Allostratigraphy is a formal, material-based stratigraphic scheme that designates mappable units based on bounding discontinuities rather than internal lithology or facies, thus allowing different lithologies deposited in different but linked environments to be included in the same stratigraphic unit. It also allows thick units of similar lithology to be divided if they contain discontinuities. The fundamental unit of allostratigraphy is the alloformation, which may be subdivided into allomembers or included in larger allogroups. There is no single scale of allounit other than they be mappable at the scale practiced in the region where the unit is defined. Allostratigraphy was initially developed by Quaternary stratigraphers but has now been applied to older stratigraphic units. This paper emphasizes Cretaceous and Quaternary examples from North America. Allostratigraphy has no requirement that a significant time-gap be present and associated with a bounding discontinuity. In studies of Cretaceous marine deposits, ash flow tuff layers (bentonites) marked an abrupt and readily identifiable lithologic discontinuity readily identifiable in outcrops and well logs that could be used for regional correlation and mapping despite no significant gap in time at the bed contacts.Allostratigraphy has some similarities with sequence stratigraphy, but does not recognize a correlative conformity and allows for designation of units using discontinuity and omission surfaces of lower rank than the unconformities emphasized in sequence stratigraphy. Allostratigraphy represents a fundamental change, relative to earlier stratigraphic methods such as lithostratigraphy, in how strata can be subdivided and named based on bounding discontinuities as opposed to the potentially more diachronous lithofacies.
