Unconformity
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Taum_Sauk_precambrian-cambrian_unconformity.jpg
An unconformity is a gap in time in rock layers, where erosion occurs while deposition slows or stops. The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were deposited and the local record for that time interval is missing and geologists must use other clues to discover the that part of the geologic history of that area. There are three types of unconformities: disconformity, nonconformity, and angular unconformity.
Disconformity
An unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.
Nonconformity
An unconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock.
Angular Unconformity
An unconformity where horizontally parallel strata of sedimentary rock is deposited on tilted and eroded layers that may be either vertical or at an angle to the overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity.