oolitic

Oolitic describes a sedimentary texture or rock dominated by ooids, which are small, roughly spherical to ellipsoidal grains formed by concentric precipitation of calcium carbonate around a nucleus. Ooids typically measure about 0.25 to 2 millimeters in diameter, with many in the 0.25 to 0.5 millimeter range. When ooids are bound together by cement, the rock is called an oolitic limestone or, if the grains are set in a sandy matrix, an oolitic sandstone.

Formation occurs in warm, shallow, well-oxygenated marine environments where seawater is highly agitated and supersaturated with calcium carbonate. In such settings, crystal growth occurs on a nucleus—often a sand grain or shell fragment—creating concentric layers as the grain is rolled by waves or currents. The process requires both sufficient ion concentration and mechanical energy to keep grains in suspension and prevent burial, allowing continual surface growth and preservation of the ooid texture. Diagenesis can modify mineralogy and cementing, producing sparry calcite or microcrystalline cement.

Modern analogs of ooidal deposits are found in tropical and subtropical seas, such as parts of the Bahamas and the Persian Gulf, where shallow, warm waters support extensive ooid formation. Oolitic textures are common in the geological record, spanning from ancient Paleozoic to Cenozoic rocks, and they remain a key indicator of former warm, agitated marine conditions.