The stratigraphic succession is the net result of sedimentary deposition, non-deposition, and erosion within constantly changing environments. Consequently, rock strata are heterogeneous and interrupted by countless discontinuities, many of which are not readily apparent. The radiometric scale of geologic time firmly supports Joseph Barrell’s theoretical conclusion that sedimentary rocks of any locality represent a small fraction of the time spanned by the formation of those rocks. Parallel evidence is supplied by the fossil record. In other words, the aggregate stratigraphic hiatus, recognized or concealed, greatly exceeds the preserved rock and fossil record.
The term unconformity (and its variants, disconformity and discordance) is employed generally for physically conspicuous stratigraphic discontinuities with inferred hiatuses. Existing confusion may be avoided if the antonym conformity is applied only to probably rare examples of strictly continuous deposits free from diastems, paraconformities, and disconformities. A preferred structural designation for indicating parallel bedding planes is concordance, whereas angularity of sedimentary contacts may be referred to as discordance.
The writer’s observations of paraconformities in craton areas of the Colorado Plateau, Rocky Mountains, Mid-Continent, Mississippi Valley, Ohio Valley, Sweden, Andes of Peru, Salt Range of India, and elsewhere have led to the conclusion that present-day configurations of erosion and sedimentation do not yet provide an obvious explanation of ancient paraconformities. There is a surprising lack of evidence of protracted subaerial erosion such as soil profiles, sinks, and channels along paraconformities. This suggests extensive planar erosion or non-deposition near, or below, sea-level for long spans of time. Evidently, relative sea-level for long intervals rarely fell far below the land surface of the cratonal areas.