DEFINITION OF REGRESSION

A retreat or seaward migration of the shoreline

caused by either a global sea-level fall, a rise in the land’s surregression

face, or a supply of sediment that is greater than the space created

for the sediment by subsidence. Global sea-level rises and

falls on different timescales depending on the cause. Changes

in ridge volume or mantle plume activity cause slow changes

to the ocean ridge volumes and slow rises or falls in sea level,

whereas changes in the volume of continental glaciers may

cause faster changes in the volume of water in the ocean. All

of these may be related to the supercontinent cycle and cause

sea-level regressions or transgressions. Local tectonic activity

may cause the land surface to rise or fall relative to a stable

global sea level, causing local regressions or transgressions.

For instance, glacial rebound may cause the land to rise rapidly

in response to the reduced load when the glaciers melt. The

shoreline of a region may retreat seaward if the sediment supplied

by a river or other system is so large that the sediment

volume fills up and overflows the space available for it to be

deposited in. Deltas form at the mouths of rivers where the

coastal subsidence can not accommodate the large sediment

flux, causing a local regression in these areas. Deltas have a

wide variety of forms, resulting from interactions between

sediment supply, subsidence, wave, and tidal action. If coastal

currents substantially rework sediment supplied by rivers and

alluvial systems, then a series of seaward prograding beach

ridges may form along a coastline experiencing a regression.

To interpret patterns of global sea-level rise and fall it is

necessary to isolate the effects of local tectonic subsidence or

uplift, and sediment supply issues, from the global sea-level

signature. This can be difficult and requires precise dating

and correlation of events along different shorelines, plus a

detailed understanding of the local tectonic and sedimentation

history. When the local effects are isolated they can be

subtracted from the global sea-level curve, and the causes of

global sea-level changes investigated.

See also DELTAS; SEA-LEVEL RISE; SUPERCONTINENT

CYCLE; TRANSGRESSION.

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