1. Transgression and Regression
2. Interpreting a Stratigraphic Sequence
3. Virtual Field Sites

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Transgression and Regression

Assume that the sedimentary formations in the stratigraphic columns (below) are conformable-the layers were deposited one right after the other.

1. Is stratigraphic column A transgressive or regressive?
2. Describe the depositional environment in which you would expect the sandstone in column A to have been deposited.
3. Describe the depositional environment in which you would expect the limestone in column A to have been deposited.
4. Is stratigraphic column B transgressive or regressive?
5. When was the water deeper, when the shale was being deposited or when the sandstone was being deposited?

Interpreting a Stratigraphic Sequence

A stratigraphic sequence from Archean through Paleozoic is described in words below. It represents the Rocky Mountain region of the Pacific Northwest. It is labeled from oldest (A, at the bottom) to youngest (L, at the top).

The lower-case letters a through k refer to the contacts between the rock layers. Each contact is the surface along which the upper layer is in contact with the rocks beneath it.

For each of the numbered rock formations in the sequence:

6. In your answer sheet, you will see questions labeled Layer A through Layer L. For each layer, name the eon (for the Precambrian rocks) or period (for the Paleozoic rocks) in which it formed.
7. For each layer, describe the major geological event (for non-sedimentary rocks) or depositional environment (for sedimentary rocks) that is suggested by the type of rock.
8. Which of the contacts (lower-case letters a through k) is an angular unconformity?
9. Which of the contacts (lower-case letters a through k) is a nonconformity?

L phosphorous-rich sandstone beds alternating with oil-rich shale beds
        k
K red beds that include sandstone with subaerial (deposited on land) dune cross-beds
        j
J thick sequence of limestone beds with Mississippian fossils
        i
I muddy limestone and dolomite with Devonian fossils
        h
H dolomite and shale with Ordovician fossils
        g
G limestone
        f
F shale with Cambrian trilobite fossils
        e
E sandstone with beds like sand in beaches and near-shore tide zones; the sandstone beds lay relatively flat across the edges of the tilted, older beds below
        d
D tilted layer of tillite-conglomerate that appears to have originated as glacial till
        c
C tilted layers of basaltic volcanic rocks inside grabens (rift valleys), with chemical compositions like rift zone volcanic rocks of the modern world
        b
B tilted beds of limestone, sandstone, siltstone, and shale (slightly metamorphosed), with a 1.2 billion year old sill that fed a lava flow within the sedimentary sequence
        a
A gneiss, schist, granite and other deep-crust metamorphic and plutonic rock of Archean age (more than 2.5 billion year radiometric age)

Virtual Field Sites

Explore the Logan Pass Virtual Field Site and answer the following questions.

10. What is the name of the set of Proterozoic rock formations that are so prominent in Glacier National Park?
11. What sedimentary structure in the pictures provides evidence of flowing water?
12. What sedimentary structure in the pictures provides evidence of mud and sand layers being deposited in shallow water that occasionally dried up?
13. What is the significance of the stromatolites?

Explore the Grinnell Glacier Virtual Field Site and answer the following questions.

14. What evidence is there that the igneous rock layer called the Purcell sill intruded underground between layers of sediment, rather than flowing out on the surface as a lava flow that was then buried beneath later sediments?
15. At Granite Park (see Week 10 Lecture), on the other side of the ridge, there is evidence which indicates that the Purcell sill intruded during the time the Belt sediments were being deposited, rather than long afterward. What is the evidence?
16. What evidence is there in the rocks of the Grinnell Glacier area of occasional high-energy water flow during Belt deposition?

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