A marine transgression is a geologic
event during which 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 regressions
may be caused by tectonic
events such as orogenies
, severe climate change
such as ice ages
adjustments following removal of ice or sediment load.
During the Cretaceous
, 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 North America
and created the Western Interior Seaway
from the Gulf of Mexico
to the Arctic Ocean
The opposite of transgression is regression
in which the sea level falls relative to the land and exposes former sea bottom. During the Pleistocene 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 Alaska
changes may indicate transgressions and regressions and are often easily identified, because of the unique conditions required to deposit each type of sediment
. For instance, coarse-grained clastic
s like sand are usually deposited in nearshore, high-energy environments; fine-grained sediments however, such as silt
muds, are deposited farther offshore, in deep, low-energy waters.
[Monroe, James S., and Reed Wicander. ''The Changing Earth: Exploring Geology and Evolution'', 2nd ed. Belmont: West Publishing Company, 1997, pp. 112 - 113 ]
Thus, a transgression reveals itself in the sedimentary column when there is a change from nearshore facies (such as sandstone
) to offshore ones (such as marl
), from the oldest to the youngest rocks. A regression will feature the opposite pattern, with offshore facies changing to nearshore ones.
[ The strata represent regressions less clearly, as their upper layers are often marked by an erosional 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.