Types of unconformities. A disconformity records a surface of erosion or nondeposition during a particular interval of geologic time. A time gap exists between the underlying and overlying strata, even though the beds dip at the same angle. An angular unconformity is a surface representing a gap in geologic time, during which older rocks initially were deposited and then deformed or tilted, next eroded (to form the unconformity), and finally covered by deposition of horizontal strata above. Thus, it is a situation in which younger sediments rest upon the eroded surface of tilted or folded older rocks (Jackson, 1997). Angular unconformities normally imply a long time interval between deposition and tilting of underlying strata, and deposition of the horizontal overlying strata.
Reprinted with permission of American Geological Institute.
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