ABSTRACT

Extensive carbonate cementation, as beach rock, is taking place in the intertidal zone of Grand Cayman. Dominant cement fabrics and mineralogies are aragonite acicular crust, clear, bladed-equant magnesian calcite crust, and magnesian calcite micrite crust. The aragonite acicular crusts and clear, bladed-equant magnesian calcite crusts were formed in intergranular pore spaces by either direct physical precipitation or secondarily as a byproduct of biologic activity. The ultrastructure of magnesian calcite micrite crusts, however, show some evidence of direct biologic participation in the cementing process. The present beach interstitial water system is mixed meteoric-marine with high mSr/Ca and low mMg/Ca. There is no evidence that the observed beach rock cement chemistry is a result f late diagenetic changes imposed by the present hydrologic system on an earlier cement. Rather, the cement chemistry reflects to a remarkable degree the chemical variations found in the present beach interstitial water system. This strongly suggests that beach rock cementation has in the past, and is currently taking place under intertidal mixed meteoric-marine conditions.