8.3: Unconformities

Unconformities – Missing Time

An unconformity represents an interruption in the process of depositing sedimentary layers. Sometimes deposition just stops. Sometimes the area is being actively eroded. Either way, sediment is not depositing so there is nothing recording the geologic events of the time.

There are three types of unconformities. More details are provided below using real-life examples, but these simple diagrams may be useful for initial understanding.

• disconformity - a gap in the geological record that occurs between parallel layers of sedimentary rock
• nonconformity - a gap in the geologic record where sedimentary rock overlies igneous or metamorphic rock
• angular unconformity - a geological feature where younger, horizontal sedimentary rock layers are deposited on top of older, tilted or folded sedimentary rock layers, creating an angular discordance between the two sets of strata

Figure \(\PageIndex{1\): Cross-sections showing the three types of unconformities. A) Arrow indicates a disconformity between two parallel sedimentary layers. B) Arrow indicates a nonconformity between an underlying igneous rock and sedimentary layer. C) Arrow indicates an angular unconformity between underlying tilted sedimentary layers and overlying sedimentary layers. Authored by Kelly Ruppert.

Recognizing unconformities is important for understanding time relationships in sedimentary sequences. We can use the rock exposed in the Grand Canyon to illustrate examples of all of the different types of unconformities.

Angular Unconformity

An example of one of the most iconic unconformities is shown in Figure \(\PageIndex{2}\) from the Grand Canyon. It is known as “The Great Unconformity.” Proterozoic rocks of the sedimentary sequence known as the Grand Canyon Supergroup, have been tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks. The specific type is an angular unconformity. The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is close to 300 million years. Tilting and erosion of the older rocks took place during this time, and if there was any deposition going on in this area, the evidence of it is now gone.

Figure \(\PageIndex{2}\): The great angular unconformity in the Grand Canyon, Arizona. The tilted rocks at the bottom are part of the Proterozoic Grand Canyon Supergroup (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 nearly 300 million years. (Credit: Doug Dolde / Doug Dolde at en.Wikipedia / Public domain. Annotated by Callan Bentley.)

Nonconformity

A nonconformity represents an erosional surface separating rocks of different types. The erosional surface will separate igneous and/or metamorphic rock from overlying sedimentary rock such as in the photo below, from a different spot in the Grand Canyon. In Figure \(\PageIndex{3}\), you see a nonconformity representing the erosional surface between 1.75 Ga Proterozoic age rock at the base of the Grand Canyon and ~545 Ma Phanerozoic age rock. Over 1 billion years separate the basement rock from the first overlying layer.

Figure \(\PageIndex{3}\): The view is centered on Sumner Butte, at southeast terminus of Bright Angel Canyon, intersecting with Granite Gorge. (The original uploader was Szumyk at Polish Wikipedia. – http://pdphoto.org/PictureDetail.php?mat=pdef&pg=5506, Public Domain, https://commons.wikimedia.org/w/inde...?curid=5883465. Annotated by Callan Bentley.)

Disconformity

A disconformity is a little harder to discern: it is not as obvious as the previous examples. Disconformities occur between parallel sedimentary layers. Several exist in the horizontal layers of the Grand Canyon. One occurs between the Muav Limestone formation which is approximately 515 Ma and the overlying Temple Butte and Redwall Limestone formations which are approximately 350 Ma and 335 Ma, respectively. This erosional surface, representing over 150 million years is well displayed in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Redwall, Temple Butte and Muav formations in the Grand Canyon. An erosional surface (disconformity) exists between both the Muav Limestone and the Temple Butte formations as well as between the Temple Butte and the Redwall Limestone formations. (United States Public Domain; National Park Service from: https://upload.wikimedia.org/wikipedia/commons/e/eb/Redwall%2C_Temple_Butte_and_Muav_formations_in_Grand_Canyon.jpg.)

Example \(\PageIndex{1}\): Unconformity Examples

Let’s take a look at some additional examples of the different types of unconformities.

Disconformity (sedimentary layers above and below the line are parallel to one another):

Angular unconformity (layers below the unconformity are tilted, layers above the unconformity are horizontal):

Nonconformity (sedimentary layers on top of an igneous unit):

More unconformities:

Figure \(\PageIndex{5}\): Angular unconformity, Telheiro Beach, Portugal, showing Triassic sandstone atop folded, eroded, Carboniferous graywacke and shale. (Photo by Joao Duarte, 2019.)

Figure \(\PageIndex{6}\): Angular unconformity in Loughshinny, Ireland, showing flat-lying Pleistocene till overlying folded limestone and shale. (Photo by Jason Loxton, 2018.)

Figure \(\PageIndex{7}\): Carboniferous / Cretaceous angular unconformity at Ense-Bremen, North Rhine-Westphalia, Germany. (Photo by Henning Bergermann; reproduced with permission.)

