Abstract:

Recent carbonate sediments were formed about 4,000-5,000 years ago in Shuiweiling and Ximao Island,Yaxian County,which consist simply of Alcyona-cea spicula.All nodes and spines are well preserved on the surfaces of these spi-cula,and no sign of transporting abrasion can be observed.They are just thesame as the alive ones both in shape and structure,but mineralogically they arequite different,making up mainly of low-Mg calcite with MgCO₃ content rang-ing from 1.9 to 4.9 mole%.The cement consists predominantly also of low-Mg calcite,which occurs in fibrous rims,micritized equant grains.These fabricforms and relic composition indicate that the primary mineral of the cement should be high-Mg calcite or arragonite formed in marine diagenetic environment.The isometric-granular calcite might be formed in a zone of underflow of fresh water,judging from its lacking cresecent and pendant forms of petrofabric which are characteristic in seepage zone.This spicula limestone was formed due to decomposition of sof issues of Alcyonacea and their consequent washout.However,the washing out should not occur before the burying because if it did,this might result in the scattering of these spicula,the contamination by impurities and the subjecting to abrasion.The later uplift of Hainan Island probably brought the approximatly cemented Alcyonacea-spicula limestone into an freshwater underflow environment,which made it subjecting further to consolidation and modification.