ECOLOGY AND TAXONOMY OF HttUmeda 257 



Table XXV. Average Skeletal Composition of Sand -size Components of 

 Some Peripheral Reef Sediments 



" Adapted from Milliman (1974) and Orme (1977). Figures are percentage skeletal com- 

 ponents in total sediments. 



growth of coral pinnacles from the lagoon floor. A scheme describing 

 the mutual influences of these accretion processes on the final composi- 

 tion of the atoll mass is given in Fig. 84. 



The mutual dependency among these various processes is clear. 

 Low sea-level will be a time of very low contribution of Halimeda to the 

 atoll mass because of the extinction of the lagoon. It will also be a time 

 of relatively low coral contribution because the pinnacles will be 

 extinguished. In times when there is little eustatic fluctuation the 

 contribution of Halimeda segments into the lagoon may be relatively 

 large, perhaps being the dominant process. But the increase of atoll 

 mass by the Halimeda segments must have an isostatic effect, thus 

 increasing the possibilities for growth of the reef ridge and the 

 pinnacles. 



The history of the atoll fabric may be investigated by drilling, and 

 the fabric revealed by drill cores from any part of the structure must 

 reflect recurrent local changes, as the site may have fluctuated from 

 pinnacle to lagoon and the sea-level may have been high or low. There 

 is likely to be a bias, however, in the record since most borings are from 

 on the reef ridges or near them. And there may be a difficulty in recon- 

 structing the actual origin of the reef fabric being examined due to the 

 changes which have taken place during diagenesis, cementation or 



