Abstract

Designation of a carbonate rock as "algal" is not informative unless the fundamental distinction is made between carbonates composed wholly or partly of the remains of skeletal calcareous algae, and cryptalgal carbonates, in which the work of noncalcareous algae is largely inferred. Some cryptalgal biolithites are not assignable to any of the established types of stromatolite. The adjective cryptalgalaminate is proposed for planar-laminated carbonate rocks bearing evidence of algal-mat activity, and the term thrombolite is proposed for non-laminated cryptalgal bodies characterized by a clotted fabric. Stromatolitic and cryptalgalaminate carbonate sediments appear to be restricted to the intertidal zone, but thrombolites and oncolites do not. Description of some cryptalgal fragmental limestones and dolomites draws attention to the sediment-forming role o,f the non-calcareous algae, which in view of widespread emphasis on the sediment-binding role, has not been given sufficient attention.

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