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Open File Report:
Seismic Reflection
Surveys |
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Karst Formation |
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Formation of Karst: A General Discussion of Nature and Origin
Many researchers have conducted in-depth studies of karst and related features.
Many of the statements herein are general statements that have been drawn
primarily from Beck and Sayed (1991) and Waltham (1989).
Sinkholes are only one part of the total landscape that is formed on
rocks that are relatively easily dissolved. The solution process also
creates unusually large pores and channels in the bedrock, including caves,
through which ground water surges to the land surface as springs. In
general the solution process begins as rainwater absorbs carbon dioxide
from the atmosphere and as it percolates downward through the soil, which
is high in CO2 generated during the decay of organic matter. This makes
the recharge water weakly acidic (carbonic acid), and it is this acid that
dissolves the limestone (CaCO3). This is a powerful process, an estimate
of 600 tons per day of CaCO3 is dissolved in the water being discharged
from Silver Springs in Ocala, Florida (Sellards, 1910). A summary of
limestone dissolution rates from sites worldwide range from 55 to 100 m3
km-2 a-1 with majority of dissolution occurring in the upper 10 meters or
in subsurface conduits (Ford and Williams, 1989).
When the slightly acidic water reaches the limestone it continues
moving downward through any interconnected pores or fractures, under the
force of gravity. As the weak carbonic acid flows downward through the
limestone, it dissolves and enlarges any pores or cracks through which it
flows. The most favored vertical path dissolves more rapidly than the
surrounding areas, because it carries more water. Because it is now
larger it can transmit more water in ever greater quantities, thus pirating
drainage from the surrounding rock mass. This "self accelerating" process
results in few greatly enlarged tubes or pipes permeating down through the
limestone, with little dissolved rock in between.
Figure 4:
Types of sinkholes and stages of development. Stages E and F
are subtypes of Stage C. (modified from Culshaw & Waltham,
1987).
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Although there are generally five different types of sinkholes
identified (Beck, 1988; Waltham, 1989;), these sinkholes are the result of
only two different processes: transport of surficial material downward along
solution-enlarged channels, or collapse of the rock roof over large bedrock
cavities (Figure 4) (Beck and Sayed, 1991). A classic solution sinkhole is
formed if the limestone is bare, or almost bare (exposed at the ground
surface). Water flows over the limestone surface and converges on dissolution
pipes and dissolves the limestone around them resulting in a bowl-shaped
depression (Fig. 4A).
If the limestone contains insoluble mineral they may be left behind
on the surface as a thin soil residue (mantle) which tends to accumulate in
the bottoms of the sinkholes. The slopes of thebasin, however, are generally
covered by residual limestone gravel or bare limestone bedrock. If a
sufficient mantle accumulates, the basin may become plugged and a
small sinkhole lake may result. As the mantle deposit thickens these overlying,
unconsolidated sediments may simply be termed "cover" and the resulting
sinkholes are cover collapse or cover subsidence sinkholes (Fig. 4E,
4F). For the sites profiled, the "cover" may consist of thin cohesive clays and
unconsolidated sands that favor cover subsidence development or thicker
cohesive clays and sands that favor cover collapse development. A
combination of collapse and subsidence processes may also occurs.
Precipitation can now percolate through the sediment and seep down to
the limestone surface. Once reaching the impremeable surface it will
migrate down the basin slope to the solution pipe that drains it deeper
into the limestone aquifer. The loose sediment directly above the solution
pipe may gradually erode and be transported down the pipe, with the aid of
infiltrating water, leaving a cavity in the soil over the pipe. If the
sediment is somewhat cohesive, such as a stiff clay, this void may grow
larger and larger over time. Cohesive strata within the overburden sediment
may cause the cavity to grow laterally, with a flat roof.
Eventually the
upward or lateral growth of the void may leave only a thin roof of soil that
is not strong enough to support its own weight; then collapse results. At
that moment, a large, gaping sinkhole suddenly appears in the ground surface:
a cover collapse sinkhole (Fig. 4F).
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