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Wakulla Springs Geology

Geologic Framework

Wakulla Spring lies within the Woodville Karst Plain (Hendry and Sproul, 1966), a low-lying area underlain by Oligocene and Miocene aged limestone overlain by a thin blanket of unconsolidated sands.

The Eocene Ocala Group is a widespread formation that is an important part of the Floridan aquifer system. However, near Wakulla Spring it lies well below all known cave passages; the top of the Ocala was encountered between 120-180 meters below the land surface in oil test wells near Wakulla (Rupert, 1988). The Lower Oligocene Suwannee Limestone unconformably overlies the Ocala Group. The Suwannee Limestone is a calcarenite containing miliolid foraminifera, mollusks, bryozoans, echinoids, and corals. Chert is common and was used by Paleo-Indians for tools and weapons. The passages in Wakulla Cave are developed in the Suwannee Limestone. The Lower Miocene St. Marks Formation unconformably overlies the Suwannee and is the uppermost bedrock unit at Wakulla Spring. The ledge above the Wakulla Spring vent is St. Marks Formation. The unit is primarly a calcilutite containing quartz sand, clay stringers and mollusks. The lower contact of the St. Marks Formation lies at approximately 27 meters water depth.

Samples collected during exploration in 1987, as well as visual descriptions, and video footage from within Wakulla Cave indicate a distinct lithologic and color change within the Suwannee Limestone at a depth of about 65 meters. Above lies a soft biocalcarenite, whereas below the contact is a harder, recrystallized dolomitic calcarenite. Rupert (1988) suggests the harder rock may have retarded additional downward dissolution in the conduits. The 3D map produced during the 1998-1999 diving project shows the floor level for much of the cave is uniform at a depth of about 90 meters (Figure 2).

During the Pleistocene, the shoreline transgressed across this area, reworking sands from older formations and depositing the sediment over the limestone. (Hendry and Sproul, 1966). Five marine terraces are recognized in Wakulla County, and Wakulla Spring lies within the Pamlico Terrace, ranging from 3 to 8 meters above sea level (Healy, 1975). The Cody Scarp (Figure 1) is an escarpment marking the boundary between the northern Tallahassee Hills and the Coastal Lowlands to the south (Rupert and Spencer, 1988). The boundary represents the ancient shoreline location.

Paleontology

The first correct identification of the enormous bones seen through the clear water on the bottom of the Wakulla Spring basin was made by Sarah A. Smith for the Tallahassee Floridian and Journal in 1850. This publication prompted a local professor to collect a significant portion of a mastodon skeleton, but the bones were lost in a shipwreck on their way to a museum on the Atlantic coast (Revel, 2002).

Additional bones were collected over the next decades. In 1930, more mastodon bones were discovered in shallow water during construction of a swimming area at Wakulla Spring (Revel, 2002). The Geological Survey was enlisted in its collection and the skeleton remains on display today in the Museum of Natural History in Tallahassee.

Dives by Wally Jenkins, Garry Salsman and their buddies in 1955-56 (see exploration section below) resulted in the discovery of mastodons, mammoths, deer, camels, giant ground sloths, bears, as well as many spear points from Paleo-Indians (Burgess, 1999). The divers used pillowcases lined with plastic bags inflated by air from their tanks to lift the heavy bones to the surface from depths up to 60 meters.

During later exploration Pleistocene mammal bones have been discovered as far back as 366 meters from the entrance.

Hydrology

Wakulla Spring is one of 33 first order magnitude springs in Florida (Scott, et. al., 2002). Average discharge from 1907-1974 was 11 m3/s. Wakulla Springs display the greatest range of discharge of any Florida spring. A minimum flow of 0.7 m3/s was recorded on June 18, 1931, whereas a maximum flow 54 m3/s [equivalent to 14,288 gal/sec] was reported on April 11, 1973 (Scott et. al., 2002).

Rupert (1988) noted the collection of fossil mammal bones located deep within the cave. This observation prompted two theories that took into account the lower Pleistocene sea level (and by extension, base level on land). Rupert suggested that mammals wandered into the dry cave entrance looking for water. Additionally, it was hypothesized that the Wakulla Spring may have been a sink where water entered the aquifer at the time. Indeed Wisenbaker (1998) reports than another Florida spring, contains an extinct land tortoise with a wooden stake stuck in its shell at 26 meters below the current water level. A less likely explanation for the presence of mastodon bones in the spring was that "In winter, mastodons crossing frozen pools broke through the ice and drowned" (Adler, 1977).

Directly north of the Wakulla area, an unconfined surficial aquifer system is found in the unconsolidated sands and gravels of the Tallahassee Hills. Recharge occurs by direct precipitation. An intermediate aquifer system lies below the surficial sediments ranging from about 15 to 46 meters thick. The bedrock contains interlayered clayey sediments, limestone, and dolomite resulting in discontinuous water-bearing zones. Water recharges into the intermediate aquifer by leakage from the surficial aquifer and from sinkhole-drained lakes (Clemens, et. al., 1998). A dramatic example of flow into the aquifer from a lake in the Tallahasee Hills occurred on September 16, 1999. Much of the 4000 acre Lake Jackson in the Tallahassee Hills suddenly drained down Porter Hole, a 5-meter deep sinkhole in the lake bed. The pit leading down from the sink, swallowed the lake's southern half in a single day. Similar drainage events have occurred in the past, as well.

