Sunday, February 13, 2011

Milankovitch Cycles and Carbonate Deposition...

Are described in this post over at Rapid Uplift, with important links to other posts, including - at Open Mind - Glacial Cycles, Part 1 and Glacial Cycles, Part 2.

Just a brief bit from Rapid Uplift:

"...Milankovitch cycles are orbital cycles which result in periodic fluctuations in the amount of solar energy received by the earth. Three types of cyclical changes in the earth's movement around the sun, eccentricity, obliquity and precession are said to determine climatic cycles on earth."...

A bit more:

"...One explanation has been that Milankovitch cycles causes growth and decay of ice sheets. This periodic locking in and release of sea water by ice sheets causes fluctuations in sea level resulting in changing water depths and cyclical deposition of sediment. A great many carbonate sequences have been explained as being deposited under the influence of sea level changes caused by such orbital forcing. The image below is one of the most famous examples of Milankovitch forced cycles.. the Triassic dolomite platform which makes up the famous dolomite mountains in Italy."

Stated in a different way, whenever you seen the alternations of sedimentary layers, in relatively thin layers, it is telling you that there were fluctuations of some sort. Milankovitch cycles (as described in the linked posts above); tectonic fluctuations of sea level and/or the local sea bottom; or fluctuations in sediment supply, which may reflect climate and/or tectonic fluctuations in the sediment source-area (on a nearby land mass).

Thicker layers represent more long-term fluctuations of the same types, as long as the alternating layers each remain within a given range of lithologies.

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