The Three Basic Types of Contacts between the Rocks

To understand the concept of nonconformity in the rock formation, it is important to mention the basic types of contacts between the rock layers.

There are three basic types of layer contacts between the rocks. These types are known as unconformities. The layer formation changes due to the transition between the layers, and the intermediate layer vanishes gradually. This is a rare event and usually happens once in a million years. The rocks produced at different times are merged to form one rock. The merged rocks can be analyzed and studied as one rock.

The three types of unconformities are angular unconformity, disconformity, and nonconformity. All these types are explained below.

Angular Unconformities: These happen when an older sediment rock has been tilted or truncated by erosion, and a new sediment rock gets deposited on the same place (eroded surface). The sequence of events has four steps. The first step is the sediment deposition, the second is the uplifting and titling, the third is the erosion, and the fourth is the new sediment deposit.

The most famous angular unconformity example is Grand Unconformity in the Grand Canyon of Arizona. In this unconformity, the sedimentary rocks of the Precambrian age are overlain by younger sedimentary rocks of the Phanerozoic age.

Disconformities: They are also an erosion surface between two sedimentary rocks. However, the lower rocks were not tilted or truncated before the deposition of the upper sediment package. The sequence of events has 3 steps. The first is the subsidence and sediment deposition, the second is uplifting, and erosion, and the third is the renewed subsidence and deposition.

The most popular example of a disconformity is Capitol Reef National Park, Utah.

Nonconformities: Unconformities that separate igneous or metamorphic rocks from the covering sedimentary rocks are known as nonconformities. Nonconformities usually indicate that erosion continued for a long time before sediments were deposited. It usually requires several kilometers of erosion before a nonconformity can form.

Its examples include the base of the Grand Canyon succession and the Wind River Gorge of Wyoming.

Nonconformity Explained

In other words, suppose, you are studying two rocks: sedimentary and metamorphic. The lower rock is metamorphic and 600 million years old and the upper rock is sedimentary and 400 million years old. You cannot differentiate between the ages of both the rocks. Even you cannot differentiate between them due to their material and structure. The only difference you will see the erosional contacts lying between both of them.

It is easier to recognize nonconformity if the nonconformity occurs on igneous/metamorphic rocks. The sedimentary rocks are layered whereas; igneous rocks do not come in layers. The minerals, metamorphic texture and a closer examination of the structure of rocks will enable you to recognize nonconformity. Let me explain via an example of how to recognize nonconformity between two rocks. The very thin layer of erosions between the younger and metamorphic rock makes them separate and it cannot be recognized easily. You can recognize it only if there are fossils between the two rocks. The layer of fossils between them can differentiate the ages of the rocks and can draw a clear difference line between them.

Nonconformity VS Disconformity

Shortly, a nonconformity occurs when sedimentary rock is lying on metamorphic rock. The contact between them can be easily recognized due to the different structure of both the rocks. The igneous rock will not come in layers whereas; the sedimentary rock is layered. The close examination of the layers will enable you to recognize the difference between them.

A disconformity occurs when two sediment rocks lie together in parallel position but they have quite different ages. Let say! One rock will be 200 million years old and the other one is 400 million years old. So, the difference between both the layers can be judged easily. The paleosol formation of the rocks shows a clear disconformity in them.

Here are a few examples of nonconformity in rocks. See below the images.

Nonconformity in Rock Formation

See the image above for nonconformity in the rock formation. It is clearly visible in the structure of the rocks that sedimentary rock is paralleled to the igneous rock and how the formation seems in two different patterns. The pattern can be differentiated only in the presence of fossils in the igneous rock.

Disconformity in Rock Formation

Here, the parallel rocks and their patterns are visible. You can easily recognize the structure of each rock. You can easily examine the sediment rocks lying parallel to differentiate their ages.