Wednesday, March 6, 2013

Jurassic fossil coral in a limestone cave

Limestone forms in several ways, some geological, for example direct exsolution of CaCO3 from seawater in appropriate physical and chemical conditions). Most are related to life's interaction with the rock cycle through biomineralisation. This is usually in a marine setting. Famous examples include the white chalk cliffs of Dover in England, formed from the accumulation of the excreta of coccolithophore (plankton with carbonate shells) eating shrimp at the bottom of a warm and shallow cretaceous sea, or the Jurassic limestone plateaux of Europe, such as the Jura mountains of France and Switzerland or the Great Oolite and Portland limestones of England (from which Bath and many of London's beautiful stone buildings are made). The Oolite is another example of a geological limestone, formed by exsolution in a wave washed lagoon type environment. Some limestones consist of entire extinct environments such as fossilised coral reefs.

Compaction and diagenesis (the complex processes of transformation of sediment into rock) often efface most obvious traces of limestone's formation, but sometimes exceptional preservation reveals features familiar from the present world.

The Caverna de las Brujas (Cave of the Witches) near Malargue, south of Mendoza in Argentina is a cave complex in a fossilised Jurassic-Cretaceous limestone reef sequence at the foot of the Andes. The name comes from an oral tradition that Mapuche shamans used to gather there for ceremonies or that 2 white captives escaped there and flew out of it. Other legends recount intense lights shining from the cave, supposedly during magical rituals.

The image is of a fossilised cave (about 3x2 metres) within the reef which has been preserved nearly intact . Those of us who snorkel or scuba dive can nearly see the light shimmering in the water, crinoids on the walls and strange fishes swimming around inside.

Such a clear and detailed glimpse of a long gone world makes it real in a way that textbooks never can. It also demonstrates that while reef building organisms might change in the periodic mass extinction events that punctuate Earth history, the structures that result can remain remarkably similar from era to era. Seeing such examples induces an awe inspiring feeling of intimate contact with deep time that is hard to cultivate in any other way, and which us poetically inclined geoscientists thrive on. It also demonstrates that in life's evolution, plus ca change, plus c'est la meme chose (otherwise known as convergent evolution, but that's another story).

The image was taken by the author during a visit to the cave.

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