Laboratory Manual for Introductory Geology 4e

EXERCISE 12.8
What Numerical Ages Can Tell Us About Sedimentary Rocks
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Radiometric dating methods are generally not helpful in dating sedimentary rocks because clastic rocks contain grains
of many different ages inherited from their source regions. Further, the minerals that form in chemical and biochemical
sedimentary rocks don’t contain elements suitable for radiometric dating. That doesn’t mean, however, that radiometric
dating cannot provide valuable information about the history of those rocks.
Two geologists disagree about the age of a nonfossiliferous sandstone that is critical to understanding the geologic
history of an area. After examining the rock with a microscope, one of their colleagues, a field geologist, observed that
among the clasts were abundant grains of zircon—a mineral that can be dated using uranium:lead isotope ratios. He
crushed 20 pounds of sandstone, separated out several hundred zircon grains (together weighing less than an ounce!),
and dated the zircons at the mass spectrometer laboratory shown above. The zircons have several different ages, but most
are clustered around specific times, as shown in the following graph.
Zircon grains (%)
20
15
10
5
600 zircon grains dated
200 300 400 500 600 700 800 900
Age (millions of years)
Before this analysis was carried out, Geologist A claimed that the sandstone is Cambrian and was folded during a
Devonian plate collision; Geologist B argued that it is Jurassic and was deformed in the Cretaceous.
(a) With your knowledge of numerical and relative dating, what can you say about the age of the sandstone based
on the ages of the zircons? Do the results support either side of the argument? Explain how you arrived at that
conclusion.
(continued)
12.4 DETERMINING NUMERICAL AGES OF ROCKS
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