the letter of the oldest unit on the bottom of the list and letter of the youngest unit on the top and fill in the rest appropriately. The order of some of the units, particularly the most recent units in number D, may be ambiguous. Mark on your list which units or events may have formed at the same time. A.

B.

C.

What kind of unconformity separates unit B from unit P? A nonconformity What kind of unconformity separates unit E from the underlying units? An angular unconformity. D.

Part 2: Photographs Answer the Questions for the following photographs, which will be provided for you in lab. 1. Carmel Valley Gravels

a. What is the name of the unit in the lower right of this photograph? Monterey Formation. b. In what depositional environment was this sedimentary rock deposited? Deep Ocean c. What is the depositional environment of the gravels? A river energetic enough

to transport those boulders. d. What kind of contact separates the unit in the lower right from the gravels? A Depositional Contact, or in even more detailed terminology, an unconformity e. Is it a conformable contact? Why or why not? No, for two reasons. First, 15 million years have passed between the deposition of the Monterey fm and the deposition of the river gravels. Second, the deep sea depositional environment of the Monterey fm is very different from the river gravel depositional environment of the younger sediments; some uplift and erosion must have occurred to bring the deep sea sediments to above sea level in order for the river gravels to be deposited on top of them. f. What is the geologic history of the rocks and sediments shown here? 1. Deposition of silica-rich mudstone or diatom-rich ooze in the deep sea to create the Monterey Formation. 2. Uplift and erosion of the Monterey Formation to above sea level. 3. Deposition of the river gravels by a river that was almost certainly a predecessor to the Carmel River. It was through the deposition of these gravel on top of an erosional surface that the unconformity was created. g. The fellow in the lower right with the red shirt on is Rich Keziran, a well-known history teacher at MPC. What do you think he is thinking as he gazes up at these rocks? Wow, I should really take an entire geology class. Then I could spend even more of my weekends gazing at rocks and thinking about their history.

2. The Grand Canyon

The point of this image is to find the angular unconformity and understand its significance in terms of the geologic evolution of the Grand Canyon. Note the two series of sedimentary rocks: the Grand Canyon Series occurs in the lower right of the photograph and is dipping towards the right. The Paleozoic series lies above the Grand Canyon series and is flat-lying. a. Draw a quick sketch that emphasizes the difference in dip between the two groups of sedimentary rocks. The lower group dips to the right of the photograph whereas the upper group has no dip; it is horizontal. b. What is the contact between the two groups of sedimentary rocks called? An angular unconformity c. What must have happened during the time period represented by this contact? Folding of the lower group of sedimentary rocks to create the dipping beds, and erosion of the lower group of rocks to create a horizontal surface for the upper group of rocks to be deposited upon. d. What is the geologic history of the rocks that comprise the Grand Canyon, as seen in this image? 1. Deposition of the lower group of rocks. 2. Folding of the lower group of rocks to create the dipping beds seen today. 3. Uplift and erosion to create the flat surface that is the angular unconformity today. 4. Subsidence 5. Deposition of the upper group of rocks. 6. Uplift 7. Erosion by the Colorado River to create the Grand Canyon.

3. Plutons in Chile The larger diagram shows a mountain-sized view of the contact, whereas the inset shows a close-up view of the contact. a. Which of the rocks, the light rock or the dark rock is younger? What evidence is seen on the close-up (the inset) to support your answer?

The lighter rock is the younger, intruding rock. On the close-up, small veins can be seetn intruding from the light rock into the darker rock. b. Where within or on the Earth did these plutonic rocks crystallize? Beneath the surface within the crust. c. What must happened after crystallization to expose these rocks at the Earth’s surface? Erosion and uplift.

4. Inclusion a. Which is older, the dark rock or the light rock? The light rock is older. b. How can you tell? The dark rock contains an inclusion of the light rock. The dark rock must be younger in order to contain an inclusion of the older light rock.

5. Garrapata Beach a. What is the origin of the sedimentary rocks in the lower half of the outcrop? Turbidites (undersea landslides) deposited on the continental slope or rise. b. Which way are these rocks dipping (the rocks in the lower half of the image)? To the left and slightly towards the photographer.

c. What is the origin of the sedimentary rocks in the top half of the outcrop? They are poorly consolidated (see the kind of erosion) beach sands; they were formed on the beach. d. What is the origin of the large boulders between the two sedimentary units? Probably got to the beach in either a small river or were eroded from a nearby granite outcrop and washed down the beach. The ocean waves subsequently rounded the boulders and deposited them on the beach. Uplift then lifted the boulders and the beach sands onto the terrace. e. What is the geologic history of this place? 1. Deposition of the turbidites. 2. Uplift and erosion. 3. Deposition of the beach sands and the big granite boulders on the wave-cut platform. 4. More uplift to create the terrace and to isolate the beach sands and the big granite boulders above the beach.

6. Contacts. a. What kind of sedimentary rock occurs above the granite? A conglomerate b. Assume the granite clast (lower image) is the same granite that occurs below the sedimentary rock. Which is older, the granite or the sedimentary rock? The granite must be older. Older rocks are always the inclusions. They are always included within younger rocks. c. In these images, there is no evidence of shearing or faulting. What kind of contact separates the sedimentary rock from the granite? A depositional contact. Since a sedimentary rock rests on a plutonic rock, the contact must be a nonconformity. d. Note that the slightly weathered granite occurs on both side if the fresh granite. Which is older, the slightly weathered granite or the fresh

granite? (Note: the difference in weathering is probably more due to a difference in composition than a difference in age.) The slightly weathered granite is older than the fresh granite. It looks like the fresh granite intruded into the slightly weathered granite. e. What kind of contact separates the slightly weathered granite from the fresh granite? An intrusive contact. f. Where did the granite form (circle one)? 1. On the surface of the Earth 2. Below the surface of the Earth? g. What had to happen to get the granite to the surface? Uplift and erosion.