Drilling operations on Plantagenet Bank, southwest of the Bermuda Islands, penetrated approximately 20 m of carbonate sediment and dolomite without reaching the substrate. To a depth of about 13 m the subsurface consists of fragments of calcareous algae, foraminifera, mollusks, and coral (listed in order of abundance). Carbonate fragments from 8 and 13 m were diagenetically altered and may be partially consolidated. Poorly ordered and Ca-rich (57 mole per cent CaCO3) dolomite, containing recognizable fragments of coralline algae, was found at 20 m.
The C13 contents of the carbonates are within the range of values observed in the sediments and calcareous skeletal fragments of marine organisms from the Bermuda Islands. The undolomitized sediment is enriched in O18 with respect to the modern sediments near the Bermuda Islands; the dolomite is enriched in O18 by 3 to 3.7 per mil with respect to associated carbonate minerals in the cuttings. These differences in O18 content may be the result of reaction of the carbonate sediment with waters enriched in O18.