A disconformity is a buried erosional or non-deposition surface or boundary between parallel rock layers of different ages. It has visible, uneven erosional surfaces and forms from erosion of underlying rock layers and later sediment deposition to create the younger rock beds.
Compared with angular unconformity and nonconformity, disconformity is harder to identify. However, considering erosional features and the fossil record, it is not impossible to notice it.
Discover more about disconformities, including some notable examples and how they form. We will also talk about how they differ from angular unconformities and nonconformities.
Conceptualization and historical background
Unconformity, postulated by James Hutton (1787-1788), is a buried surface of erosion or no deposition between two rock sequences of different ages. It represents a duration of no rock record or hiatus.
Hutton didn’t use the term unconformity. Also, in his description, he referred to angular type, as seen at Siccar Point, Isle of Arran, and Jedburgh.
Later, Robert Jameson applied the term unconformity. Today, several unconformities exist, notably angular, disconformity, and nonconformity. Also, there are less common ones like paraconformity, blended, and buttress unconformities.
What is disconformity?
Disconformity is an unconformity with visible, uneven, or irregular erosional features between roughly parallel rock layers of different ages. The younger upper strata and the older lower one will have the same orientation, i.e., strikes and dips, and you will see signs of erosion between the rock beds.
Like others, it represents a buried erosional or non-depositional surface resulting in a measurable rock record gap. However, erosion and weathering play a larger role considering the erosional or weathering features noticeable.
Initially, disconformity was known as parallel, erosional, or nonangular disconformity. Please don’t confuse it with paraconformity, as both have a parallel rock layer between the breaks. However, paraconformity has little or no visible erosional features, making it difficult to identify without studying the respective strata ages.
In the field, you will identify a disconformity by checking for buried visible erosional features between rock layers. These include reliefs, weathering profiles, solution features, stream channels, soil horizons/paleosols, karts, pebbles, etc. Also, there may be missing, or large strata breaks between layers of different ages.
Besides the visible erosional features, studying fossils will further help reveal this unconformity. For instance, if some rock sequences lack certain fossils, there is a geologic time or rock record gap. Such a gap will indicate a disconformity in the sedimentary record. Of course, knowledge of stratigraphic sequence, biostratigraphic zones, and geochronological data will help.
How does a disconformity form?
Disconformities form due to rising or falling sea levels and land uplifting or subsidence without tilting. These events will result in erosion or a non-deposition period.
Assume the study area was under the sea where the deposition was active and sedimentary rocks formed over a long period of time. Then the sea level fell, or the land was uplifted.
When the sea level falls, including regression or land uplift, without tilting, sediment deposition will cease, and erosion will occur. Erosion and weathering will remove some deposited rock layers and create visible, irregular, erosional features.
This period of no deposition and erosion can go on for millions to billions of years, and there will be no rock record. Why? No sediment deposition means no new rock layers, and erosion removes some existing layers.
After a certain time, if the sea level rises, transgresses, or the land subsides, sedimentation will occur on the eroded surface. And since no tilting happens, new younger sediments will be deposited roughly horizontally, just as the law of original horizontality states.
These long periods of erosion, followed by sediment deposition, cause a gap in the geologic timeline or rock records. This essentially tells you how disconformity occurs.
Examples of disconformities
There are many examples of disconformities across the world. Notable ones include the following:
1. Grand Canyon Devonian Temple Butte Formation
The Grand Canyon is one site with a disconformity. It occurs between older Mauv limestone (Cambrian) with deep channels and younger upper Redwall Limestone (Mississippian age). It has a geologic time gap that resulted in the missing Ordovician and the Silurian sequences.
Also, the Grand Canyon has angular unconformities and nonconformities.
2. North Arizona’s Bidahochi and Chinle Formations
The Bidahochi (4-8 million years old Mio-Pliocene age) and Chinle Formations (208-225 million years Upper Triassic Chinle) feature a disconformity.
3. Southeastern Wyoming
There is a disconformity between Precambrian (older) and Mississippian Madison Limestone (younger strata) due to erosion, not a non-deposition reason.
4. Cutler Group in Utah
Cutler Formation or Cutler Group in Utah is another example. It has Kaibab Limestone overlaying Organ Rock Shale. The Organ Rock Shale underwent erosion before the deposition of the upper strata.
We cannot mention all the disconformities since there are many, including one with a 165-million-year gap in Montana. It is between Cambrian age rock and the Mississippian Madison Limestone.
How does disconformity differ from nonconformity?
Disconformity and nonconformity are types of unconformities formed primarily by erosion. Their difference lies in rock type between the two rock beds of different ages. Also, in one, old and younger rock strata are parallel. So, what are they?
A disconformity is an unconformity between parallel rock beds of different ages with visible erosional surfaces. Both rocks are sedimentary, and some erosion features include reliefs, stream channels, solution features, paleosols, old karts, etc., which make them identifiable.
In contrast, nonconformity separates upper younger sedimentary and older metamorphic or intrusive igneous rocks. It represents rocks of different kinds, i.e., upper younger sedimentary rocks that overlie on older crystalline metamorphic or intrusive igneous rocks. Therefore, it is easy to identify and indicates a long erosion period before the deposition of sediments.
Angular unconformity vs. disconformity
These unconformities have sedimentary rock layers of different ages above and below the contact. However, the orientation of the upper and lower strata differ, and strikes and dips vary.
In angular unconformity, the younger rock beds are almost horizontal, while the older ones tilt at an angle at the boundary or unconformity. In contrast, disconformity will have both older and younger strata parallel visible eroded surfaces at their contact or boundary.
Angular unconformity suggests that initially formed sedimentary rocks tilted, folded, or deformed, then were exposed to erosion before the younger rock beds formed.
References
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- Disconformity involving the Bidahochi and Chinle Formations of northern Arizona. AZGS. The University of Arizona. (2018, June 11). Retrieved March 8, 2023, from https://dev.azgs.arizona.edu/photo/disconformity-involving-bidahochi-and-chinle-formations-northern-arizona
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