HYDROGEOLOGY

Geologic units of varying permeability underlie southeastern Florida from land surface to depths between about 150 to 400 feet. These units constitute the unconfined surficial aquifer system, which is the principal source of potable water used in southeastern Florida (Fish and Stewart, 1991). In Miami-Dade County, a highly permeable part of the aquifer system has been named the Biscayne aquifer (Parker, 1951; Parker and others, 1955). Underlying the Biscayne aquifer are two semiconfining units that occur above and below the gray limestone aquifer (fig. 2). This study focuses on the Biscayne aquifer and the semiconfining unit above the gray limestone aquifer (fig. 2). The geology and hydrology of the study site have been previously reported by Causaras (1987), Fish and Stewart (1991), Solo-Gabriele and Sternberg (1998), Nemeth and others (2000), and Cunningham and others (2004b).

Sequence Stratigraphy

In this report, sequence stratigraphic relations are described for the Pleistocene carbonate rocks that form the Biscayne aquifer. Critical to defining this sequence stratigraphy was delineation of the lithofacies of the Miami Limestone and Fort Thompson Formation. Sixteen lithofacies comprise the upper Fort Thompson Formation and Miami Limestone (Cunningham and others, 2004b). Eleven of these facies form important stratal components in the Fort Thompson Formation and Miami Limestone at the Levee 31N study area. Cunningham and others (2004b) defined these facies to include: (1) peloid grainstone and wackestone; (2) peloid wackestone and packstone; (3) gastropod floatstone and rudstone; (4) mudstone and wackestone; (5) pedogenic limestone (laminated calcrete, massive calcrete, and root-mold limestone); (6) skeletal grainstone and packstone; (7) pelecypod floatstone and rudstone; (8) sandy pelecypod floatstone and rudstone; (9) touching-vug (Lucia, 1999) pelecypod floatstone and rudstone; (10) sandy, touching-vug pelecypod floatstone and rudstone; and (11) quartz sandstone and skeletal sandstone (fig. 3).

Delineation of Vertical Lithofacies Successions and High Frequency Cycles

Figure 3. Conceptual hydrogeologic column for the Levee 31N study area. Ground-water flow classes of the Biscayne aquifer defined by Cunningham and others (2004b). [larger image]

Vertical lithofacies successions (VLSs) are the smallest set of genetically related lithofacies that form the fundamental building blocks of the Biscayne aquifer. Individual high-frequency cycles (HFCs) are defined by single or repetitive VLSs (fig. 3). Kerans and Tinker (1997) indicated that HFCs are comparable to the parasequences of Van Wagoner and others (1988). Marine flooding surfaces occur at or near the upper and lower boundaries of each VLS and HFC. A marine flooding surface is a surface separating younger from older strata across which water depth abruptly increases (Van Wagoner and others, 1988). All bounding surfaces of HFCs contain evidence of alteration by subaerial exposure; however, surfaces bounding VLSs and associated host rock do not always display such evidence of subaerial exposure (fig. 3). Alveolar textures (Goldstein, 1988) in host rock below upper bounding surfaces of VLSs in the Fort Thompson Formation provide evidence for subaerial exposure in some instances (fig. 4). Nonetheless, surfaces bounding VLSs with or without evidence of subaerial exposure are flooding surfaces (fig. 3). Subaerial exposure surfaces bounding HFCs are regional in extent across the southern part of the Florida platform (Perkins, 1977), with those unique to VLSs having an indeterminate extent. In this study, VLSs are related to a single rise and fall in relative sea level in the less than 10- to 100-kiloyear duration range (fifth order and higher). The HFCs develop during fifth-order 10- to 100-kiloyear sea-level cycles (Kerans and Tinker, 1997). As shown in figure 3, the HFCs of the Miami Limestone and Fort Thompson Formation are equivalent to the Quaternary (Q) units of Perkins (1977), which have been interpreted to be the result of eustatic sea-level oscillations (Perkins, 1977; Multer and others, 2002; Hickey, 2004). Bundled VLSs might be related to allocyclic forces such as high-frequency, sub-Milankovitch sea-level cycles (Locker and others, 1996; Mundil and others, 1996) or tectonically driven sea-level changes (Galli, 1991). Alternatively, bundled VLSs may be associated with repeated autocyclic progradation and marine flooding of paralic environments that include freshwater marshes and ponds, or migration of shallow-water mud mounds and islands similar to those in Florida Bay (Enos and Perkins, 1979). Most VLSs and HFCs can be correlated using unique attributes such as the vertical arrangement of lithofacies at the cycle scale, diagenetic overprints, and the vertical order of the stacking of VLSs and HFCs. The one-dimensional sequence stratigraphic framework or fingerprint of each well permitted discrete correlation of VLSs and HFCs and the hydraulic interconnection between wells (well-to-well connection of corresponding ground-water flow classes).

