If we want to understand the geological history of a location we need to look at the rocks in that location. But if we want to understand a region, we need to correlate the rocks between different locations so that we can meld the individual histories of the different locations into one regional history.
Matching Up Rock Layers
Superposition and cross-cutting are helpful when rocks are touching one another and lateral continuity helps match up rock layers that are nearby. To match up rocks that are further apart we need the process of correlation. How do geologists correlate rock layers that are separated by greater distances? There are three kinds of clues:
Distinctive Rock Formations
1. Distinctive rock formations may be recognizable across large regions (Figure below).
The famous White Cliffs of Dover in southwest England can be matched to similar white cliffs in Denmark and Germany.
2. Two separated rock units with the same index fossil are of very similar age. What traits do you think an index fossil should have? To become an index fossil the organism must have (1) been widespread so that it is useful for identifying rock layers over large areas and (2) existed for a relatively brief period of time so that the approximate age of the rock layer is immediately known.
Many fossils may qualify as index fossils (Figure below). Ammonites, trilobites, and graptolites are often used as index fossils.
Mucrospirifer mucronatus is an index fossil that indicates that a rock was laid down from 416 to 359 million years ago.
Microfossils, which are fossils of microscopic organisms, are also useful index fossils. Fossils of animals that drifted in the upper layers of the ocean are particularly useful as index fossils, since they may be distributed over very large areas.
A biostratigraphic unit, or biozone, is a geological rock layer that is defined by a single index fossil or a fossil assemblage. A biozone can also be used to identify rock layers across distances.
3. A key bed can be used like an index fossil since a key bed is a distinctive layer of rock that can be recognized across a large area. A volcanic ash unit could be a good key bed. One famous key bed is the clay layer at the boundary between the Cretaceous Period and the Tertiary Period, the time that the dinosaurs went extinct (Figure below). This widespread thin clay contains a high concentration of iridium, an element that is rare on Earth but common in asteroids. In 1980, the father-son team of Luis and Walter Alvarez proposed that a huge asteroid struck Earth 66 million years ago and caused the mass extinction.
The white clay is a key bed that marks the Cretaceous-Tertiary Boundary.
- A single rock unit contains the story of the geology of that location. To understand the geology of a region, scientists use correlation.
- To correlate rock units, something distinctive must be present in each. This can include an index fossil, a unique rock type, a key bed, or a unique sequence of rocks.
- A key bed can be global. An example is the iridium layer that was deposited at the time of the Cretaceous-Tertiary extinctions.
Use this resource to answer the questions that follow.
Leaf fossils in Lapilli tuff
1. Where were these leaf fossils found?
2. Why is this volcanic ash unique?
3. How is the lapilli tuff formed?
4. Why is this lapilli tuff important?
5. What does the lapilli tuff allow scientists to do?
1. What features must the iridium layer that dates to around 66 million years ago have to be a key bed?
2. Why are microfossils especially useful as index fossils?
3. What is the process of correlation?