Allostratigraphy classifies strata based on bounding discontinuities (unconformities) rather than lithology, fossils, or time. Allostratigraphic units (alloformations, allomembers, allogroups) are bounded by mappable discontinuities. This classification system (NACSN, 1983) is particularly useful in Quaternary geology and sequence stratigraphy where unconformities are the primary correlatable surfaces, independent of the lithology of the enclosed strata.