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.
Key Features That Can Be Used to 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.
|Cycles of Transgression and Regression outcrop.
Dalian, Table Leg Rock Photo By Raimondo Restelli
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).
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.
- 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.
- 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).
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.