The Stone of Brutein

On a busy street in the middle of the City of London, hiding behind a metal grille, is an old piece of unremarkable oolitic limestone (see http://on.fb.me/1WHE33i) that embodies some of the oldest legends in the British Isles, and is supposedly the talisman that ensures London’s continued survival and prosperity. We know that it was placed there by the hand of man, since the nearest outcrop of this rock is some 100km away in Kent, but not how long it has been there.

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Ooids • Sedimentary Spheres

While these dark gray spherical clasts could be mistaken for some type of fossil, they’re actually ooids, which are chemically precipitated sedimentary grains.

Ooids form around a nucleus of a mineral grain or shell fragment. Concentric layers composed of calcium carbonate precipitate out and progressively coat the nucleus, forming a pattern almost like tree rings. In the bottom right photo, you can clearly see the center nucleus and the individual surrounding concentric layers that compose the ooid.

Generally, ooids form in shallow wave-agitated water. These higher-energy marine settings allow for accretion to occur on all sides of the grain and for their well-polished appearance. However, ooids are also known to form in non-marine settings, such as the Great Salt Lake.

Grains that are less than 2 mm in diameter are termed ooids, while those larger than 2 mm are termed pisoids. Accordingly, ooids cemented together form the sedimentary rock oolite, and pisoids form the rock pisolite.

Specimens of unknown age from Banff, Alberta, Canada

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The Sand Between Your Toes

Whilst treading along the picture-perfect white beaches found throughout the tropics, you may not be aware that the sand beneath you is made of tiny fragments of dead carbonate organisms. These carbonate-sand beaches are very common, but in some rare places such as the Joulter Cays in the Bahamas, the sand that the lucky tourists walk on is in fact not even sand at all. The picture above shows the tiny grains known as ooids which form the pristine beaches found in these rare parts of the world.

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One of my favorite sedimentary rock features, ooids. These are small, rounded grains of calcium carbonate, usually 1-2 millimeters in diameter. 

Ooids are like coated sand grains. They form in areas where carbonate minerals are readily precipitating, such as warm, tropical oceanic waters. They need a core to start off growth, usually a sand sized grain of sediment or shell fragment. As those grains are tossed around by the waves, typically in a beach or near shore setting, newly formed carbonate minerals grow around the edges. The action of the waves rolls the grains, mostly keeping them from sticking together and allowing them to grow on all sides, forming these nice little spheres.

You’ll sometimes find entire rocks made of these known as oolitic limestones, and in fact there’s even a small town named Oolitic in Indiana (although residents always told me they pronounced the word different, with a long O sound in the name of the town).


Image credit: https://flic.kr/p/8EsGdG

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More Mysterious Ooliths

We recently posted an article about ooliths (http://on.fb.me/1yWGLbD), and mentioned that one criteria for formation of ooids is a rather active sedimentary environment, one in which waves or currents can roll them around and around and around, and thus induce them to grow symmetrically to form their spherical shapes. 

Now imagine another environment. One where sedimentary deposition is sooooo slow that it takes ~5000 years to accumulate a single 5 cm layer. We’ve written of this environment in the past as well, the most boring environment on Earth, the Abyssal Plains (goo.gl/S8UUDq).

Now imagine – wait, this is hard. Another sort of rock in which ooids are found within formations formed from the abyssal ooze. Ummm… how can we form ooids, that apparently require an active sedimentary environment, in the Earth’s most inactive sedimentary environment?

Several speculations on how these silicious ooids could form include:
– Could there be an upwelling of current within the abyssal zone that lasts thousands of years? Perhaps near a hydrothermal field near a spreading ridge?

– Could we cheat, and say that these must have been originally formed as limestone ooliths, but these limestone ooliths were then transposed (through plate action such as near a subduction zone) to such great depths that the calcite dissolved and was replaced by silica (that is, chert?)

In the field area where this particular sample was found, there are great thick deposits of abbysal ooze (radiolarian cherts) that occur in association with spreading ridge formations. Alas, since this sample is from a tectonic mélange zone, it could also have come from a carbonate that was partially subducted into the depths where calcite dissolves…

Oh dear. I suppose there might be some way to solve this mystery, but then it wouldn’t be a mystery any more, would it? 

Please feel free to add your own speculations.

Annie R

Photo: mine (a scanned image of the rock, actually) from sample found by D. Ghikas within Vourinos Ophiolite sole mélange.

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