Retrogradation

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.

See also

• Progradation – Growth of a river delta into the sea over time
• River delta – Silt deposition landform at the mouth of a river
• Aggradation – Increase in land elevation due to the deposition of sediment
• Marine transgression – Geologic event in which sea level rises relative to the land
• Marine regression – Geologic event in which sea level falls relative to the land
• Sedimentology – Study of natural sediments and their formation processes
• Stratigraphy – Study of rock layers and their formation
• Sequence stratigraphy – Study and analysis of groups of sedimentary deposits
• Sediment transport – Movement of solid particles, typically by gravity and fluid entrainment