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Jim Flocks
  This section provides an overview of the hydrogeologic framework and associated seismic reflection characteristics of the different geologic units encountered in these profiles. Site specific information is discussed in the appropriate sites' section.

The deepest geologic units that were encountered by the seismic profiling are the Ocala Group carbonates of Eocene age. These carbonates are primarily limestones (CaCO3) that were deposited between 40 and 28 million years ago (mya). Over time, sections of the rock have been recrystallized into dolomite CaMg(CO3)2 and chert (SiO2). The limestone are generally tan to cream, highly fossiliferous lime mud, packstone to wackestone (Scott, 1992). Numerous fossils, especially, occur in the upper part of the unit and are used for formation identification. A well developed secondary porosity related to dissolution along joints and fractures makes this a highly productive water bearing unit. The Ocala Group comprise the upper Floridan aquifer in north central Florida. A well developed karst was sculpted on the limestone surface when it was exposed before Miocene time (24 mya). Evidence of the karst can be seen in the seismic records.

The Oligocene age (28 mya - 24 mya) Suwanee limestone occurs only as scattered outliers. These sediments were either completely eroded or were not deposited on the topographically high areas. This unit is included in the upper Floridan aquifer system.

  Figure 11A
Figure 11: Seismic profile (A, above) with line drawing (B, below) show a high amplitude seismic-reflection signature that may represent a subsurface collapse feature with associated subsidence. See Figure 6 for location.
Figure 11B
The seismic reflection records from these pre-Miocene units may exhibit a hummocky character at the upper surface. The limestone pinnacles, solution pipes and collapse features can truncate the horizontal reflectors of the overlying units. Cavities, fissures and other dissolution features may appear as hyperbolas or possibly produce no return (blank areas). There is little documentation of HRSP over known caves, however features were observed in the Orange Lake and the St. Johns River data that may be water filled voids. The carbonates of the upper Floridan aquifer are generally massive, with few horizontal internal reflectors. The areas that have been converted to dolomite or chert, however, may produce strong reflections that can be seen in the records (
Fig 11).

Sediments that were deposited during Miocene time (24 m ya -5.3 m ya) in north central Florida include the various formations of the Hawthorn Group. These sediments were first described by Dall (1892) at a site several miles northeast of Orange Lake near the town of Hawthorn. The lithology is highly variable and contains uncommon minerals such as palygorskite, phosphate, and opaline cherts (Scott, 1988). Impermeable massive clay and dolomite units that can prevent the vertical movement of ground water occur in the Hawthorne. Interbedded with the impermeable units are sands, sandy clays and fractured carbonate units that can be water producing zones. These units are referred to as the Intermediate aquifer system (Miller, 1986). Except where thin or breached by karst, the Hawthorn group is the main semiconfining unit to the Floridan aquifer. For all of the lakes that were profiled, the surface of the Floridan aquifer lies lower than the lake bottom, thus, if the semiconfining unit is breached water will flow from the lake into the Floridan aquifer. A catastrophic example of this occurred in the late 1800's when a sinkhole collapsed and drained the former Alachua Lake thereby creating Paynes Prairie (Pirkle and Brooks, 1959).

The interbedded nature of the Hawthorn sediments can provide good reflecting surfaces for acoustic signals. These layers appear on the seismic records as parallel bands. Folds, faults and facies changes can be recognized as bends, lateral and vertical discontinues, and truncations of the bands by other reflectors.

Overlying the Hawthorn Group are quartz sands, clayey sands and clays of Plio-Pleistocene age (5.3 mya to present). These occur as a surface veneer a few tens of feet (~10 m) thick or as a elongate ridges that may be over one hundred feet (~30 m) thick. The ridges expressions of relict shorelines created during Pleistocene interglacial periods (Cooke, 1945; White, 1970). Two prominent ridges are present in the lake sites that were profiled.

One of the ridges is named the Fairfield Hills (White, 1970). This ridge flanks the western shoreline of Orange Lake. The Fairfield Hills, in part, controls the morphology of Orange Lake by acting as a physical barrier to westward growth of the lake (Pirkle and Brooks, 1959). The sands from the hills have been reworked and deposited in Orange Lake, thereby filling depressions and creating a relatively flat bottom. From borings by Roland (1957), it does not appear that there is a thick lake-wide accumulation of these sands on the bottom and the sand accumulations on the lake bottom cannot be readily identified in the seismic records. The accumulations are relatively thin or they have mixed with clay and are so compacted that they only produce multiple bottom reflections (discussed later).

Lake Lowry, Lake Magnolia and Kingsley lake are flanked on the west by the Trail Ridge deposits. The lakes are underlain and surrounded by Citronelle sediments (Clark, 1964) which consist of a relatively thick section of unconsolidated to semi-consolidated quartz sands, clayey sands, and gravels. The Trail Ridge sands are above an elevation of 149 feet (45 m) and are mined commercially for heavy minerals used in paints and abrasives. This unconsolidation coupled with complete saturation enhances the filling process when sinkholes collapse or sediment is washed into the lakes by surface runoff. Generally the sands are seismically transparent but clay stringers or cementation may provide reflecting horizons.

The Plio-Pleistocene sands comprise the surficial aquifer system. This system is generally unconfined, has variable lateral continuity and can either provide recharge to or from the lakes. Water levels in the system are highly dependent on rainfall.

Structural features, such as fractures and faults, were created during post Eocene movement in the Florida peninsula. The first episode occurred in Late Oligocene through Early Miocene and the second episode occurred in Early Pliocene and into Early Pleistocene (Williams and others, 1977). This movement created the feature referred to as the Ocala Uplift. A deeper feature, the Peninsular Arch, has also caused faulting and fractures. The associated weakening of the rocks provided optimum conditions for dissolution and creation of the karst we see today.

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