diachronism

In geology, a diachronism (Greek dia, "through" + chronos, "time" + -ism), or diachronous deposit, is a sedimentary rock formation in which the material, although of a similar nature, varies in age with the place where it was deposited.

Typically this occurs as a result of a marine transgression or regression, or the progressive development of a delta. As the shoreline advances or retreats, a succession of continuous deposits representing different environments (for example beach, shallow water, deeper water) may be left behind. Although each type of deposit (facies) may be continuous over a wide area, its age varies according to the position of the shoreline through time.