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Geological Analysis of Sedimentary and Volcanic Rock Deposition in the Appalachian Seaway

Sediment distribution patterns in the Seaway

1. Does the pattern of sediment distribution in the Seaway support the principle that grain size generally decreases from the margin of a sedimentary basin to its center? Explain.

2. What evidence is that sediment transport to the Seaway was not uniform along the western coastline of the Acadian Highlands?

3. What probably caused this pattern of localized sediment delivery to the Seaway?

4. Rocks with different grain size are not arranged in simple bands parallel to the coastline of the Acadian Highlands, as would be expected if sediment deposition was driven solely by distance to shoreline. What phenomena could produce such intra-basin transport of sediment? Explain.

B. The principle of Lateral Continuity: Sedimentary & volcanic rocks are laid down in layers that are usually much greater in lateral extent than in thickness. illustrates this relationship. It also shows that beds do not always extend completely across a basin, as is the case with the sandstone layer. On the left side of the basin, there are three strata: a basal shale, then the sandstone layer, and on top of that another layer of shale. On the right there is only a single thick shale layer

5. Using the Principle of Superposition, identify the relative geologic ages of the three beds on the left of side of the basin.

Oldest                       Intermediate                                          Youngest

6. What is the geologic age of the thick shale bed on the right relative to the three beds on the left?

Lithostratigraphic Correlation: The physical characteristics of strata can often be used to correlate these strate in geological time. Figure 4, on the next page shows how this can be done using the example above. Note that in Figure 4 the dashed correlation lines are drawn to connect equivalent horizons in each outcrop. Note also that the sandstone is indicated as pinching out (disappearing) between outcrops 2 and 3 because there is no evidence of it at outcrop 3. 

7. Explain why the sandstone bed may not extend across the entire basin (hint: remember that environment controls the character of sediment deposited at any point in a basin, and that sandstone is coarser grained than shale).

8. Apply your response above to interpreting the reasons why the different rock types are distributed in the northern Appalachian Seaway.

9. Correlate the three outcrops in Figure 5 using correlation lines as in the example on the previous page.

10. Is more than one interpretation possible? If so, is one more reasonable than the others?Explain.

11. These outcrops are from different parts of the same depositional basin. Why would the sequence of strata be different in each one?

12. Examine the upward sequence of rock type just above the inset. Describe the upward change in grain size.

13. Is this observation consistent with the “fining upward” rule?

14. Examine the upward sequence of rock type at the extreme right margin of the Figure 6C. What type rock would be exposed in an outcrop of this section of the basin?

15. Suppose we had independent confirmation that the geologic age of the T=1 limestone bed is Silurian (425 ma), and that the age of the T=2 limestone bed is Late Cretaceous (75 ma). What does that indicate about the nature of the horizon (timeline T=2) that separates them?

16. The contact between the two limestones beds represents what type of unconformity?

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