A nonconformity is?

• A. A gap in the geologic record caused by erosion
• B. A type of unconformity in which layered sedimentary rocks lie on an erosion surface cut into igneous or metamorphic rocks
• C. A surface where sedimentary rocks overlie igneous or metamorphic rocks
• D. A fault boundary

Answer: (C)

Nonconformity is a boundary where younger sedimentary rocks sit on top of much older non-sedimentary rocks, such as igneous or metamorphic basement rocks, after those basement rocks were exposed at the surface and eroded. That combination—sedimentary overlying crystalline rocks—is the defining feature.

The best way to express it is a surface where sedimentary rocks overlie igneous or metamorphic rocks, because it directly states the essential relationship at the contact. It emphasizes the transition from the old crystalline basement to the younger sedimentary cover, which is what nonconformity is all about.

A gap in the geologic record caused by erosion describes unconformities in a general sense, but it doesn’t specify the rock types at the contact. A surface where sedimentary rocks overlie igneous or metamorphic rocks, i.e., the nonconformity, is distinct from a fault boundary, which is a different structural feature.