|
|
|
Sand
Hill Lake (Lake Lowry)
|
|
|
|
Subsurface
Characterization
|
|
Subsidence
features observed from profiles show two areas identified
as large (1000 m) subsidence sinkholes (Track
Map). One forms the northwest section of the lake
and the other is in the southeast area. Profile A-A’
illustrates a large combined subsidence feature that includes
buried and active subsidence (Type 2). This cross section
shows a variety of depositional fills including cross
bedding and onlapping fill. Also found in each profile
( A-A’, B-B’)
is the characteristic pattern of fill and subsidence that
indicates rapid subsidence activity and hiatus with slow
deposition of sediment. Each of the sinkholes may have
developed independently and coalesced over time. Lake
Lowry has many of the same characteristic cover-subsidence
characteristics found in Orange
Lake, but they are much larger.
|
A
ground water well located on the northwest shore of Lake
Lowry is used to monitor the Floridan aquifer, indentified
as well C-0439
on cross section A-A’ (Index Map A). The
natural gamma log of the well indicates the top of the
Floridan aquifer is at -57 feet NGVD or approximately
80 ms on the seismic data. The majority of resolvable
data on the seismic profile is above 20 ms and so it cannot
be determined if the entire confining unit is breached.
|
|
The
mechanical processes that result in lake development are
a slumping or subsidence of underlying clastics or carbonates,
and a clustering of sinkholes. Two factors that effect lake
formation are karst development in host limestone and thickness
of unconsolidated overburden (the confining unit). If the
host limestone is highly karstic then the probability of
collapse is greater than in areas of less karst. Thickness
of overburden is the other controlling factor. A slight
surface depression will form over a collapse in an area
with a thick unconsolidated overburden (ten’s to 100
m). As the unconsolidated material fills the depression
left by solution, there is little or no accommodation space
for lake formation. A larger surface depression will form
if the same collapse were to occur with two meters of overburden,
thereby creating accommodation space for lake formation.
|
|
|
|
