Principles of relative dating in geology
The flat-lying rocks at the top are Paleozoic (540 to 250 Ma).
The boundary between the two represents a time gap of nearly 300 million years.
The image is about 7 m across.) [SE photo] An unconformity represents an interruption in the process of deposition of sedimentary rocks.
Recognizing unconformities is important for understanding time relationships in sedimentary sequences.
An example of an unconformity is shown in Figure 8.8.
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.
There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6.
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.
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.
Figure 8.6a A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawaii.