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Geology of the Pacific Northwest
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Week 8 Lab Assignment
  1. Labeling Cenozoic Geologic Structures of the Pacific Northwest
  2. Answering Questions about Cenozoic Geologic Structures of the Pacific Northwest
  3. Virtual Field Sites

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I. Labeling Cenozoic Geologic Structures of the Pacific Northwest

Your online resources for Part I of this lab include:

It is very important that you know the difference between a geologic map and a cross-section. Your colored geologic map of Washington state, for example, is a geologic map. It looks down on the Earth's surface from above. The different formations of rock (such as lower Tertiary sedimentary rock, lTs) and unconsolidated sediment (such as Quaternary glacial till, Qg) are exlained in the map legend on the right, in age order. The geologic formations, and geologic structures such as faults, are shown on the map itself.

A cross-section is not a geologic map. A cross-section is a sideways view of a slice of the Earth's interior. For example, the pre-drawn cross-sections for this lab are sideways views of the Earth sliced open along the Okanogan and Metamorphic core complexes with the Republic graben in-between (cross-section A), the Chiwaukum graben (cross-section B), and the Olympic Mountains accretionary prism (cross-section C).

In this lab, all the instructions of faults and rock formations and geologic structures that you are to label in the cross-sections should ONLY be labeled in the cross-sections, NOT on the geologic map.

The geologic map of Washington state that you will send in for this lab will only have three simple line segments drawn on it, and three letters, and NOTHING ELSE (except your name). The line segments on the geologic map are labeled A, B, and C and show on the map where the cross-sections cross their respective structures.

  1. On a copy of your geologic map of Washington state, figure out just where the cross-sections A, B, and C go. Carefully draw straight lines on the map showing where those cross-sections are located and label them A, B, and C. Don't forget to send a copy of your geologic map with the three lines on it, along with the cross-sections on their separate page.

    On the geologic map of Washington state, only draw those three lines and write the letters A, B, and C . (Also put your name on the map, of course.) Do NOT draw cross-sections on the geologic map.

    On a map, you are looking down from above. On a cross-section, you are looking in from the side.

    Thus, the perspective of the geologic map, a view looking down, vertically, onto the face of the Earth, cannot be the background on which to draw a cross-section, which is a view looking sideways, horizontally, at a slice of the Earth. The top edge, the top line, of a cross-section, is the only part of a cross-section that can be seen on a geologic map.

    That is why you should not put a cross-section on a map.

    Instead, what you put on the geologic map for each cross-section is the "line of cross-section," a simple, straight line, a line showing where the cross-section slices across the geology shown on the map.

The work you perform on cross-sections A, B, and C (below), adding labels and half-arrows, is to be done on your print-out of the simplified cross-sections.

Cross-section A runs west (left-hand side) to east (right-hand side). Cross-section A runs across the Okanogan fault, Okanogan metamorphic core complex, Republic graben, Kettle metamorphic core complex, and Kettle fault.

The Okanogan fault is a detachment fault that dips (slopes downward) to the west. The Kettle fault is a detachment fault that dips (slopes downward) to the east.

On your printed copy of cross-section A, perform the following:

  1. Along the Okanogan fault, draw half-arrows showing the direction of motion of the rocks on each side of each fault. Do the same thing along the Kettle fault.
  2. Label the bottom end of the Okanogan detachment fault ODF and the bottom end of the Kettle detachment fault KDF.
  3. Above cross-section A, write labels and arrows clearly showing the locations along the top of the cross-section of the Okanogan metamorphic core complex (OMCC), Republic graben, and Kettle metamorphic core complex (KMCC).
  4. Inside cross-section A, use the appropriate two-, three-, or four-letter abbreviations from the map legend to label the rock units in the:
Cross-section B represents the Chiwaukum Graben. The line of the cross-section crosses the graben 10 miles northwest of Wenatchee. Cross-section B runs from southwest (the left-hand end) to northeast (the right-hand end).

