Nonconformity is unconformity where younger sedimentary rock beds overlie older intrusive igneous or metamorphic rocks. It is a contact plane created when younger sediments cover the erosional surface of older metamorphic or plutonic rocks.
Occurrence of nonconformities implies some Earth movement, i.e., land uplift and a long period of erosion and weathering. Later, the eroded metamorphic or plutonic surface subsides, and younger sediments deposit and form rock beds.
This discussion covers nonconformity, including what it means in geology, how it forms, and its significance. We will also give you examples of nonconformities with images and how they differ from disconformity.
Conceptualization
An unconformity is a buried erosional and/or no depositional surface between rock beds or masses of different ages. It represents a hiatus or a gap in the geologic record with no deposition or removal of rock mass by erosion.
Through sketches, early geologists like Nicolaus Steno (1638–1687), John Strachey (1671–1743), and Jean Etienne Guettard (1715–1786) tried to make sketches. However, they vaguely could explain its significance.
Later, James Hutton (1726–1797), one of the fathers of geology, postulated the idea of unconformity. He was referring to angular unconformity but didn’t use the word unconformity.
Later, the term was applied. Today, there are several types of unconformities like angular unconformity, disconformity, and nonconformity. Also, there are others like paraconformity, buttress, and blended unconformities.
Back to our subject matter, Eliot Blakwelder (1880-1969) first suggested nonconformity in 1909. He proposed its usage to mean contact between sedimentary and underlying igneous or metamorphic rocks of different ages.
However, nonconformity was a synonym for angular unconformity. So, instead, heterolithic unconformity was suggested. Heterolithic is Greek for unlike rock. Later, the term nonconformity was agreed upon and not heterolithic unconformity.
What is nonconformity in geology?
Nonconformity in geology (Earth science and related areas) is the contact surface between younger sedimentary beds and eroded older intrusive igneous or metamorphic rocks. Essentially, it is unconformity separating younger sedimentary rock strata from older plutonic or metamorphic rocks.
Nonconformity describes contact between different rock types, i.e., younger sedimentary and older plutonic or metamorphic. It doesn’t focus on the orientation of the rock mass below or above the unconformity. Sometimes, you will find tilted upper, younger sedimentary beds.
For instance, in the Grand Canyon, on the nonconformity between Vishnu Basement and Unkur Group, the Unkur Group is tilted. Unkur is the younger sedimentary that overlays older igneous and metamorphic rocks on the Vishnu Basement.
Lastly, nonconformities are easily recognizable due to lithological variations, i.e., crystalline metamorphic or intrusive igneous and sedimentary.
Igneous intrusions vs. nonconformity
Please note that nonconformity isn’t the same case as where igneous rock intrudes, such as in the formation of batholiths or dikes. Such a scenario will represent a cross-cutting relationship: the younger cuts, the older structure, feature, or rock. Also, you are likely to notice baked margins on the neighboring rocks.
That is not all. You can also apply the law of inclusions to know which of the two rocks is older. For instance, if it is older, you find rock fragments of the upper sedimentary layer in the intrusion.
Lastly, igneous intrusions are one scenario where we cannot apply the law of superposition, i.e., the younger strata or rock mass overlies an older one.
Always underlying rock texture
The assumption that nonconformity takes a long time may be trivial if you don’t consider crystalline intrusive igneous or metamorphic rocks. For instance, magma flows into the ocean, or you have submarine eruptions. Then, sedimentation begins immediately.
The sedimentary rock above the igneous rocks that have just solidified will have the same age as the rocks. Luckily for these cases, the resultant igneous rocks will not be crystalline.
How does nonconformity form?
Plutonic or metamorphic rocks form deep in the Earth’s crust. Therefore, the formation of nonconformity indicates substantial Earth’s crust uplift and erosion went on for a long time before the subsequent subsidence and burial by the younger strata.
At the start, intrusive igneous or metamorphic rocks formed deep in the Earth’s crust. Magma crystallized deep within the crust to form intrusive rock or intense pressure/heat of the overlaying strata, creating metamorphic rocks.
