Changing Paradigms in Carbonate Diagenesis

Goldstein, Robert H.¹, Jennifer A. Roberts¹, Luis A. Gonzalez¹ (1) University of Kansas, Lawrence, KS

New approaches to studies of the ancient, studies of modern hydrologic systems, experimental work, and quantification of biological processes are revolutionizing conceptual models for carbonate diagenesis. Common approaches for predicting diagenetic alteration focus on modern analogs and low-temperature fluids, where fluid flux and meteoric-, marine-, evaporated-marine-, or mixed-fluids are regarded to be of primary importance. Although of value, this approach has proven too simplistic, leading workers to over-emphasize the importance of meteoric diagenesis in controlling porosity distribution. Studies show, however, that much dissolutional pore space, previously thought to form in low-temperature meteoric water, forms in other settings. Seawater can no longer be viewed as a constant either, with different diagenetic products depending on age and setting. Mixing ratios between marine and fresh waters do not appear to exert the primary control on diagenetic product, and mixed systems yield various cement mineralogies or dissolution. Evaporation of seawater provides a chemical and hydrologic drive lacking in many simple marine systems, and low-temperature evaporated fluids are commonly responsible for diagenetic products ascribed to other processes. Alternatively, processes and products of low-temperature diagenesis may be tied to microbial activity, position of the water table, fluid drive, and other factors controlling saturation state.

Finally, early work in high-temperature carbonate diagenesis focused on burial conditions, but evidence of fluid flow is apparent, some reducing and some enhancing porosity. The criteria for identifying injection of warm fluids into cooler rocks (hydrothermal systems) is still developing, and once these systems are reliably identified in ancient systems, there will be more success in predicting distribution of hydrothermal enhancement of porosity from tectonic and hydrologic setting.