Historical Geology/Ooids and oolite

Ooids from a beach on Joulter's Cay, Bahamas.

In this article we will discuss the formation of ooids, and how to recognize oolitic limestone in the geological record.

Ooids and oolite

An ooid consists of a nucleus (a fragment of shell, a grain of sand, or whatever) around which layers of minerals are deposited to form roughly spherical grains.

These grains are typically between 0.25mm and 2mm in diameter; in fact, some authors use a different term for ooids of different sizes, but in this article we shall use the word "ooid" as a catch-all term.

Although there are a number of minerals which can form ooids, in this article we are interested in ooids formed from calcium carbonate, and from now on we shall confine our discussion to them. Such ooids are typically formed in water rich in calcium carbonate (for obvious reasons) and for preference warm shallow water agitated by waves.

Today ooids are to be found in a number of locations with warm shallow water, including the Bahamas, Shark Bay in Australia, and the Persian Gulf, all of which are marine sites; but they are also sometimes found in inland waters such as the Great Salt Lake in Utah.

Oolite, somewhat magnified.

Like other small grains, ooids can be cemented together to form a kind of rock. Rock formed from Calcium carbonate ooids is, by definition, a form of limestone, and is known as oolitic limestone or oolite. The term oolith may also be used as a term either for the rock or for an individual ooid.

Oolite: how do we know?

It is very easy to distinguish limestone formed from ooids. Magnified, it looks like it is made of ooids, as shown in the top picture to the right. (The limestone in this photograph and the one below is from the Carmel Formation in southern Utah.)

Photomicrograph of the grains composing oolite, in cross-section.

This resemblance might be fortuitous, but any such conjecture is laid to rest by looking through a microscope at a cross-section of oolite, as shown in the lower picture to the right.

Not only are the grains the right size and shape to be oolites, but in cross-section one can see the nuclei around which they formed and the growth patterns typical of oolites. Plainly, then, what we are looking at is ooids cemented together to form oolite.