Cycles of Transgression and Regression outcrop
Cycles of Transgression and Regression outcrop

Transgression occurs when the ocean basins have more quantity of water than their capacity. It can also occur when the land starts sinking into the sea. It results in a flood that is known as transgression.

In the same way, the term regression works oppositely. When the water level in the ocean basins is at a lower level than their capacity, the sea starts to expose the previously covered lands. The submerged seafloor is exposed as a result. This process is known as regression. All the changes we see are horizontal rather than vertical.

A Marine Transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water (or decreasing in capacity). 

Transgressions and regressions may be caused by tectonic events such as orogenies, severe climate change such as ice ages or isostatic adjustments following removal of ice or sediment load. In either case, sea water rises farther up onto land than it did before.



Transgression
Transgression
 
In this case we will have deeper sea sediments (shales and limestones) being deposited on top of continentally-derived beach sediments (sand). This forms a sequence (from bottom to top) of:  sandshale limestone. A maximum transgression occurs where the finest sediments reach the farthest landward.

Regression 

In this case, continental sediments are being deposited farther out to sea than they once were. Therefore, we see a sequence (from bottom to top) of:  limestone shale sandstone. A maximum regression occurs where the coarsest sediments reach the farthest seaward.

Remember: This process can occur where not all the rock types are present (due to an unconformity) or where coarser sediment is deposited from farther inland. In other words, you don’t always have to have a limestone and you can have conglomerates and breccias landward.
 
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