Lithofacies

Lithofacies are the fundamental descriptive rock components of this study. Fifteen lithofacies delineate the sedimentary rocks that form the Fort Thompson Formation and Miami Limestone at the Northwest Well Field. If a rock was composed of a significant amount of quartz sand grains, but less than 50%, then "sandy" is used as the prefix to the lithofacies type. "Touching-vugs," a prefix to a lithofacies type, refers to vuggy porosity that forms an interconnected pore system (Lucia, 1999). The 15 lithofacies include: (1) peloid packstone and grainstone, (2) peloid wackestone and packstone, (3) Planorbella floatstone and rudstone, (4) pedogenic limestone (laminated calcrete, massive calcrete, and root-mold limestone), (5) mudstone and wackestone, (6) laminated peloid packstone and grainstone, (7) skeletal packstone and grainstone, (8) sandy skeletal packstone and grainstone, (9) coral framestone, (10) pelecypod floatstone and rudstone, (11) sandy pelecypod floatstone and rudstone, (12) touching-vug pelecypod floatstone and rudstone, (13) sandy touching-vug floatstone and rudstone, (14) skeletal quartz sandstone, and (15) conglomerate (Fig. 3 and Fig. 4). Cunningham et al. (2004b) contains detailed descriptions of these lithofacies and inferred environments of deposition, although some lithofacies terminology and definitions of depositional environments are modified herein. For example, the gastropod floatstone and rudstone lithofacies used in Cunningham et al. (2004b, 2004c) has been renamed Planorbella floatstone and rudstone in this study.

Figure 3. Idealized fifth-order cycles (Table 2) for the Fort Thompson Formation and Miami Limestone. Note the relations between lithofacies, depositional environments, porosity (pore classes), and groundwater-flow types. In terms of aquifer characterization, important points are that the most porous and permeable parts (pore classes) of the two upward-shallowing cycles are just above the flooding surface in the lower part of the cycles, and the aggradational cycles have a generally regular vertical distribution of vuggy porosity throughout the cycles. For additional information about this figure, please contact Kevin Cunningham at kcunning@usgs.gov. [click on images above for larger versions]

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Figure 4. Conceptual hydrogeologic column for the Northwest Well Field including ages, major depositional environments, groundwater-flow types, pore classes, lithofacies, cyclostratigraphy, Q-units of Perkins (1977), formations, and hydrogeologic units. For additional information about this figure, please contact Kevin Cunningham at kcunning@usgs.gov. [larger image]

Depositional Environments

Five major carbonate depositional environments characterize the Fort Thompson Formation and Miami Limestone in the study area (Fig. 3, Fig. 4, and Fig. 5). In a generally regressive succession, these include: (1) platform margin to outer platform, (2) open-marine platform interior, (3) restricted platform interior, (4) brackish platform interior, and (5) freshwater terrestrial environments. Rock fabric and texture, faunal constituents, sedimentary structures, and relation to surfaces bounding vertical lithofacies successions were the basis for interpretation of the five depositional environments for the rocks of the Fort Thompson Formation and Miami Limestone. In the study area, all five depositional environments were recognized in the Fort Thompson Formation (Fig. 5); however, only the open-marine platform interior environment is representative of lithofacies contained in the Miami Limestone.

Figure 5. Conceptual facies model showing relations between major lithofacies, depositional environments, and pore classes for the Fort Thompson Formation at the Northwest Well Field (Fig. 1 and Fig. 2). Five main depositional environments include platform margin to outer platform, open-marine platform interior, restricted platform interior, brackish platform interior, and freshwater terrestrial (ponds and marshes) environments. The platform margin to outer platform depositional environments are a notable exception because they are representative of only the lowermost high-frequency cycle (HFC2a) of the Fort Thompson Formation (Fig. 4). Four photographs of characteristic slabbed-core samples from the Fort Thompson Formation illustrate some of the carbonate textures and diagenetic features associated with key lithofacies. Each incremental color change represents 1 cm on the scale bars. [larger image]

