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Open File Report:
Seismic Reflection
Surveys |
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Regional Geology |
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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.
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| 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. |
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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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