Bank barriers are the major structural reef type in the eastern Caribbean, yet their internal structure and formation process are poorly understood. Twenty-six core holes drilled along the northern St. Croix barrier and on a transect combined with a ship-channel section through the southern bank barrier have shown that this is a bar-type structure which consists of a series of patch reefs within a carbonate sand and rubble matrix capped by reef-crest and fore-reef facies. Locally, reef linearity and location are controlled by a 4-m limestone step of Pleistocene age.

These bank reefs apparently were initiated by shelf-current deposition of carbonate traction load in a bar configuration. Scattered corals--Porites porites, Diploria spp., Acropora cervicornis, and dominantly Montastrea annularis--acted to stabilize the bank. Framework-dominated patch reefs developed at scattered locations and depths as the bank grew upward. As the reefs built into shallow water, carbonate production increased, eventually coalescing them into a continuous reef crest.

Core return shows that the structure is characterized by little cementation. Cement occurs in localized lenses within the structure just as it does along the present reef surface. Framework construction is concentrated in patch-reef, reef-crest, and fore-reef facies. This type of construction produces highly permeable structures with extremely high local porosity. Extensively cemented caps occur only in areas having low accumulation rates and in areas of high energy.

Bank-barrier reefs are probably also important in the Indo-Pacific in exposed non-atoll situations, but their occurrence is generally not recognized. Descriptions of Mesozoic and Cenozoic reefs suggest that the bank reef is an extremely important but little understood type in the fossil record also.

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