5.2: Unconformities

Types of Unconformities

 There are three types of unconformities: nonconformity, disconformity, and angular unconformity.

A nonconformity occurs when sedimentary rock or volcanic lavas are deposited directly on top of crystalline intrusive igneous rocks or metamorphic rocks. In order for this situation to arise, considerable erosion must have occurred in order to bring the metamorphic or igneous intrusive rocks to the surface. In the block diagram at the top of the (Figure \(\PageIndex{1}\)), the formation of a nonconformity is demonstrated in three panels. The first panel, a magma has intruded the lower layers of a group of horizontal sedimentary rocks. In the second panel, uplift and erosion at a later time have removed the horizontal sediments from above the (now) crystalline rock and exposed it's surface to the atmosphere. In the third panel, subsidence has led to deposition of new horizontally layered sedimentary rocks above the original crystalline igneous rock. The nonconformity is the boundary between the older igneous rock and the newly deposited sedimentary layers above it.

Figure \(\PageIndex{1}\): Block diagrams showing the formation of a nonconformity, which is created when an intrusive or metamorphic rock is eroded and then new layers are deposited atop. The nonconformity separates the older eroded crystalline rocks from the younger horizontal strata above. Modified from the original work by Bradley Deline, is licensed under CC BY-SA.

An angular unconformity can be recognized where horizontal sedimentary strata or volcanic lavas are deposited on a terrain of deformed sedimentary or volcanic strata that have been tilted, folded, and/or faulted so that they are no longer horizontal. The formation of an angular unconformity is depicted in Figure \(\PageIndex{2}\). On the left,layered horizontal sedimentary (or volcanic) rocks have been deposited. In the second panel, uplift, tilting and erosion at a later time have produced a sequence of tilted layers that outcrop at the surface. In the third panel, subsidence has led to the deposition of new horizontally layered sedimentary rocks above the older tilted strata. The angular unconformity is the boundary that separates the tilted layers from the horizontal sedimentary layers above. 

Figure \(\PageIndex{2}\): Block diagrams showing the formation of an angular unconformity, which is created when layered sedimentary rocks or volcanics are tilted and eroded and new horizontal sedimentary or volcanic layers are deposited atop; the angular unconformity separates the tilts strata from the younger horizontal strata above. Modified from the original work by Bradley Deline, is licensed under CC BY-SA.)

A disconformity is created when either non-deposition or erosion took place. In other words, layers of rock that could have been present, are absent. The time that could have been represented by such layers is instead represented by the disconformity. A disconformity is an unconformity that occurs between parallel layers of strata indicating either a period of no deposition or a period of erosion. They can be recognized by subparallel surfaces and sometimes layers of distinctive sediments above and/or below the disconformity. The formation of a nonconformity is depicted in the in the lower row of Figure \(\PageIndex{3}\). On the left, layered horizontal sedimentary (or volcanic) rocks have been deposited. In the second panel, uplift and erosion at a later time have removed a portion of the original sequence of layers, and moved the older layers closer to the surface, producing a sequence of horizontal layers that outcrop at the surface. In the third panel, subsidence has led to the deposition of new horizontally layered sedimentary rocks above the older horizontal strata. The disconformity is the subparallel line that separates the older horizontal layers from the horizontal sedimentary layers above. 

Figure \(\PageIndex{3}\): Block diagrams showing the formation of a disconformity, which is created when layered sedimentary rocks or volcanics are uplifted and eroded and new horizontal sedimentary or volcanic layers are deposited atop; the disconformity is subparallel to the horizontal layers above and below. Modified from the original work by Bradley Deline, is licensed under CC BY-SA.)

 

Query \(\PageIndex{1}\)

The Grand Canyon

The Grand Canyon in Arizona is a natural geologic laboratory that can illustrate the major Principles of Cross-cutting Relationships, Superposition, and Original Horizontality, as well as the importance of recognizing unconformities in the study of a geologic region. Figure \(\PageIndex{4}\) shows a simplified cross-section of the rocks exposed on the walls of the Grand Canyon. These units are summarized in the table as well. Numbers in the images correspond to numbered units in the table and the text that follows.
 

