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COLIN 



Fig. 10 Upper: Aerial view of the windward reef in the area of the Odum and Odum (1955) transect north of Japtan Island. The 

 ocean is to the right and the lagoon to the left. The reef flat and associated reefs lie in the middle of the photograph. The photo- 

 graph was taken while flying over the island of Japtan: Chinimi Island is visible with Ananij Island behind It. A normid surf is 

 breaking on the windward reef with the lagoon margin very calm. Lower left: View of the zone of small coral heads on the Odum 

 and Odum transect, depth approximately 1 m. Lower right: Junction of large coral head zone with the sand shingle zone of the 

 Odum and Odum transect. 



water circulation patterns. Johannes and Gerber (1974) 

 illustrated a simplified cross section of reef near the tran- 

 sect of Odum and Odum (1955). 



In the Odum and Odum (1955) study area, the bottom 

 slopes gradually lagoonward from the encrusting zone. Indi- 

 vidual coral colonies grow upward to a level limited by low 

 water. In some corals the central portion of the colonies 

 are dead with the sides continuing to thrive, producing 

 structures known as "microatolls" (Fig. 12). These have 

 been examined further on Enewetak reefs by Highsmith 

 (1979) and will be commented on later. Often a distinct 

 lagoonward edge to the reef flat pavement exists, and in 

 many places, water flowing across the reef flat has eroded 

 away and undercut the sediment beneath this lagoonward 

 edge (Fig. 12). This has caused the reef flat pavement to 

 collapse or buckle in places. This is most evident in areas 

 where reef flat rips pass the edge of the pavement. The 

 swift currents combined with the effects of dropping off 

 the pavement have scoured deep potholes (as deep as 4 to 

 5 m) down into the sediments. The pavement is usually 

 undercut on these edges. 



The shallow reefs of the northern lagoon are p>oorly 

 known. From Engebi west to Bokoluo, the reef Is broad, as 

 much as 1 to 1.5 km across, unlike southern reefs. Its 



zonatlon can be seen In aerial photographs but has not 

 been investigated In detail. There Is a reef flat about 100 

 m wide, then a broad (to 1 km) shallow area with coral 

 heads. This coral head area on the west side of Engebi 

 was examined. There were large microatolls of Pontes 

 lutea and acroporld corals on a level sandy bottom. 



To the west of Bokoluo lies the open expanse of the 

 northwest reef tract. It runs fairly straight to the northwest 

 corner of the atoll at the West Spit. The gentle arc of the 

 northwest reef is about 1.5 to 1.7 km across from the 

 ocean to the deepening lagoon. From aerial photographs 

 there appear to be four major zones: (1) a reef flat, (2) a 

 coral head zone, (3) a clustered coral head zone, and (4) a 

 patch reef zone. The reef flat Is estimated to be about 

 150 m across, merging with a deeper coral head zone 

 toward the lagoon. The coral head zone appears about 

 800 m across and Is complex, with three visible com- 

 ponents to It. The middle one-third of the coral head zone 

 appears deepest, whereas the lagoonward one-third 

 appears shallow. The density of coral heads in this area Is 

 high. Density data from photographs Indicate there are at 

 least 500,000 coral heads in this "coral head zone" 

 between Bokoluo and the West Spit. There is scarcely any 

 open sand of more than a few tens of meters between any 



