We all know someone who just doesn’t conform to our expectations.
Believe it or not, rocks can do that, too. Learn about unconformable rocks in this lesson.
An unconformity is the contact between sedimentary rocks that are significantly different in age or between sedimentary rocks and older, eroded igneous or metamorphic rocks.
Unconformities represent gaps in the geologic record, periods of time that are not represented by any rocks.Unconformities happen for two reasons: sediment deposition stopped for a considerable time and/or existing rocks were eroded prior to being covered by younger sediment. There is no single time span represented by an unconformity. It depends on how long erosion occurred or for how long deposition ceased.
Some unconformities are easier to identify than others. For example, the contact between a very old granite and a younger sandstone is pretty obvious. On the other hand, figuring out whether two limestone beds are significantly different in age might require more investigation.
Types of Unconformities
The type of unconformity (in other words, what we call it) is based upon what rock types are involved, whether it is the result of erosion or no deposition, and whether older sedimentary rock layers were tilted prior to being eroded.
Let’s look at some different types in more detail.Angular UnconformityAn angular unconformity is the result of erosion of tilted layers of sedimentary rock. The erosion surface is buried under younger, horizontal layers of sedimentary rock. Hutton’s Unconformity at Siccar Point, Scotland, is probably the most famous one, where tilted beds of eroded sandstone are covered by horizontal beds of younger sandstone.DisconformityA disconformity also involves erosion of sedimentary rocks.
But here, the older rock layers were not tilted before they were eroded. In the least obvious case, the disconformity (the erosion surface) is parallel to the layering of both rock layers. Those can be difficult to identify and usually require dating of the rocks using fossils or some other method in order to know with certainty that the two rock layers are substantially different in age.In many instances, the erosion surface is irregular and the unconformity undulates somewhat, cutting across the rock layers.
Those types of disconformities are more easily recognizable.ParaconformityA paraconformity is the contact between two individual layers (or beds) in a sedimentary rock that were deposited at different times, but no erosion happened. The time gap represented is typically very short (keeping in mind that geologically-speaking, ‘short’ might mean hundreds or thousands of years). Paraconformities are usually very difficult to identify because of the short time involved and lack of an erosion surface.NonconformityWhen igneous or metamorphic rocks are eroded and then covered by younger sedimentary rocks, the contact is called a nonconformity. One of the most famous of these is found in the Grand Canyon, where the oldest sedimentary rocks are more than a billion years younger than the 1.6 billion-year-old metamorphic rocks on which they rest.
Nonconformities typically represent the greatest gaps in the rock record due to the great age of the eroded igneous or metamorphic rocks.
Let’s review. Unconformities are gaps in the geologic rock record. They are surfaces of contact between older rocks and younger sedimentary rocks, formed due to erosion or lack of sediment deposition over extended periods of time. There are different types of unconformities such as angular unconformity, disconformity, paraconformity, and nonconformity.
After completing this lesson, you should be able to:
- Define unconformities
- Describe four different types of unconformities