Unconformity

An unconformity is a buried erosional or non-depositional 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 layer, but the term is used to describe any break in the sedimentary geologic record. The significance of angular unconformity (see below) was shown by James Hutton, who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland.^[1]^[2]

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 preserved in a region or were subsequently eroded before the next deposition. 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. It is a kind of relative dating.

Disconformity at Horni Pocernice, Czech Republic

Disconformity (at the hammer) between underlying Mississippian Borden Formation and overlying Pennsylvanian Sharon Conglomerate, near Jackson, Ohio

There is a billion-year gap in the geologic record where this 500-million-year-old dolomite nonconformably overlies 1.5-billion-year-old rhyolite, near Taum Sauk Hydroelectric Power Station, Missouri.

Nonconformity at Ratssteinbruch near Dresden, Germany

Hutton's angular unconformity at Siccar Point where Famennian age (371–359 Ma) Devonian Old Red Sandstone overlies Llandovery age (444–433 Ma) Silurian greywacke^[14]

Angular unconformity of Triassic rocks overlying steeply-tilted Carboniferous rocks at Praia do Telheiro, Portugal

Angular unconformity between the underlying Dockum Group and the overlying Exeter Sandstone at Steamboat Butte in the valley of the Dry Cimmarron, New Mexico

Angular unconformity in Jingtai County, China

Angular unconformity in pyroclastic rock layers erupted by Chimborazo volcano, Ecuador

Geological unconformity, Camelback mountain, Arizona, showing deposition of Chattian sandstone (right) on Precambrian granite (left).

Great Unconformity

U.S. Bureau of Mines Dictionary of Mining, Mineral, and Related Terms published on CD-ROM in 1996.