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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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Crescent Beach Spring
near St. Johns County, Florida

Introduction | Subsurface Characterization

Subsurface Characterization

Crescent Beach
Crescent Beach Spring

Numerous transects across Crescent Beach spring were conducted in 1994 to acquire HRSP of the subsurface. Two examples of the profiles are shown as A-A' and B-B'. Unfortunately the navigational fixes attached to the digital seismic data have been lost, so horizontal scale of the profiles and their geographic location cannot be determined at this time. Another survey of the sink using Side-Scan Sonar, HRSP and a fathometer were done in 1998.

The seismic profiles (A-A' and B-B') show numerous strong, parallel reflections from about 30 m (98 ft) to 60 m (197 ft) below sea level. Gamma log profiles (Index Map H), interpreted from Gamma counts acquired from inland wells drilled within eight kilometers of the spring show numerous peaks in gamma counts at the base of the Hawthorn Group. These peaks are at similar depths to the strong reflections in the seismic profiles. The package of reflections highlighted by an orange background (A-A' and B-B') exhibits a slightly more "noisy" characteristic than adjoining reflections and may represent different lithologic or stratigraphic parameters. The series may correlate with the higher frequency gamma count peaks seen in gamma log profiles SJ0798 and SJ0171 (see gamma log cross-section) between -21 m (-70 ft) and -30 m (-100 ft). Below 60 m (197 ft) the strong reflections diminish in the seismic profile (not shown). This change in acoustic return may represent the top of the Ocala Limestone. In the gamma profiles this surface is indicated by a blue line and ranges from an elevation of -53 m NGVD (71 ms or -170 ft) at SJ0151 to -69 m NGVD (92 ms or -220 ft) at SJ0162 (see gamma log cross-section) . Though there is insufficient data to confirm the identity of one reflecting horizon as the top of the Ocala Limestone, it is most likely around 70 ms (53 m, 174 ft) on the seismic data.

Seismic Proflie A-A'
Profile A-A'

What is readily apparent in the HRSP examples is the very large (~1 km) subsidence feature evidenced by the downwarped reflections within the Hawthorn Group. Discontinuities in the horizontal reflections (yellow vertical lines) may represent stress fracturing associated with the downwarping. Meisburger and Field (1976) identify this large subsidence feature as a pronounced fold. Popenoe and others (1984) identified the top of the Eocene on the downward flexure of the fold to be between -47 m (-150 ft) in the undisturbed section to about -75 m (-240 ft) at the deepest part. Karst-related dissolution at depth and subsequent near-surface subsidence might be another explanation, rather than a structural fold. The area highlighted by a green background (A-A' and B-B') appears to contain offlap and cross-bedded reflections that may represent fill when the depression was exposed.

Profile B-B'

The downwarped reflections of the Hawthorn Group are truncated at about 22 m depth, shown by the blue dashed line in the example profiles. This surface, and a second one near surface (red dashed line), may represent erosional surfaces related to sea level low stands. The area highlighted by a red background shows a second depression with offlap- and cross-bedded fill. This feature may represent an area of resumed subsidence following the first sea level cycle. It may also be an incised fluvial channel with fill occupying the topographic low created by the original subsidence event. These sequences of truncation surfaces and fill may be remnants of the last two sea level cycles, the parallel reflections overlying these sequences being the most recent marine deposition.

3-D Model of Bathymetry at crescent Beach Sink

At the sea floor the spring vent appears to be independent of the large subsurface subsidence feature. Although their relationship at depth is not resolvable from the seismic profiles, their formation is probably a manifestation of major dissolution (mega-void) within the underlying limestone. The vent incises the most recent marine sequence, which suggests that the vent is recent. The north (left) flank of the sink is higher in elevation than the south flank (B-B'). Sediment removal from the vent may be accumulating on the prevailing down-current side of the vent. Another possibility is that the vent may occupy a fault line, with the southern flank being a down dropped block. Popenoe and others (1984) mapped numerous downward flexures and fractures traces along the northeast coast of Florida. The reflections within the Hawthorn Group show some minor displacement. There is some definite discontinuity in the reflections below the sink (B-B') which could represent the breach within the Hawthorn and the migration pathway for the freshwater discharge. At depth in the seismic profiles the signal does appear to be slightly more chaotic than the neighboring acoustic return (outlined with yellow dashed line). This effect could be from noise in the signal caused by the sink itself, or it could be a zone of recrystallization or more advanced karst development within the Ocala Limestone. Removal of limestone by dissolution may have created a cavity and caused subsequent roof collapse and fill of the void. Increased dissolution within the Ocala Limestone would be the cause and effect of the fluid-migration pathway related to the spring vent and freshwater discharge at the sea floor.

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)