diachronous

Diachronous is an adjective used in geology, stratigraphy, and related earth sciences to describe features whose ages are not the same across different locations. The term derives from Greek dia- meaning through or across and chronos meaning time, and is contrasted with synchronous, which means occurring at the same time everywhere.

In stratigraphy, a diachronous unit or boundary does not have a single uniform age across its extent. The base or top of a formation can be older in one region and younger in another due to differences in deposition, erosion, or shoreline migration. As a result, the same rock unit may represent different moments in time depending on where it is measured. Diachronous surfaces are common with transgressive or regressive sequences, where changes in sea level or sediment supply shift the timing of deposition along a coastline. Determining diachronous behavior involves cross-regional correlations, fossil assemblages, and radiometric or relative dating to establish age differences along the unit.

Implications include challenges for regional geological correlation and the mapping of stratigraphic units. Recognizing diachronous boundaries helps geologists understand spatial changes in depositional environments and improves interpretations of basin history and resource distribution.

In other disciplines, diachronous can describe processes that do not operate synchronously across locations, though in linguistics the related term diachronic is more commonly used to describe language change over time.