Below the surficial and intermediate aquifers lies the Floridan aquifer system that is a major carrier of water and extends through much of the northern part of the state. In the Woodville Karst Plain the Floridan aquifer is comprised of the St. Marks Formation, Suwannee Limestone and Ocala Group. Transmissivities are high and range from 5,000-125,000 feet squared per day (Pratt et. al., 1996). Recharge comes from downward leakage from the intermediate aquifer in the Tallahassee Hills and through sinkholes (Hendry and Sproul, 1966). The Woodville Karst Plain also forms a recharge area via direct rainfall and through sinkholes (Scott, et. al., 1991). Four streams sink underground, also contributing to recharge, though one re-emerges again (Clemens, et. al., 1998).

Regional groundwater flow of the Upper Floridan aquifer is to the southeast across Wakulla County (Figure 3). In the Woodville Karst Plain the Floridan aquifer is unconfined and no low-permeability units lie between the surface and carbonate aquifer units (Lane, 2001).

Using uranium isotopes, Cowart, et. al. (1998) determined that the primary source of Wakulla Springs is southward flowing Floridan aquifer water. Further, they used strontium isotopic ratios to conclude that the spring water has not come solely from local recharge, but rather comes from water having been in contact with Floridan aquifer bedrock for considerable time. However, Katz (1998) reports that the shallow and deep ground water was recharged during the past 30 years based on tritium age dating.

The conduit flow of groundwater in the Wakulla Spring cave and the Woodville Karst Plain is remarkably complex considering the relatively uniform pieziometric surface of the area (Figure 3). Kincaid (1999) reports that a groundwater divide occurs 1 to 2 km inside the main tunnel of Wakulla Spring. The divide marks a divergence between water flowing north to the Wakulla Spring vent and water flowing approximately along the regional groundwater gradient south, presumably to the Spring Creek Springs Group of thirteen submarine springs. This flow divergence is perplexing because one would not expect diffuse aquifer percolation to supply water to a tunnel approximately 30 meters in diameter.

The Spring Creek Springs Group has pulsating changes in flow where the surface of the water alternates between flat quiescence and boiling surface turbulence (Lane, 2001). The alternating surges generally last for several minutes and thought to be do to flushing through complex, tortuous passages. Since the Spring Creek Springs Group is likely to be connected, at least indirectly, with the nearby Wakulla Spring, much remains to be learned about the flow within the Woodville Karst Plain.

The importance of understanding the sources of water for Wakulla Spring is apparent when considering the clarity of water discharging from the spring. The vertigo-inducing clarity of the water in the 19th and early 20th centuries has diminished during the last few decades. One of the first reports of low visibility came in 1894 where "The water has been stirred up by the heavy rains, and we could only see down 80 feet" [24 meters] less than the maximum of 125 feet [38 meters] (Savery, 1998). Reports in 1945 and 1946 by the commercial operation run of Ed Ball noted that visibility was affecting the ability to run glass-bottom boat tours and turn a profit. The visibility is diminished primarily from tannic and humic acids in surficial swamp or river waters that enter the karst. Particulate matter may also contribute to lower visibility, but the dark color of the tannic water is a bigger problem. Savery (1998) quotes long time boat guides stating that dark water has become a more frequent, longer duration problem at Wakulla. Records of dark water indicated water visibility was poor for 58% of the time over the last 12 years and that the poor visibility is correlated to rainfall events. The best chance for crystal clear water is during the dry months of May and June.

Another significant problem with water quality at Wakulla Springs is the increase in nitrates, presumably the result of runoff from fertilizers used on lawns and agriculture. Simultaneous has been the introduction of exotic algae, particularly hydrilla. A virtual explosion of algae growth has choked much of the Wakulla Spring basin and river in recent years, probably enhanced by the high nitrate levels. The state park has attempted to manage the hydrilla using mowers and divers to remove as much as they can, but it has been a losing battle. Recently, the park applied a herbicide to the spring basin to improve visiblity of the spring by glass bottom boat tours and to regain a healthy ecologic balance in the river system (Ritchie, 2002). It is hoped that native plants re-establish themselves and that the hydrilla will not be re-introduced.

Speleogenesis

Little work has been conducted on the speleogenesis of Wakulla Spring cave system considering its magnitude. The reason is probably because the cave system is entirely flooded and averages approximately 88 meters water depth. Rupert (1988) took advantage of recent exploration to do rudimentary description and analysis.

Chen, et. al. (1998) studied the development of the Woodville Karst Plain, including its offshore extension. They examined the regional structural geology, sea-level fluctuations, climatic change, and groundwater flow and proposed that karstification began approximately 9,000 years B.P.

Kincaid (1999) developed a model for the origin of Wakulla Spring. He defined Wakulla as a branching, flow-dominated, saturated cave, and described a four-stage sequence of development. Speleogenesis development began through self-initiation. Small, random variations in permeability created positive feedback loops. The up-gradient process was based on geochmical feedback. Down-gradient, hydrodynamic processes governed the feedback loop and was enhanced by corrosion from mixing of waters. The largest conduits were developed in the cave when sea level was lower during the Pleistocene. Discharge occurred in springs that are presently submarine, while recharge entered through a sinkhole that now is the current Wakulla Spring.

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