retrogradation

thumb | right|Scheme describing a retrogradation depositional pattern of coastal (shore or deltaic) sediments: t1-2-3 are consecutive sedimentation times. Ideal lithostratigraphic sections along the depositional system are reported. Retrogradation is the landward change in position of the front of a coastal depositional system (such as a river delta) with time. This occurs when the mass balance of sediment into a delta or a beach system is such that the volume of incoming sediment is less than the volume of the delta that is lost through subsidence, sea-level rise, and/or erosion. As a result, retrogradation is most common: during periods of sea-level rise which results in marine transgression. This can occur during major periods of global warming and the melting of continental ice sheets. with extremely low sediment input.

Retrogradation may occur also in carbonate platforms during phases of sea-level rise, when the increment exceeds carbonate inputs from the biological community. In such case there is a landward shift of the reef facies onto preceding back-reef or lagoonal facies, while fore-reef or even basinal facies develop above the preceding reef system.