Absaroka sequence

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The Absaroka sequence was a cratonic sequence that extended from the end of the Mississippian through the Permian periods. It is the unconformity between this sequence and the preceding Kaskaskia that divides the Carboniferous into the Mississippian and Pennsylvanian periods in North America.

Like the Kaskaskia sequence, Absaroka sedimentary deposits were dominated by detrital or siliclastic rocks. The first sediments were deposited near the continental margins, particularly near the Ouachita and Appalachian highlands. Characteristic of the strata from this time are cyclothems: alternating marine and non-marine strata indicative of changes in sea-level, probably due to cyclic glaciation in the Southern Hemisphere.

In North America, the waters of the Absaroka sequence regressed westward as the highlands to the east steadily eroded. Restricted oceanic circulation in the west led to extensive evaporite formation. By the end of the period, the regression was complete, and the marine strata were superseded by extensive red bed deposition.

[edit] Transgressive-Regressive Units

These cycles of sea level change have been divided into at least six magnitudes of order.[1] Each order is a span of years that become smaller. The smallest unit is described as a punctuated aggradational cycle (or PAC) and is between 225,000 and 100,000 years of sediment. [2] In the field, these units are usually one to five meters thick and contain several different rock units. These units show quick changes in sea level that were controlled by climate change due to glaciers. The fifth order is often called a cyclothem and may contain several PAC sequences and generally represent about 500,000 years.[1] Again, glaciation seems to be the cause of the cyclic nature of the strata. The first four orders, though are controlled more by continental shifts and orogenic cycles.[3] The first order cycle being the Absaroka, the second being the Carboniferous Period, and third order being rock formations or groups which are generally 8 to 10 million years long. The fourth order might be controlled by large scale glaciation and global ocean patterns over the course of a million years. [4]

[edit] References

  • Monroe, James S., and Reed Wicander. The Changing Earth: Exploring Geology and Evolution, 2nd ed. Belmont: West Publishing Company, 1997. ISBN 0-314-09577-2(pp. 544-9)
  1. ^ a b 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.
  2. ^ 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.
  3. ^ Vail, P.R., Mitchum, R.M., Jr., and Thompson, SIII, (1977). Seismic stratigraphy and global changes of sea level, PArt 4: Global cycles of relative changes of sea level in Payton, C.E., ed., Seismic stratgraphy; applications to hydrocarbon exploration. American Association of Petroleum Geologists Memoir 26, p. 83-97.
  4. ^ Chang, K.H., (1975). Unconformity-bounded stratigraphic units. Geological Society of America Bulletin, v. 86, p. 1544-1552.