Prominence of ichnologically influenced macroporosity in the karst Biscayne aquifer: stratiform “super-K” zones


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January 2009


A combination of cyclostratigraphic, ichnologic, and borehole geophysical analyses of continuous core holes; tracer-test analyses; and lattice Boltzmann flow simulations was used to quantify biogenic macroporosity and permeability of the Biscayne aquifer, southeastern Florida. Biogenic macroporosity largely manifests as: (1) ichnogenic macroporosity primarily related to postdepositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems (Ophiomorpha); and (2) biomoldic macroporosity originating from dissolution of fossil hard parts, principally mollusk shells. Ophiomorpha-dominated ichnofabric provides the greatest contribution to hydrologic characteristics in the Biscayne aquifer in a 345 km (super 2) study area. Stratiform tabular-shaped units of thalassinidean-associated macroporosity are commonly confined to the lower part of upward-shallowing high-frequency cycles, throughout aggradational cycles, and, in one case, they stack vertically within the lower part of a high-frequency cycle set. Broad continuity of many of the macroporous units concentrates groundwater flow in extremely permeable passageways, thus making the aquifer vulnerable to long-distance transport of contaminants. Ichnogenic macroporosity represents an alternative pathway for concentrated groundwater flow that differs considerably from standard karst flow-system paradigms, which describe groundwater movement through fractures and cavernous dissolution features. Permeabilities were calculated using lattice Boltzmann methods (LBMs) applied to computer renderings assembled from X-ray computed tomography scans of various biogenic macroporous limestone samples. The highest simulated LBM permeabilities were about five orders of magnitude greater than standard laboratory measurements using air-permeability methods, which are limited in their application to extremely permeable macroporous rock samples. Based on their close conformance to analytical solutions for pipe flow, LBMs offer a new means


Applications, Aquifer, S Arthropoda, Atlantic Coastal Plain, Biogenic Processes, Biscayne Aquifer, Broward County Florida, Burrows, Carbonate Rocks, Computed Tomography Data, Cores, Crustacea, Cyclostratigraphy, Data Processing, Digital Simulation, Florida, Ground Water, Hydraulic Conductivity, Hydrostratigraphy, Ichnofossils, Invertebrata, Karst, Karst Hydrology, Lattice Boltzmann Method, Malacostraca, Mandibulata, Miami-Dade County Florida, Mollusca, Ophiomorpha, Permeability, Petroleum, Porosity, Reservoi

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Geological Society of America Bulletin, Vol. 121, no. 1-2 (2009-01).