Interplay of Water-Rock Interaction Efficiency, Unconformities, and Fluid Flow in a Carbonate Aquifer: Floridan Aquifer System
Harris Cander, 1995. "Interplay of Water-Rock Interaction Efficiency, Unconformities, and Fluid Flow in a Carbonate Aquifer: Floridan Aquifer System", Unconformities and Porosity in Carbonate Strata, David A. Budd, Arthur H. Saller, Paul M. Harris
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The middle Eocene Avon Park Formation comprises shallow subtidal skeletal limestones and dolomitized peritidal limestones that underwent several periods of unconformity-related exposure during the Cenozoic. The porous limestones are typical of the Tertiary Floridan aquifer system, which has both high interparticle, matrix porosity and a conduit flow system comprising karst zones, caves, vugs, channels, and bedding planes. Avon Park limestones have retained most primary porosity (ϕ = 20 to 30%) and Eocene marine-like geochemical compositions, despite being exposed to flushing by meteoric groundwater during these long-lived unconformities. The marinelike geochemical compositions indicate low water/rock ratios during mineralogical stabilization to calcite. The most common diagenetic product in the limestones is isopachous bladed calcite cement that precipitated during intraformational unconformities or immediately after deposition. The limestones are in oxygen, carbon, and strontium isotopic disequilibrium with modern Floridan aquifer groundwater (limestone: δ180 = -1.0 to +1.0%o, PDB; δ13C = 0 to +2.0%o; 87Sr/86Sr = 0.70777 ; Sr = 400 ppm; dilute groundwater: δ180 = -0.5%o, SMOW; δ13C = -5 to -14%0; 87Sr/86Sr = 0.7081 to 0.7089). Based on geochemical modeling, quantitative estimates of the number of pore volumes that have reacted with Avon Park limestone compared to the number of pore volumes that have flowed through the rocks indicate that the long-term efficiency of water-rock interaction is less than 0.002%.
In contrast to the matrix limestone, late-stage, conduit-lining coarse calcite cements in the Avon Park are in isotopic and elemental equilibrium with modern Floridan aquifer groundwater, indicating precipitation at extremely high water/rock ratios and interaction efficiency (late calcite: δ180 = -3.3%o, PDB; δ13C = -7.0%o; 87Sr/86Sr = 0.70875; Sr = 15–20 ppm). The radiogenic 87Sr/86Sr composition of these calcite cements indicates that they contain Sr of Middle Miocene age or younger. The contrasting data from the matrix limestones and the conduit-lining cements indicate that the two fluid-flow systems give rise to two different diagenetic systems in the same aquifer. The matrix system is characterized by low efficiency with products precipitated at low water/rock ratios; the conduit system is characterized by high water-rock interaction efficiency and products precipitated at extremely high water/rock ratios. The conduit system has active diagenesis, where large mass transfer of calcium carbonate is occurring and the matrix system is relatively inert. The response of the Avon Park Formation to unconformity-related diagenesis can be interpreted based on the Eocene age of matrix cements, the post-Middle Eocene age of conduit-lining cements, and the timing of long-lived regional unconformities. During periods of subaerial exposure asociated with intraformational and early postdepositional unconformities, the conduit system was poorly developed and the matrix system was the locus of water-rock interaction; the dominant product was intra- and interparticle calcite cement precipitated in near-equilibrium with the host limestone. During the later-stage, long-lived exposure associated with regional unconformities (Late Oligocene, Late Miocene, and throughout the Pliocene-Pleistocene), the conduit fluid-flow system developed and focused both fluid flow and water-rock interaction out of the matrix and into the conduits; the dominant product became coarse cavity-lining calcite cement, precipitated in equilibrium with the groundwater. Today, the conduit system has active diagenesis where large mass transfer of calcium carbonate is occurring and the matrix system is relatively inert.
The history of water-rock interaction in the Avon Park Formation suggests that as diagenesis in carbonate platform limestones evolves, a conduit fluid-flow system may develop in response to meteoric diagenesis during long-lived unconformity-related exposure. In these systems, the conduit porosity system overtakes the matrix porosity system as the locus of diagenesis and carbonate mass transfer. In so doing, the conduit system serves to limit diagenesis in the matrix and preserve matrix porosity. Results of this study indicate that the type of fluid-flow system(s) must be considered, as well as the fluids and rocks, when interpreting carbonate rock-water interaction and porosity modification below unconformities.