Figure \(\PageIndex{8}\): Angular unconformity marked where tilted layers of Neogene-aged sandstone and shale overlain by Pleistocene gravels (with clam-bored boulder bed just above the unconformity surface). Bonus: a layer of recent colluvium overlies the gravels! (Photo by Seth Finnegan; reproduced with permission.)

Figure \(\PageIndex{9}\): Angular unconformity marked where Jurassic-aged Morrison Formation (shale and sandstone) is folded and eroded, overlain by Quaternary sandstone and conglomerate. (Photo by Elizabeth Bollen; reproduced with permission.)

Figure \(\PageIndex{10}\): Nonconformity between Neoproterozoic andesite (~570 Ma) and overlying Triassic sandstone (~210 Ma). (Photo by Jack Matthews; reproduced with permission.)

Figure \(\PageIndex{11}\): The Knox unconformity (a disconformity bounding the Sauk and Tippecanoe epeiric sequences) exposed on Isle La Motte, Lake Champlain, Vermont. Providence Island Formation of the Beekmantown Group (tidal dolomite of the Middle Ordovician) is overlain by lower Day Point Formation (also Middle Ordovician) from the Chazy Group, which is sandy limestone and carbonate sandstone. Note the karstic dissolution of the lower unit at the unconformity surface. (Photograph by Matthew Clapham; reproduced with permission.)

Figure \(\PageIndex{12}\): The “Great Unconformity” (really a nonconformity) north of Durango Colorado. 1.7 Ga Baker’s Bridge granite overlain by late Devonian Ignacio quartzite. (Photograph by Cody Mason; reproduced with permission.)

Figure \(\PageIndex{13}\): Nonconformity in Gallinas Canyon, New Mexico. (Photo by Howell Bosbyshell; reproduced with permission.)

Figure \(\PageIndex{14}\): A paleo-hill of Jurassic sandstone, overlain by Paleocene volcanics mainly basalts. It was quarried for its rock used to make the approach to a local lighthouse on the Isle of Mull. (Photo by James Westland; reproduced with permission.)

Figure \(\PageIndex{15}\): Angular unconformity at El Chalten, Argentina. Jurassic volcanic strata overlie Paleozoic meta-sedimentary strata. (Photo by Donna Anderson; reproduced with permission.)

Figure \(\PageIndex{16}\): Angular unconformity marked by dipping Paleocene interbedded sands and muds with overlying horizontal Eocene conglomerates. Bighorn Basin, Wyoming. (Photo by Amanda Owen; reproduced with permission.)

Figure \(\PageIndex{17}\): Probably one of the best examples of an angular unconformity in Southern England. An unconformity is a buried erosional surface separating strata of different ages; it indicates that sediment deposition was not continuous. In this unconformity at the very eastern end of the Mendips, younger strata of sedimentary Jurassic Inferior Oolite rest upon the eroded surface of tilted older Carboniferous Vallis Limestone. (Reproduced via a CC-by license; Photo by Vieve Forward, https://www.geograph.org.uk/photo/4165184)

Figure \(\PageIndex{18}\): At Chippewa Falls, Wisconsin, there is an unconformity shows a \(\approx\)1.8 Ga saprolitized trondhjemite (granitoid) nonconformably overlain by the Cambrian-age Mt. Simon Sandstone. (Photo by Scott Clark; reproduced with permission.)

Figure \(\PageIndex{19}\): Tilted Pleistocene terrace deposits over tilted Miocene Astoria Formation creating an angular unconformity. The latter was deposited between episodes of Columbia River flood basalts reaching the coast, which was the case with Otter Rock, the large headland in the mid-ground. (Photo by Lockwood DeWitt; reproduced with permission.)

Figure \(\PageIndex{20}\): This is an upside-down unconformity (specifically a disconformity)! The rock above is ophiolitic seafloor breccia (160 Ma). The sediments at the bottom are Late Jurassic. Within the sediments there are also turbidites with inverted grading. The whole package has been flipped by tectonics (we are in the reverse limb of a major fold here). (Photo by Samuele Papeschi; reproduced with permission.)

Figure \(\PageIndex{21}\): A monocline overprinting an unconformity. Proterozoic basement rocks (purple, low in canyon) are overlain by the Chinle Formation, with the Wingate Sandstone as the cliff-forming unit. Colorado National Monument. (Photo by Robert Mahon; reproduced with permission.

Figure \(\PageIndex{22}\): Angular unconformity in the Grand Canyon. Lower tilted layers are Neoproterozoic strata of the Grand Canyon supergroup (various sedimentary rock and some lava flows). The cliff-forming unit extending horizontally across the field of view is Cambrian-aged Tapeats Sandstone. (Photo by Zoltán Sylvester; reproduced with permission.)

