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Location of seismic profiles collected from the North and
Central area of Orange Lake. Click on the numbers in red to view
Figures 18,
19, and
20. |
The north and central areas of Orange Lake are geologically similar and will be combined
and discussed as one area (Fig. 6). The lake bottom
and subsurface of this area can be
described as relatively smooth with similar small subsidence features throughout the area.
This area encompasses the largest percentage of the lake and the bottom is covered with a
sandy-clay sediment most likely deposited by run-off from the surrounding shorelines. The
north and central part of Orange Lake proved to be a difficult area to collect geophysical
data. The composition of the surficial sediments and shallow water depth produced strong
multiples that masked much of the geologic data
(Fig. 18). A multiple is an artifact of the
acquisition process and occurs when sufficient energy is reflected from a horizon to the
water surface and back to the lake floor. This reflected energy appears in seismic profile
as a mirror image or multiple of the horizon that produced the original reflection. The data
shown in Figure 18
may indicate the early stages of a subsidence sinkhole due to the low
slopes of the sides. The central area is actively subsiding and/or collapsing and slumpage on the southwest flank is seen as
high angle faults.
Figure 19 shows a high angle fault or small
scale subsidence. The multiples have masked the structure below 4 milliseconds.
The features in this section of the lake are generally small (1 to 10 meters across) and
tend to be isolated
(Figs. 19, 20).
This is different than the features in the southeast area
that tend to be grouped together to form larger (>100 meters) features.
Limited by the poor quality data collected from 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. Seen in the seismic data were surface features and
shallow horizons (< 2m) that indicated there were a number of subsidence sinkholes in this
area. The subsidence features are of similar size, approximately 40 m across, and have
disturbed the surface sediments.
The features observed in the southwestern section of the North and Central area adjacent to
Samsons point are noteworthy for two reasons. The first is the presence of high angle faults
that cut through the lake bottom confining unit
(Figs. 18, 19).
This indicates either the
break has occurred in recent years or the less likely case that there has been no sediment
deposited on the lake bottom. This type of breach has the highest potential to provide a
pathway for leakage or accelerate the development of a collapse sink. The plasticity of the
clays may allow the break to suture however, and leave the bottom intact.
Figure 20 shows
the continued presence of strong multiples and recent or active small-scale subsidence.
These small subsidence features (3 to 5 m) are common throughout the area and typically
have a slight surficial
depression associated with their presence. These may represent plugged solution pipes.
The second noteworthy characteristic of the North and Central area is the clustering of
features near Samsons Point. Figure 15
shows the location of disturbances of the subbottom
at Orange Lake. Most of the nearshore area between Samsons Point and Boardman show some
form of subsidence features.
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