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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 Harris
Lake County, Florida

Introduction | Subsurface Characterization

Subsurface Characterization

Lake Harris
Lake Harris. Black likes show profile locations.
Multiples persist in the seismic profiles throughout the central portions of the lake. This is characteristic in lakes where the bottom sediments are hard sands or rock. Not enough data was collected to produce contours of subsurface features, additional data is necessary to provide adequate coverage. Scott (1988), estimates the top of the Hawthorn Group to be greater than 15 m (50 ft) above mean sea level in a nearby core. Lake level at the time of the survey was 19 m (63 ft) NGVD. This would suggest that the lakes that occupy the interstices of the sand hills in this area are floored within the Hawthorn Group, which contains phosphatic sands, limestone and dolomites. In two places the acoustic return is obscured by noise, or "gassed out" (C-C'). This could indicate an accumulation of organic material in the bottom sediments which acts to disperse the signal. Profiles A-A' and B-B' show areas where a reflective horizon can be seen dipping away from the surface. Associated with this is a subsidence depression in the lake bottom. The feature resembles that of a type 2 feature, although little or no infilling is visible in the record. Another possibility is that the dipping horizon could represent a down-faulted or rotated block that has subsided into a large collapse structure at depth. However, multiples and noise obscure the record so that if any deeper, influencing structures are present they are not visible.

Seismic Proflie A-A'
Profile A-A'

Seismic Proflie B-B'
Profile B-B'

Seismic Proflie C-C'
Profile C-C'

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

Gamma logs indicate the top of the Hawthorn Group to be near the surface from seven meters depth to the west of the lake. The blue horizon from the seismic profiles may correlate with this contact (A-A', B-B' and Index Map F). Profile C-C' shows an example of small scale lake bottom subsidence within the lake that could be considered an active sink. No influencing features below the subsidence depression can be seen because of the persistent multiples, although dissolution within members of the Hawthorn Group is probably occurring. The subsidences are similar in size to the numerous small sinks visible to the south of the lake and trending to the northwest. It is possible that the lake bottom subsidences represent a lakeward extension of this karst trend.

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)