Florida Karst III

Conduit morphology

Observations from over 300 maps of Florida caves, both vadose and phreatic, suggest that conduits enlarge along both vertical and horizontal planes. Whereas some conduit cross-sections develop "fissure" or "tabular" morphologies; a combination or"plus-sign" morphology is more common, and may even be the signature morphology of conduit cross-sections in Florida. Visual inspection correlates vertical extensions of the conduits to fractures. Horizontal extensions can be pervasive and laterally continuous. Their origin is unclear; however, it is likely that their existence is related to water-table positions, lithologic variability, or a feedback combination of the two.

Directional anisotropy

Development of conduits along fractures is strikingly visible from cave maps in plan-view. These conduits produce directional anisotropy in groundwater flow. Rose diagrams of conduit lengths for 14 individual caves in the Ocala area reveal repeating fracture orientations; however, the length of conduit can be heavily weighted in one direction. No strong correlation is visible between these anisotropic rose diagrams and major lineaments, hypothesized basement faults, hillslope directions, or presence of Miocene cover, although the evidence is suggestive that all have an influence. Further interpretation will require orders of magnitude more data. A combined rose diagram for all caves shows that the regional signal is an underlying pattern of sub-orthogonal NW-SE and NE-SW fractures. These field measurements of fractures confirm the results of remote sensing studies of photo-linears by Vernon (1951), and Littlefield et al. (1984).

Layered heterogeneity

Survey data of vadose and phreatic conduits in west-central Florida suggest that conduit development is restricted to specific horizons. In some quarry highwalls, it appears as though specific horizons have been virtually removed, creating a laterally continuous network of voids (LaFrenz et al., 2003). Lateral dissolution is evident in a map of Blue Springs in Volusia County. Flow to many springs is provided by conduits formed along horizontal planes. Many of these spring systems are fed from depth by multiple horizons of conduit development. Core records mention bit drops and recovery of silica sand from depths greater than 300 feet in the unconfined aquifer in Hernando and Citurs Counties in west-central Florida.

Conduit integration

Data from cave maps imply that the caves do not represent an integrated conduit network in the Floridan Aquifer System. Exceptions include river-type systems such as the Woodville Karst Plain. Many caves in Florida consist of a network of passages that end in ever-narrowing fissures and/or sediment fills and collapses.

The disjunct nature of conduits within the Floridan Aquifer System is supported by one of several conclusions from a numerical study of the Silver Springs and nearby spring-sheds. The GeoTrans (1988) report states that regionally extensive and hydraulically connected fractures or solution features cannot be present within the aquifer. Inclusion of these features prevents calibration of the model to known potentiometric values. Rather, the model supports fractures and conduits that are discontinuous and heterogeneous. One possible source of this heterogeneity is presented by Back and Hanshaw (1970) who hypothesized that flow retardation in the Floridan Aquifer may be a result of siliciclastic residuum, eroded from the overlying Miocene Hawthorn, filling solution cavities. Indeed this is observed in some caves and core records.

The implication of disconnected conduits within the Floridan Aquifer System is that input and output points within the aquifer are not directly connected. Unconnected input and output point would cause flow retardation in the aquifer. Flow retardation is consistent with seasonal fluctuations in water levels within the aquifer near Silver Springs found by Phelps (1992) and higher than expected potentiometric gradients observed by Back and Hanshaw (1970). As a counter example, the Yucatan karst aquifer has a well-integrated conduit system and exhibits very little hydraulic gradient (Back and Hanshaw, 1970).

References

Back, W., Hanshaw, B. B., 1970, Comparison of chemical hydrology of the carbonate peninsulas of Florida and Yucatan, Journal of Hydrology, vol. 10, pp. 330-368.

Phelps, G. G., Hydrogeology, water quality, and potential for contamination of the upper Floridan Aquifer in the Silver Springs ground-water basin, central Marion County, Florida, Water-Resources Investigations Report 92-4159, United States Geological Survey, 69 pp.

GeoTrans, 1988, Hydrologic investigation of the northern portion of the Southwest Florida Water Management District, northern district model project, phase II final report, 151 p.

LaFrenz, W. B., Bulmer, W. H., Jamilla, S. V., O'Neal-Caldwell, M., 2003, Characteris-tics and development of shallow solution features in thinly mantled karst, Alachua and Levy Counties, Florida, in (L. Florea, H. L. Vacher, and E. A. Oches, eds.) Karst Studies in West Central Florida: USF Seminar in Karst Environments, South-west Florida Water Management District, pp. 21-37.

Littlefiled, J. R., Culbreth, M. A., Upchurch, S. B., Stewart, M. T., 1984, Relationship of Modern Sinkhole Development to Large-Scale Photolinear Features, 1st Multidisciplinary Conference on Sinkholes (B. F. Beck, ed), pp. 189-195.

Vernon, R. O., 1951, Geology of Citrus and Levy Counties, Florida, Florida Geological Survey Bulletin 33.

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