Changing
Paradigms in Carbonate Diagenesis
Goldstein, Robert H.1,
Jennifer A. Roberts1, Luis A. Gonzalez1 (1)
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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California