Oolitic limestones are whitish to cream-colored limestones composed of sand-sized (1/16 to 2 mm in size), well rounded, concentrically-layered calcite or aragonite grains called oolites (a.ka. ooliths and ooids). Oolites form by rolling back and forth on a shallow seafloor, or sometimes on a shallow lakebed, by wave action. Oolites are forming today on the Bahamas Platform and in Great Salt Lake, Utah. The technical geologic term for most oolitic limestones is “oolitic grainstone”.
Uncertainty exists about the specifics of the origin of oolites. Some researchers conclude that oolites form by completely inorganic chemical precipitation of CaCO3 from seawater around some nucleus (a tiny shell or skeletal fragment or sediment grain). Other researchers conclude that the presence of bacterial films on oolite grain surfaces play a significant role in the precipitation of CaCO3 layers. However, the undoubted presence of bacteria does not necessarily indicate a biogenic origin for oolites - bacteria are everywhere.