Unconformity


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Related to Unconformity: disconformity

unconformity

[¦ən·kən′fȯr·məd·ē]
(geology)
The relation between adjacent rock strata whose time of deposition was separated by a period of nondeposition or of erosion; a break in a stratigraphic sequence.

Unconformity

 

(geology), the occurrence of relatively young strata of rock on the erosion surface of older strata. There are two main types of unconformities: parallel and angular. In parallel unconformities (disconformities), strata of different ages lie parallel to one another and are separated only by an erosion surface—a stratigraphic break. In angular unconformities (disconformities), strata of different ages are not only separated by an erosion surface but dip at different angles; as a result, the ends of the old strata abut the base of the younger overlying stratum. Angular unconformities are formed when previously deposited strata have been tilted or folded and then the upper parts of the strata destroyed (by marine erosion, for example) and layers of a new stratum deposited on the erosion surface.

The study of unconformities is of great importance in elucidating the history of the movements and deformations of the earth’s crust. Analysis of unconformities also helps date magmatic activity, ore formation, and other geological processes.

References in periodicals archive ?
This cover tectonically undisturbed rests over an unconformity that is Bajocian in age and forms the base of the Middle Jurassic succession (Dieni et al.
The Shublik shale layer and Sadlerochit Group, layers of sandstone and shale, are interrupted by the Permian unconformity, which divides the Ellesmerian into upper and lower sequences.
Occurring worldwide, the Great Unconformity juxtaposes old rocks, formed billions of years ago deep within the Earth's crust, with relatively young Cambrian sedimentary rock formed from deposits left by shallow ancient seas that covered the continents just a half billion years ago.
The upper contact of the Horton Bluff Formation is almost everywhere an angular unconformity with the overlying Cheverie Formation; thus the angle of tilting of the Horton Bluff Formation and the extent of erosion also contributed to thickness variations.
Figure 3 is a structural cross-section along Post Oak Creek showing the angular unconformity between the horizontal Warrensburg Sandstone and an excellent section of underlying dipping Desmoinesian Series sedimentary rocks.
In the Australian case, the presence of an unconformity clearly indicates a land surface reaching above sea level.
The first-priority issue for proposed ODP drilling in the Newfoundland Basin is to ascertain the origin of the "U" unconformity and the nature of underlying crust.
The alteration zone is a 25-metre thick interval of clay minerals at and below the unconformity.
The Cigar Lake and Jabiluka deposits are among the largest unconformity uranium deposits ever discovered.
The Sibley Basin is the least explored for unconformity uranium deposits in Canada, mostly due to a lack of the necessary technology.
Garnet porphyroblasts from upper amphibolite facies show a textural unconformity defined by inclusion-free rims, which coincides with changes in the garnet composition for both major a nd trace elements, including Cr.