Unconformity: Types of Unconformities

Unconformities are geological features that represent gaps in the rock record, indicating periods of erosion or non-deposition.

Unconformities are geological discontinuities that represent gaps in the sedimentary record. They occur when sedimentation is interrupted or when previously deposited sediments are eroded away. If there is an interruption in sedimentation, such that there is a measure able gap in time between the base of the sedimentary unit and what lies beneath it, then we say that the contact is unconformable.

An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the sedimentary geologic record.

What Causes Unconformity

Erosion: If the land surface is eroded, the underlying rocks will be exposed. Over time, these exposed rocks may be weathered and then covered by new sediments.

Subsidence: If the land surface subsides, it may be below sea level, allowing sediments to be deposited on top of older rocks.

Folding: If the land surface is folded, the rocks may be tilted or even overturned. This can create a break in the rock record where the older rocks are now on top of the younger rocks.

Regional uplift: If an area of the Earth's crust is uplifted, the land surface will be raised above sea level. This can cause erosion to occur, exposing the underlying rocks.

Climate change: If the climate changes, it can affect the rate of deposition. For example, if the climate becomes drier, there will be less sediment deposited. This can create a gap in the rock record.

Tectonic activity: Tectonic activity, such as earthquakes and volcanoes, can also cause unconformities. For example, an earthquake can cause the land surface to be uplifted or tilted, creating a break in the rock record.

Types of Unconformities

Angular Unconformity

Angular unconformity is a type of unconformity that occurs when younger sedimentary rocks are deposited on an eroded and tilted surface of older sedimentary rocks. This means that the older rocks were tilted and eroded before the younger rocks were deposited. Angular unconformities are a clear indication that there was a period of uplift and erosion between the deposition of the two sets of rocks.

Angular Unconformity. Chimborazo volcano, Ecuador

Because of the angular discordance at angular unconformities, they are quite easy to recognize in the field. Their occurrence means that the sub-unconformity strata were deformed (tilted or folded) and then were truncated by erosion prior to deposition of the rocks above the unconformity. Therefore, angular unconformities are indicative of a period of active tectonism. (Figure 1. b).

Angular unconformities are found worldwide in various geological settings. Examples include Hutton's Unconformity, the Grand Unconformity, and the unconformity at Siccar Point.

Characteristics of angular unconformities:

The older layers are tilted, and the younger layers are horizontal.
There is an angular discordance between the two sets of layers.
The eroded surface may be marked by scour features, such as channels or grooves.
The eroded surface may be covered by a paleosol, which is a soil horizon that formed from weathering prior to deposition of the overlying sequence.

Nonconformity

A nonconformity is a type of unconformity that separates sedimentary rocks from older igneous or metamorphic rocks. It represents a period of time during which no sediments were deposited, and the older rocks were exposed at the surface and eroded. Nonconformities are typically marked by a surface of erosion, as indicated by scour features, or by a paleosol, which is a soil horizon that formed from weathering prior to deposition of the overlying sequence.

Nonconfirmity at Ratssteinbruch near Dresden, Germany

For example, the unconformity between Cambrian strata and Precambrian basement in the Grand Canyon is a nonconformity (Figure 1. c). Grand Unconformity: The Grand Unconformity in the Grand Canyon is a well-known example of a disconformity. It marks a vast gap in the geologic record, spanning nearly 1.8 billion years, between Precambrian metamorphic rocks and overlying Paleozoic sedimentary rocks.

Disconformity

A disconformity is a type of unconformity that occurs when there is a gap in the geologic record due to erosion or non-deposition. It represents a period of time during which no sediments were deposited. Disconformities are typically marked by a parallel surface between the overlying and underlying rocks, and there is little apparent erosion. They are often identifiable by the presence of a paleosol, which is a soil horizon that formed from weathering prior to deposition of the overlying sequence (Figure 1. a).

Unlike an angular unconformity, where the older rocks are tilted, the older and younger rocks in a disconformity are parallel to each other.

disconformity unconformity representing about 165 million years worth of missing time.
Quaternary gravel over Jurassic red shale (Salina Canyon, Utah, USA)
Sevier County, central Utah, USA
Photo: James St. John

Disconformities are formed due to two primary causes:

Erosion: Erosion can remove previously deposited sediments, creating a gap in the geologic record. This erosion can occur due to various factors, such as uplift, exposure to wind or water, or changes in sea level.
Non-deposition: Even without erosion, a disconformity can form if no sediments are deposited for a period of time. This can happen if the area is not submerged in water or if the conditions are not suitable for sediment accumulation.