Figure \(\PageIndex{4}\): Cross section of Grand Canyon geologic units. A schematic cross-section of the strata exposed in the Grand Canyon illustrating very general geological relationships between the Precambrian Basement Rocks, the Precambrian Grand Canyon Supergroup rocks, and the overlying horizontal Paleozoic rocks of the Grand Canyon. This image is modified from "Grand Canyons three sets of rocks" by US Park Service, is licensed under public domain.

 

Simplified summary of strata and geologic relationships for the Grand Canyon as shown in Figure \(\PageIndex{4}\).

Unit Number in Figure \(\PageIndex{4}\)	Geologic Unit/Formation and Unconformity with approximate age gap	Description	Layer thickness in feet	Approximate Age in millions of years (Ma)
1	Kaibab Formation	Cliff forming, dominantly limestone that is Permian in age.	350	270
2	Toroweap Formation	Easily eroded, interbedded shale and limestone unit that is Permian in age.	250	273
3	Coconino Sandstone	Cliff forming cross-bedded sandstone, Permian in age.	300	275
4	Hermit Formation	Slope forming shale and siltstone unit that is Permian in age.	300	280
5	Supai Group	Slope forming shales and siltstones of Pennsylvanian age capped by ridge forming sandstone that is Permian in age. A 5-35 Ma disconformity separates it from the unit below.	1000	285-315
6	Surprise Canyon Formation	Limestone that is Mississippian in age. A 20 Ma disconformity separates it form the unit below.	0-75	320
7	Redwall Limestone	Cliff forming limestone that is Mississippian in age; a 45 Ma disconformity separates it from the unit below.	500	340
8	Temple Butte Formation	Dolomitic unit that is separated from unit below by a 120 Ma disconformity.	0-50	385
9	Muave Limestone	Cliff forming limestone that is the upper part of the Cambrian Tonto Group.	450	505
10	Bright Angel Shale	Slope forming shale unit.	350	515
11	Tapeats Sandstone	Basal cliff forming sandstone of the Tonto Group; unconformable with all units of the Grand Canyon Supergroup across a 215 Ma angular unconformity (the Great Unconformity)	0-200	575
12	Sixtymile Formation	Tilted upper shale unit of the Proterozoic Grand Canyon Supergroup. Separated form the underlying unit by a >30 Ma disconformity.	200	<740
13	Tilted Chuar Group	Tilted shale unit of the Proterozoic Grand Canyon Supergroup; disconformable with the underlying unit. The disconformity is 130-160 Ma.	5200	740-770
14	Nankoweap Formation	Tilted shale unit of the Proterozoic Grand Canyon Supergroup; disconformable with underlying unit. The disconformity is 200-300 Ma.	370	900
15	Unkar Group	Tilted shale and basal limestone unit of the Proterozoic Grand Canyon Supergroup; nonconformable with underlying basement rocks. The disconformity is 380-470 Ma.	6800	1100-1200
16, 17, 18	Vishnu Basement	Schists (#16), Gneisses (#17) and younger granites (#18); schists are in nonconformable contact with overlying Grand Canyon Supergroup rocks.	Unknown Thickness	1680-1840

 

In the lowest parts of the Grand Canyon, deformed Proterozoic igneous and metamorphic crystalline rocks form the "basement". The principle of cross-cutting relationships reveals the sequence of events in the basement rocks: the metamorphic schist (#16) is the oldest rock formation and the cross-cutting granite intrusion (#17) is younger. The Elves Chasm gneiss (#18) is intermediate to these units because it cuts the older schist and is intruded by the younger granite. 

These ancient crystalline basement rocks (1680-1840 Ma in age) are unconformably overlain by the tilted Grand Canyon Supergroup (a series of layered sediments), which is roughly 1200-740 Ma in age. The nonconformity separating these groups of rocks could represent as much as 480 Ma of Earth’s history! We know that the basement rock were brought to the surface by erosion and/or faulting, but we don’t know much else from this section alone!

The overlying Grand Canyon Supergroup is a tilted sequence of limestones, shales and siltstones that is approximately 12570 ft thick. These units, which are all parallel to one another, are broken by three disconformities: the oldest separates the Unkar Group (#15) from the Nankoweap Formation (#14) and accounts for 200-300 Ma; the next is a gap of approximately 130-160 Ma between the Nakoweap and the overlying Chuar Group (#13); a final discontinuity is a gap of at least 30 Ma separating the Chuar Group from the overlying Sixtymile Formation (#12). Each of these disconformities could represent a period of non-deposition or deposition, followed by erosion. It’s impossible to know without further study and it’s impressive to keep in mind that we are in the dark about as much as 490 Ma of geologic time during the late Precambrian in this area!

