ROCK-DOMINATED DIAGENESIS OF LACUSTRINE MAGNESIAN CALCITE MICRITE

Textural and compositional attributes of dolomite and calcite within L ower Cretaceous Peterson Limestone micrite indicate that these phases coprecipitated during stabilization of precursor magnesian calcite in a largely closed diagenetic system. Uniform difference in the isotopic compositions of contained carbonate phases over a range of whole rock compositions reflect equilibrium partitioning of O-18 and C-13 betwee n dolomite and calcite. Diagenetic dolomite is enriched in O-18 by 4.4 o/oo and in C-13 by 1.8 o/oo relative to co-existing calcite, values which are discernibly different from expected open system mixing trend s, and which are in general agreement with high temperature fractionat ion data extrapolated to sedimentary temperatures as well as with calc ulated deltaO-18 dolomite-calcite from Holocene dolomites.