Drilling on Eniwetok Atoll, Marshall Islands, revealed the presence of olivine basalt beneath shallow-water limestone of Eocene age at a depth of 4,154 feet.
Two holes were put down on opposite sides of the atoll: F-1 on the northwest where a guyot (flat-topped seamount) adjoins the atoll at 700 fathoms, and E-1 on the southeast where no guyot is present. In F-1, hard basement rock was struck at 4,610 feet, but no sample recovered. In hole E-1, unweathered basalt cuttings were obtained from a depth of 4,154 feet and solid basalt core from 4,208 to 4,222 feet.
Each hole penetrated several hundred feet of soft, Quaternary reef limestone before entering a thick, Tertiary section of similar rocks. The Tertiary rocks are mostly limestones with minor amounts of dolomite and dolomitic limestone. Some of the limestones are made up of clay-size and silt-size particles and are carbonaceous. Most of the section is soft or weakly consolidated. The two holes are similar to depths of 1,400 feet; below this point there are striking differences in lithologic characters, organic constitution, and the distribution of hard rock and cavities.
The drilling data indicate that the atoll is a thick cap of limestone resting on the summit of a volcano that rises 2 miles above the floor of the ocean.