A surface exhibiting evidence of an abrupt increase in water depth, separating younger from older strata. The surface may also display evidence of minor submarine erosion. It forms in response to an increase in water depth and typically bounds parasequences. In sequence stratigraphic terminology, it replaces the older, more generic term "trangressive surface," although it is not a strict equivalent.
This term is also used as a short version of the terms maximum flooding surface or marine flooding surface.