Engineering geology


Basic types of contacts

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 known as Unconformities. When there are transitions between the layers in a particular gap of time, the layer formation changes and the intermediate layer vanishes gradually. It happens in millions of years and the rocks produced at different times are pushed jointly. It seems to form one rock. During the study of the rock structure, it can be analyzed well and so, the structure of two rocks pushing together now as a one can be understood.

There are three types of Unconformity known as Disconformity, angular unconformity and nonconformity.

The third basic type of unconformity is a nonconformity, in which younger sedimentary strata overlie older metamorphic or intrusive igneous rocks. Just as angular unconformities and some disconformities imply crustal movements, so too do nonconformities.

Weathering and erosion

Intrusive igneous masses and metamorphic rocks originate far below the surface. Thus, for nonconformity to develop, there must be a period of uplift and erosion of overlying rocks.

Once exposed at the surface, the igneous or metamorphic rocks are subjected to weathering and erosion, then undergo subsidence and renewed sedimentation.


In two rocks, the erosional contacts lying parallel to the surfaces of both the rocks; 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.

Nonconformity, Teton Range, Wyoming
Slikovni rezultat za disconformity"

Disconformity between massive Coconino Sandstone and thinner bedded Hermit Shale, Grand Canyon National Park

Adapted from “Geologydegree“; Photos: M.Miller; D. Sherman

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