Then, there was a land uplift, considering the great depths where plutonic and metamorphic rocks formed, considering the amount of erosion required to expose intrusive or metamorphic rock deep in the Earth’s crust.
This uplift exposed the uplifted area to erosion, removing kilometers of the overlying rocks. This erosion exposed crystalline plutonic or metamorphic rocks, which weathered and eroded for a long time.
What finally happened was the subsidence of land, or the sea level rose, covering the eroded surfaces of plutonic and metamorphic rocks. Sediments began depositing on the eroded intrusive igneous and metamorphic rock surfaces, forming the younger sedimentary strata.
From the above, we can conclude that:
- Nonconformities represent a long period of erosion. Why? Because of the great depths where intrusive igneous rocks or metamorphic rocks form.
- The uplift was substantial, and it lasted for a long time. Otherwise, much erosion wouldn’t happen.
Significance of nonconformities
Like other unconformities, it is important to understand what they are, correctly identify them, and know how they form. Why?
First, it will help to know past events, such as plate tectonic activities like land metamorphism, changes in sea levels, etc. These are the same events that result in the formation of unconformities.
Secondly, it will help stratigraphers and paleontologists get accurate relative ages of fossils or geological structures and features. Otherwise, they would be wrong without knowing there was a gap in the rock record that may have lasted billions of years.
Lastly, some nonconformities are associated with oil wells, aquifers, and other minerals. So, they are important in the exploration of these resources.
Examples of nonconformities with images
There are many examples of nonconformities. Some of the famous ones include the following:
1. Red Rocks Park, west of Denver, Colorado, USA
The Red Rocks Park, west of Denver, Colorado, USA, is a classic example of nonconformity. It has tilted hematite-rich conglomerates, sandstones, and arkoses of Pennsylvanian Permian’s Fountain Formation, overlying migmatites and granitic gneisses of late Paleoproterozoic’s Idaho Springs Formation. This Precambrian-Pennsylvanian nonconformity has an age gap of about 1.4 billion years.
2. Grand Canyon
Grand Canyon is another place you will get an example of nonconformity, angular conformity, and disconformity. There are two instances of nonconformities, i.e., one is between the Tonto Group of sedimentary rocks (1.4–1.1Ga), sitting on 1.75Ga Vishnu Basement igneous and metamorphic rocks. Another one is between Unkur Group (1.25 Ga) and Vishnu Basement.
3. Ute Trail, Manitou Springs, Colorado, USA
Ute Trail, Manitou Spring, Colorado, USA, has a Precambrian-Cambrian nonconformity, i.e., the Sawatch Sandstone overlying Pikes Peak Granite. This unconformity has a geologic age gap or hiatus of about half a billion years.
4. Wind River Canyon Wyoming
Another notable example of nonconformity is the Great Unconformity in Wind River Canyon. Cambrian Flathead Sandstone, 520- 505 million years, overlays 2.7 billion years of Precambrian Granite and Gneiss. This example has a hiatus of over 2 billion years.
Igneous intrusions vs. nonconformity
Please note that nonconformity isn’t the same case as where igneous rock intrudes, such as in the formation of batholiths or dikes. Such a scenario will represent a cross-cutting relationship: the younger cuts, the older structure, feature, or rock. Also, you are likely to notice baked margins on the neighboring rocks.
That is not all. You can also apply the law of inclusions to know which of the two rocks is older. For instance, if it is older, you find rock fragments of the upper sedimentary layer in the intrusion.
Lastly, igneous intrusions are one scenario where we cannot apply the law of superposition, i.e., the younger strata or rock mass overlies an older one.
Nonconformity vs. disconformity
Nonconformity is unconformity where young sedimentary rock overlies older, eroded intrusive or metamorphic rock. In contrast, disconformity is an unconformity between sedimentary rock sequences with visible erosional features.
References
- Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical Geology (15th ed.). McGraw-Hill Education.
- Wyatt, A. R. (2005). Unconformities. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5). Elsevier Academic.
- Borradaile, G. J. (2015). Understanding geology through maps. Elsevier.
- Lutgens, F. K., Tarbuck, E. J., & Tasa, D. (2018). Essentials of geology (13th ed.). Pearson.
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.