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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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Wekiva River

Introduction | Subsurface Characterization

Subsurface Characterization

Wekiva River
Wekiva River: Black lines represent profile locations. Numbers represent well locations.
As often encountered during surveys of rivers in this area, the data quality is generally poor. The common problem is the rivers are shallow and acoustic echoes from the river banks add to noise in the signal which obscure any subsurface features. Profiles A-A' and B-B' are two examples where some subsurface features can be seen. Profile A-A' shows a subsurface depression at depth that has been filled, similar to a type 3 karst feature. Throughout the profile several type 4 dissolution features appear to be present. Horizontal reflections at the nearsurface (green lines) may represent more recent fill. Profile B-B' exhibits a persistent reflection (blue line) throughout the profile that may represent a horizon within the Hawthorn Group or the contact between the Group and overlying undifferentiated sands and clays. The left side of the profile shows an area of disturbance in the subsurface, where type 4 dissolution features may be present. These features are characterized as distinct parallel and sub-parallel reflections with a higher amplitude than the surrounding material and may represent filled solution pipes. It appears that some subsidence has occurred with subsequent fill to the present day river bottom, however, the discontinuities at depth do not reach the present day river bottom and may not affect the more recent fill.

Seismic Proflie A-A'
Profile A-A'

Seismic Proflie B-B'
Prolfie B-B'

The natural gamma log cross section A-A' shown on Index Map E shows the elevation of the top of the Ocala Limestone in wells OR0652 (-40 m [-130 ft] NGVD) and L-0037B (-43 m [-140 ft] NGVD), below resolvable depth in the seismic records. The areas of disturbance seen in the profiles may represent areas of subsidence within the Hawthorn Group as it accommodates dissolution in the underlying limestone. The gamma log cross sections show how variable the sand and clay units within the Hawthorn Group and overlying undifferentiated sediments are. Peaks in the gamma logs are not laterally continuous and the thickness varies considerably.

An additional cross section comes from three wells drilled on the small islands in the Wekiva River (E-E', Index Map E). Again, the variability within the post-Tertiary units is apparent, note the high peak in well S-0866 at 3 m (9 ft) NGVD. One mile north in well S-0864, this peak is completely missing as are all of the other clay rich beds in S-0866. The log for S-0864 shows 17 m (55 ft) of clean material, probably sand, below which is a clay unit that may correlate with the blue line in profile B-B' and represent the top of the Hawthorn Group. The gamma peaks in well S-0866 show that the Hawthorn Group may extend to about -6 m (-20 ft) to -8 m (-25 ft) NGVD. These logs probably do not represent the total drilled depth of the wells since they do not penetrate the Ocala Limestone, yet surface flow and water quality are indicative of the Floridan aquifer. Depth to the Ocala is inferred to be at about -32 m (-105 ft) NGVD (~44 ms), beyond the resolvable depth in the seismic profiles in this area.

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)