ABSTRACT

Holocene algal cup reefs in Bermuda, locally called "boilers," are presently being cemented by fibrous and fine-grained "mud textured" magnesium calcite to produce a rock with low porosity and permeability. As much as 50 percent of the resulting rock consists of cement and internal sediment. Internal composition varies from pelleted ostracod- and foraminifer-rich sediment to oolites and pisolites. Such sediments occur in vugs measuring from a few mm to caves up to more than 1 m across. These submarine caves are formed by overgrowth of sheet-like layers of coralline algae and the hydrozoan Millepora sp. (constructional formation), and by organic enlargement of cave walls due to boring organisms (destructional formation). Such caves could easily be mistaken for subaerially formed caves, and the internal oolites and pisolites might be mistaken for fresh-water cave pearls.

Syncementation fractures up to 1.5 m in width cross-cut one of the "cup' reefs. Fractures are believed to result from instability due to organic growth which commonly produces overhanging ledges along the margins of these reefs.