Micritic envelopes are commonly believed to develop from the lime-mud infillings of circumferential perforations made in skeletal particles by algae. Because “algae” are usually taken to mean photosynthetic algae, the corollary has been proposed that micritic envelopes would not appear on carbonate skeletal particles originating in waters too deep for active growth of photosynthetic algae. Extensions of this “photosynthetic-algae-only” origin of micritic envelopes implies that: (1) the disappearance of envelopes with depth could mark a useful paleobathymetric datum; and (2) preservation of outlines of skeletal particles should be much better in “shallow”-water carbonates than in “deep”-water carbonates.
We doubt the validity of the depth-limiting concept of the origin of micritic envelopes for two reasons: (1) petrographic study of a large number of thin sections thought to be representative of various paleo-depth levels does not indicate any systematic variations in the preservation of the outlines of skeletal particles; and (2) molluscan skeletal debris displaying micritic envelopes and effects of borings identical with those generally ascribed to the activities of photosynthetic algae have been found in an aragonite-cemented sandstone from the Atlantic shelf where geologic evidence indicates that the water at time of boring was too deep for active growth of photosynthetic algae.
Accordingly, we re-emphasize the fact that organisms other than photosynthetic algae can bore into calcareous materials. In particular, fungi, bacteria, and heterotrophic algae (whose distribution does not depend on the penetration of light into seawater) are known to be active borers. Bacteria can be eliminated as producers of bores having diameters greater than approximately 10 μ. Fungi create boring structures with radiating or branching patterns or bores with nonuniform size. Some fungal bores appear to be indistinguishable from excavations known to have been bored by photosynthetic blue-green algae. At present we do not know how to distinguish the results of boring activity of photosynthetic algae from those of algae living het-erotrophically.