Explain how reliable numerical dates are determined for layers of sedimentary rock.
Although reasonably accurate numerical dates have been worked out for the periods of the geologic time scale (see Figure 1), this is not an easy task.
The primary difficulty in assigning numerical dates to units of time is the fact that not all rocks can be dated by using radiometric methods. For a radiometric date to be useful, all the minerals in the rock must have formed at about the same time. For this reason, radioactive isotopes can be used to determine when minerals in an igneous rock crystallized and when pressure and heat created new minerals in a metamorphic rock.
However, samples of sedimentary rock can only rarely be dated directly by radiometric means. Although a detrital sedimentary rock may include particles that contain radioactive isotopes, the rock’s age cannot be accurately determined because the grains composing the rock are not the same age as the rock in which they occur.
Rather, the sediments have been weathered from rocks of diverse ages.
Radiometric dates obtained from metamorphic rocks may also be difficult to interpret because the age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock initially formed.
Instead, the date might indicate any one of a number of subsequent metamorphic phases.
If samples of sedimentary rocks rarely yield reliable radiometric ages, how can numerical dates be assigned to sedimentary layers? Usually a geologist must relate the strata to datable igneous masses, as in Figure 2.
In this example, radiometric dating has determined the ages of the volcanic ash bed in the Morrison Formation and the dike cutting the Mancos Shale and Mesaverde Formation. The sedimentary beds below the ash are obviously older than the ash, and all the layers above the ash are younger. The dike is younger than the Mancos Shale and the Mesaverde Formation but older than the Wasatch Formation because the dike does not intrude these Paleogene-age rocks.
From this kind of evidence, geologists estimate that the last part of the Morrison Formation was deposited about 160 million years ago, as indicated by the ash bed.
Further, they conclude that the Paleogene period began after the intrusion of the dike, 66 million years ago. This is just one example of literally thousands that illustrate how datable materials are used to bracket the various episodes in Earth history within specific time periods. It shows the necessity of combining laboratory dating methods with relative dating principles applied to field observations of rocks.