Learn About Sinkholes: The Mantled Karst of West Central Florida

In mantled karst terrane, the buried carbonate rock is fur-rowed and pitted. When the covering deposits subside into the underlying depressions, sinkholes and a hummocky to-pography result.

Where karst processes affect rocks that are covered by relatively insoluble deposits, the presence of buried karst features forms a distinctive type of terrain known as mantled karst. In mantled karst regions, the carbonate units are not exposed at land surface, but their presence may be indicated by sinkholes and the hummocky topography that results when the covering deposits take the shape of the underlying depressions. The mantled karst of west-central Florida has resulted in a number of distinct geomorphic regions (White, 1970; Brooks, 1981), including several lake districts with numerous lakes created by subsidence of overburden into the buried karst surface. In other areas, especially where the mantling deposits are thick, the buried karst surface is not reflected in the topography.

Sinkhole formation is related to the thickness and composition of the overlying materials.

The mantled karst of west-central Florida has been classified into four distinct zones on the basis of the predominant type of sink-holes (Sinclair and Stewart, 1985). The type and frequency of sink-hole subsidence activity have been correlated to the composition and thickness of overburden materials, the degree of dissolution within the underlying carbonate rocks, and local hydrologic conditions. Three general types of sinkholes occur: dissolution sink-holes depressions in the limestone surface caused by chemical erosion of limestone; cover-subsidence sinkholes formed as over-burden materials gradually infill subsurface cavities; and cover-collapse sinkholes also formed by movement of cover materials into subsurface voids, but characteristically formed more abruptly.

In the northern part of the region a thin (0 to 30 feet thick) mantle of highly permeable sediments overlies the carbonate rock. Rain water moves rapidly into the subsurface, dissolving the carbonate rock, and dissolution type sinkholes tend to develop. The slow dissolution of carbonates in these terranes has little direct impact on human activity (Culshaw and Waltham, 1987).

The type, location, and frequency of sinkhole subsidence in the Southwest Florida Management District of west-central Florida have been related to the type and thickness of overburden materials.

TYPE AND THICKNESS OF OVERBURDEN

FREQUENCY OF SINKHOLES

TYPE OF SINKHOLES

Thin; highly permeable

Generally few

Dissolution; cover-subsidence; cover-collapse

30 to 200 feet thick; permeable sands are dominant

numerous

Cover-subsidence-occur slowly; cover-collapse- usually induced

30 to 200 feet thick; more clayey

Very numerous

Cover-collapse-occur abruptly

Greater than 200 feet

Few

Cover-collapse-large diameter and deep

To the south, the overburden materials are generally thicker and less permeable. Where the overburden is 30 to 200 feet thick, sinkholes are numerous and two types are prevalent, cover-subsidence and cover-collapse. Where permeable sands are predominant in the overburden, cover-subsidence sinkholes may develop gradually as the sands move into underlying cavities. Where the overburden contains more clay, the greater cohesion of the clay postpones fail-ure,and the ultimate collapse tends to occur more abruptly.

In the southernmost part of the region, overburden materials typically exceed 200 feet in thickness and consist of cohesive sediments interlayered with some carbonate rock units. Although sinkhole formation is uncommon under these geologic conditions, where sinkholes do occur they are usually large diameter, deep, cover-collapse type.