Identifying Marine Transgression and Regression

Identifying Transgression and Regression in Sedimentary Outcrops
Cycles of Transgression and Regression outcrop.
Dalian, Table Leg Rock Photo By Raimondo Restelli

Transgression and regression are two important concepts in geology that describe how the shoreline of a body of water moves over time. Transgression occurs when the shoreline moves seaward, while regression occurs when the shoreline moves landward.

Transgression and regression can be identified in sedimentary outcrops by looking for changes in the type and composition of the rocks. During a transgression, the shoreline moves seaward and deeper water sediments are deposited. During a regression, the shoreline moves landward and shallow water sediments are deposited.

Identify Transgression and Regression

Here are some of the key features that can be used to identify transgression and regression in a sedimentary outcrop:

  • Changes in rock type: During a transgression, deeper water sediments, such as shale and limestone, are deposited over shallow water sediments, such as sandstone and conglomerate. During a regression, the opposite pattern is observed.
  • Changes in rock composition: During a transgression, the composition of the sediments becomes finer grained. During a regression, the composition of the sediments becomes coarser grained.
  • Changes in sedimentary structures: During a transgression, sedimentary structures such as cross-bedding and ripple marks are typically absent. During a regression, these sedimentary structures are typically present.

By looking for these key features, geologists can identify transgression and regression in sedimentary outcrops. This information can be used to understand the history of the Earth's oceans and to predict where future transgressions and regressions might occur.

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 How to Identify Transgression and Regression in a Sedimentary Outcrop
 

Transgression is the advance of the sea over land areas, resulting in the flooding of previously dry land.

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. 

Causes

Transgressions can be caused by a number of factors, including:

  • Sea level rise: This can be caused by melting glaciers and ice sheets, or by thermal expansion of the oceans due to climate change.
  • Subsidence: This is the sinking of the land, which can be caused by tectonic forces or by the weight of sediments.

Effects

  • Shoreline Shift: The shoreline moves landward, leading to the inundation of coastal regions.
  • Sedimentation: Marine sediments are deposited over terrestrial sediments, creating a sequence known as a transgressive systems tract. This can lead to the formation of transgressive deposits such as marine shales overlying coastal sands.
  • Facies Changes: The depositional environments shift landward, with coastal and shallow marine facies overlaying deeper marine facies.

Regression 

Regression How to Identify Transgression and Regression in a Sedimentary Outcrop?

 

Regression is the retreat of the sea from land areas, exposing previously submerged areas. 

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. 

Causes

Regressions can be caused by a number of factors, including:

  • Sea level fall: This can be caused by the growth of glaciers and ice sheets, or by contraction of the oceans due to climate change.
  • Uplift: This is the  rising of the land, which can be caused by tectonic forces or by  isostatic rebound (the rebound of the land after the weight of glaciers  and ice sheets is removed).

Effects

  • Shoreline Shift: The shoreline moves seaward, leading to the exposure of former sea floors.
  • Sedimentation: Terrestrial sediments are deposited over marine sediments, creating a regressive systems tract. This can result in the formation of regressive deposits such as fluvial sands overlying marine shales.
  • Facies Changes: The depositional environments shift seaward, with deeper marine facies being replaced by shallow marine and terrestrial facies.
Transgressions and regressions can have a significant impact on the environment and on human populations. Transgressions can lead to flooding, erosion, and the displacement of people. Regressions can expose new land for agriculture, but they can also lead to the loss of coastal wetlands and fisheries.

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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