RADIAXIAL FIBROUS CALCITES AS LOW-MAGNESIAN CALCITE CEMENT PRECIPITATED IN A MARINE-METEORIC MIXING ZONE

Abstract

Radiaxial fibrous calcite (RFC) has previously been interpreted as a marine or replacive cement. Study of the Dongjeom Formation (Early Ordovician), Korea, shows that RFC can form in marine-meteoric mixing zones as a low-magnesian calcite (LMC) cement. RFC in the shallow-marine Dongjeom Formation occurs in arenaceous limestones at the top of a transgressive facies overlying a regressive facies. It shows well-developed growth zonation, and lighter oxygen isotope values and more radiogenic strontium isotope ratios than those of Early Ordovician marine calcite. Such petrographic and chemical evidence indicates that the RFC was precipitated as a primary LMC cement in a marine and meteoric mixing zone. Owing to the unique environment of formation, the Dongjeom RFC is characterized by growth zonal fabric comprising alternating subzones, which may indicate precipitation from varying fluids. In addition, this study documents the importance of substrate for development of RFC. Early 'nucleation' for RFC occurred mainly on microcrystalline skeletal grains and internal sediments, whereas on homogeneously altered substrates, thin-coated banding structure developed, ultimately forming coarse crystalline spar. This suggests that microcrystalline substrates are preferred sites for nucleation of RFC.