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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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Karst Development & Characterization
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Identification of Karst Features from Seismic Patterns
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Lake Magnolia
Clay County, Florida

Introduction | Subsurface Characterization
Lake Magnolia
Lake Magnolia. Black lines show profile locations.

Subsurface Characterization

Profile A-A' (below) shows the basic character of Lake Magnolia, which appears to be comprised of a single depression. The characteristics of this lake are very similar to Kingsley Lake, Blue Pond and several other lakes in the region. The subbottom was disturbed during the subsidence then covered and infilled similar to Types 2 and 6 karst features described in Fig, 16. In the northwestern corner of the lake is a buried block that has rotated and slumped into the sink (Type 3 karst feature).

Profile A-A'
Profile A-A'

Profile B-B'
Profile B-B'

The main depression is continuous across the lake in both of the predominant traverse directions. Profile A-A' from Lake Magnolia shows a singular subsidence that is shallow nearshore with onlapping fill on the northwest flank. The central portion of the lake has an undisturbed surficial layer that is acoustically transparent and is possibly composed of high-organic sediments termed "gyttja". The undisturbed nature of the surficial sediments implies that there has been little to no recent subsidence. In this case, as the sink became plugged, the lake developed a shallower and flatter bottom due to the infilling associated with runoff and eolian processes.

Correlation of gamma logs from the boreholes to contacts seen in the seismic records is tenuous. Log C-0451 (see Index Map A and gamma log cross-section B-B') is from a well approximately 1 km west of Lake Magnolia and log C-0439 is from a well located on the northwest shore of Sand Hill Lake (see Index Map A and gamma log cross-section A-A'). The units identified from the gamma logs are the clay confining units of the Hawthorn Group and the top of the Ocala Limestone. The blue horizon in profile A-A' has been interpreted as a reflection near the top of the Hawthorn Group.

Sinkhole lakes can be delineated into a progressive sequence of lake formation based on geomorphic types (see Sinkhole Lake Evolution & Effect of Urbanization). 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 Magnolia is used as the type description for transitional phase or middle-age lakes. 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 (fractures and solution pipes), allowing the sinkhole to reactivate and revert to an active subsidence phase. 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 02, 2016 @ 11:25 AM (JSS)