GEOL 101L_Geologic Maps
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California State University, Fullerton *
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101L
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Geology
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Feb 20, 2024
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GEOL 101: Geologic Maps
Name:
Geologic Map of Yellowstone National Park
In this lab exercise, you will practice reading and extracting information from a geologic
map.
Required
Materials:
-
1972 Geologic Map of Yellowstone National Park (pdf posted on Canvas)
-
The Geologic Time Scale (pdf posted on Canvas)
Introduction:
This is a 1:125,000-scale map compiled from larger-scale mapping by multiple USGS geologists. It is a good first example of a geologic map, using the conventions and symbols discussed in the last few weeks to convey information about the lithologies, ages/stratigraphic relationships, and some structural features present in the area. As you examine this map, refer to
the Explanation at the right-hand side for details of the ages, lithologies, and names of geologic units, explanations of what the various symbols mean, and so on.
A bit about the geology of the Yellowstone area (edited from a lab by Professor Peter Bird at UCLA): The rocks of the Yellowstone area were deformed during the Laramide orogeny, from latest Cretaceous to Eocene time. Accompanying the last stages of the Laramide orogeny was the first outpouring of volcanic rocks: the Absaroka Supergroup, which is well exposed in the Absaroka Mountains along the eastern side of the park. A second, distinct group of volcanics now buries all previous features in the central and southwestern part of the park. Although their ages are all late Pliocene-Quaternary on this map, they are part of a swath of Neogene volcanism that began about 16 Ma in southeastern Oregon/southwestern Idaho, and gradually propagated northeastward along the present Snake River Plain. The Yellowstone volcanic center is considered “supervolcano,” with major eruptions 2.2 Ma (2500 km
3
), 1.2 Ma (280 km
3
), and 0.62 Ma (1000 km
3
), spreading ash layers across large fractions of North America. The removal of so much magma from the magma chamber caused the roof to collapse, forming the elliptical Yellowstone caldera, partly occupied by Yellowstone Lake. Fainter outlines of older calderas (such as Island Park caldera) can be found to the southwest. Most geophysicists believe that there is still magma present at shallow depths (2-8 km) below the park, with abundant hot springs and geysers (“Old Faithful” the most famous) supporting this idea. Extensive circulation of hot groundwater containing sulfuric acid has altered many of the rhyolites to "yellow stone".
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Assignment:
1.
(Consulting the Geologic Time Scale): What level of division of geologic time is the Quaternary (Eon, Era, Period, Epoch, or Age)?
2.
(Consulting the Geologic Time Scale): What range of absolute time (in millions of years before present, represented as “Ma”) does the Quaternary span?
3. What are the two main lithologies of volcanic rocks erupted during the Quaternary?
4. Are these types of volcanic rocks felsic, intermediate, or mafic in composition?
5. What is the dominant lithology (rock ID) of volcanic rock in the older, Absaroka Supergroup?
6. Is this type of volcanic rock felsic, intermediate, or mafic in composition?
_______________________________________________________________________________
Note:
Older than the Absaroka Supergroup are sedimentary rocks (sandstone, siltstone, shale, limestone, and dolostone/dolomite) formed during the Paleozoic and Mesozoic.
7.
The oldest of these sedimentary rocks date from what Period of the Geologic Time Scale? (note that the Explanation divides these sedimentary rocks into “Northern” and “Southern” parts of the park; check both halves to find the oldest sedimentary rocks of the entire park).
______________________________________________________________________
Note:
Yellowstone National Park contains all three rock types. In addition to the igneous and sedimentary units considered above, there is also metamorphic basement, which is older than all other rocks in the map area.
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8. What lithologies (rock ID) compose this metamorphic basement?
_____________________________________________________________
Note:
Not all of the igneous rocks on this map are extrusive; some are intrusive, including many individual dikes. Dikes are planar, or sheet-like, bodies of igneous rock. They form when magma intrudes along planar cracks or fissures that are forced open in the surrounding rock. Examine the “Tia” dikes, surrounded by map unit “pЄg,” in the northwestern part of the map: they are along “The Crags,” at approximately N45°50’, W110°55’ (latitude and longitude are indicated every 10’ on the edges of the map).
9. What type of contact are these Tia/pЄg contacts? (Think about the types of conformities)
_____________________________________________________________
Note:
Approximately 25 km southeast of The Crags is Gibbon Hill, a rounded, isolated peak rising hundreds of feet from its surroundings. Curiously, the entire rounded peak, but none of the surrounding landscape, is made of map unit “Qpo.”
10.
What geologic feature is Gibbon Hill? (hint: read the unit description in the explanation)
______________________________________________________________________________
Note:
Northwest of The Crags, along the western edge of the map, is a contact between units pЄg/Єp.
11. What type of contact is this? (be as specific as possible)
_______________________________________________________________________________
Note:
Now look at the southeastern corner of the map, at Two Ocean Plateau and The Trident. No strike-and-dip symbols are present to indicate the orientation of bedding but note that the contacts between the Tertiary units (Tl, To, Twi) almost exactly follow/have the same
pattern as the topographic contour lines.
12. Based on this pattern, what is the approximate orientation of these Tertiary units in this area?
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Note:
At the southwestern end of Two Ocean Plateau, near the southern edge of the map, there is an outcrop of unit Mm, with a measured bedding orientation plotted.
13.
What direction is bedding striking in this outcrop of unit Mm? What direction is it dipping? What angle is it dipping?
14.
What type of contact is the contact between Mm and Twi at this location? (be as specific as possible)
15.
What is the dominant lithology of unit Kh? (the first lithology listed under unit Kh in the Explanation)
16. What is the dominant lithology of unit Kmt?
_______________________________________________________________________________
Note: Part of the topographic basemap at Chicken Ridge is a heavy, long-dashed path, which continues north of Chicken Ridge, where it is labelled “Continental Divide,” and also southeast of Chicken Ridge, to the aptly named Two Ocean Plateau. This long-dashed path marks the position of the Continental Divide.
17.
What is the Continental Divide? (If you don’t know, do a quick internet search)
18.
Yellowstone Lake (the largest body of water on the map, formed in the depression of Yellowstone Caldera) drains northward as the Yellowstone River. Which ocean does this water eventually flow into?
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19.
Heart Lake, just northwest of the folds considered earlier, drains southward as the Heart
River, joining the Snake River near the southern edge of the map. Which ocean does this water eventually flow into?
______________________________________________________________________
Note: East of Chicken Ridge is a fault, running north-south from Yellowstone Lake to the southern edge of the map. This fault originated as one kind of fault, but was subsequently
reactivated as a different kind. This is quite common, because old fault planes provide pre-existing planes of weakness which can be utilized during subsequent phases of deformation.
20. What kind of fault was this originally, and what symbols are used to convey this on the map?
21.
What kind of fault was this most recently, and what symbols are used to convey this on the map?
22. Which side of the fault are the “bar and ball” symbols on: the hanging wall, or the footwall?
23. What is the youngest
map unit that is
cut by this fault?