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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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Lake Harney
Volusia/Seminole, Florida

Introduction | Subsurface Characterization

Subsurface Characterization

Lake Harney.
Lake Harney. Black lines show profile locations.
Seismic profiles from Lake Harney show a good example of subsurface karst imaging (A-A', B-B'). Profile A-A' shows a deep reflective surface (red line) with apparent subsidence. This feature is similar to a type 3 feature described in the explanation. This subsidence influences the integrity of the overlying strata, as shown in subsequent collapse across another reflective surface (blue line). Profile B-B' shows another deep collapse structure (red line). Within this subsidence, horizontal reflections onlap the steeper sides of the structure. This may represent fluvial or aeolian infilling of the depression. This type of infilling may have also occurred in the shallower depression shown in profile A-A', as evidenced by the patterned texture of the acoustic signal from the overlying material. This pattern could represent foresets or cross-bedding, as opposed to collapse-type infilling, which typically returns a noisier or chaotic signal. Comparison of the deeper subsidence structures between profiles A-A' and B-B' may provide insight into the timing relationships between collapse in the host rock and subsidence in the overburden. The deeper structure in profile A-A' does not appear to have the infilling seen in the B-B' subsidence. Also, the overburden in profile A-A' appears to be more disrupted than that overlying the subsidence in profile B-B'. The infilling might indicate that the deeper subsidence seen in profile B-B' may be a relic sinkhole that was aerially exposed and filled prior to accumulation of the overburden. The structure in profile A-A' represents a continued subsidence, controlled by the deeper featured. The disruption in the overburden in A-A' further supports continued subsidence, whereas the overburden in profile B-B' appears to be undisturbed.

Profile A-A'
Profile A-A'

Profile B-B'
Profile B-B'

Distribution of Features
Lake Harney Distribution of Features
(noted from seismic profiles)

Gamma logs to the west of Lake Harney show the top of the Ocala Limestone at about 12 m below mean sea level along the southern portion of the lake (Index Map E). This depth correlates well with the red reflections seen in the profiles. The deep structures may represent collapse in the Ocala. Likewise, the blue reflections in the profiles may represent material in the Hawthorn Group. High angle reflections (orange and gold) may represent stress in the overburden and indicate breaches through the otherwise impermeable Hawthorn sediments.

Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180 U. S. Department of the Interior | U.S. Geological Survey
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Updated December 05, 2016 @ 11:25 AM (JSS)