Dating rock strata

17 Feb

Older classifications relied on the Ural Mountains stratigraphy. proposed a worldwide stratigraphy of the Permian Period made up of four series/epochs: the Uralian, the Chihsian, the Guadalupian, and the Lopingian.

In the early 2000s, work by Jin and others resulted in the stratigraphy currently accepted by the International Commission on Stratigraphy.

It affected many groups of organisms in many different environments, but it affected marine communities the most by far, causing the extinction of most of the marine invertebrates of the time.

Some groups survived the Permian mass extinction in greatly diminished numbers, but they never again reached the ecological dominance they once had, clearing the way for another group of sea life.

For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.

Permian fossils that have been used as index fossils include brachiopods, ammonoids, fusilinids, conodonts, and other marine invertebrates, and some genera occur within such specific time frames that strata are named for them and permit stratigraphic identification through the presence or absence of specified fossils.

* Dates from the International Commission on Stratigraphy's International Stratigraphic Chart, 2009.

By looking at other outcrops in the area, our geologist is able to draw a geological map which records how the rocks are related to each other in the field.

From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.