Prominence of Ichnologically-Influenced Macroporosity In the Karst Biscayne Aquifer, Southeastern Florida: Stratiform “Super-K” Zones

A combination of cyclostratigraphic, ichnologic, and borehole geophysical analyses of coreholes; tracer-test analyses; and lattice-Boltzmann methods (LBMs) were used to characterize and quantify biogenic macroporosity and permeability of the Biscayne aquifer. Biogenic macroporosity largely manifests as: (1) ichnogenic macroporosity primarily related to post-depositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems (Ophiomorpha); and (2) biomoldic macroporosity originating by dissolution of fossil hard parts, principally mollusk shells. Ophiomorpha-dominated ichnofabric contributes most to shape the hydrologic characteristics of the Biscayne aquifer in a 345-km² 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 stack vertically within the lower part of a high-frequency cycle set. Broad continuity of many macroporous units concentrates groundwater flow in extremely permeable passageways. Ichnogenic macroporosity represents an alternative pathway for concentrated groundwater flow that differs considerably from standard karst flow-system paradigms that describe groundwater movement through fractures and cavernous dissolution features.

Permeabilities were calculated using LBMs applied to computer renderings assembled from X-ray computer tomography scans of various biogenic macroporous limestone samples. The highest simulated LBM permeabilities are 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 of obtaining accurate permeability values for such materials.

We suggest that the stratiform ichnogenic groundwater flow zones have permeabilities even more extreme (~2-5 orders of magnitude higher) than the Jurassic “super-K” zones of the giant Ghawar oil field. The flow zones of the Pleistocene Biscayne aquifer provide examples of ichnogenic macroporosity for comparative analysis of its origin and evolution in other carbonate aquifers, as well as petroleum reservoirs.