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Thin-section of a grainstone with calcitic ooids; Carmel Formation, Middle Jurassic, of southern Utah, USA.
Grainstone (Brassfield Formation near Fairborn, Ohio, USA). Grains are crinoid fragments.

Under the Dunham classification, grainstones are grain-supported carbonate sedimentary rocks that contain no micrite. The spaces between grains are filled with sparry cement.