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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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Cowpen Lake
Clay County, Florida

Introduction | Subsurface Characterization

Subsurface Characterization

Cowpen Lake
Cowpen Lake. Black line represents profile locations.
The acoustic signal in Cowpen Lake as a whole is relatively weak. This is shown in the example profiles (A-A', B-B' and C-C' below). Factors which contribute to a noisy or weak acoustical return in the lakes of this study area include proximity of hardbottom (limestone) to the sediment surface, accumulation of organic debris on the lake bottom, shallow water depths and proximity, steepness and irregularity of the shoreline.

In Cowpen Lake, the nearby and irregular shoreline could create interference (noise) in the signal, and the marshy area in which it resides could produce organic-rich surficial sediments which dampen the return. As a result, little can be seen in the seismic profiles below about 10-12 m. The lake bottom shows an undulating surface marked by localized subsidence less than tens of meters in width. Accumulations of material is imaged near surface in the bathymetric lows (red lines, profiles B-B' and C-C').

Profile A-A'
Profile A-A'

Profile B-B' Profile C-C'
Profle B-B' Profile C-C'

This could represent fill from the surrounding sand hills. In the subsurface, high frequency reflections occasionally can be seen (orange dashed lines, A-A' and B-B'). These may represent dissolution-type features or disturbed bedding and could indicate breaches in the overburden. The contact between the top of the Hawthorn Group and overlying undifferentiated fill is interpolated to be around 12 m (39 ft) below lake level (Index Map A). This contact is difficult to detect in the seismic profile because of the signal noise and multiples, but the disturbed bedding at depth would indicate breaches in the confining Hawthorn Group. The top of the Ocala Limestone is estimated to be around 20-30 m (67-98 ft) below lake level (~0 ft NGVD), but is obscured in the profiles. Solution of the limestone at depth could produce dissolution type features which transport material downwards and can create the smaller subsidence areas seen at the surface.

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 May 06, 2013 @ 09:24 AM (JSS)