|
 |
|
|
 |
Open File Report:
Seismic Reflection
Surveys |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| |
Southeast Area |
 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
 |
|
|
|
|
|
|
|
| |
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.
|
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: 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:
- Horizon A collapses and is infilled by Fill A forming Horizon B
- the collapse continues deforming Horizons A and B and Fill A
- a hiatus in collapse allowing Fill B and Horizon C
- 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.
top of page
 |
next section
|
