Methods

Core samples from eight borings were selected and analyzed for whole-core permeability by Core Laboratories, Inc. The full-diameter (2-in) pieces were measured (using air flow) for permeability in two horizontal directions and one vertical direction while in a Hassler rubber sleeve. Porosity was determined using a helium pycnometer. The unconsolidated quartz-sand rich sediments recovered beneath Tree Island 3AS3 were measured for permeability using a plastic-sleeve technique at Core Laboratories, Inc. Results of 36 measurements from Tree Island 3AS3 are summarized in Figure 8.

Permeability of Limestone Units

For the Tree Island 3AS3 cores, 14 samples from the limestone units provided reliable permeability data (summarized in Figure 8). The permeability is highly variable, and likely related to differences in lithofacies type and degree of subaerial diagenesis. The higher permeability values appear to be related to lithofacies that have a relatively low percentage of lime mud (for example, micrite), a relatively high percentage of carbonate grains, and have undergone pervasive diagenetic leaching of skeletal grains. The moldic and vuggy porosity formed in these carbonate-grain-rich lithofacies (wackestone, packstone, and grainstone) produces extremely high effective porosity and permeability often in excess of 1 Darcy. Conversely, the lithofacies rich in lime mud (for example, micrite associated with mudstone and wackestone depositional textures) are often dense and well cemented with very low effective porosity. Furthermore, the formation of well-cemented subaerial exposure horizons (calcrete and soil breccias) may also generate intervals with very low permeability.

Tree Island 3BS1 has 27 samples with reliable permeability data (summarized in Figure 9). Similar to the 3AS3 core samples, a wide range of permeabilities was recorded in 3BS1 cores. The moldic and vuggy lithofacies produced extremely high permeabilities, usually in excess of 1 Darcy, and some in excess of 5 Darcies. The more moderate permeability values appear to be associated with the finer grained, well-cemented, mud-rich (micrite-rich) lithofacies; subaerial exposure horizons; and well-sorted, fine, skeletal-sand lithofacies lacking moldic or vuggy porosity.

The relationship of horizontal to vertical permeability in the two study areas indicates an anisotropic pore structure. In the 3BS1 core data, nearly half (11/23) of the measurements with both vertical and horizontal permeability have a greater vertical than horizontal value (see Figure 9). Likewise, the 3AS3 core data show a split horizontal-vertical preference in permeability (7 of 13 measurements). The higher vertical permeabilities could be related to semivertical dissolution features, such as solution-enlarged root molds, desiccation cracks, and burrows.

The presence of well-developed calcrete horizons and dense, well-cemented soil breccias within or at the Q-unit bounding surfaces would probably restrict direct vertical exchange of ground water. The competency and lateral continuity of these low-permeability units are unknown at this site, but are likely an important component in the vertical exchange of fluids.

Permeability of Underlying Siliciclastics at the 3AS3 Tree Island

The permeability of these quartz sands and mixed carbonates is generally in the moderate (200-500 md) and high (>500 md) range, as shown in Figure 8. These values are generally consistent with unconsolidated mixed sand- and mud-sized material. However, because the packing of grains may have been rearranged during shipping of the core samples to the lab, it is possible that the permeability measurements are not representative of in-situ conditions (S. Krupa, South Florida Water Management District, oral commun., 2001). The mud content generally controls the amount of permeability in samples of this type. The relatively permeable nature of these siliciclastic sediments may provide a shallow source of ground water to this tree island system. The amount of hydraulic communication and ease of vertical exchange is probably dictated by the lateral continuity of the low-permeability mud-rich lithofacies (mudstone, calcretes) in the overlying limestone section.