Superposition dating

Rated 4.83/5 based on 848 customer reviews

The tilted rocks at the bottom are part of the Proterozoic Grand Canyon Group (aged 825 to 1,250 Ma).

The flat-lying rocks at the top are Paleozoic (540 to 250 Ma).

[SE] The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted.

An example of this is given in Figure 8.2.2, which shows three different sedimentary layers.

For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it (Figure 8.2.1).

Figure 8.2.1a A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawaii.

A different, absolute dating method would need to be used to do that. Stratigraphy is the study of how layers of earth are deposited and overlap, leaving patterns in the soil. Superposition just means that the upper layers of stratigraphy are newer than the lower layers.

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.

The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is close to 300 million years.

But what happens if something moves the soil around, like a farmer’s plow or an animal burrowing into the earth? A good way to think about superposition is to imagine a messy desk, full of four weeks of mail!

Say one day you need to find a letter from three weeks ago in those layers of envelopes, how will you know where to look in the pile when there are two weeks’ worth of newer mail on top?

Leave a Reply