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Orange Lake - Southeast Area

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Jim Flocks
Location of seismic profiles collected from the Southeast area of Orange Lake. Click on the numbers in red to view Figures 11, 14, 16, and 17.
Figure 11 Figure 14 Figure 16 Figure 17 The southeastern area of Orange Lake at present has a water depth of one to three meters. The primary Formation of Karst Features within this area are cover subsidence sinkholes and associated fissures (fractures/solution pipes) (Fig. 4). Pirkle and Brooks (1959) describe how openings in the lake bottom have resulted from solution of the upper part of the Ocala limestone with slumpage of the Hawthorn sediments to close the holes. At times slumpage of Hawthorn sediments takes place simultaneously with solution of the Ocala. Other, less common Formation of Karst Features have been identified, such as buried sinkholes, faults, and features tentatively identified as subsurface cavities.

A subsidence structure found along the southern shore measured approximately 400 m in diameter (Fig. 14). The surface expression apparent on the lake bottom is only a slight depression. In the seismic profile and the line drawing interpretation of Figure 14, shallow faults are seen in the unconsolidated cover sediments. There may be more faults present but are below the resolution of this data and are not differentiated The unconsolidated sediments cover a high-amplitude reflection (Horizon HL) possibly produced by the density differential of consolidated sediments. Horizon HL forms a relatively bowl-shaped depression filled by the cover sediments which onlap HL along the edges of the depression. Within this consolidated unit are faults and buried subsidence sinkholes. This cover subsidence sinkhole was not formed by a single sinkhole or type of sinkhole, but a combination of subsidence and buried subsidence features. This example shows several high-angle faults in which most have very little vertical displacement and none that recognizably displace the surficial sediments of the lake bottom. Leakage of lake waters to the aquifer in these and similar areas would be probably controlled by the permeability of the cover sediments or by faults that penetrate the lake floor. Of the features identified in the southeast area, very few appeared to have broken through the lake bottom thereby providing a pathway for enhanced leakage. The features are buried by either original deposition of Hawthorn clays or clay and sand deposited on the lake bottom during the development of the lake.

  Figure 15
Figure 15: Location of areas noted from seismic profiles where subsidence or collapse has disturbed the subbottom of Orange Lake.
Buried subsidence features are present throughout the southeast area of the lake (Fig. 15, at right). Figure 16 is a seismic profile and line drawing interpretation of an inactive buried sinkhole. This feature, smaller than the sinkhole described in Figure 14 (400 m), is approximately 100 m in diameter. A simple interpretation for this complex feature is that the sinkhole subsided and was subsequently filled by sediment seen as onlapping reflectors within the depressions. A complex explanation would include that an initial sink formed and differential subsidence occurred after the subsidence of the original sink. Subsidence features like these are common within the southeast area of Orange Lake. Buried by less than one meter of sediment, this sinkhole is an example of the composite mature subsidence sinkhole shown in Figure 2. These are indicative of a quiescent area with little or no subsidence occurring at present.

A limited number of depressions found in the southeastern area may be linked to more recent sinkhole formation. An example shown in Figure 17, seismic profile with line drawing, is a relatively small subsidence sinkhole with minimal overlying cover. The sink is less than 50 m in diameter and unlike figures 14 and 16, it is a single sinkhole without other features superimposed. A pattern of disturbed horizons implies active subsidence with intervening periods of deposition. This pattern would form in stages:

  1. Horizon A collapses and is infilled by Fill A forming Horizon B
  2. the collapse continues deforming Horizons A and B and Fill A
  3. a hiatus in collapse allowing Fill B and Horizon C
  4. collapse continues the process
This process while geologically important precludes any communication between the lake and aquifer unless a complete collapse occurs to flush the sediment plug from the sink.

The detection of underground caves or cavities by geophysical methods is an inexact technique. Few examples of cavities are shown in seismic profile are found in the scientific literature, but an attempt has been made to identify unverified seismic characters in Orange Lake as a cavity or collapse structure associated with the cavity. Figure 11 shows such a seismic example, approximately 11 m (15 milliseconds) below the lake bottom, that is a high-amplitude seismic reflection directly below a small buried sinkhole (approximately 50 m across). It is speculated that the high-amplitude seismic reflection represents rubble or debris from the partial cover collapse through a solution pipe into a cavity. This is a gradual sinkhole development that may occur where a clay stratum overlies the limestone and is below the water table (Beck, 1988; Beck and Sayed, 1991). As support is eroded from beneath the clay by erosion down the solution pipe, the clay becomes loose and may absorb more water. This loose, water-saturated clay may "sag" or collapse to the floor of the void. Similar seismic signatures are found throughout the southeast area of Orange Lake and are not thought to be artifacts of the sampling system. Alternatively, these high amplitude reflectors may represent sections of the limestone that has been replaced by chert.

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