Perform the following on cross-section B.
  1. Draw half-arrows on each of the two faults, showing the relative offset directions of rocks on each side of each fault.
  2. Inside cross-section B, use the appropriate two-, three-, or four-letter abbreviations from the map legend to label the rock units in the:

Cross-section C represents the Olympic Peninsula. Cross-section C runs across the Olympic Peninsula from west (the left-hand end) to east (the right-hand end).

The line of cross-section C that you draw on the geologic map should be drawn so that it crosses five of the steeply dipping, high-angle faults, faults that have triangular teeth on them on the geologic map.

(But not in a cross-section - do not draw geologic map symbols such as triangular teeth on fault lines in a cross-section. In a cross-section, only draw half arrows alongside faults to show which way the body of rock on each side of the fault moved relative to the rock on the other side.)

Because you will be drawing your line of cross-section C across five faults, you will need to add two more faults to cross-section C, drawing them parallel to the three faults already there.

The spacing of the faults will not accurately match the spacing, the east-west distance, between each of the faults on the geologic map, which is not a good procedure for drawing a cross-section accurately, but that's OK in this case, where the goal is to produce a generally representative cross-section of the geologic structures across the Olympic Peninsula.

Cross section C begins on its west (left) end in a green rock unit and ends on its right (east) end in a brown rock unit. In-between, it crosses a bluish green rock unit that has been faulted repeatedly.

Ignore any yellow Qg in the area of cross section C. The yellow Qg unit is just a thin veneer of recent, unconsolidated glacial sediment. Ignore the yellow Qg and focus on the bedrock instead, which extends to great depth.

Perform the following on cross-section C.

  1. Draw half-arrows on each of the five faults, showing the relative offset directions of rocks on each side of each fault.

    NOTE: As stated above in bold letters, you will need to draw in two more faults yourself on cross-section C, parallel to the faults already drawn.

  2. Inside cross-section C, use the appropriate two-, three-, or four-letter abbreviations from the map legend to label the rock units in the:

II. Answering Questions about Cenozoic Geologic Structures of the Pacific Northwest

  1. What type of stress (tension, horizontal shear, or compression) is expressed by the structures of cross-section A?
  2. During which geologic epoch did the geologic structures in cross-section A form?
  3. The name of the main rock formation in the Republic graben is (choose one of the following): Sanpoil Volcanics, Columbia River Basalts, Chuckanut Sandstone
  4. The Chumstick Formation is the main rock unit in the Chiwaukum graben (cross-section B). The Chumstick formation is of the geologic age known as (choose one) Miocene, Eocene, or Cretaceous.
  5. The Chumstick Formation consists of (choose one) clastic sedimentary rocks, chemical sedimentary rocks, or volcanic rocks.
  6. The depositional environment of the Chumstick Formation was (choose one) rivers and streams draining from rising mountains down steep slopes into a tectonic basin, tropical reefs surrounding oceanic islands, or wind-blown sand dunes in a hot desert basin.
  7. The type of fault illustrated in cross-section C, for the Olympic Peninsula, is called a (choose one) normal fault, reverse fault, or strike-slip fault.
  8. To form the faults shown in cross-section C, the crust must be subjected to the type of stress known as (choose one) tension, horizontal shear, or compression.

III. Virtual Field Sites

Explore the Chuckanut Coast Virtual Field Site and answer the following questions.

  1. What evidence is there in the Chuckanut Formation that indicates the climate was warmer at the time of Chuckanut sediment deposition than it is in the same area now? Be specific.
  2. Is the Chuckanut Formation an accreted terrane or an overlap deposit? (An overlap deposit is a sedimentary formation deposited on accreted terranes, after the terranes have been accreted.)
  3. In what epoch of the Tertiary Period was the Chuckanut Formation deposited?

Explore the Paradise Virtual Field Site and answer the following questions.

  1. What evidence is there that the climate of the Mt. Rainier area was colder 10,000 to 20,000 years ago than it is now? Be specific.
  2. What specific type of rock has Mt. Rainier erupted the most of? (Hint: You have a sample of it in your Pacific Northwest Geology Rocks kit.)
  3. Explain how Mt. Rainier is related to plate tectonics.

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Geology of the Pacific Northwest
Lab Assignment 8
updated: 6/17/13