Lake Johnson
Clay County, Florida
Introduction Subsurface Characterization
Subsurface Characterization
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| Lake Johnson. Black line indicates profile location. |
The geologic units present at Lake Johnson are typical of northeast Florida stratigraphy. The surface material consists of Plio-Pleistocene sand hills. The lakes within these hills define the surficial aquifer system of the region. The natural gamma logs from two wells south of Lake Johnson (Index Map A, wells C-0453 & C-0457, gamma log cross-section A-A') indicate that these sands are present above +28.1 m (90 ft) NGVD on the western side below Lake Johnson and may be missing or only a few feet thick on the eastern section of the lake.
Below the sands, the clay and sandy clay of the Hawthorn Formation can be identified in the gamma logs by the sequence of high peaks between +28.1 m (90 ft) and -21.8 m (-70 ft) NGVD in well C-0453 (see gamma log cross-section A-A'). Most of the imagable area of the seismic profiles is within the Hawthorn Formation. Below -21.8 m (-70 ft), the Eocene carbonates of the Ocala Limestone are identified by the extremely low counts (less than 20 counts per second) on the natural gamma logs. These depths are below the imagable areas of the seismic profiles.
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Distribution of features
(noted from seismic profiles). |
Acoustic data from Lake Johnson is generally poor due in part to a strong lake bottom multiple, signal attenuation and technical difficulties. Parallel, horizontal reflections are present above 12 ms (8.7 m below lake surface) that may represent in situ and transported Plio-Pleistocene sands (see Profile A-A' below). Below 12 ms, only "windows" of interpretable data are present. Sections of the data can be resolved to approximately 32 ms (10 m). High angle reflections suggest collapse of material into a large sink (Type 1, A-A') and smaller, concave reflections that suggest subsidence into smaller sinks or dissolution pipes (Type 4, A-A').
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| A-A' |
Lake Johnson appears to have formed by the coalescing of many collapse sinkholes. After the initial collapse of an individual sinkhole, the sides have eroded into the central portion of the sink, flattening the banks and filling the center. A very steep-sided, deep sink is located near the entrance of the adjacent Gold Head Branch State Park and may represent the younger stages of this process. The area where the seismic profiles were run is relatively flat bottomed and shallow, indicating a more mature feature. Flow into Lake Johnson with no outflow indicates direct recharge to the aquifer.
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U. S. Department of the Interior | U.S. Geological Survey
