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Open File Report:
Seismic Reflection
Surveys |
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Southwest Area |
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Loction
of seismic profiles collected
from the Heagy-Burry vicinity of Orange Lake. Click on the numbers in red to view
Figure 21 and
22. |
The third area delineated within Orange Lake is the southwestern area.
This area consists of a broad flat bottom with a bathymetry very similar to
southeast, north, and central areas except for the collapse sinkhole near
the southwestern shore. The lake bottom caused multiples that may have
masked features at depth. There are primarily two differences between the
southwest and other areas: (1) there were very few locations where the
subbottom had been disturbed by subsidence and/or collapse
(Fig. 15), and;
(2) this is the only area where collapse sinkholes have totally breached
the confining unit (Figs.
21, 22).
It is possible that other features are
present but access was limited by aquatic weeds and could not be profiled.
Near the southwestern shoreline adjacent to Heagy-Burry Park and the
boat ramp are the relatively large doline discussed above. Observations
reported by S.C.U.B.A. divers as mentioned earlier indicate there is a
downward flow of water from the lake into one of the sinks (Spechler, 1992).
This report and documented studies (Pirkle and Brooks, 1959) indicate there
is direct hydraulic connection with the Floridan aquifer. It was reported
that during the 1956 drought, drainage of Orange Lake was caused by the
flushing-out of the plug from this doline, allowing the lake to drain.
In Figures 21 and
22, the steep slope and fault
blocks of the northwestern
doline are shown, the mass movement of faults would open pathways for water
to migrate along the fractures and displaced blocks to the aquifer.
Sediment slumps moving down the slope into the doline are seen in Figure 21,
this is a part of the natural process of sediment transport into the doline
to form a plug. Fault block movement and collapse structures are shown in
Figure 22. Included in this
Figure and Figure 23 (below) are associated depressions
that are evidence of adjacent cover subsidence sinkholes.
A problem occurs when collecting data over the high-angle sloped sides
of a relatively small sinkhole in that the sides reflect sound waves to
produce side-reflections similar to the multiple or mirror images previously
discussed above. These side reflections are seen in the data in Figures
21
and 22.
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Figure 23:
Bathymetry (in milliseconds) of the sinkhole complex in
southwestern Orange Lake at Heagy-Burry Park. Part A is
a contour map of the lake bottom. Part B is a 3-D computer
model of the bathymetry. Location of the sinkhole is
shown in Figure 6. |
A three dimensional model of the sinkhole complex at the Heagy-Burry
park area is shown in
Figure 23. This model was
constructed by gridding the
two-way travel time for the lake bottom reflector. Over 400 points from
the seismic data were used to grid the contours. Two collapse sinkholes
and two adjacent subsidence features are evident from this model. In this
model a slight ridge separates the two structures.
It is not clear at this time if the features that the model shows are
remnants from the sinkhole that was observed in 1956 or are features that
have formed since that time. Past reports (Roland 1957, Jessen 1972)
indicate that a single hole approximately 63 meters (200 feet) in diameter
was exposed. There is no documentation of the original extent. A temporary
sandbag and earth dam was emplaced around the hole and subsequently collapsed
into the hole. Large quantities of fill dirt, a storage tank, and junked
vehicles were also put into the hole. There was no evidence in the seismic
data of buried vehicles or tanks.
Figure 24: Bathymetry of the sinkhole complex
superimposed on a 1990 aerial photograph of southwestern
Orange Lake at Heagy-Burry Park. White areas show the collapse
sinkholes, the cover subsidence features are shaded. Location
of the contours relative to the shoreline are approximate. |
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Remnants of the dam, and hence the most northern boundary of the 1956 sink, can presently be seen about 10 meters offshore of the park.
Figure 24 is a schematic of
the sinkhole contours superimposed on an enlarged aerial photograph of the
Heagy-Burry Park area. Horizontal distance on the contour plot are
referenced to a known horizontal distance (length of the boat dock) on the
photograph. Though more accurate survey location data is needed to
precisely reference the features, the schematic is a close approximation
for illustrative purposes. The Figure shows the extent of the collapse and
subsidence features within this area, including a subsidence feature on
land in the southeastern portion of the photograph. There is an area east
of the boat ramp (Fig. 24) that is continually subsiding and fill is
periodically put there to fill the depression. Though the limits of the
sinkhole that was observed in 1956 are not known, it is clear that this
area represents a sinkhole complex that is still quite active and expanding.
Orange Lake is a far more complex feature than previously considered
by investigators. The lake is primarily a clay basin catchment overlaying
a soluble and permeable bedrock of limestone. There are many small
(1 to 10 m, Fig. 20) and medium
(10 to 50 m, Fig. 18) cover subsidence
features throughout the lake
(Fig. 15). In the southeast area singular
small to medium features are closely spaced, forming larger areas (<100 m,
Fig. 14) of subsidence. The north
and central section of the lake also
contains small subsidence features, but these tend to be isolated from each
other. An exception occurs in the concentration of features seen near
Samsons Point. The largest and most important features in the lake are
the collapse sinkholes along the southwestern shore at Heagy-Burry Park.
These features provide a conduit for exchange between the lake and the
Floridan aquifer. These features, combined with the variable potentiometric
surface and lake levels, produce a very dynamic hydrologic and geologic
system.
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