Video Transcript
Unconformities can sometimes be hard to identify in nature. Which of the following, if present, can provide evidence for the presence of an
unconformity in nature? (A) A sudden change in fossil content. (B) Large-scale graded bedding. (C) Large-scale cross-bedding. (D) Rocks of the same age. (E) Thick beds of fine-grain sandstones.
An unconformity is an erosional or nondepositional surface that represents a gap in
the geological record. This means a period of time where no sediment has been deposited or erosion has taken
place removing sediment. An unconformity is represented by a surface between two different sets of rocks. Sometimes it is easy to identify an unconformity, such as an angular unconformity,
where the layers of two sets of sedimentary rocks are nonparallel. However, when the rocks have no apparent change in dip, such as a disconformity,
where the layers of rock are parallel to each other, the unconformity can be hard to
identify. In these cases, geologists must use other pieces of evidence to help identify where
an unconformity exists in nature.
Fundamentally, an unconformity represents two different sets of rocks that are
different ages. So evidence that can help date the rocks can be useful when identifying an
unconformity. An example of this is to study the fossil content of rocks. Fossils can help to date rocks because we can assume the age of the rock is the same
as the age of the fossil it contains. In this example, the lower bed contains trilobites, which are Devonian, and the upper
bed contains ammonites, which are Jurassic. The Devonian fossils are tens of millions of years older than the Jurassic
fossils. The significant age gap between the beds that contain ammonites and the beds that
contain trilobites suggest the presence of an unconformity.
So, a sudden change in fossil content can provide evidence for the presence of an
unconformity in nature.
