Transgressions and Regressions
Sea-Level Changes
Sea-level change is a natural part of Earth history. At times, sea-level has risen by as much as a few hundred meters, creating shallow seas that covered the interiors of the continents. There have also been times when sea-levels have dropped by hundreds of meters, exposing even the continental shelves to the air. Sea-level changes can be grouped into two broad categories:
Local sea-level changes:
- Tectonic uplift and subsidence may result in parts of a coastline either rising above sea-level (locally falling sea-level) or falling below sea-level (locally rising sea-level).
- Sediment compaction will result in coastal areas slowly sinking below sea-level (locally rising sea-level) much like in the modern Mississippi delta region.
World-wide (eustatic) sea-level changes:
- Climatic changes resulting in growth of continental ice sheets will cause global sea-levels to drop.
- Climate changes resulting in melting of continental ice sheets will cause global sea-levels to rise.
- Global warming will heat the upper several hundred meters of ocean water, causing a thermal expansion of the water and a global sea-level rise.
- Periods of increased plate tectonic activity at the mid-ocean ridges will cause the volume of the ridges to increase, thereby displacing ocean water and causing global sea-level rise.
Transgressions and Regressions
When sea level rises, the coastline migrates inland. We call this process Transgression. For example, as a sandy beach migrates inland, it will cover the marsh sediments that were originally behind it. Similarly, the original beach sediments will then be covered by deeper water sediments, thereby creating a record of the transgression. See the illustrations below.
When sea level falls, the coastline migrates oceanward. We call this process Regression. For example, as a sandy beach migrates oceanward, it will cover the deeper-water sediments that were originally in front of it. Similarly, the original beach sediments will then be covered by marsh sediments, thereby creating a record of the regression. See the illustrations below.
The above two diagrams are taken from Marshak (2001).
Transgressive-regressive sequences in the Pennsylvanian rocks in western Pennsylvania may be attributed to a combination of global sea-level changes resulting from changes in polar ice sheet volume and local sea-level changes resulting from sediment compaction in the delta systems.
Second and third order transgression-regression cycles in the Pennsylvanian section of southwestern Pennsylvania. From Busch and Brezinski (1984).
References
Busch, R. M., and Brezinski, D. K., 1984, Stratigraphic analysis of Carboniferous rocks in southwestern Pennsylvania using a hierarchy of transgressive-regressive units: Field Trip Guidebook for the Eastern Section Meeting of the American Association of Petroleum Geologists. 104 p.
deV. Klein, G., and Kupperman, J. B., 1992, Pennsylvanian cyclothems: Methods of distinguishing tectonically induced changes in sea level from climatically induced changes: Geological Society of America Bulletin, v. 104, p. 166-175.
Marshak, S., 2001, Earth: Portrait of a Planet, W. W. Norton & Company, New York, 735 p.
http://people.hofstra.edu/faculty/j_b_bennington/research/cyclothems/cyclothems.html