Figure \(\PageIndex{23}\): An angular unconformity created as Pleistocene terrace deposits formed over tilted deltaic deposits of the Eocene Coaledo Formation, Shore Acres State Park, Oregon. (Photo by Lockwood DeWitt; reproduced with permission.)

Figure \(\PageIndex{24}\): Nonconformity in the Grand Canyon. Lower rock unit is Mesoproterozoic schist (gray) and granite (pink). The cliff-forming unit extending horizontally across the top of the field of view is Cambrian-aged Tapeats Sandstone. (Photo by Zoltán Sylvester; reproduced with permission.)

Figure \(\PageIndex{25}\): Nonconformity in the Grand Canyon. Lower rock unit is Mesoproterozoic schist (gray) and granite (pink). The cliff-forming unit extending horizontally across the top of the field of view is Cambrian-aged Tapeats Sandstone. (Photo by Zoltán Sylvester; reproduced with permission.)

Figure \(\PageIndex{26}\): At Gribun on the Isle of Mull is an angular unconformity. The underlying dipping rocks are Neoproterozoic psammites (meta-sandstones). There are horizontal Triassic conglomerates above. Most of the boulders on the shore are the conglomerate. Higher up, but not seen in this photo, there are also Paleocene basalts. (Photo by James Westland; reproduced with permission.)

Figure \(\PageIndex{27}\): The classic Scottish angular unconformity between Dalradian metasedimentary slates/phyllites/metalimestones and the overlying basal conglomerate of Late Devonian Old Red Sandstone. This exposure is on the Isle of Kerrera. Gylen Castle ruin for scale. (Photo by Roderick Brown; reproduced with permission.)

Figure \(\PageIndex{28}\): Basal Cambrian arenites (sandstones) on Ercall Granophyre (an igneous rock, like a granite) creating a nonconformity. Ercall quarries, Shropshire. (Photo by Jon Radley, University of Birmingham; reproduced with permission.)

Figure \(\PageIndex{29}\): Angular unconformity near Cody, Wyoming. The lower unit is Triassic Chugwater Formation; the upper unit is Quaternary terrace gravels. (Photo by Ben Edwards; reproduced with permission.)

Figure \(\PageIndex{30}\): A gorge in Shades State Park, Indiana shows the Mansfield Sandstone (Pennsylvanian) lying unconformly over limestones and siltstones of the Mississippian aged Borden Group (creating a disconformity). Glacial waters cut into the sandstone, creating these gorges, exposing the strata. (Photo by Troy Simpson; reproduced here with permission)

Figure \(\PageIndex{31}\): Gently dipping Triassic strata over vertical Carboniferous layers, Rainy Bay, Nova Scotia, Canada creating an angular unconformity. (Photo by Jon Noad; reproduced with permission.)

Figure \(\PageIndex{32}\): 7,000-year-old Mazama Ash Bed overlying steeply dipping Carboniferous limestone creating an angular unconformity. Exshaw, Alberta, Canada. (Photo by Jon Noad; reproduced with permission.)

Figure \(\PageIndex{33}\): A nonconformity exposed at Deer Creek in the Grand Canyon. The waterfall emerges from the contact between lower Mesoproterozoic basement rocks and overlying Cambrian Tapeats Sandstone. (Photo by Logan Wren Raming; reproduced with permission.)

Figure \(\PageIndex{34}\): The Kentland Disturbance Geologic Area is a theorized impact site where Ordovician and Silurian carbonate strata were forced to the surface by the impact. A quarry has existed here for a hundred years excavating rock aggregate and exposing folded and tilted strata with localized faults and telltale shattercones. The upended strata were then scraped over by Pleistocene glaciation, leaving a thin veneer of glacial deposits on top creating an angular unconformity. (Photo by Troy Simpson; reproduced here with permission.)

Figure \(\PageIndex{35}\): Chapman's Drive nonconformity, with Table Mountain Group sandstones (~450 Ma) overlying Peninsula Granite (~630 Ma). Cape Town, South Africa. (Photo by Evelyn Mervine; reproduced with permission.)

Figure \(\PageIndex{36}\): Annex Hill State Park in Minnesota shows two unconformities: an angular unconformity between Archean banded iron formation and overlaying Cretaceous shale, and a disconformity between the shale and overlying Pleistocene till.

Key Terms

• angular unconformity - a geological feature where younger, horizontal sedimentary rock layers are deposited on top of older, tilted or folded sedimentary rock layers, creating an angular discordance between the two sets of strata
• disconformity - a gap in the geological record that occurs between parallel layers of sedimentary rock
• nonconformity - a gap in the geologic record where sedimentary rock overlies igneous or metamorphic rock
• unconformity - a gap in the geologic record that represents an interruption in the deposition of sedimentary layers