Transgression and regression

Transgression and regression are two of the most common phenomena of the geologic record. The changes in sea level that they represent significantly impact the interpretation of Earth's history and the reconstruction of ancient environments. Understanding these phenomena in the geologic past is essential if present sea-level changes are to be dealt with effectively.

Global Processes

Transgression and regression are two phenomena by which sea-level changes are recognized. The two are not processes as such but rather are the effects of changes in sea level. Transgression is any rise of the sea over a land area (including the continental shelf) or any change in the physical marine conditions involving a progression into a deeper water environment. Regression is a withdrawal of the sea from any land area or any change resulting in a progression from deep water to shallow water or nonmarine environments. The concepts of transgression and regression are themselves independent of a causal mechanism. They are the results of the processes that can cause sea-level change.

Transgression and regression are very diverse phenomena resulting from many different processes. Some of these processes involve an actual change in global sea level or eustacy. Because transgression and regression are effects, however, they can be produced by a local process rather than a global one. It is essential to understand the various processes involved in transgression and regression by examining first the global processes and then the local ones.

The global, or eustatic, processes resulting in transgression and regression cause a change in the volume of seawater on the continents. Such global changes are accomplished by two major processes: first, an actual change in the volume of the global seawater supply and, second, a change in the volume or holding capacity of the ocean basins. A change in the amount of seawater would not seem an easy task to accomplish. The Earth's environment is essentially a closed system. However, small amounts of water are added from volcanism and human activity, which is insufficient to account for wide fluctuations in global sea level. Even if enough new water could be added to the system to account for a sea-level rise, the problem of getting rid of large amounts of water to account for a sea-level fall remains. Rather than adding or subtracting new water, redistributing existing water at the Earth's surface seems more likely to change the volume of seawater. One way that may be accomplished is by the growth and melting of the earth's continental glaciers.

Glacial ice is different from sea ice. Sea ice forms in high latitudes over the open ocean. As such, there is a constant connection with the oceans and a seasonal interchange as the ice melts in the summer and freezes in the winter. Glacial ice, however, forms from accumulated snow on the continents. Once a glacier grows, the snow that forms glacial ice is no longer available to the oceans. It is locked on the continents for the duration of the glacial episode. As glaciers grow, more water is locked within them. As glaciers melt, the meltwater is available to the oceans. The effect on sea level can be dramatic. Sea level will fall during a glacial advance (an “ice age”). During a glacial melt, the sea level will rise. This sea-level mechanism has characterized the geologic record of sea-level change over the past 20 million years and perhaps longer. It has also operated during various other times in Earth’s history as long as continental-scale glaciation existed.

Another process by which transgression and regression can occur is by a change in the geometry of the ocean basins. The ocean basins are the ultimate reservoir of standing water on Earth. The depths in these basins are great, and it would seem that a change in the volume would not have a significant effect on sea level. Yet if the volume of the ocean basins decreases, for example, the excess water must go somewhere. When that happens, a flooding of the continents will occur. Once the volume of the ocean basins increases, the water will withdraw from the continents. The actual mechanism in the ocean basins is related to heat flow from the Earth's interior at the mid-ocean ridges. As the ridges become more volcanically active, heat flow increases; the high heat flow causes an increase in the elevation of the mid-ocean ridge. This increase in elevation causes a decrease in the volume of the ocean basins, resulting in a transgression. As volcanic activity decreases, heat flow decreases, and the elevation of the mid-ocean ridge drops, increasing the volume of the ocean basins. This results in a regression. Sea-level changes brought about by this process produce large-scale fluctuations, resulting in flooding not only on the continental shelves and the coastal plains but also on the continents' interiors. Much of the geologic record contains rocks deposited in seas in continental interiors. This process may result in transgressions where the ancient sea level could be as much as 300 meters above the present sea level. This process is not the dominant process in the twenty-first century. The last sea-level transgression related to this process was during the early Paleocene epoch (about 66 to 58 million years ago). The sea reached from the Gulf of Mexico up the Mississippi Valley into present-day southern Illinois and as far west as the Dakotas.

