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Intraclast Grainstone

Many limestones contain mixtures of different grain typcs. In 127 probable ooids can bc scen (lower lcfl), although conccnlric lamination is noi clearly visible in thc photograph, and bioclasts are present (c.g. shell fragmems on the right hand sidc). However, the dominant grains in the field of view are the intraclasts - composite grains in which individual components were deposited, cementcd together and then reworked. Intraclasts in this rock contain abundant angular quartz grains, the Iow relief of the quartz conlrasting with the high relief of the sparitc cement.

Peloid Grainstone

In 128 many of the grains are madę of structureless micrite and are thus peloids. A few grains such as the shell fragment in the lower right of the figurę are bioclasts with micrite coatings. These are fonned by micritization—a process involv-ing boring of the shell by microbes and infilling of the borings with micrite. The micrite-filled borings can just be madę out at the margin of the shell fragment.

Intraclast Grainstone, Peloid Grainstone

128 Peloid grainstone in plane-polarized light. Locality: Upper .lurassic. Morocco (x 13 ).

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