Allochem

Ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah, USA. Largest is 1.0 mm in diameter.

Allochem is a term introduced by Folk^[1] to describe the recognisable 'grains' in carbonate rocks. Any fragment from around ½mm upwards in size may be considered an allochem. Examples would include ooids, peloids, oncolites, pellets, fossil or pre-existing carbonate fragments. Fragments are still termed allochems if they have undergone chemical transformations - for example if an aragonite shell were to dissolve and be later replaced by calcite, the replacement would still be deemed an allochem.^[2]

The allochems are typically embedded in a matrix of micrite (lime mud) or sparry calcite.

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