Ooids: A review
Abstract
Ooids are spherical or subspherical carbonate grains characterized by an internal concentric structure and average dimensions ranging mainly from 0.25 to 1.00 mm. In Recent, unaltered ooids it is possible to distinguish a variable nucleus (mud aggregate, bioclastic grains, quartz grains, etc.) and an external cortex formed by concentric envelopes of variable thickness and number. Chemically ooids are formed almost completely of CaCO 3 and may be distinguished as (a) concentric ooids with cortical envelopes in which the individual crystals are randomly or tangentially arranged, and (b) radialfibrous ooids with cortical envelopes in which the individual crystals have a radial arrangement. While the former ooids always appear to be aragonitic, the latter may be formed by aragonite as well as by high-Mg calcite. These oolitic structures were formerly considered in general as the typical products of shoal environments, but in recent years a more complex view has developed and different environmental conditions are envisaged for different ooids. Ooids are known in the geological past from the Precambrian onward, but they are now always calcitic. Generally they show a complex structure where concentric and radial structures coexist. This combination has for a long time been interpreted as a result of diagenetic transformation of oolitic grains with original tangential substructure. Recent works tend to recognize close analogies between ancient radial-fibrous ooids and Recent unaltered structures with similar crystalline arrangement.
- Publication:
-
Earth Science Reviews
- Pub Date:
- 1980
- DOI:
- 10.1016/0012-8252(80)90053-7
- Bibcode:
- 1980ESRv...16..319S