Figure 4. Hydrogeologic section B-B' showing schematic relations of geologic formations, hydrologic units, lithology, and sequence stratigraphy in the surficial aquifer system along Levee 31N. Line of section B-B' is shown in figure 1. Top of the section is the top of the Miami Limstone and ground level is not shown. Please note: this image is quite large (18"x18"). The best format for viewing this image is PDF, but you will need the free Adobe Acrobat Reader in order to view the PDF. Formats: jpg (204 KB), tif (6.6 MB), PDF (18.7 MB)

High-Frequency Cycles of the Levee 31N Study Area

Stratigraphic research conducted since the late 1970s has improved the resolution of the sequence stratigraphic framework of the Fort Thompson Formation and Miami Limestone in southeastern Florida as reported by previous investigators. Perkins (1977) first divided the two rock formations into five unconformity-bound or Q units (fig. 3). Galli (1991) further delineated the Fort Thompson Formation as a single depositional sequence containing eight parasequences defined “as sequences representing higher frequency, short-term phases of sedimentation.” Harrison and others (1984) and Multer and others (2002) subdivided the Q4 and Q5 units of Perkins, and Multer and others (2002) refined ages of some of the Q units (Q3, Q5e, and post Q5e). Droxler and others (2003) and Hickey (2004) discussed various correlation scenarios of Q-units of Perkins (1977) and Multer and others (2002) to late Pleistocene interglacial marine isotope stages. Cunningham and others (2004b) recognized two subaerial exposure bounded units within the Q3 unit of Perkins (1977), which may be related to a sea-level fluctuation discussed by Multer and others (2002). In the vicinity of the Levee 31N study area, only HFC2 to HFC5 corresponded to the Q2 to Q5 units of Perkins (1977) as shown in figure 3. The four HFCs are each bound at the upper surface by laminated calcretes (fig. 3) correlated throughout southeastern Florida (Perkins, 1977; Multer and others, 2002). Preliminary mapping of the sequence stratigraphy of the Fort Thompson Formation in north-central Miami-Dade County suggests that the Q1 unit of Perkins (1977) is not present in the Levee 31N study area. In fact, this unit may onlap onto the top of the Tamiami Formation east of the study area.

