|
 |
|
|
 |
Open File Report:
Seismic Reflection
Surveys |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| |
Karst & Sinhole Features |
 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
 |
|
|
|
|
|
|
|
Description of Karst and Sinkhole Features in Seismic Profiles from Orange Lake
Centered approximately 29° 28' N, 82° 11' W, in Alachua county Florida,
Orange Lake is presently a relatively shallow, irregular-shaped lake with
much of the shore covered by freshwater marsh. The physiographic development
of Orange Lake and the immediate surrounding area is linked to the erosion of
Hawthorn sediments, exposing, in numerous places, the Ocala limestone of
Eocene age (Pirkle and Brooks, 1959). Pirkle and Brooks (1959) reported
that the Ocala limestone is exposed or is under only a thin cover of
sediment throughout most of the region and during the lake lowstand of
water in 1956, Ocala limestone was exposed in the southwestern part of
Orange Lake.
Except for the dolines (collapse sinkholes) near the southwest shore,
the surficial sediments of the lake bottom are organic and/or organics
mixed with sand and clay transported into the lake by storm run-off and
streams (Roland, 1957). This organic-clayey-sand layer is underlain by
semiconfining clays (Hawthorn Group). It was suggested by Pirkle and
Brooks (1959) that these sediments were impermeable and when the water
table drops the lake level is held above the ground water level. Under
sufficient hydrostatic pressure this material will fail and be flushed into
solution channels in the limestone. Once the outlets are opened, the lake
water will adjust to the level of the water table in the limestone unless
the sinks again become plugged. In contrast, water-table lakes with bottoms
developed in permeable materials and with no bottom cover of impervious
sediments will rise and fall with ground-water level.
Historically, the water level in Orange Lake has fluctuated drastically.
Such an occurrence was documented in 1956 when the lake area was reduced
from 16,000 to less than 5,000 acres. The drop in water level occurred as
the Orange Lake basin, that had been filled by abundant rain, was subjected
to a long dry period with a gradual lowering of the potentiometric surface
of the Floridan aquifer. These conditions may have triggered the flushing
out or collapse of the doline at the southwestern edge of the lake.
Breaching of the confining units provides direct hydraulic communication
with the Floridan aquifer. The thickness and continuity of confining units
are crucial to preventing leakage from the lake to the Floridan aquifer.
This lake was divided into three areas
(Fig. 6) based on distinct
karstic features identified from seismic profiles. The primary sinkhole
features found within Orange Lake are various stages of cover subsidence,
cover collapse, and buried sinkholes (Figs. 2,
4).
top of page
 |
next section
|
