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Karst
Development and Characterization
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Karst
Development
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Karst
topography is created by a
chemical dissolution process when groundwater circulates
through soluble rock (Fig.
9). Carbon dioxide from the atmosphere is fixed or
converted in the soil horizon to an aqueous state, where
it combines with rainwater to form carbonic acid, which
readily dissolves carbonate rock. Root and microbial respiration
in the soil further elevates carbon dioxide partial pressure,
increasing acidity (lowering pH). In tropical and subtropical
regions such as Florida, abundant vegetation, high rainfall
and high atmospheric CO2
values favor the rapid dissolution of the preexisting
limestone.
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Karst
features develop from a self-accelerating
process of water flow along well-defined pathways. As
the water percolates downward under the force of gravity,
it dissolves and enlarges any pore or fracture in the
rock through which it flows. These pathways also include
bedding planes, joints, and faults (Fig.
10). Enlarging the fracture allows it to carry more
water, which increases the dissolution rate. As the fracture
gets larger and transmits more water, it begins to pirate
drainage from the surrounding rock mass. This process
will create areas where the rock is highly eroded with
very little dissolution around it, creating a very jagged
appearance to the substructure.
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Water
will continue to percolate downward until it reaches the
water table, below which all pore space is occupied by water.
Since the rock is saturated with water at this point, water
circulation is not as rapid and dissolution rates slow (the
dissolution potential of the water is expended over time).
However, the water table itself fluctuates up and down as
a result of seasonal change, drought conditions and groundwater
removal. This movement creates a zone of preferential dissolution
along the zone of fluctuation. Over time this process creates
pathways in the rock near the water table and provides a
very efficient means to transport water. During wet cycles,
the potentiometric surface of the confined aquifer may be
higher than the ground surface. This allows water to flow
through breaches in the confining unit and flow at the surface
as springs.
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Water
table fluctuations also occur on larger spatial and temporal
scales related to sea-level change. Numerous sea-level
cycles have occurred with lowstands extensive enough to
expose most of the Florida peninsula and create extensive
karst environments. These episodes of paleokarst development
have been documented (Randazzo,
1972; Randazzo
and Zachos, 1984) and can be identified in the rock
record as cycles of shallowing-upward sequences. Geologic
evidence includes the presence of evaporite deposits which
form during the shallow phase of sea-level cycle. A diagenetic
end-member of evaporites is hydrogen sulfide which, when
oxidized to sulfuric acid, will greatly enhanced the carbonate
dissolution process (Hines,
1997). The presence of paleokarst in the subsurface
may influence the development of modern day karst.
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