Pore class I commonly includes the lower part of many of the upward-shallowing cycles within the Fort Thompson Formation and upper aggradational subtidal cycle of the Miami Limestone, where the porosity and permeability are highest (Fig. 3 and Fig. 6; Table 3). Characteristic lithofacies associated with pore class I are (1) touching-vug pelecypod rudstone and floatstone, (2) sandy touching-vug pelecypod rudstone and floatstone, (3) peloidal packstone and grainstone, (4) coral framestone, and (5) laminated peloid packstone and grainstone lithofacies (Table 3). Pore types commonly associated with specific lithofacies include solution-enlarged fossil molds up to pebble size, irregular vugs of uncertain origin, and molds of burrows or roots, or irregular vugs surrounding casts of burrows or roots (Fig. 7). Touching-vugs are the most common type of effective porosity in this class, but conduit porosity also occurs as bedding-plane vugs and uncommon cavernous vugs (Cunningham et al., 2006). Atabular three-dimensional geometry regionally characterizes the touching-vug flow zones, which are constrained between cycle boundaries, based on porous-zone mapping in the Lake Belt area (e.g., Cunningham et al., 2004b, 2004c, 2006). Therefore, accurate cycle correlation can produce a realistic linkage of permeable or preferential groundwater flow zones. Groundwater flow in the touching-vug flow zones should not be conceptually viewed as the movement of groundwater through a system of large-scale pipes or underground stream conduits, but more of a stratiform passage formed by coalescence of vugs into a mostly tortuous path for the movement of groundwater flow from vug to vug (Fig. 3). Figure 6 best exemplifies the stratiform distribution of pore class I, notably in digital optical borehole images where the darkened area at the base of high-frequency cycle HFC2e2 represents touching-vug porosity (Fig. 7). Cunningham et al. (2004b, 2006) showed that pore class I has the highest porosity and permeability of the three pore classes defined herein.

Figure 7. Digital image of a borehole wall that spans a highly porous and permeable stratiform groundwater flow zone at the base of highfrequency cycle HFC2e2 in injection well G-3816 (Fig. 4 and Fig. 6). Note intraburrow and interburrow porosity at arrows. The dashed line marks the boundary and flooding surface that separates high-frequency cycles HFC2d and HFC2e2. [larger image]

Typically assigned to pore class II are the (1) skeletal packstone and grainstone, (2) sandy skeletal packstone and grainstone, (3) pelecypod floatstone and rudstone, (4) sandy pelecypod floatstone and rudstone, and (5) skeletal quartz sandstone lithofacies (Table 3). The first four lithofacies listed commonly occur in the upper part of upward-shallowing subtidal cycles and the middle part of the upward-shallowing paralic cycles (Fig. 3), and the last lithofacies is uncommon. Interparticle and separate-vug porosity characterize these lithofacies, which yield groundwater movement through vug-to-matrix-to-vug connections (Lucia, 1999). Diffuse-carbonate groundwater flow (cf. Shuster and White, 1971; Thrailkill, 1976) characterizes movement of groundwater in areas of the Biscayne aquifer characterized by pore class II (Fig. 3).

Usually assigned to pore class III are (1) mudstone and wackestone, (2) Planorbella floatstone and rudstone, (3) peloid wackestone and packstone, (4) conglomerate, and (5) pedogenic limestone lithofacies (Table 3). The first two lithofacies commonly cap upward-shallowing paralic cycles, and the third and fourth are representative of the lower aggradational subtidal cycle of the Miami Limestone (Fig. 3). Porosity types common to this pore class include thin, semivertical solution pipes, and fossil molds. The matrix porosity and permeability of these lithofacies are low (Table 3), and the solution pipes (small-scale conduits) and fossil molds are typically unconnected. Thus, these lithofacies tend to retard groundwater movement and are conceptualized as leaky, low-permeability units (Fig. 3). On a local scale, however, pore class III can comprise bedding-plane vugs, which may have sheet-like geometry and could represent major conduits that are highly permeable.