Limestone formation

[Cave_Troll]

I've always understood that limestone was basically the fosilised remains of coral reefs, sea creatures and the like. "millions of little sea creatures die and their skeletons and shells form the limestone", is what we always tell the kids.
but...
http://news.bbc.co.uk/1/hi/sci/tech/4664741.stm is a news article about fosilised organisms that lived 550million years ago.
"Frondose vendobionts died out before the Cambrian explosion about 540 million years ago, when animals with bones and shells appeared. "
so what was the limestone they were fosilised in, formed of, if it was formed at least 10million yeears before animals with bones and shells appeared?

 

[RobinGriffiths]

Hi Cave_Troll.

Not a geologist, but I did do several of the earth science courses whilst studying with the Open University a few years ago.

Two common forms of limestone not formed of animal skeletons are oolitic limestone and micrite or lime mud. Micrite is actually the most common form of limestone.

Oolitic limestones consist of cemented ooids. Ooids are small spheres formed under strong wave or current action where lime either precipitates directly to or is accreted from small particles onto a nucleus forming a concentrically layered sphere. Microbes are believed to be implicated in the formation of ooids. Oolitic limestones are found in South Wales and on Mendip.

Micrite or lime mud is fine grained and deposited under quiet conditions. 'Modern' micrites are primarily derived from the breakdown of calcareous algae skeletons. Calcareous mats of cyanobacteria would have been common before Cambrian times. Micrite can also precipitate directly from seawater.

A google search on Proterozoic Micrites brings back several interesting articles on early limestones.

The following is a nice link on the classification of limestones:

http://csmres.jmu.edu/geollab/Fichter/SedRx/Carbonate.html


Cheers,

Robin