Upper Cretaceous shallow-marine sediments from Vega Island, Antarctica, contain five major authigenic phases; glauconite, pyrite, a zeolite mineral of the clinoptilolite-heulandite group, chlorite, and calcite. The framework sediment composition changes from quartzose at the base of the measured succession to volcaniclastic at the top. The petrogenesis of individual samples reflects the local controls on diagenesis of depositional environment and sediment composition, combined with the effects of burial to no more than 1 km. Calcite cements are the most abundant precipitates. Marine carbonate cements include acicular and other fringing cements that commonly are present within bioclasts. Early-burial micritic to sparry calcite cements include both concretionary and nonconcretionary forms. Burial calcites occlude residual porosity, replace detrital grains and form veins with fibrous and cone-in-cone textures. The stable-isotope composition of the carbonate cements is very variable, with delta 18 O ranging from 0.60 per thousand to - 19.93 per thousand PDB and delta 13 C of - 1.33 per thousand to - 28.09 per thousand PDB. The stable-isotope data reflect the initial conditions of mineral precipitation in oxic and anoxic marine pore waters, together with the effects of subsequent fluid/rock interaction through both recrystallization and cementation. The latest precipitates, thought to have been formed during over pressuring, define a vertical field for burial calcite on an isotope cross plot, suggesting that late fluids responsible for cementation and alteration of earlier precipitates had negative delta 18 O and contained carbon with variable delta 13 C. The oxygen values are compatible with either influx of high-latitude meteoric water or intense fluid-rock interaction with reactive volcanic detritus, or a combination of the two processes. The very variable carbon signatures probably reflect dissolution of bioclasts and earlier diagenetic precipitates. Only by identifying possible end-member compositions for both early and late diagenetic precipitates can most of the isotopic data be interpreted correctly.