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One of the objectives of the 1950 expedition to Bikini carried on by the Scripps Institution of Oceanography and the U. S. Navy Electronics Laboratory was to investigate previously located flat-topped seamounts (guyots of Hess, 1946) in an area 600–1100 miles west of Hawaii. Five of these seamounts were surveyed by echo sounder, dredged, and cored, and were found to be peaks on a great submarine range—the Mid-Pacific Mountains—which extends from Necker Island in the Hawaiian Islands to near Wake Island.

The flat-topped guyots are submerged to between 700 and 900 fathoms. The sides are concave upward with slopes about 20° near the tops. The profiles are symmetrical with breaks in slope at 720–1150 fathoms. Rounded sand grains, pebbles, cobbles, and boulders of olivine basalt were dredged from the tops and across the breaks in slope. Sandstone and reef coral were dredged together at one station. Dredge hauls across the breaks in slope on the tops of two guyots brought up an integrated Cretaceous (Aptian-Cenomanian) fauna of reef coral, rudistids, stromato-poroids, gastropods, pelecypods, and an echinoid; these have affinities with the faunas of the Tethyan Province around the Gulf Coast and adjacent regions. In a core taken in a basin near one guyot, the basaltic gravel layers contained Upper Cretaceous (Campanian-Maestrichtian) Foraminifera. Paleocene and Eocene Foraminifera occur on the flat tops of four guyots.

The evidence indicates that in Cretaceous time the guyots were a chain of basaltic islands. These islands were wave-eroded to relatively flat banks on which a reef coral-rudistid fauna found lodgment and grew into reefs on and among the erosional debris. They never became fully developed atolls. The guyots were submerged during the Cretaceous to below the zone of reef-coral growth; finally they sank to present depth. The submergence is thought to have been due to regional subsidence of the sea bottom resulting for the most part from isostatic adjustments and subcrustal forces. A minor part of the submergence can probably be attributed to increase in ocean volume, sedimentation, and the compaction of soft sediments.

In general the findings support Darwin’s Subsidence Theory for the formation of atolls; they furnish evidence for a deep Cretaceous Pacific Ocean; and they refute the hypothesis of transoceanic “sunken continents” used to explain faunal migrations, at the same time suggesting the possibility of “island stepping stones”.

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