carbonate in calcite. High-magnesium calcite is defined as containing 

 greater than four mol percent MgCO in calcite. The methods of Chave 



(1954) and Lowenstam 

 and calcite 



(1954) determined the percentages of aragonite 



The submerged beachrock at the two stillstand levels are extremely 

 well cemented with a tan carbonate cement. The cement, which consists 

 of macrocrystalline high-magnesium calcite and aragonite, contains 

 coral fragments, coralline algae, Foraminifera, sponge spicules, 

 fragments of gastropod and pelecypod shells and scattered carbonate 

 and noncarbonate grains. Marine organisms have intensely bored the 

 outer part of the cement. 



Cement from beachrock on the present shoreline occurs mineralogically 

 either as aragonite or, as in two samples, as a mixture of aragonite 

 and high-magnesiiom calcite. In the contemporary beachrock, most of 

 the clasts contain a thin 50-100 micron rim of acicular calcite 

 crystals oriented perpendicular to the clast surface. The space 

 between the clasts with acicular rims may be open, or may be filled 

 with a microcrystalline cement. Marine organisms have also intensely 

 bored the present day beachrock. Mineralogically, the cements of the 

 submerged and recent beachrock consist of unstable forms of calcite 

 that will revert with time to calcite and low-magnesium calcite. 



Cement Aragonite 



wt % 



Present beach 30-100 



24' 8-50 



43' 13-46 



All values + 10% 



The cements in the submerged beachrock are similar in mineralogy and 

 MgCO content, but are reduced in aragonite as compared with the 

 contemporary beachrock. The aragonite reduction may be due to marine 

 organisms removing the initial aragonite cement by repeated boring; 

 filling of the borings with fecal pellets and clastic debris, followed 

 by lithification and precipitation of high-Mg calcite as described by 

 Alexandersson (1969) . All of these processes may have reduced the 

 aragonite content. No evidence of biogenetic structures (such as 

 algal layers) or recrystallization was observed in the submerged 

 beachrock cement. 



AGE OF BEACHROCK 



Radio-carbon age determinations have been made by the U. S. Geological 

 Survey on two samples of carbonate cement from the 24-foot beachrock 



VI-78 



