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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).

Figure 4

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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