William J. Iams

Cup-shaped reefoid structures and associated shoreline features found along the south shore of Bermuda have been studied with particular regard to documentation of form, structure, biota and location. The considerable similarity between these reef forms and biologically produced structures in the Mediterranean (Blanc and Molinier, 1955; Peres and Picard, 1952), on the western coast of Florida (Shier, 1969) and at several other localities is noted.

Cup reefs are bioconstructional mushroom-shaped calcareous formations exhibiting four distinct morphologic sections: i) base; ii) sidewalls (stem); iii) lip; iv) central depression. These structures are formed principally by the constructive efforts of: i) encrusting vermetid gastropods, ii) crustose and nodular coralline algae, iii) an adnate species of foraminifera, and iv) a hydrozoan cnidarian. Accessory contributors are anthozoan cnidaria, barnacles, serpulids and ectoproct polyzoans. Interstices in the rock are often filled by detrital material composed to a great extent of algal fragments. This fill is cemented by a cryptocrystalline, high-magnesian calcite cement. There is a direct correlation between depth below the living surface rock and the degree of infilling and cementation.

Cup reefs are found from very near shore (sometimes attached to Pleistocene rocky headlands) to approximately 200 meters offshore (Southwest Breaker). The cup reefs are, in some cases, well aligned in rows closely paralleling the trend of the islands, that is, northeast to southwest. They may be found in depths from 2 to 30 feet.

The origin of the cup reefs has never been satisfactorily resolved. Early workers attributed them to Pleistocene erosional remnants. Present indications lead the author to suggest a predominantly constructional nature with subsurface Pleistocene rock influencing only initial location (hence, alignment) and perhaps being present as a core material in the basal portion of the cup reef. The form, surface geometry and size of the cup reefs is controlled by the organisms which contribute to the calcareous structure and by prevailing hydrologic and bathymetric conditions.

Keywords:
Pages:
Supervisors: N. Ginsburg