Retrogradation
Encyclopedia
Retrogradation is the term for the landward change in position of the front of a river delta
with time. This occurs when the mass balance
of sediment into the delta 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:
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
with time. This occurs when the mass balance
Mass balance
A mass balance is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique...
of sediment into the delta is such that the volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
of incoming sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
is less than the volume of the delta that is lost through subsidence
Subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
, sea-level rise, and/or erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
. 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 warmingGlobal warmingGlobal warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
and the melting of continental ice sheets. - with extremely low sediment input.
See also
- ProgradationProgradationIn sedimentary geology and geomorphology, the term progradation refers to the growth of a river delta farther out into the sea over time. This occurs when the mass balance of sediment into the delta is such that the volume of incoming sediment is greater than the volume of the delta that is lost...
- River deltaRiver deltaA delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
- AggradationAggradationAggradation is the term used in geology for the increase in land elevation due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of material that the system is able to transport...
- Marine transgression
- Marine regressionMarine regressionMarine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously exposed land....
- SedimentologySedimentologySedimentology encompasses the study of modern sediments such as sand, mud , and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary...
- StratigraphyStratigraphyStratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
- Sequence stratigraphySequence stratigraphySequence stratigraphy is a branch of geology that attempts to subdivide and link sedimentary deposits into unconformity bound units on a variety of scales and explain these stratigraphic units in terms of variations in sediment supply and variations in the rate of change in accommodation space...
- Sediment transportSediment transportSediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...