Disconformities are characterized by several distinguishing features:

Parallel bedding planes of the overlying and underlying rocks
Signs of erosion, such as scour marks or channels
Paleosols, a soil horizon that formed during the period of erosion or non-deposition

Paraconformity

A paraconformity is a type of unconformity that represents a period of non-deposition in the geologic record. It occurs when younger sedimentary rocks are deposited on a horizontal surface that represents an interruption in the deposition of sediments. This surface, known as a paraconformity surface, is typically marked by a parallel contact between the overlying and underlying rocks, with no apparent erosion.

Paraconformities are characterized by a parallel surface between the overlying and underlying rocks, suggesting no significant erosion before deposition.

Paraconformity Vs Disconformity

Paraconformities and disconformities are both gaps in the geologic record, but they differ in their surface characteristics and how they are formed. Paraconformities lack significant erosion before deposition, while disconformities often exhibit an erosion surface.

Buttress Unconformity

A buttress unconformity (also called onlap unconformity) is a type of unconformity that occurs when younger sedimentary rocks are deposited against an older, topographically irregular surface. This results in the younger rocks abutting or "buttressing" the older rocks, creating a distinct angular relationship between the two sets of layers.

Buttress Unconformity. Copyright: © Marli Bryant Miller

Characteristics of Buttress Unconformity

Onlapping: The younger sedimentary rocks terminate at the edge of the older rocks, forming a buttress-like shape.
Angular Discordance: There is no angular discordance between the younger and older rocks, meaning they are parallel.
Erosion Surface: The surface of the older rocks may exhibit signs of erosion, indicating a period of non-deposition before the younger sediments were deposited.

Note that a buttress unconformity differs from an angular unconformity in that the younger layers are truncated at the unconformity surface (Figure 2. d).

Examples include the buttress unconformity at Grand Falls, Arizona, the one in Ogden Canyon, Utah, and the one in the Appalachian Basin.

Blended unconformity

A blended unconformity is a type of unconformity where the beds above and below the unconformity are not parallel, but the erosion surface is not clearly defined.

Blended unconformities are often associated with environments where erosion and deposition occur simultaneously, leading to a gradual mixing of sediments.

Characteristics of blended unconformity:

Gradual transition between younger and older rocks
Intermixing of sediments or materials from both formations
Lack of a clear boundary between the younger and older rocks
May occur in environments with slow and continuous erosion and deposition
Can be challenging to identify

Figure 1 the Principal Types of Unconformities:
(a) Disconformity, (B) Angular Unconformity
(C) Nonconformity, (D) Buttress Unconformity.

Recognize an unconformity in the field

Well, if it is an angular unconformity or a buttress unconformity, there is an angular discordance between bedding above and below the unconformity. A nonconformity is obvious, because crystalline rocks occur below the contact.

Some Features Used to Identify Unconformities: (a) Scour Channels in Sediments,
(B) Basal Conglomerate, (C) Age Discordance From Fossil Evidence, and (D) Soil Horizon or Paleosol.

Disconformities, however, can be more of a challenge to recognize. If strata in the sequence are fossiliferous, and you can recognize the fossil species and know their age, then you can recognize a gap in the fossil succession.

Commonly, an unconformity may be marked by a surface of erosion, as indicated by scour features, or by a paleosol, which is a soil horizon that formed from weathering prior to deposition of the overlying sequence. Some unconformities are marked by the occurrence of a basal conglomerate, which contains clasts of the rocks under the unconformity.

Recognition of a basal conglomerate is also helpful in determining whether the contact between strata and a plutonic rock is intrusive or whether it represents a nonconformity.

Unconformity Photos

Angular unconformity found on the coast of Portugal at Telheiro Beach.
Copyright by: Gabriel Gutierrez-Alonso

Angular Unconformity in the Caledonides at Siccar Point (Scotland).

Conclusion

Understanding and identifying unconformities are crucial in deciphering the geological history of an area. They provide insights into past tectonic events, sea-level changes, and the overall evolution of the Earth's surface over time. Each type of unconformity provides clues about the geological history of an area, including the timing of deposition, deformation, and erosion events. By studying unconformities, geologists can reconstruct past environments and understand the complex processes that have shaped Earth's surface over millions of years.