Platform Margin to Outer Platform

The four lithofacies that distinguish the platform margin to outer platform environments include coral (Montastrea) framestone, conglomerate, sandy skeletal packstone and grainstone, and sandy pelecypod floatstone and rudstone lithofacies. The grainy lithofacies contain amphistiginids, which prefer areas of reef growth in the platform margin of the modern south Florida platform, and patch reefs and nearby environments not far (possibly a few kilometers) from the platform margin (Rose and Lidz, 1977). The platform margin to outer platform environments are a notable exception because they only occur at the base of the Fort Thompson Formation in the study area. Rocks representative of these environments overlie quartz sand and quartz-sand-rich limestone of the Tamiami Formation (Fig. 2), which Cunningham et al. (2006) suggested indicate a middle-ramp environment. Common to the sand and limestone of the Tamiami Formation are amphistiginids, globigerinids, and other planktonic foraminifers indicative of relatively deep water. Together, the rocks of the uppermost Tamiami Formation and basal Fort Thompson Formation are consistent with upward shallowing.

Open-Marine Platform Interior

Overlying the platform margin to outer platform depositional facies in the lowermost Fort Thompson Formation is open-marine platform interior depositional facies, further suggesting upward shallowing and platform progradation within the lower Fort Thompson Formation (Fig. 4). For the Fort Thompson Formation, lithofacies characteristic of the open-marine platform interior depositional environment include touching-vug pelecypod floatstone and rudstone, sandy touching-vug pelecypod floatstone and rudstone, skeletal packstone and grainstone, and sandy skeletal packstone and grainstone lithofacies. Common to these lithofacies are benthic foraminifers (soritids, archaiasinids, and peneroplids) that suggest deposition in an open-marine platform interior, similar to the modern platform interior of southern Florida that is seaward of the present-day islands of the Florida Keys (Rose and Lidz, 1977; Lidz and Rose, 1989). These lithofacies are commonly highly burrowed, suggesting deposition below a fair-weather wave base in a lower shore-face zone. Mollusks present in samples from the pelecypod-rich lithofacies are suggestive of the outer estuary to shallow-marine platform interior environments of Florida Bay (Table 1).

Two lithofacies, peloid wackestone and packstone, and peloid packstone and grainstone, characterize the Miami Limestone (Fig. 3 and Fig. 4). Burrowing of these lithofacies is pervasive, suggesting deposition below a fair-weather wave base in a lower shore-face zone. A benthic foraminiferal assemblage dominated by archaiasinids, soritids, and peneroplids in the peloid wackestone and packstone lithofacies is consistent with deposition in an open-marine platform interior (Rose and Lidz, 1977; Lidz and Rose, 1989). Schizoporella bryozoan are commonly present in both lithofacies. The two lithofacies correspond to the bryozoan facies of Hoffmeister et al. (1967), which they interpreted to represent an open-marine shelf lagoon. Later, both Perkins (1977) and Evans (1984) indicated deposition of the bryozoan facies was on an open-marine platform.

Restricted Platform Interior

Characteristic of the restricted platform interior environment is typical pelecypod floatstone and rudstone, sandy pelecypod floatstone and rudstone, skeletal packstone and grainstone, and sandy skeletal packstone and grainstone lithofacies. Miliolids commonly dominate the benthic foraminiferal assemblage of the lithofacies, which is consistent with deposition in a restricted platform interior. Lidz and Rose (1989), and Rose and Lidz (1977) noted that miliolid-dominated benthic foraminiferal assemblages are common in restricted areas of modern Florida Bay. These lithofacies are commonly highly burrowed, suggesting deposition below a fair-weather wave base in a lower shore-face zone.

Brackish Platform Interior

The mudstone and wackestone lithofacies commonly distinguishes the brackish interior platform environment. This lithofacies is principally micrite and has an abundance of the benthic foraminifer Ammonia and smooth-shelled ostracods. Charophytes, the benthic foraminifer Elphidium, and the freshwater gastropod Planorbella are less commonly present. Other types of benthic foraminifers are not common. Modern Florida Bay sediments with large populations of Ammonia and Elphidium, and containing few other foraminiferal species, are indicative of a brackish platform interior (Rose and Lidz, 1977; Lidz and Rose, 1989).

Freshwater Terrestrial

The Planorbella floatstone and rudstone lithofacies characterizes a freshwater terrestrial environment. This micrite-rich lithofacies commonly contains abundant Planorbella, smooth-shelled ostracods, and charophytes. Interpretation indicates deposition of the Planorbella-rich beds in freshwater ponds or marshes (Galli, 1991).