1. How many unconformities exist in each column? Note that it is not the same number, necessarily, for each column!

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1 Geology 103 Name(s): Lab 5: Sequence stratigraphy Introduction: In the previous lab, you were able to illustrate Walther s Law by showing how transgressive rock sequences mirrored the offshore facies sequence. In this lab, you are going to create a correlated cross-section of the Mesozoic stratigraphy of the Colorado Plateau, a diagram similar to your combined crosssection from the previous lab. Objective: To illustrate the principle of correlation to define the stratigraphy of a region from only a few rock outcrops. Further, to infer the scope and sequence of worldwide transgressive sequences. On the attached sheet are the stratigraphic columns for Canyonlands and Capitol Reef National Parks and Colorado National Monument, as well as the explanation for the symbols used. 1. How many unconformities exist in each column? Note that it is not the same number, necessarily, for each column! On the Capitol Reef National Park stratigraphic column, the named units can be divided into the following time periods: the Triassic is represented by the Cutler, Moenkopi and Chinle formations; the Jurassic is represented by the Wingate, Kayenta, Navajo, Carmel, Entrada, Curtis, Sumerville, Morrison and Cedar Mountain formations and the Cretaceous is represented by the Dakota and Mancos formations. From the accompanying handout, you know that the Absaroka transgressive sequence ended in the early Jurassic and the Zuni transgressive sequence began in the early Jurassic. 2. What should exist in the rock record that separates transgressive sequences? Hint: If it s not a transgression, then what s the ocean doing, and how is this generally represented in the rock record? 3. Do you see evidence of your answer to the previous question in the three stratigraphic columns?? In particular, what is the problem with the Absaroka sequence at Capitol Reef NP, especially during the Triassic? (This is a major problem with sequence stratigraphy: figuring out which unconformities are the important ones)

2 4. With a pair of scissors, cut out the three columns (don't cut off any of the fossil symbols) and arrange them on another sheet of paper such that the columns are oriented west to east in the proper arrangement (see the attached region map to get the order right). The spacing between the columns should be roughly proportional to the spacing of the areas on the map. Tape or paste them down to the paper, taking care not to get tape in between the columns (you're going to write in between). Draw a mileage scale at the bottom. 5. Correlate the different units by drawing tie-lines (lines representing the contacts between units) between the columns. For instance, draw a line from the contact of the Dakota/Mancos in Capitol Reef NP to the contact of the Dakota/Mancos in Canyonlands NP; then draw a line from the contact of the Dakota/Mancos in Canyonlands NP to the contact of the Dakota/Mancos in Colorado NM. Continue to do this for as many of the units as you can. Note that due to the unconformities not all tie lines will exist. You will have to use the rock type, the fossils found and key beds to perform the correlations. You may wish to color certain layers so that you can trace them better across the region. Show terminations of beds as sideways V's; in other words, some layers will pinch out between two stratigraphic columns. 6. Note that the Mancos mudstone (shale) exists at the top of each of the three columns. Give two reasons why there is such a differing thickness of this unit between the three areas (note that these two reasons are true for all formation thickness differences) 7. As you look at your completed correlated cross-section, in general, which direction was the deeper ocean? How do you know? Hint: find a specific formation and visualize what facies changes you ought to see as the ocean gets deeper.

3 8. The Wingate, Navajo and Morrison formations are terrestrial sandstones, with cross-bedding literally ten meters tall. What is a possible depositional environment for them? What type of sandstone (arenite, arkose or greywacke) should they be to confirm your suspicions? 9. Notice there are a few more national parks in the area, like Grand Canyon NP. What prevents us from extending the sequence stratigraphic column to the North Rim of the Grand Canyon? Hint: Look at the Cedar Breaks to North Rim of the Grand Canyon cross-section poster. 10. Why is there not as much limestone in the cross-section you constructed compared to the Paleozoic era record of the Grand Canyon (see the Grand Canyon cross-section on the poster)? 11. Estimate the thicknesses (in feet) of the Absaroka and Zuni sequences in Capitol Reef National Park (use the scale provided on the cut-out sheet). How would you expect the thicknesses of the sequences in Zion National Park to compare (look at the map for relative geographic positioning)? Explain your reasoning.

4 12. Look at the poster with the Cedar Breaks to North Rim of the Grand Canyon cross-section, and estimate the thicknesses (in feet) of the Absaroka and Zuni sequences in Zion National Park (or nearby). Were you correct in your prediction in the previous question? 13. From the same poster, note that there seems to be quite a slope between Kanab, Utah (middle of the poster), and the North Rim of the Grand Canyon. a. Estimate the elevation (in feet) of the top of the Kaibab Formation near Kanab. b. Estimate the elevation (in feet) of the top of the Kaibab at the North Rim. c. Using the horizontal scale given and the fact that 1 mile = 5280 feet, determine the distance (in feet) between Kanab and the North Rim. d. Calculate the slope (in degrees) of the top of the Kaibab Formation between Kanab and the North Rim. The formula for calculating the angle of the slope in degrees is: angle = tan -1 (rise/run) where the rise and the run should be measured in the same units. e. Is this a particularly steep slope? Therefore, how much secondary deformation has there been of the Kaibab Formation? The origin of the uplift of the Colorado Plateau has been a mystery since the second director of the US Geological Survey, John Wesley Powell, rafted down the Colorado River through the Grand Canyon in 1869 and wondered at the forces that might uplift this thickness of rocks. This area was hundreds of kilometers east

5 of the Farallon Plate subducting under the North American Plate at the time of its uplift. There were also few volcanoes in this area during that time. 14. Go Scott Bennett s Tectonic Evolution of the Colorado Plateau chapter and draw a cross-section that illustrates how the Colorado Plateau may have uplifted with so little tilt to its rocks. bennett/publications_files/ch3_grandcanyontectonics.pdf; also the video at may be helpful. Is there a single explanation that satisfies all geologists? 15. What is the earliest time that this uplift could have occurred, and how do you know? So what orogeny (give the name) created this uplift? 16. Estimate the amount (in feet) of the uplift. Hint: Look on the Cedar Breaks to North Rim poster; examine the Cedar Breaks NM area and look for an offset formation across a fault.

Geologic Time Grand Canyon National Park

Chapter 8: Geologic Time & the Grand Canyon Geologic Time Grand Canyon National Park Colorado Plateau nearly-horizontal rocks lifted feet above sea level River drains Mtns Who was John Wesley Powell? Label