Types of Unconformities

There are three basic types of contacts in geology

Depositional contacts: These are the most common type of contact and occur when one rock layer is deposited on top of another. The contact between the two layers is typically sharp and there is no evidence of deformation.
Intrusive contacts: These contacts occur when molten rock (magma or lava) intrudes into preexisting rock. The contact between the intrusive rock and the preexisting rock is typically irregular and there may be evidence of deformation.
Fault contacts: These contacts occur when two blocks of rock have been displaced relative to each other along a fault plane. The contact between the two blocks is typically smooth and there may be evidence of fracturing.

Depositional contacts are the most common type of contact because they are the result of the normal process of sedimentation. Intrusive contacts are less common because they require the intrusion of igneous rock into preexisting rock. Fault contacts are the least common type of contact because they require the movement of rock along a fault plane.

Unconformity

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.

In this article, we consider in more detail the nature and interpretation of depositional contacts.

Depositional unconformities

Relatively continuous sedimentation in a region leads to the deposition of a sequence of roughly parallel sedimentary units in which the contacts between adjacent beds do not represent substantial gaps in time. Gaps in this context can be identified from gaps in the fossil succession.

Best road cut ever Quite possibly. Fall out from several episodes of activity at Chimborazo volcano, Ecuador

The boundary between adjacent beds or units in such a sequence is called a conformable contact.

Figure 1 the Principal Types of Unconformities: (a) Disconformity, (B) Angular Unconformity

The rock formation above shows an angular unconformity found on the coast of Portugal at Telheiro Beach.
Copyright by: Gabriel Gutierrez-Alonso

Bedding in the two units is parallel, and the contact between these two units is gradational.
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.

Figure 2 the Principal Types of Unconformities:
(C) Nonconformity, (D) Buttress Unconformity.

Unconformable contacts are generally referred to as unconformities, and the gap in time represented by the unconformity (that is the difference in age between the base of the strata above the unconformity and the top of the unit below the unconformity) is called a hiatus.

Figure 3 Angular Unconformity in the Caledonides at Siccar Point (Scotland).
The Hammerhead Rests on the Unconformity, Which Is Tilted Due to Later Deformation.

In order to convey a meaningful description of a specific unconformity, geologists distinguish among four types of unconformities that are schematically shown in Figures 1&2 and defined in the Table.

Unconformities represent gaps in the rock record that can range in duration from thousands of years to billions of years. Examples of great unconformities, representing millions or billions of years, occur in the Canadian shield, where Pleistocene till buries Proterozoic and Archean gneisses. In the classic unconformity between Paleozoic sedimentary rocks and Precambrian gneisses is shown and many introductory geology books show this contact in the Grand Canyon.

It is a special experience to put your finger on a major unconformity and to think about how much of Earth’s history is missing at the contact. Imagine how James Hutton felt when, in the late eighteenth century, he stood at Siccar Point along the coast of Scotland (Figure 3), and stared at the Caledonian unconformity between shallowly dipping Devonian Red Sandstone and vertically dipping Silurian strata and, as the present-day waves lapped on and off the outcrop and deposited new sand, suddenly realized what the contact meant. His discovery is one of the most fundamental in field geology.

Figure 4 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.

Types of Unconformities

There are several types of unconformities:

Angular Unconformity

At an angular unconformity, strata below the unconformity have a different attitude than strata above the unconformity. Beds below the unconformity are truncated at the unconformity, while beds above the unconformity roughly parallel the unconformity surface. Therefore, if the unconformity is tilted, the overlying strata are tilted by the same amount. 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. If the beds below the unconformity are folded, then the angle of discordance between the super- and sub-unconformity strata will change with location, and there may be outcrops at which the two sequences are coincidentally parallel (Figure 1. b).

Buttress Unconformity

UnconformitiesA buttress unconformity (also called onlap unconformity) occurs where beds of the younger sequence were deposited in a region of significant predepositional topography. Imagine a shallow sea in which there are islands composed of older bedrock. When sedimentation occurs in this sea, the new horizontal layers of strata terminate at the margins of the island. Eventually, as the sea rises, the islands are buried by sediment. But along the margins of the island, the sedimentary layers appear to be truncated by the unconformity. Rocks below the unconformity may or may not parallel the unconformity, depending on the pre-unconformity structure. Note that a buttress unconformity differs from an angular unconformity in that the younger layers are truncated at the unconformity surface (Figure 2. d).

Nonconformity

Nonconformity is used for unconformities at which strata were deposited on a basement of older crystalline rocks. The crystalline rocks may be either plutonic or metamorphic. For example, the unconformity between Cambrian strata and Precambrian basement in the Grand Canyon is a nonconformity (Figure 2. c).

Disconformity

At a disconformity, beds of the rock sequence above and below the unconformity are parallel to one another, but there is a measurable age difference between the two sequences. The disconformity surface represents a period of nondeposition and/or erosion (Figure 1. a).

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. This can happen if the beds above and below the unconformity are deposited in the same environment, but there is a gradual change in the sediment type.

How do you recognize an unconformity in the field today

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.

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.