Numerous inclined projections resembling stalactites but indicating a marine origin occur on the ceiling of a submarine cave in the Belize barrier-reef platform near Columbus Cay (3 km from the outer edge of the barrier reef and 21 km from the mainland). The accreting "pseudostalactites" consist largely of Vermiliopsis serpulid tubes and varying amounts of magnesium calcite cement, which is present either as a matrix or as a coating on the upper surfaces of the inclined projections. The opening of the cave is 10 m long and less than 3 m wide; it breaches the roof at a depth of 17 m. A short distance inside and up to at least 40 m from the opening (the limit of our observations), the ceiling is covered by a field of closely packed pseudostalactites more than 30 cm thick. A distinct transition was observed from large club-shaped forms (30 cm wide at the point of maximum development), present about 10 m from the cave opening, to pencil-thin projections at the 40-m limit. Characteristically, the pseudostalactites incline toward the cave opening at about 40 to 60° near the opening and are almost horizontal at the limit of observation. The magnesium calcite cement (15 mole% MgCO₃), which commonly constitutes more than half of a pseudostalactite, exhibits the dentate crystals, peloidal textures, and knobby surface relief recognized in submarine cements from other reef areas. These traits and the results of oxygen and carbon-isotope analyses discount any influence of fresh water. Marine planktobacteria associated with the undisturbed surfaces of c ment accumulation suggest that bacteria are an active factor in the precipitation of this submarine cement.

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