Rock fabric and texture, faunal constituents, sedimentary structures, and relation to surfaces bounding vertical lithofacies successions were the basis for our definitions of five principal depositional environments for the Pleistocene Fort Thompson Formation and Miami Limestone, the major lithologic components of the Biscayne aquifer. The five depositional environments are (1) platform margin to outer platform, (2) open-marine platform interior, (3) restricted platform interior, (4) brackish platform interior, and (5) freshwater terrestrial environments. Vertical lithofacies successions, which have stacking patterns that reoccur, fit within high-frequency cycles. Upward-shallowing subtidal cycles, upward-shallowing paralic cycles, and aggradational subtidal cycles define three types of ideal high-frequency cycles. Fundamental to the identification of an upward-shallowing paralic cycle is a capping micrite-rich carbonate lithology indicative of deposition in a paralic transitional realm between subaqueous brackish and terrestrial environments. Grouping of high-frequency cycles based on vertical cycle patterns produced two cycle sets, one progradational (Fort Thompson Formation) and another aggradational (Miami Limestone).

There is a predictable vertical pattern of porosity and permeability within the three ideal cycles, because the distribution of porosity and permeability relates directly to lithofacies. Fifteen major lithofacies of the Fort Thompson and Miami Limestone have been assigned to one of three pore classes (I, II, and III), as shown in Table 3. Pore class I commonly includes the lower part of upward-shallowing cycles within the Fort Thompson Formation and an upper aggradational cycle of the Miami Limestone. Vug-to-vug groundwater flow is most typical of pore class I. Conceptualization of the touching-vug flow is movement of groundwater through a stratiform passage formed by coalescence of vugs into a mostly tortuous path. Less common in pore class I is conduit groundwater flow through beddingplane vugs and cavernous conduits. Pore class II commonly occurs in the upper part of the upward-shallowing subtidal cycles and middle part of the upward-shallowing paralic cycles. It is principally composed of interparticle and separate-vug porosity and characterized by diffuse-carbonate groundwater flow through vug-to-matrix-to-vug connections. Micrite-dominated lithologies distinguish pore class III, which commonly caps upward-shallowing paralic cycles and occurs throughout much of a lower aggradational cycle of the Miami Limestone. These lithologies tend to retard groundwater movement and are conceptualized as leaky, low-permeability units.

Zones of stratiform, high (but variable) permeability occur within many individual cycles and comprise preferential groundwater flow zones. Highly permeable zones commonly occur just above flooding surfaces in the lower part of upward-shallowing subtidal and paralic cycles. Aggradational subtidal cycles are either mostly high-permeability zones or leaky, low-permeability units. In the study area, groundwater flow within high-permeability zones is through a secondary pore system of stratiform touching-vug porosity principally related to molds of burrows and pelecypods, and to interburrow vugs. Movement of a dye-tracer pulse observed using a borehole fluid-temperature tool during a conservative tracer test indicates heterogeneous permeability. Advective movement of the tracer appears to have been most concentrated within a thin stratiform flow zone contained within the lower part of a high-frequency cycle, indicating a distinctly high relative permeability for this zone. Borehole flow-meter measurements corroborate the relatively high permeability of the flow zone. Identification and mapping of such high-permeability flow zones are crucial to conceptualization of karst groundwater flow within a cyclostratigraphic framework.

The cyclostratigraphic approach taken herein demonstrates (locally) that its combined use with borehole geophysical logs is valuable to the development of an accurate conceptual hydrogeologic model. The one-dimensional cyclostratigraphic framework, or fingerprint, of each well permitted discrete correlation of vertical lithofacies successions and high-frequency cycles, and the well-to-well connection of corresponding high-permeability zones between wells. The concepts should be useful for providing a framework for regional-scale triple- and dual-porosity groundwater-flow and solution-transport numerical simulations. Many karst aquifers occur in cyclic platform carbonates, so the cyclostratigraphic approach is applicable to many areas in the world (e.g., Hovorka et al., 1996, 1998; Ward et al., 2003; Cunningham et al., 2004b, 2006). Applications include well-head protection at well fields, design of tracer studies, solute-transport modeling of contaminants, saltwater intrusion modeling and monitoring in coastal areas, engineering design of underground barriers to seepage and tunnels, delineation of storage zones for aquifer storage and recovery projects, and providing a conceptual framework for development of next-generation simulations of regional karst groundwater flow.