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.
- 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.
- 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 data from this area indicates the sinkhole complex
is not stable and is actively subsiding
- 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
- 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
- Additional ground water monitoring sites are needed to support
predictive ground water modeling
- Ground penetrating radar and/or land
based seismic reflection surveys should be run around the southern shore
to identify the underground drainage system.
- 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
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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