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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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Introduction
Methods
Regional Geology
Karst Development & Characterization
Sinkhole Lake Evolution & Effect of Urbanization
Identification of Karst Features from Seismic Patterns
Summary
Acknowledgements
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Drayton Island (Lake George)
Putman County, Florida

Introduction | Subsurface Characterization
Drayton Island (Lake George)
Drayton Island (Lake George). Black lines show profile locations.

Subsurface Characterization

Three example seismic profiles near Drayton Island show several different types of geologic characterization present within the St. Johns River Valley. Karst development in the underlying limestone is accompanied by fluvial-type incised channels, occupying areas of subsidence caused by the loss of material at depth. Profile A-A' below shows relatively mature karst development in the limestone, represented by the red dashed line at 30 to 45 m (92 to 148 ft). Gamma log profiles from four wells surrounding the northern portion of Lake George (P-0410, V-0346, M-0149 and M-0021; Index Map D) show a highly fluctuating upper contact to the Ocala Limestone. Depths to limestone range from greater than -61 m (-200 ft) below sea level southwest of Drayton Island, to -30 m (-100 ft) to the west, to -15 m (-50 ft) at the lake's eastern shoreline. The variability and range are consistent with the contact represented by the red dashed line on profiles A-A' and B-B'. In profile A-A', a fluvially-incised channel (light brown line) appears to reside over one of the more pronounced depressions in the karst surface. Multiple incisions appear within the channel (orange line) with fill (purple lines). Channel development was apparently terminated and a planing surface (green line) is overlain by a more recent depositional event (solid red lines). This sequence can be correlated to spikes in the gamma counts at -12 m (-40 ft) below sea level (P-0410, V-0346 and M-0149, Index Map D), suggesting a fluvial source, possibly a Pleistocene flooding surface and estuarine deposition, as seen elsewhere within the St. John's Offset (Brooks and Merrit, 1981). These low-angle reflections are also truncated (dark blue line) and what appear to be recent, riverine deposits occupy the nearsurface of the profile. On the right side of the profile there appears to be another drop in the limestone surface which is also occupied by a channel incision (green lines), but most of this feature is obscured by noise in the seismic record.

Seismic Profile A-A'
Profile A-A'

Seismic Profile B-B'
Profile B-B'

Seismic Profile C-C'
Profile C-C'

Profile B-B' exhibits similar fluctuations to the karst surface, with another incised channel taking shape (brown and orange lines) before being obscured by noise in the record. The truncation surface and subsequent depositional event represented by the solid red lines in profile A-A' are not as readily apparent. It is possible that the orange lines in profile B-B' may be correlative with this depositional event. The more recent hiatus (dark blue line) and overlying fluvial deposits are consistent in both profiles. The relationship of these incised channels to subsidence in the underlying geology is probably geomorphologic; channel development occurred within previously existing depressions and was not necessarily concurrent to karst development.

The shape of the channel incisions and the nature of their fill are similar to buried incised channels observed in seismic profiles acquired from the nearshore shelf environments of the Gulf and Atlantic coasts. The feature outlined in profile C-C' is characteristic of karst-type subsidence rather than a fluvial incision. Again the deepest red reflection may be correlative to the top of the Ocala Limestone, the overlying reflections may represent subsequent subsidence in the sediments of the Hawthorn Group. Reflections exhibit subsidence up to the near-surface, suggesting the karst feature in this profile post-dates the fluvial deposition shown in the previous two profiles. The uppermost subsurface reflections (dark blue line) is again overlain by high frequency, parallel reflections which may be representative of recent fluvial deposition.

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 May 06, 2013 @ 09:24 AM (JSS)