Unconformity
From Wikipedia, the free encyclopedia
An unconformity is a buried erosion 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, but the term is used to describe any break in the sedimentary geologic record.
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 that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus. There are three types of unconformities: disconformity, nonconformity, and angular unconformity.
Contents |
[edit] Disconformity
An unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. A paraconformity is a type of disconformity in which the separation is a simple bedding plane; i.e., there is no obvious buried erosional surface.(AGI, 366)
[edit] 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.
[edit] Angular Unconformity
An unconformity where horizontally parallel strata of sedimentary rock are 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.
[edit] Reference:
- American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.
