Marine lithification of reef rock and rhodolites at a fore-reef slope locality (-50m) off Bermuda

Abstract

At a depth of 50 meters the fore-reef slope at the northern margin of the Bermuda platform consists of solid reef rock interspersed with sand patches and channels. Coralline algae are the predominant framebuilders, the only common coral being Montastrea cavernosa. On the rippled sand abundant red algal nodules (rhodolites) occur with diameters up to 12 cm. Both reef and rhodolites show various stages of alteration and lithification by repetitive boring, secondary framebuilding, internal sedimentation and cementation. All processes are marine. Aragonite cement occurs only within the in situ deposited primary framework, and is probably a very early, post mortem precipitate. Magnesian calcite is the only common cement on all other substrates. Practically all porosity and permeability is destroyed within approximately 1500 years. Net accretion rates in the rhodolites are in the order of 0.05 mm/year. With local current patterns providing a suitable pumping mechanism, the very slow accretion rates are thought to be the main reason for the pervasive nature of diagenetic alteration on this fore-reef locality. Rhodolites of this type may be useful palaeo- environmental indicators for deep (more than 50 m) tropical waters.