Friday, December 12, 2008

Ooids in Oolites

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Another nice example from carbonate sedimentology, specifically carbonate grains. In this example, that I have from one of the quarries that I am documenting, the ooids are almost white even in a fresh exposure surface (the image may not show it that well though). I put a few comments into the image. A few ooids can be seen very well rounded, some others are broken and the core nucleus can be seen. The diameter of the ooids is 0.5 to 1.0 mm. The shell fragments are part of the frequent shell layers that occur within the Hauptrogenstein-Formation. I am unable to decide without a thin section what kind of ooids these are - meaning if tangential or radial, one or multiple laminae or if micritised or not.


Ooids with shell fragments

Marine ooids occur in intertidal and subtidal marine environments like shoals, tidal bars or tidal deltas. They usually form in wave-agitated water or in areas of strong currents in generally warm temperatures. Other controlling factors are the presence of nuclei (i.e. small siliciclastic input), sufficient agitation to move grains (waves or current), supersaturated water and a process of water renewal - among others of course. Very important is the long underestimated influence of organic matter and the activity of microbes.

I would call it an oolitic packstone with shell fragments. BTW, the image approximately resembles a bedding plane surface - more or less. Otherwise you could see that the shells are in a distinct layer.

For more information take a look at the excellent USC Sequence Stratigraphy Web.

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