ABSTRACT

Three fossiliferous concretions of the Fox Hills Formation (Upper Cretaceous) of North Dakota display similar diagenetic histories. Petrographic and geochemical data from each concretion have yielded the following diagenetic sequence: 1) bioturbation by marine benthos; 2) partial phosphatization of glauconite, fecal pellets, and shell material; 3) sulfate reduction and pyrite formation; 4) siderite precipitation; 5) radial-fibrous calcite precipitation, and 6) equant calcite precipitation. Although the occurrence of open-marine faunas and glauconite indicates that concretion sediments were deposited under marine conditions, geochemical data suggest that marine pore fluids were replaced by brackish and meteoric water. As the Fox Hills Formation is part of a regressive, marginal marine equence, it is likely that the transition from marine to freshwater is recorded in concretion paragenesis.

Early diagenetic minerals indicate oxidation of organic matter at shallow burial depths under normal marine conditions. Locally reducing conditions, produced initially by degradation of molluscan soft parts and later by degradation of organic material in crustacean and annelid fecal pellets, occurred at shallow burial depths. Alteration of iron silicates (glauconite) produced pore fluids enriched in Fe²⁺. Due to the abundance of reactive organic matter, subsequent reduction of marine sulfate yielded a small amount of disseminated pyrite. Following the removal of H₂S, elevated Fe²⁺, pH, and bicarbonate concentrations soon exceeded siderite saturation and siderite was precipitated. Elevated Ca and Mg compositions and marine ¹⁸O values of siderite cements indicate that pore fluids were essentially reduced marine fluids. Depleted ¹³C values suggest that siderite precipitated in an environment dominated by the oxidation of organic matter.

Isotopic compositions from the earliest and latest carbonate phases constrain intermediate phases and allow comparison of environments of precipitation. Carbon and oxygen isotopic data from skeletal aragonite provide an estimate of the isotopic composition of marine carbonate. The latest phase of equant calcite cement is characterized by invariant ¹⁸O, coupled with highly variable and light ¹³C. This characteristic isotopic signature and occurrence in overlying fluvial sediments suggests a meteoricphreatic origin for equant spars. Isotopic values of radial-fibrous calcite lie between those of marine (aragonite, siderite) and freshwater (equant calcite) ph ses and represent a complex mixing of marine and meteoric pore fluids. The paragenetic sequence of carbonate cements records a gradual decrease in ambient fluid Mg, Fe, and Mn content during shallow burial. Such decreases are consistent with the influx of oxidizing meteoric fluids.