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Answers to Chapter 19 Review Questions
Xenoliths of basalt within a granite must be older than the granite according to the principle of inclusions.
(a) At both disconformities and paraconformities the beds above and below are parallel, but at a disconformity there is clear evidence of an erosion surface (the lower layers have been eroded). (b) A nonconformity is a boundary between sedimentary rocks above and non-sedimentary rocks below while an angular unconformity is a boundary between sedimentary rocks above and tilted and eroded and sedimentary layers below.
A useful index fossil must have survived for a relatively short period (e.g., around a million years), and also should have a wide distribution so that it can be used to correlate rocks from different regions.
The granitic rock F has been dated to 175 Ma. The wood in layer D is approximately 5,000 years old, so we can assume that layer D is no older than that, although it could be as much as a few hundred years younger if the wood was already old when it got incorporated into the rock.
Layer C must be between 5,000 y and 275 Ma.
The unconformity between layer C and rock F is a nonconformity.
The granite (F) was eroded prior to deposition of C, so it’s likely that layer B was also eroded at the same time. If so, that makes the boundary between C and B a disconformity.
The last magnetic reversal was 780,000 years ago, so all rock formed since that time is normally magnetized and it isn’t possible to distinguish older rock from younger rock within that time period using magnetic data.
William Smith was familiar with the different diagnostic fossils of the rocks of England and Wales and was able to use them to identify rocks of different ages.
The last age of the Cretaceous is the Maastrichtian (71.2 to 66.0 Ma) and the first age of the Paleogene is the Danian (66.0 to 61.6 Ma).