Ideal Cycles in the Levee 31N Study Area

Paralic cycles and subtidal cycles are two types of ideal cycles that occur in the Pleistocene limestone of the Biscayne aquifer. These cycles have similarities in that both shallow upward; however, subaerial exposure features may or may not be evident at the upper surface of the paralic cycles, which typically are capped by freshwater deposits (fig. 3). Incomplete paralic cycles are capped by brackish or restricted marine deposits (fig. 3). Evidence of tidal flat deposition, such as algal laminations and mud cracks (Shinn and others, 1969), are rare occurrences in the paralic cycles. The middle part of the paralic cycles can be composed of skeletal packstone or grainstone lithofacies, whereas the lower part typically is either touching-vug pelecypod rudstone or floatstone or sandy, touching-vug pelecypod rudstone or floatstone. Common benthic foraminifers (archaiasinids, soritids, and peneroplids) in the lower and middle units are consistent with relatively open-marine shelf deposition (Rose and Lidz, 1977). Upper brackish beds have an abundance of the benthic foraminifera Ammonia and smooth-shelled ostracodes; less commonly, the beds contain charophytes, the benthic foraminifer Elphidium, and gastropods that can include Planorbella. Rose and Lidz (1977) suggested modern Florida Bay sediments that contain large populations of Ammonia and Elphidium and that contain relatively few total species are indicative of a brackish interior environment. Upper freshwater strata are composed of gastropod floatstone and rudstone containing Planorbella (commonly in abundance), smoothed-shelled ostracodes, and charophytes and were probably deposited in a freshwater marsh or pond (Galli, 1991).

Subtidal cycles represented by HFC4 and HFC5 may be restricted to only the Miami Limestone (fig. 3). These subtidal cycles are described in detail by Cunningham and others (2004b).

Pore System

Vacher and Mylroie (2002) described the Biscayne aquifer as an eogenetic karst aquifer system that best fits a dual-porosity conceptual model. Pore system type in the Biscayne aquifer is related to lithofacies, and has a predictable vertical distribution within fundamental cycles. Each carbonate lithofacies, and its typically associated pore system type, has been assigned to one of three ground-water flow classes defined by Cunningham and others (2004b): (1) horizontal conduit flow; (2) diffuse-carbonate flow; and (3) leaky, low-permeability flow (fig. 3). Holocene sediments of the Biscayne aquifer have been assigned to the low-permeability peat, muck, and marl ground-water flow class (Cunningham and others, 2004b). Discussion of the pore system of the Biscayne aquifer is restricted to the Fort Thompson Formation. Details of the Miami Limestone pore system are discussed in Cunningham and others (2004b).

Characteristic lithofacies associated with the horizontal conduit ground-water flow class are touching-vug pelecypod rudstone and floatstone or sandy, touching-vug pelecypod rudstone and floatstone; both are common in the lower parts of paralic cycles of the Fort Thompson Formation. These lithofacies typically are characterized by large fossil molds, solution-enlarged fossil molds, or vugs with shapes and spatial relations, suggesting they are either molds of burrows or voids that surround casts of burrow molds. Accordingly, the lower part of many of the paralic cycles is the most porous and permeable. Porous-zone maps in north-central Miami-Dade County suggest a tabular three-dimensional geometry (Cunningham and others, 2004b) for these zones. Therefore, an accurate correlation of cycles produces a realistic linkage of permeable or preferential flow zones. Ground-water flow for the horizontal conduit flow class is conceptualized as ground water flowing from vug to vug in a pore system characterized by touching vugs (Lucia, 1999, p. 26 and 31). Ground-water flow associated with this ground-water flow class is not through pipes or underground streams, but along a passage (typically tabular in shape) formed by touching vugs that act as a major route for ground-water flow.

The middle part of the ideal paralic cycle of the Fort Thompson Formation is characterized by a skeletal packstone or grainstone lithofacies, which are associated with the diffuse-carbonate ground-water flow class. These lithofacies are characterized by interparticle, intraparticle, and irregular separate vug pore space that facilitates ground-water movement through vug-to-matrix-to-vug connections.

Gastropod floatstone and rudstone or mudstone and wackestone lithofacies typically cap the paralic cycles, which are associated with the leaky, low-permeability ground-water flow class. Secondary porosity common to these lithofacies includes bedding plane vugs, thin semivertical solution pipes, and gastropod molds. The matrix porosity of these lithofacies is relatively low, and the gastropod molds and solution pipes are localized and typically separated. Bedding plane vugs that have a sheet-like geometry, however, could represent significant conduits for moving large volumes of ground water.