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Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

Subsurface Characterization of Selected Water Bodies in the St Johns River Water Management District, Northeast Florida

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Sand Hill Lake (Lake Lowry)
Clay County, Florida

Introduction | Subsurface Characterization

Subsurface Characterization

Sand Hill Lake (Lake Lowry)
Sand Hill Lake. Black lines so location of profiles.
Subsidence features observed from profiles show two areas identified as large (1000 m) subsidence sinkholes (see Track Map). One forms the northwest section of the lake and the other is in the southeast area. Profile A-A' (below) 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.

Profile A-A'
Profile A-A'

Profile B-B''
Profile B-B'

A ground water well located on the northwest shore of Lake Lowry is used to monitor the Floridan aquifer, identified as well C-0439 on cross section A-A' (see Index Map A and gamma log cross-section A-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.

Sinkhole lakes can be delineated into a progressive sequence of lake formation based on geomorphic types (Sinkhole Evolution). The progression begins with the initial collapse, forming a sinkhole. The depression may be open or, if a portion of the depression is below the water table, it may be filled with water. As sediments are washed into the depression, the sinkhole becomes plugged. The process continues until the sinkhole is buried.

Lake Lowry is in the transitional phase (middle age), when the lake becomes partially or completely plugged, the lake begins to develop a shallower and flatter bottom. During this phase the plug may be flushed into the karst (faults, fractures, and/or solution pipes), allowing the sinkhole to reactivate and revert to an active subsidence phase, described above as subsidence activity and hiatus with slow sediment deposition (A-A' and B-B'). This may occur several times until sediment accumulates faster than dissolution of the underlying limestone.

Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

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Updated December 05, 2016 @ 11:25 AM (JSS)