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Ooid Grainstone

Ooids often exhibit a radial structure as well as concentric laminations. A good examplc is shown in 123. Nuclei of the ooids are micritic grains and many individ-ual smali rounded micritic grains (peloids) are also present, particularly at the top of the figurę. Micrite is so fine-grained that individual crystals cannot be distin-guished and it appears very dark brownish-grey in thin section. An unusual feature of this sedimcnt is the presencc of broken ooids (c.g. left hand side of figurę). The sediment is grain-supported and is cemented by sparite.

Ooid Packstone

Ooids are often not well-preserved and 124 shows a limestone in which ooids are micrite with faint concentric laminations. The sediment is grain-supported and mostly cemented by sparite. However, carbonate mud matrix is present, seen espe-cially in the right hand part of the figurę. This makcs it a packstone rathcr than a grainstone according to the classification on page 113.

Ooid Grainstone, Ooid Packstone

123 Ooid grainstone in plane-polari/ed light. Locality: Upper Jurassic, Provence, France (x 19).

124 Ooid packstone in plane-polari/ed light. Locality: Middle Jurassic, England (x 20).

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