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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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North East Florida Atlas
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Regional Geology You are at the Regional Geology section of the NE Florida Atlas
Karst Development & Characterization
Sinkhole Lake Evolution & Effect of Urbanization
Identification of Karst Features from Seismic Patterns
Summary
Acknowledgements
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Regional Geology

In north-central Florida, sinkholes at the surface are generally related to the dissolution of two host limestone units, the Ocala and Suwannee Limestones (see stratigraphic column, Fig. 4). The Eocene-age Ocala Limestone was deposited between 40 and 28 million years ago (ma). Over time, sections of the rock have been recrystallized into dolomite, CaMg(CO3)2. Ocala Limestone is generally tan to cream, highly fossiliferous lime mud preserved as packstone to wackestone (Scott, 1992). Above the Ocala Limestone is the Oligocene age Suwannee Limestone (28 ma-24 ma) only present as scattered deposits in low topographic areas and typically absent on the topographically high areas. Figure 5 (below) shows the surface distribution of these units.

Surface stratigraphy of the Floridan Peninsula.  Approximate limits of the Hawthorn Group, along with structural controls.
Figure 5. Surface stratigraphy of the Floridan Peninsula. Map from Lane, 1994. Click on the image for a larger version. Figure 6. Approximate limits of the Hawthorn Group, along with structural controls. Contour intervals in feet. Modified from Scott, 1988. Click on the image for a larger version.

Overlying the Ocala and Suwannee Limestones is the Hawthorn Group (Miocene 24-5.3 ma). This group is composed of massive impermeable clay and dolomite units. Interbedded with these impermeable units are sands, sandy clays and fractured carbonate units (Miller, 1986). Except where thin or breached, the Hawthorn Group is the main semiconfining unit to the Floridan aquifer in north-central Florida. The thickness, stratigraphic position and confining nature of the Hawthorn Group determines the form and function of sinkholes. The Hawthorn Group is absent from the structural highs such as the Ocala Uplift to the east of the study area and the Sanford High (Fig. 6) . It maintains a thickness of 9 to 18 m (30-60 ft) across the St. Johns Platform and thickens to over 46 m (150 ft) over the Jacksonville basin (Mallinson and others, 1994). In most of the sites profiled, the potentiometric surface of the Floridan aquifer lies below lake surface. This condition creates a downward gradient which allows water to permeate through the Hawthorn Group sediment from the lake into the limestone units below. Additionally, breaches in the Hawthorn Group allow enhanced surface groundwater interaction. An example of catastrophic breaching occurred in the late 1800s when a sinkhole collapsed and drained the former Alachua Lake, thereby creating Paynes Prairie (Pirkle and Brooks, 1959b).

Quartz sands, clayey sands and clays of Plio-Pleistocene age (5.3 ma-30 ka) overlie the Hawthorn Group and occur as a surface veneer ~10 m (33 ft) thick or as elongate ridges that may be over 30 m (98 ft) thick. The ridges are expressions of relict shorelines created during Pleistocene interglacial periods (Cooke, 1945; White, 1970). These ridges and related features that developed during the Plio-Pleistocene sea-level cycles form the current physiography of the Floridan Peninsula (Fig. 7). This physiography is highly perforated by karst terrain.

Faulting within the deeper sediments of Florida have long been a source of controversy (Scott, 1997). Faulting occurred during the late Oligocene to early Miocene and again through the Pliocene to early Pleistocene. Williams and others (1977) suggest the movement created the Ocala Uplift (Fig. 6). Pirkle and Brooks (1959a) believe the uplift was a pre-Hawthorn Group occurrence. Opdyke and others (1984) suggest uplift was due to isostatic rebound in response to loss of the carbonate load by dissolution processes, they reported that at least 12 x 108 cubic meters (4 x 1010 cubic feet) per year of limestone are lost from peninsular Florida. This loss, over a period of 38 ka years could result in a rebound of 33 m (108 ft). A deeper and older (early Cenozoic, 60 ma) feature, the Peninsular Arch, has also caused faulting and fractures. The associated weakening of the rocks provides optimum conditions for dissolution and formation of karst.

Physiographic regions of Florida. Physiographic provinces of Florida.
Figure 7. Physiographic regions of Florida. Modified from Randazzo and Jones (eds.), 1997. Click on the image for a larger version. Figure 8. Physiographic provinces of Florida. Modified from Scott, 1988. Click on the image for a larger version.

The term "fault" as used in this report refers to vertical displacement or discontinuities occuring at a specific site. Primarily, these are faults resulting from sediment slumping into a sinkhole depression, or tension faults. No large scale faults that can be mapped regionally and reflect a tectonic origin were identified.

From the surface geology and physiographic regions, Scott (1988) delineated physiographic provinces for the peninsula. Provinces included in the study are shown above (Fig. 8).

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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