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  Conclusions You are at Seismic Reflection Surveys - Conclusions
Jim Flocks
  The use of high-resolution seismic tools to investigate shallow fresh water lakes or karsts terrain has in the past had limited success. Utilizing relatively new digital technology has increased the potential success of these geophysical systems. The surveys conducted in Orange, Kingsley, Lowry and Magnolia Lakes, and the Drayton Island area of St. Johns River have shown the potential application of these techniques in understanding the formation of individual lakes, thus aiding in the management of these natural resources. This paper is primarily a descriptive report of the data and interpretations collected from these lakes and river.

Orange Lake, the primary focus of the study, is a shallow, flooded plain that was formed essentially as an erosional depression in the clayey Hawthorn formation. The lake can be arbitrarily divide into three areas according to the distinct karstic features present in each area of Orange Lake (southeast, north-central, and southwest). The primary karstic features identified in various sizes and stages of development were cover subsidence, cover collapse and buried sinkholes.

The karst features within the southeast area are mostly cover subsidence sinkholes and associated features. Many of the subsidence features found are grouped in proximity to form larger composite sinkholes, some larger than 400 m in diameter. The size of these composite sinkholes and number of buried subsidence sinkholes separate the southeast area from the others. The probability of lake waters leaking to the aquifer in this area is probably controlled by the permeability of the cover sediments or by faults that penetrate the lake floor.

The lake bottom and subsurface of north and central area are relatively smooth with similar small subsidence features throughout the area. Limited the poor quality data of this area, only relatively few features could be identified and these features were identified by their similarity in structure to features found in better quality data. The main features identified were subsidence sinkholes that have no cover sediments overlying them, implying that the sinks have been actively subsiding. The most active area is located near Samsons Point. Near Samsons Point, there are high angle, faults or fractures that penetrate the lake bottom. These features have the highest potential for leakage from the lake to the Floridan aquifer if the clays have not been sutured.

There are primarily two differences between the southwest and other areas: (1) the features found towards the central part of the lake are small in scale (1 to 10 m across) and tend to be singular structures compared to the southeast area where features combined to form larger sinkholes (>400 m). Very few of either type of feature was observed in the southwest area, and; (2) the southwest area is the only area where collapse dolines that penetrate to the Floridan aquifer were found. The deep sinkholes have associated fault blocks and slumps identifiable within the holes and are separated by a slight ridge.

Orange Lake is a complex geologic feature that is primarily a clay basin catchment overlaying a soluble and permeable bedrock of limestone. It has many small (1 to 10 m) to medium (10 to 50 m) cover subsidence dolines throughout the lake. In the southeast area singular small to medium features are closely spaced forming larger areas (<100 m) of subsidence. The largest and most important features in the lake are the collapse sinkholes along the southwestern shore that provide a conduit for exchange between the lake and subsurface aquifer. These features combined with the variable water table produce a very dynamic hydrologic and geologic system.

Seismic profiles from the other several selected lakes and river were compared to the Orange Lake profiles. These were examples of deeper lakes constructed of one large subsidence or a coalescing of sinkholes to form one large sinkhole. Aside from the difference in scale the basic characteristics of the subsidence sinkholes were very similar.

The potential applications of the data that were collected are varied and numerous. The USGS is currently enhancing the acquisition and processing techniques related to this rapidly expanding technology. Each site that was surveyed has a particular focus for the SJRWMD and concerned citizens. The following are recommendations for ways to either maximize the use of the data or for additional work to confirm or refine the interpretations.

  1. As more seismic data is collected from sites in the lake area around Keystone Heights, an analysis to assess correlation of subbottom conditions of lakes that exhibit wide ranges of lake stage fluctuations to lakes that remain relatively stable should be done. This could provide insight into factors other than rainfall that effect the lake stages.
  2. A controversial management alternative for Orange Lake that is being considered involves plugging or isolating the sinkhole in the southwest area by Heagy-Burry park. The data collected from there suggests that this area is a "sinkhole complex" rather than one isolated hole. Other areas of the lake and surrounding land areas show evidence of collapse and subsidence activity. Though there is no method available to predict the geohydrologic impact of isolating the complex, strong consideration should be given to the following:
    1. The data from this area indicates the sinkhole complex is not stable and is actively subsiding
    2. The collapse features have penetrated the confining unit to the Floridan aquifer. The water that is flowing through this breach recharges the Floridan aquifer and is filling solution cavities to the level of the potentiometric surface. Subsurface stability is enhanced by the fluid filled voids. Reducing the recharge to the Floridan aquifer could reduce the potentiometric surface enough to empty these voids thereby increasing the chances of sinkhole formation both in the lake and on the land
    3. Detailed mapping of sinkholes in the land area surrounding Orange Lake should be done to identify the karst system that the complex is a part of. This should include mapping from existing aerial photography and field surveys
    4. Additional ground water monitoring sites are needed to support predictive ground water modeling
    5. Ground penetrating radar and/or land based seismic reflection surveys should be run around the southern shore to identify the underground drainage system.
  3. Data collection and analysis should be continued in the St. Johns River. These results should be evaluated with water quality and borehole geophysical data and included in numerical ground water flow models for the area.

The authors would like to express their thanks to the Governing Board of the St. Johns River Water Management District (SJRWMD), Barbara A. Vergara, and Douglas A. Munch of SJRWMD, for their continuing support of high resolution seismic reflection studies within the District. We also want to thank Jerry and Brenda Harris, owners of Sportsmans Cove fish Camp at Orange Lake and Mr. F. W. Strickland, owner of Strictland's Landing on Kingsley Lake, for the generous use of their facilities and personal knowledge of the lakes. We would also like to recognize Dana Wiese (USGS) for operating the seismic equipment and Shane Dossat (SJRWMD) for his support in the field.

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