Calcified algal filaments are a prominent and probably diagnostic feature of the extensive syngenetic submarine cementation in recent Bermudan reefs. The filaments occur within millimeter- to decimeter-size cavities of the reef frame to depths of at least 12 m; they range from 20 to 300µ in diameter and are up to 5 mm long. The filaments are worm-like threads with lateral branching and commonly form an irregular meshwork which lines or fills the cavities; the meshwork commonly incorporates fine sediment that is in turn cemented.

SEM photographs show that the original filaments are enveloped by crusts, 5-150µ thick, of bladed to fibrous, outward-growing crystals that bear rhombs at their terminations. The internal casts of the filaments are inward-growing equant and bladed crystals. X-ray diffraction analyses indicate that the crystals are magnesium calcite with 16 mol % MgCO₃.

The filaments remaining after decalcification are Ostreobium sp., a widely distributed boring green algae characterized by variations in diameter of the filaments from 1-20µ, by a lack of transverse partitions, and by local swellings of the filaments. Individual filaments show both the boring habit and incrustation with magnesium calcite.

Whether the outward-growing crust around the filaments is the result of vital activities of the alga, photosynthesis, or boring, and whether the filaments provide an active organic matrix or a passive substratum to localize calcification have not been determined yet. However, it is clear that this type of cementation is submarine and early diagenetic; hence, similar calcified filaments in ancient reefs provide a criterion for and measure of early diagenesis.

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