The entire package of Precambrian Grand Canyon Supergroup Rocks and basement rocks in the lower part of the Grand Canyon are unconformably overlain by a thick sequence of horizontal Paleozoic rocks which make up much of the Grand Canyon. Where these rocks rest on the tilted Grand Canyon Supergroup units, this "Great Unconformity" defines an angular unconformity representing a gap of approximately 215 Ma Figure \(\PageIndex{5}\).

 

Figure \(\PageIndex{5}\): The great angular unconformity in the Grand Canyon, Arizona. Horizontal Paleozoic sedimentary rocks rest atop tilted Proterozoic layers. The tilted rocks at the bottom are part of the Proterozoic Grand Canyon Group (aged 825 to 1,250 Ma). The flat-lying rocks at the top are Paleozoic (540 to 250 Ma). The boundary between the two represents a time gap of hundreds of millions of years. This work by Steven Earl, is licensed under CC-BY.

In some areas, horizontal Paleozoic rocks rest directly upon the basement units, forming a nonconformity, which can be seen between the Vishnu Schist (basement) and the overlying Tapeats Sandstone Figure \(\PageIndex{6}\).

Figure \(\PageIndex{6}\): View from Hopi Point of the Great Unconformity, with the base of the Cambrian Tapeats Sandstone truncating steeply foliated and veined late Paleoproterozoic to early Mesoproterozoic Vishnu Basement Rocks. A nonconformity separates the Vishnu basement and Tapeats Sandstone. A white line shows the location of this nonconformity in several areas. This work by Martha House, is licensed under CC BY-SA 4.0/ a derivative from the original work.

The Paleozoic rocks of the Grand Canyon are horizontal limestones, shales, siltstones, sandstones and some dolomites with an overall thickness of roughly 3500-3825 ft. and spanning approximately 300 Ma. In the figure and table, the other layers on the walls of the Grand Canyon are numbered with #1 (the Permian Kaibab Formation) being the youngest and the last to form, and #11 (the Cambrian Tapeats Sandstone) the earliest of this group. This sequence illustrates the Principle of Superposition. The Colorado River carves through the Colorado Plateau, exposing the horizontal strata. The layers can be followed from one side of the canyon to the other, illustrating the Principle of Lateral Continuity. These rock strata have been barely disturbed from their original deposition, except by a broad regional uplift. 

Most of these rocks are conformable with one another except for units immediately above and below the cliff-forming Red Wall Limestone. This prominent unit (approx. 500 ft thick) is bound by disconformities above it and below, and the relatively thin unit above (the 320 Ma Surprise Canyon Formation) and below (the 385 Ma Temple Butte Formation) are, in turn, separated from older and younger units by disconformities. Together, these four disconformities (two that bound the Surprise Canyon Formation and two that bound the Temple Butte Formation) account for as much as 225 Ma of geologic time (160 Ma below the Red Wall Limestone and as much as 65 Ma above it). This is a significant fraction of the entire period of the entire 300 Ma represented by the Paleozoic rocks of the Grand Canyon!

A closeup of a younger disconformity in New Mexico gives an idea of what these features look like in the field. Notice the parallel layers of sedimentary rocks and the slightly angled boundary between the Chinle Group (lower) and the Entrada Sandstone above (Figure \(\PageIndex{7}\)). Close examination of surfaces like this sometimes reveals pebble layers or evidence of surface exposure (oxidation, for example). 

Figure \(\PageIndex{7}\): This image shows an exposure of Rock Point Formation of the Chinle Group capped by Entrada Formation sandstone in which an erosional disconformity separates the horizontal sedimentary layers. The Entrada Sandstone is a Jurassic formation, separated from the lower unit by the regional J-2 disconformity, a gap of about 45 million years in the geologic record. This work by Martha House, is licensed under CC0/ a derivative from the original work.

Video: When a Billion Years Disappeared

This video dives into the discovery of the Great Unconformity and discusses the significance of the missing time it represents.

Query \(\PageIndex{2}\)

Query \(\PageIndex{3}\)

References

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