Chapter 8 Measuring Geological Time 8.
For example, the principle of superposition states that sedimentary layers are deposited in sequence, and, unless the entire sequence has been turned over by tectonic processes or disrupted by faulting, the layers at the bottom are older than those at the top. The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included.
For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it Figure 8. The lava flow took place some time after the diorite cooled, was uplifted, and then eroded. Hammerhead for scale [SE] Figure 8. The pieces of shale were eroded as the sandstone was deposited, so the shale is older than the sandstone.
An example of this is given in Figure 8. The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than that layer.
But the faults do not appear to continue into the coal seam, and they certainly do not continue into the upper sandstone. So we can infer that coal seam is younger than the faults because it disrupts themand of course the upper sandstone is youngest of all, because it lies on top of the coal seam.
The coal seam is about 50 cm thick. Dark grey metamorphosed basalt 3.
A 50 cm wide light-grey felsic intrusive igneous dyke extending from the lower left to the middle right — offset in several places Using the principle of cross-cutting relationships outlined above, determine the relative ages of these three rock types. The near-vertical stripes are blasting drill holes.
The image is about 7 m across. Recognizing unconformities is important for understanding time relationships in sedimentary sequences.
An example of an unconformity is shown in Figure 8. The Proterozoic rocks of the Grand Canyon Group have been tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks.
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The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is close to million years. Tilting and erosion of the older rocks took place during this time, and if there was any deposition going on in this area, the evidence of it is now gone. The flat-lying rocks at the top are Paleozoic to Ma. The boundary between the two represents a time gap of nearly million years.
Relative Dating Practice 2015