Stratigraphic Cycles

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Stratigraphic Cycles refer to the transgressive-regressive sequences bounded by unconformities in the stratagraphic record on the cratons. These cycles represent a large scale eustasy cycle since the Cambrian period with further sub-divisions of those units. [1]

Contents

[edit] Divisions

Startigraphic Cycle Orders
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 Appalachian Basin and Cordilleran Basin of North America. Eventually these sequences were correlated in Russia and South America.[3] The transgressive-regressive units show gaps in the rock records which indicate times of continual erosion and very little deposition. Local tectonics did not play a role in these global events, rather, a world-wide rise and fall of sea level.

Smaller orders in stratagraphic cycles have also been proposed. Fifth-order cycles and sixth order cycles have also been described in much of the Absaroka sequence. The time scale is much smaller and instead of Wilson cycle controlled sea-level change, these shorter cycles were controlled by glaciers (also called glacio-eustasy).[4] [5]

[edit] First-Order Cycles

This cycle is mostly 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 Pre-Cambrian to the late Cambrian, late Devonian to the Triassic-Jurassic border, and since the Miocene until the present time, the earth was an "icehouse", with icesheets covering the poles. In the intervening years, the earth was a "greenhouse", with high global temperatures and elevated atmospheric CO2. Volcanic activity was also high in the greenhouse years.[3] These long periods of continental emergence helped produce changes in ocean currents and the distribution of atmospheric heat.

[edit] 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 magma being produced at mid-ocean ridges.[6] During times of increased sea floor spreading, more magma is being produced and the volume of the ocean basins are displaced by this. This would result in a higher sea level. This increased in magmatic activity corresponds to increased mantle activity and the earth's magnetic field.[7]

Another theory, is that earth's true polar wander occurs over a long period of time. The tectonic plates of the earth would move relatively faster due to imbalance of continents near the poles. This was true during the Cambrian Period, but the same event also happened approximately 65 million years ago but not as severely. [8]

[edit] 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]

[edit] Fourth Order Cycles

[edit] Event Stratigraphy

[edit] References

  1. ^ Sloss, L.L., (1963). "Sequences in the Cratonic Interior of North America." Geological Society of America Bulletin. 74: 93-114.
  2. ^ Vail, P.R., et al., (1977). "Global Cycles of Relative Changes in Sea Level." American Association of Peteroleum Geologists Memoirs. 26:83-98.
  3. ^ a b c Prothero, D.R., and Schwab, F., (2004). Sedimentary Geology. W.H. Freeman, New York: 323.
  4. ^ 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.
  5. ^ 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.
  6. ^ Hallam, A., (1963). "Major Epierogenic and Eustatic Changes since the Cretaceous and Their Possible Relationship to Crustal Structure." American Journal of Science, 261:397-423.
  7. ^ Sheridan, R.E., (1987). "Pulsation Tectonics As the Control of Long-Term Stratigraphic Cycles." Paleoceanography, 2: 97-118.
  8. ^ Mound, J.E., and J.X. Mitrovica, (1998). "True Polar Wander As a Mechanism for Long-Term Sea-Level Variation. Ann. Geophysics. 16:57.