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Tuesday, June 04, 2013

Hydrogeology of Wakulla Cave, Floridan Aquifer, north Florida
- from Kincaid, T.R., 1999, Morphologic and Fractal Characterization of Saturated Karstic Caves, Ph.D. Dissertation, University of Wyoming
Figure 1 (Click on the image for an expanded view)
Map of Wakulla cave and surrounding topographic features, Floridan aquifer, north Florida. Topography is from the USGS Crawfordville East, FLA. (1972) topographic quadrangle. Regional hydraulic gradient is south toward the Gulf of Mexico. A local ground water divide crosses the conduits and is marked by a broad zone of low ground water velocities. The southern conduits convey ground water toward the Gulf of Mexico while the northern conduits convey ground water to Wakulla spring.
Wakulla cave is comprised of a dendritic network of conduits of which 8,770 m have been surveyed and mapped (Figure 1). The largest conduit, labeled A and O on Figure 1 trends south from the spring / cave entrance for over 5.5 km (Jablonski, personal communication) of which 3.2 km have been mapped. Five secondary conduits labeled B, C, D, K, and M on Figure 2-5, measure 4.3 km in combined length and intersect the larger conduit. Eleven smaller conduits measuring 1.2 km in combined length connect to the secondary conduits. Most of the conduits have not been fully explored (Jablonski, personal communication).
The conduits are characterized as long tubes. The diameter and depth of any tube is consistent through space but individual or joining tubes can be divided by larger chambers of varying geometries. Conduit width is consistently larger than the height and the trends of the conduits do not follow noticeable fractures.
Water velocities in the smaller conduits were variable but sufficiently high to impede swimming against the flow. An average velocity of approximately 1 m/sec was estimated by calculating the rate at which a resting diver was propelled in the down-gradient direction. Water velocities in the largest conduits were smaller but remained large enough to propel a resting diver. The distribution of water velocities throughout a cross-sectional profile of the conduits is commonly variable wherein the water moves slower along the upper part of the conduit than through the center of the conduit. Scallop marks on the cave walls were observed throughout the cave and indicate the persistence of large ground water through-flow velocities for an extended period of geologic time.
The pattern of ground water flow through Wakulla cave is complicated by the fact that the largest conduits trend down-gradient from the spring discharge point. Flow directions compiled from observations made by cave divers indicate the presence of a ground water divide that crosses the cave between conduits D and M (Figure 1). Reduced ground water velocities encountered in conduit A after the junction with conduit D indicate that the divide is better characterized as a broad zone that crosses the cave at a distance of between 1 and 2 km from the spring. Variations in ground water velocities observed at different times throughout the divide zone indicate that the location of the divide fluctuates.
Direct connections to surface water sources are indicated in conduits A, K, M, and O by water clarity reductions that occur in as little as one day following local storms or prolonged regional precipitation (Jablonski, personal communication). Even during periods when turbid water discharges at the spring, clear water was typically observed discharging into conduit A from conduits B, C, and D (Jablonski, personal communication). Uranium isotopic compositions measured by Macesich and Osmond (1989) support that observation by showing that conduit A has a surface water signature while conduit B has a signature typical of regional ground water.
Genesis (Hypothesis)
Wakulla cave is a branching flow-dominated cave that has developed in the Floridan aquifer under the Woodville karst plain of north Florida. In terms of the proposed model, the cave has evolved into a flow-dominant stage wherein the conduits connect recharge sources with a spring. The geologic processes active on the Florida platform that have affected the development of Wakulla cave include, deposition of carbonate and siliciclastic rocks, sea level fluctuations, and subsequent erosion of the siliciclastic rocks that had provided an upper confining layer to the Floridan aquifer.
The Florida platform extends for more than 200 km into the present Gulf of Mexico and is defined by the 300 ft bathymetric contour (Figure 2). Carbonate sedimentation on the northwestern part of the platform was continuous between mid-Cretaceous and late Oligocene times (Scott, 1992). During the Miocene Epoch, siliciclastic sediments were transported onto the platform by rivers and distributed by longshore and other currents (Scott, 1992). Subsequent subaerial exposure resulted in the evolution of the carbonate rocks into the Floridan aquifer for which the overlying Miocene siliciclastic rocks, described as the Hawthorne Formation, provided a laterally extensive confining layer.
Sea level fluctuations have caused transgressions and regressions over the Florida platform several times since the deposition of the Hawthorne Formation. Submarine springs on the Florida platform (Figure 2) indicate that karstification had established preferential flow routes through the Floridan aquifer prior to the last sea level rise.
Landward erosion of the Hawthorne Formation exposed the underlying carbonates to infiltration of surface waters and caused the development of karstic ground water flow systems. The erosional boundary of the Hawthorne Formation creates a prominent topographic scarp known as the Cody Scarp (Figure 2). Surface drainage across the scarp and onto the exposed carbonates creates a zone of extensive karstification marked by numerous caves, sinkholes, and disappearing rivers (Ceryak, 1981; Spangler, 1981; Scott, 1981; Lane, 1986). The zone of karstification has followed the landward migration of the scarp leaving remnant caves and sinkholes on the evolving low carbonate land surface. Fossil and archeological artifacts discovered inside Wakulla cave but close to the spring discharge (Figure 3) indicate that the shaft that currently localizes Wakulla spring is one of these remnant sinkholes.
These facts combined with the present pattern and flow characteristics in Wakulla cave indicate that the largest conduits in the cave developed during a period of lower sea level and more seaward extent of the confining layer. Discharge from the aquifer was through the present submarine springs. Recharge was through a sinkhole that is currently Wakulla spring and was localized by the paleo-extent of the confining layer. The paleo-flow system caused dissolution of rock from the aquifer that culminated in the development of a flow-dominated cave that connected the sinkhole with the down-gradient spring(s). Precise timing of the cave development cannot be constrained beyond any of the periods of lower sea level that post dated the deposition of the Hawthorne formation during Miocene time.
Following the retreat of the confining layer and sea level rise, regional hydraulic gradients reduced forcing the evolution of the flow system into the current condition. The sinkhole that was the predominant source of recharge evolved into Wakulla spring. New sinkholes and karstic depressions like the Leon Sinks sinkholes developed farther up-gradient in the proximity of the retreating confining layer. The smaller conduits in Wakulla cave developed as a consequence of dissolution along flow paths connecting the newer sinkholes with Wakulla spring.
The original conduits comprising Wakulla cave remained in the aquifer however, and provided preferential flow paths that connected the newly formed Wakulla spring with the down-gradient springs that were drowned by the rise in sea level. A ground water divide has developed across the cave where flow converging on the conduit from the northern part of the Woodville Karst Plain discharges at Wakulla spring while flow from the southern part of the region enters the conduit and flows farther down-gradient to the submarine springs.
Figure 2. (Click on image for an expanded view)
Map of the Florida peninsula delineating the lateral extent of the Florida platform at the 300 ft water depth contour and showing the location of submarine springs relevant to the hydrology of the Woodville Karst Plain, north Florida. Location of Wakulla spring is shown relative to the approximate present position of the Cody scarp which marks an extensively karstified region in the transition zone between unconfined and confined conditions in the Floridan aquifer. Map is from Lane (1986).
Figure 3. (Click on image for an expanded view)
Location of fossil and archeological artifacts near the entrance to Wakulla cave at Wakulla spring, north Florida. Fossil and artifact locations are from Olsen (1958). Cave profile is from data collected by the Woodville Karst Plain Project cave divers.
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