Midway and Kure islands are the world's northernmost atolls but
have flourishing algal-coral reefs with typical coral reef structures. An almost
circular outer reef and a broad, shallow (< 5 meters deep), sediment-built lagoon
terrace surround the deeper parts of each lagoon (maximum depths: Midway,
21 meters; Kure, 14 meters). Unconsolidated carbonate sand and gravel form islands
along the southern margins of the atolls. Patch reefs form a series of intersecting
ridges partly covered by sediment. Emergent parts of older presumed reef rock,
built primarily of coralline algae, extend about 1 meter above sea level; they are
especially well developed on Midway and are present but less conspicuous on Kure.
Sediment grain size decreases lagoonward. Carbonate -gravel and coarse sands
predominate on the reef flats, on the seaward sides of the islands, and on the lagoon
terrace. Fine carbonate sands and silts cover the deeper parts of the lagoon bottom.
Major sediment-contributing organisms are (in order of abundance): coralline
algae, corals, foraminifers, and mollusks. Halimeda is nowhere a major constituent.
Most sediment grains deposited in the lagoons are reef-derived, primarily from the
windward reefs on the northeastern margin. There is no distinctive lagoonal sediment
facies
