254 



L. HILLIS-COLINVAUX 



tEEF INFtUENCE - FUNAFUTI ATOll 



(BAT* FKOM HINOC, KOYAL SOCIgTr, 1304) 



n/N«rvri isijino 



LAGOON eoniNC- 



- Approilmotaly Vrt miln - 



R.N& 

 Oeetmbtr, 1957 



100 

 120' 

 140' 

 160' 

 180' 

 200' 

 220' 

 240' 



IS I 



; 'Slight 

 percentage" 



.?^ 



S. X. 



20% Corolt 

 S0% Unconiolldoted 

 skeletal debris 



'"•^"^Flnt Calcite-cemented 

 k^l skeletal debris 



Practically all orogonite gone 

 First coral costs and moulds 



, Calcite - cemented skeletal 

 debris, Corals rore 



Fig. 82. Cross-section of Funafuti Atoll, Ellice Islands, showing some of the results 

 of the cores raised by the Coral Reef Committee of the Royal Society (1904). Halimeda 

 segments were the chief constituent for the first 60 feet below the lagoon floor. 

 They also were the predominant component of the main boring between 652-660 feet, 

 but this depth is not included. (From Ginsburg et al. (1963), original data from 

 Judd (1904) and Hinde (1904); reproduced with permission.) 



extends the data to various forms of peripheral reef, where Halimeda 

 segments can collect in channels, lagoons and behind sills. 



At Funafuti the Halimeda-rich stratum of unconsolidated sediment 

 ended with an unconformity which was almost certainly the subaerial 

 exposure surface which had been weathered during the last eustatic 

 lowering of sea-level. In the lower parts of this boring, under the uncon- 

 formity, only one-third of the recognizable parts were Halimeda, the 

 other two-thirds being foraminiferans (Fig. 82). 



If we accept the Funafuti, Bikini and Enewetak results as correct 

 in suggesting that lagoonal accretions of sediment are likely to be very 

 largely Halimeda segments, then it is reasonable to postulate that the 

 ultimate origin of the mass of an atoll depends on the relative rates of 

 accretion on the lagoon floor and along the reef ridges. We may write : 



A^={H,L + E,R^)t 



where A^ is the atoll mass, ^g is the deposition of Halimeda segments 



