|Cycles of Transgression and Regression outcrop|
Transgression and RegressionThe energy at a point on the continental shelf is a function of the water depth. Therefore, with any change in sea level, the water depth at a point on the shelf will change introducing a new energy regime. This means a different facies will be deposited.
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.
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: sand ► shale ►limestone. A maximum transgression occurs where the finest sediments reach the farthest landward.
Marine 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.
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.