View source for Earth:Marine transgression

{{short description|Geologic event in which sea level rises relative to the land}}
[[File:Duinkerketransgressies.jpg|thumb|upright=1.35|Maps of transgression and regression at the Belgian coast]]

A '''marine transgression''' is a [[Earth:Geology|geologic]] event during which [[Sea level|sea level]] rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling with water or decreasing in capacity. Transgressions and [[Earth:Marine regression|regressions]] may be caused by tectonic events such as [[Physics:Orogeny|orogenies]], severe [[Earth:Climate change (general concept)|climate change]] such as ice ages or isostatic adjustments following removal of ice or sediment load.

During the [[Earth:Cretaceous|Cretaceous]], [[Earth:Seafloor spreading|seafloor spreading]] created a relatively shallow Atlantic basin at the expense of deeper Pacific basin. That reduced the world's ocean basin capacity and caused a rise in sea level worldwide. As a result of the sea level rise, the oceans transgressed completely across the central portion of [[Earth:North America|North America]] and created the [[Earth:Western Interior Seaway|Western Interior Seaway]] from the Gulf of Mexico to the [[Earth:Arctic Ocean|Arctic Ocean]].

The opposite of transgression is [[Earth:Marine regression|regression]] in which the sea level falls relative to the land and exposes former sea bottom. During the [[Earth:Pleistocene|Pleistocene]] [[Earth:Last Glacial Period|Ice Age]], so much water was removed from the oceans and stored on land as year-round glaciers that the ocean regressed 120 m, which exposed the Bering land bridge, between [[Earth:Alaska|Alaska]] and [[Earth:Asia|Asia]].

==Characteristic facies==
[[File:Offlap & onlap EN.svg|thumb|upright=1.35|Cross-sectional diagrams illustrating the shift of sedimentary facies during transgression (onlap) and regression (offlap)]]
[[Earth:Sedimentary rock|Sedimentary]] [[Earth:Facies|facies]] changes may indicate transgressions and regressions and are often easily identified, because of the unique conditions required to deposit each type of [[Earth:Sediment|sediment]].  For instance, coarse-grained clastics like sand are usually deposited in nearshore, high-energy environments; fine-grained sediments however, such as [[Earth:Silt|silt]] and [[Chemistry:Carbonate|carbonate]] muds, are deposited farther offshore, in deeper, lower energy waters.<ref name=Monroe>Monroe, James S., and Reed Wicander.  ''The Changing Earth:  Exploring Geology and Evolution'', 2nd ed.  Belmont:  West Publishing Company, 1997, pp. 112 - 113  {{ISBN|0-314-09577-2}}</ref>

Thus, a transgression reveals itself in the sedimentary column when there is a change from nearshore facies (such as [[Earth:Sandstone|sandstone]]) to offshore ones (such as [[Earth:Marl|marl]]), from the oldest to the youngest rocks.  A regression will feature the opposite pattern, with offshore facies changing to nearshore ones.<ref name=Monroe/>  The [[Earth:Stratum|strata]] represent regressions less clearly, as their upper layers are often marked by an [[Earth:Erosion|erosion]]al [[Earth:Unconformity|unconformity]].

These are both idealized scenarios; in practice identifying transgression or regressions can be more complicated.  For instance, a regression may be indicated by a change from carbonates to shale only, or a transgression from sandstone to shale, and so on.  Lateral changes in facies are also important; a well-marked transgression sequence in an area where an epeiric sea was deep may be only partial farther away, where the water was shallow.  One should consider such factors when interpreting a specific sedimentary column.

==See also==
*{{annotated link|Earth:Sea level rise|Sea level rise}}
*{{annotated link|Earth:Ingression coast}}
*{{annotated link|Earth:Marine band (geology)}}
*{{annotated link|Earth:Raised beach|aka=Marine terrace}}
*{{annotated link|Earth:Sequence stratigraphy}}
*{{annotated link|Biology:Submerged forest}}

== References ==
{{Reflist}}

{{coastal geography}}
{{Geologic Principles}}

[[Category:Geological processes]]
[[Category:Sedimentology]]
[[Category:Coastal geography]]

{{Sourceattribution|Marine transgression}}