Field Geology and Structures Assignment 5
Field Geology and Structures
Field Assignment 5
Due: Friday October 24, 2008
Brittle and Ductile Structures (2)
Stop 1: Pokamoonshine Mountain (Figure 1 and Map 1). Pokamoonshine is a
granitic and charnockitic gneiss. It is part of the Rocky Branch Complex
(Whitney and Olmsted, 1993). Throughout the face of the mountain you will
observe sills of mafic granulite. Study these sills carefully.
Figure 1: Face of Pokamoonshine from HW 9, looking west.
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Field Geology and Structures Assignment 5
Map 1: Map of Pokamoonshine. The blue line is 2,000ft. North is vertical.
Contour interval is 10 meters.
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Field Geology and Structures Assignment 5
1. What evidence do you see of faulting along the face of the mountain?
2. Studying the offsets, what are the fault types?
a. Draw generalized block diagrams to show how you arrived at your
decision concerning the fault types.
b. If you have decided on two or more possible fault types, how
would you test to see the type of fault?
3. Study the joints (many can be seen on outcrops along the road). Describe
the orientation of these joints.
a. Do you observe joint sets?
b. Are the joints parallel?
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Field Geology and Structures Assignment 5
i. Parallel joints suggest tension. Tension is due to negative
stress (σ3<0 and σn<0 ). From your observations, is it
reasonable to assume there was tension in the region?
ii. What are the causes of the tension?
iii. Consider the Mohr’s Diagram. Show on the Mohr’s
Diagram the orientation of the fracture plane resulting from
tension. What is the shear stress on this plane?
Stop 2: Split Rock Falls is described as a “Zero Displacement Crackle Zone”
(ZDCZ) (Bohlen et al., 1989). ZDCZ’s are highly fractured rocks that denote
faulting. Unlike faults, ZDCZ’s show no displacement. Study the outcrop on the
west side of the road and view the falls. Becarefully going down the trail near
the falls! It is very steep and may be slippery. You chould be able to make all
you observations from the safety of the trail.
1. Describe the fractures you observed
a. Are the orientations about the same or different through out the
area?
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Field Geology and Structures Assignment 5
b. Does the frequency of fractures change?
2. Using the map 2, draw a cross section of the valley, perpendicular to the
valley across Split Rock Falls.
a. Does the cross section suggest anything about the origin of the
valley?
b. If you believe the valley is structurally controlled, does it follow
that other valleys are structurally controlled in the area?
c.
How would you test this hypothesis?
Stop 3: Cascade Lakes between Pitchoff Mountain and the Cascade Range (Map
3). The Cascade Lakes form along a fault. The Cascade Range is made up of
anorthosite, while Pitchoff Mountain is syenite. The anorthosite is younger than
the syenite.
1. Construct a cross section perpendicular to the axis of the valley. Compare
this cross section with the cross section you drew for stop 2.
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Field Geology and Structures Assignment 5
a. Are the cross-sections similar?
b. Where would you estimate the location of the fault in this cross
section?
c. What is your best guess as to the type of fault? On what do you
base this guess.
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Field Geology and Structures Assignment 5
Map 2: Map of Split Rock Falls. The blue line is 2,000 ft. North is vertical.
Contour interval is 10 meters.
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Field Geology and Structures Assignment 5
Map 3: Map of Cascade Lakes. The blue line is 2,000 ft. North is vertical.
Contour interval is 10 meters.
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Field Geology and Structures Assignment 5
References Cited:
Bohlen, S.R., J.W. Valley, P.R. Whitney, 1989. The Adirondack Mountains—A
Section of Deep Proterozoic Crust: Field Trip Guidebook T164, American
Geophysical Union: Washington, D.C., 63 pp.
Whitney, P.R. and J.F. Olmstead, 1993. Bedrock Geology of the Au Sable Forks
Quadrangle, Northeastern Adirondack Mountains, New York, Map and Chart
Number 43, New York State Museum: Albany.
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