Our triple-porosity conceptual model of the karst Biscayne aquifer contains a series of vertically stacked interlayered (1) diffuse- carbonate flow zones, (2) touching-vug flow zones, and (3) less common conduit flow zones. Leaky, low-permeability zones are interbedded with some of the flow zones. Both high-permeability and low-permeability zones occur within the context of high-frequency cycles (Fig. 3 and Fig. 4). Visual examination of cores and digital borehole images (Fig. 5 and Fig. 6), quantification of porosity and permeability in cores (Table 3), computed porosity from digital borehole images (Cunningham et al., 2004b), and temperature and flow-meter logs (Fig. 8 ) indicate that permeability of the Biscayne aquifer is heterogeneous. The touching-vug flow in pore class I is mostly constrained to zones of solution-enlarged burrows, interburrow vugs, moldic fossils, root molds, or vugs between root casts that overprint repeating vertical arrangements of lithofacies within stacked high-frequency cycles. Touching-vugs characterize these secondary dissolution features, which coalesce to form tabular-like stratiform zones of vug-to-vug groundwater flow. The size, shape, and spatial distribution of touching-vug porosity within the Biscayne aquifer can be mapped within the context of the high-frequency cyclostratigraphic framework because they commonly occur in the lower part of the high-frequency cycles above flooding surfaces, which facilitates well-to-well interpolation of highly porous zones. These highly porous zones commonly occur at the base of the paralic and subtidal upward-shallowing cycles of the Fort Thompson Formation and throughout the uppermost subtidal aggradational cycle of the Miami Limestone (Fig. 3, Fig. 4, and Fig. 6).

Small-scale interparticle and separate-vug porosity mostly contributes to the diffuse-carbonate groundwater flow (flow through pore class II) in the Biscayne aquifer (cf. Shuster and White, 1971; Thrailkill, 1976; Martin and Screaton, 2001). These two pore types relate to specific lithofacies, and diffusecarbonate flow is the principal type of flow in the middle of ideal paralic upward-shallowing cycles and the upper part of ideal subtidal upward-shallowing cycles of the Fort Thompson Formation (Fig. 3). Cunningham et al. (2004b) showed that in the upper Biscayne aquifer, median values of core-scale air permeabilities from the middle (diffuse) part of paralic upwardshallowing cycles are about one order of magnitude less than the lower part of these cycles, where the pore system is principally touching-vugs. However, the relative difference in median permeability values between diffuse-carbonate and touchingvug flow zones must be even greater because recovery of intact core samples has never been accomplished for the most porous and permeable parts of touching-vug zones at the base of cycles, where the limestone is fragile and always broken up during drilling. Thus, no laboratory measurements exist for core-scale permeabilities from the most permeable part of touching-vug flow zones, precluding comparison of core-scale touching-vug flow-zone permeabilities to known permeability values of core samples from diffuse-carbonate flow zones.

We propose that karstic development of the highly permeable zones at the base of upward-shallowing cycles of the Fort Thompson Formation relates to cyclostratigraphy and Pleistocene sealevel history. Figure 3 shows that the vertical arrangement of lithofacies and pore classes are linked within the context of ideal high-frequency cycles. Formation of secondary porosity was likely produced by meteoric water flowing through the limestone of the Fort Thompson Formation during its emergence into the vadose zone, caused by periodic lowstands in sea level that span Pleistocene glacial maximums (Perkins, 1977). It is possible that these episodic vadose events promoted aggressive dissolution of carbonate grains and depositional textures in the lower part of cycles due to perched, concentrated, downdip flow of meteoric water above flooding surfaces. We hypothesize the focusing of low-gradient, lateral flow of meteoric water above flooding surfaces was due to the presence of relatively low-permeability lithologies at cycle tops, which underlie the flooding surfaces and act as baffles or barriers to downward vertical drainage.