Local Processes

While the global sea-level changes would seemingly result in uniform transgression or regression worldwide, that is not apparent when the sea-level record at any given place is studied. The reason for that is the effect of local processes on sea level. Many transgressions and regressions are local phenomena and may not be recognizable in other regions.

Two local processes that control sea level are sedimentation and tectonics. By operating in coastal regions, these two processes can mask the global trend of sea level. Sedimentation is the process whereby sediment is deposited by water or wind. Sedimentation can affect local sea levels if the accumulation rate exceeds the global sea-level rise. As the sediment builds up in the coastal regions, it displaces the ocean, thus “filling in” the former sea floor. A regression has occurred because an area that was once a part of the sea is now land. This process is operating in the modern environment. The modern Mississippi Delta in southern Louisiana is a perfect example of this phenomenon. In this area, sedimentation rates are very high. Although the global sea level is rising as the remaining continental glaciers melt, new land is being created around the Mississippi Delta. At this location, sediment is accumulating faster than the sea level is rising. Thus, a regression at the delta is taking place. Away from the delta, the sea level is still rising, and marine waters are flooding more land each year in southern Louisiana. Therefore, even though the Mississippi Delta is a site of high sedimentation rates and locally affects sea-level change, that is not enough to alter the global process.

Tectonism is another local process by which the pattern of sea-level change can be controlled. Although tectonics is a global process, the effects of this process can be very localized, and some include phenomena not related to global tectonics at all. For sea-level change, the processes of interest are uplift and subsidence. Uplift and subsidence in a coastal region can be related to many causes, but the result rather than the process is important to transgression and regression. If the uplift process is more rapid than the global sea-level rise, a regression will be apparent. Similarly, if subsidence exceeds the global sea-level fall, a transgression will be the interpretation. These two examples illustrate the extremes of the tectonic effects, but many variations exist. In the modern environment, the global sea level is rising, but the tectonic setting controls the apparent change in any specific area. An example is the Baltic Sea coast in Sweden, where the uplift resulting from glacial rebound is not reversing the global sea level trend but is slowing the apparent rate of sea-level transgression. Tectonic factors are among the most challenging effects to consider when examining transgression and regression because of the geographic variability and the nature of the tectonic processes.

Transgression and regression are seemingly simple phenomena: Sea level is either rising or falling. Yet the contribution to the observed pattern by local processes is significant and, in some cases, is the dominant effect observed. To gain meaningful insight into the global pattern of sea-level change from local data, a thorough understanding of the geologic history of the geographic region being studied is essential.

Transgression and regression appear to be very dramatic and perhaps even catastrophic when considering their effects. Nevertheless, both must be kept in the proper time context. Transgression and regression are not unique phenomena in the geologic record. Sea level has constantly fluctuated throughout most geologic time, and there is very little evidence that there were times of global sea-level stasis. Transgressions and regressions generally occur over long periods. While some sea-level changes, such as glacially induced ones, occur over a relatively short span of geologic time (several hundred thousand years), many occur over millions of years. Thus, the observed effect of sea-level change may appear minor unless viewed over long intervals of geologic time. In the twenty-first century, much attention has been paid to transgression and regression due to their ability to contribute to coastal erosion, landscape transitions, and the displacement of human populations, which are all main concerns of global climate change. Anthropogenic interventions have had both negative and positive impacts on transgression.

Principal Terms

continental shelf: the margin of the continents, usually covered by shallow seas

eustacy: any change in global sea level resulting from a change in the absolute volume of available seawater

ocean basins: the large worldwide depressions that form the ultimate reservoir for the earth's water supply

regression: the retreat or withdrawal of the sea from land areas and the evidence of such a withdrawal

transgression: the extension of the sea over land areas and the evidence of such an advance

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