REGRESSION, MARINE

A seaward shift in the position of a shoreline. Evidence of a marine regression in a succession of sediments or sedimentary rocks may be preserved in the form of non-marine deposits overlying marine; an upward shallowing of facies; or, in terrestrial environments, a change in facies type or facies architecture that might indicate, for example, entrenchment of a river channel. A forced regression is caused by mechanisms of allocyclic change that bring about an actual sea-level change at the coast, such as a eustatic sea-level fall (i.e. absolute sea-level change) or a decrease in tectonic subsidence, or actual uplift (a relative sea-level change). A progradational regression occurs through mechanisms of autocyclic change under conditions of constant sea level, such as the accumulation of sediment leading to progradation of a shoreline, and can occur during a period of eustatic sea-level rise if the sediment accumulation rate is sufficient. One of the challenges in interpreting sedimentological evidence of environmental change is to separate these complex relationships amongst controls on depositional environments represented in the form of regressions and transgressions in order to reconstruct the record of sea-level change through the geological record.

[See alsoeustasy, sequence stratigraphy]