Abstract

The 77 atolls, 35 banks, 21 lagoonless coral islands, 5 reef-encircled volcanic islands, and 2 raised “phosphate” islands in the Gilbert, Marshall, and Caroline groups extend from 132° to 177° East Long, and from 15° North to 3° South Lat. The seamounts are located on three types of submarine trends: The Gilberts, Ralik and Radak chains of the Marshalls, and eastern Carolines lie on gently arcuate trends with each member perched as an individual; the west-central Carolines evidence no regional alignment or arrangement but lie on a broad rise 1000 fathoms high; Yap and Palau are small island arcs. The submarine slopes of the seamounts average from 25° to 35° for the upper 1000 fathoms.

The Gilbert atolls are abnormally shallow and manifest slight regional tilting and excessive variation of reef width from windward to leeward. Aranuka has a bank at lagoon depth that extends several miles beyond the western reef margin.

The Marshall atolls exhibit remarkably uniform reef widths and lagoon depths. Kwajalein has a lagoon area of 663 square miles. Aur is tilted to the south and evidences no detrital filling or leveling; Eniwetok has a current that affects only one fourth the lagoon, yet it is the same depth throughout.

In the Carolines, Zohhoiiyoru, Kapingamarangi, Oroluk, Nomwin, and Murilo have abnormal lagoon floors that condemn detrital leveling; Ngatik, 87 fathoms, and Nukuoro, a “two-story” atoll, in turn condemn detrital filling. In the west-central Carolines none of the banks lie under more than 50 fathoms, and Gray Feather and Mogami, the two largest, are separated by a narrow shallow passage with rim reef growth on each side, evidence incompatible with the sheath-like coral growth that should accompany major subsidence. The reef-encircled volcanic islands have truncated spurs and cliffed shorelines; reef widths and lagoon floors are normal.

The accordance of lagoon and bank depths, the uniformity of reef width, the absence of inboard detrital slopes, pass notches, and outboard deltas, the general absence of coral causeways or piers, the angularity of shape, the demonstrated inefficacy of detrital filling and leveling, and the presence of truncated spurs and cliffed shore lines all indicate seamount truncation during the Pleistocene.

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