Wednesday, February 2, 2011

Milankovitch Cycles and Carbonate Deposition

Over at Open Mind, Tamino has written a series of excellent posts on Milankovitch climate cycles and how they influence the growth and decay of ice sheets.

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.

Read the posts (Part 1, Part 2, Part 3, Part 4).

These climate cycles are of great interest in my field of carbonate sedimentology. Ancient shallow water carbonate basins are often made up of  exquisitely layered strata with the same sediment type recurring at intervals throughout the sequence. The origin of this rhythmic deposits have been a subject of great debate and discussion.

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.

Source: The Latemar Controversy

The other popular explanation and which may well be correct in many instances as well is that shallow marine carbonate systems generate cyclicity by self organization i.e in these systems there is an inbuilt interplay between water depth, currents and sediment production and deposition. The locales of sediment production and deposition keep shifting in response to local thresholds resulting in the same conditions appearing and disappearing from any one particular region. The result is cyclical carbonate strata which are hard to distinguish from orbitally forced cyclical sediment.

This debate over the generation of carbonate cyclicity continues.. but the interest in not just academic. There are differences in the three dimensional geometry and internal structure of sediment bodies built up by orbitally forced cyclicity versus those built up by local thresholds.

Cycle thickness, lateral continuity of facies, the extent of sea level fall and the duration of exposure to fresh water are some of the differences between the two systems that will influence the pattern and movement of fluids in the basin which may result in differences in the patterns of porosity and permeability, two properties of great economic importance.

A petroleum geologist looking at a hydrocarbon lined pore space in a rock chip or thin section and wondering whether this porosity extends all across the basin will have to think big.. of large scale facies patterns, of cycles upon cycles, of fluid flow, of sea levels rising and falling, of continental configurations and paleo-geography... and even the movement of the earth across the heavens..

That's what makes geology such a fascinating subject of study..

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