Stratigraphic cycles
Stratigraphic cycles refer to the
Divisions
Type[2] | Other Terms | Duration (in millions of years) |
---|---|---|
First-order | Supercycle | 200-400 |
Second-order | Sequence or Synthem | 10-100 |
Third-order | Mesothem | 1-10 |
Fourth-order | Cyclothem | .2-.5 |
The division of these sequences were originally constructed in the
Smaller orders in stratigraphic cycles have also been proposed. Fifth-order cycles and sixth order cycles have also been described in much of the
First-order cycles
This cycle is most likely caused by the break-up and formation of super-continents. The earth went through major climatic swings over the course of 200 to 400 million years. From the late
Second-order cycles
There are two competing arguments for second-order sea-level changes. The first states the sea level can be affected by the number of and the volume of the
Another theory, is that earth's true
Third-order cycles
This order of sea-level change has yet to be fully explained. It was originally thought that glaciers controlled these sea-level changes. But glaciers form and retreat far too rapidly, only tens of thousands of years instead of over a million years. Instead, short-term changes in earth's surface due to volcanics and tectonic events could change global sea levels over a million years. This change to earth's shape could produce "bulges" or "sags" that contribute to ocean level fluctuations.[3]
Fourth order cycles
Again, there are two competing theories for what controls fourth order cycles. Often called cyclothems, the relative short time period in which individual layers of rock are never more than 1 million years. Glaciers are capable of causing quick changes in sea level that can show up in the rock record. This mechanism has been proposed for many of the Carboniferous-aged coal deposits producing in some regions, such as North America, cycles of seashore advance and retreat of approximately 600 miles along the shallow slopes of the continental margins.
Event stratigraphy
This can refer to accumulation of sediments in one specific event. This event could be a large storm, landslide, volcanic eruption, or flood. The thickness of the bed could sometimes be over 50 feet (15 m) in depth. The uniform (or often the erratic) nature of the sediments in relation to the surrounding sediments is the only clue that a particular bed might have been deposited in a single event. A sandstone, for instance, that is well-sorted, contains erratic fossils (like
See also
- Milankovitch cycles, a set of forth to seventh-order (0.02-0.4 ma) climate cycles potentially resulting in stratigraphic cycles particularly during glacial climates
References
- ^ Sloss, L.L., (1963). "Sequences in the Cratonic Interior of North America." Geological Society of America Bulletin. 74: 93-114.
- ^ Vail, P.R., et al., (1977). "Global Cycles of Relative Changes in Sea Level." American Association of Peteroleum Geologists Memoirs. 26:83-98.
- ^ a b c Prothero, D.R., and Schwab, F., (2004). Sedimentary Geology. W.H. Freeman, New York: 323.
- ^ Busch, R. M., and Rollins, H. B. (1984). Correlation of Carboniferous strata using a hierarchy of transgressive-regressive units. Geology, v. 12, p. 471-474.
- ^ Anderson, E.J., and Goodwin, P.W., (1980). Application of the PAC hypothesis to limestones of the Helderberg Group. Society of Economic Paleontologists and Mineralogists, Eastern Section Guidebook, p. 32.
- Hallam, Anthony, (1963). "Major Epierogenic and Eustatic Changes since the Cretaceous and Their Possible Relationship to Crustal Structure." American Journal of Science, 261:397-423.
- ^ Sheridan, R.E., (1987). "Pulsation Tectonics As the Control of Long-Term Stratigraphic Cycles." Paleoceanography, 2: 97-118.
- ^ Mound, J.E., and J.X. Mitrovica, (1998). "True Polar Wander As a Mechanism for Long-Term Sea-Level Variation. Ann. Geophysics. 16:57.