Act#10-Laramide_Orogeny-1
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University of Colorado, Boulder *
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1040
Subject
Geology
Date
Dec 6, 2023
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docx
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Act#10 – The Laramide Orogeny
GEOL 1040
Learning Goals:
1.
Extract useful information from a geologic map
2.
Apply your knowledge of folds, faults, rock characteristics, and the geologic time scale to
deduce when the Rocky Mountains were uplifted. Also recall the conclusions we reached in
Activities 1 and 2 about when the mountains were formed.
To answer all questions, look at your geologic time scale and at the map, legend, and cross-sections
on the
Geologic Map of the Masonville Quadrangle, Larimer County, Colorado
. 30 pts total.
1.
Locate the Fletcher Hill Fault on Masonville map cross-section A-A’:
a.
Which is the hangingwall, the left side or the right side? (1)
b.
What kind of fault is it (i.e. normal, reverse, strike-slip). Explain how you can tell (2)
c.
What kind of stress does this type of fault accommodate (extension, compression, or
horizontal sliding)? (2)
d.
Cross-section A-A’ goes from southwest (point A) to northeast (point A’). Using that
orientation information, in what direction was the stress you deduced in part c) applied
(i.e. direction of extension, compression, or horizontal sliding)? (2)
e.
What is the oldest possible age of movement on this fault? Give your answer as the most
specific portion of a period that you can. Explain the evidence that allowed you to deduce
this. (Hint: you will want to use the map legend to determine the rock age) (3)
f.
Given the period and the specific portion of that period you deduced in e), give the oldest
age of fault movement in millions of years. (1)
2.
Locate the Milner Mountain Fault on Masonville map cross-section B-B’:
a.
All of the rocks from pink to green are sedimentary.
What type of rock structure exists
immediately to the left of the fault? Explain how you were able to tell. (3)
b.
What type of rock structure exists immediately to the right of the fault? (2)
c.
What kind of stress do structures of the types you listed in a) and b) accommodate
(extension, compression, or horizontal sliding)? (2)
d.
Cross-section B-B’ goes from southwest (point B) to northeast (point B’). Using that
orientation information, in what direction was the stress you deduced in part c) applied
(i.e. direction of extension, compression, or horizontal sliding)? (2)
e.
What is the oldest possible age of formation of the structures you identified in parts a) and
b) above (give your answer as the most specific portion of a period that you can)? Explain
the evidence that allowed you to deduce this. (Hint: you will want to use the map legend to
determine the rock age) (3)
f.
Given the period and the specific portion of that period you deduced in e), give the oldest
age of structure formation in millions of years. (1)
3.
Now synthesize your discoveries from questions 1 and 2. Explain what stress field created
these structures, when the structures were formed (from the information contained on this
map you can only deduce the maximum age of the tectonic event), and what type of topography
is typically formed by this type of stress field. Use
only
the clues you have gathered from this
exercise, not other information from this class or elsewhere.
(6)
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