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Orange Lake - Southwest Area

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Jim Flocks
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.
Figure 21 Figure 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.

  Figure 23
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
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.
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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Updated May 06, 2013 @ 09:24 AM (THF)