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 MgCO

3

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.