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Paleokarst in Iowa

Red ball iconPaleokarst in Iowa

by Robert M. McKay


Following heavy rain, a muddy torrent of water laden with dislodged soil, twigs and leaves rushes down a gully and spills over a lip of rock, disappearing into a dark echoing hole. Crashing to the bottom of the pit, the flow continues outward and downward along narrow passageways until some of it slows to a standstill in a large subterranean room rapidly filling with turbid water and debris.

This description characterizes a geologic process that takes place in "karst" landscapes. Such landscapes are typically underlain by shallow limestone, and surface drainage is connected to subterranean cavern systems via sinkholes and vertical openings in the rock. Today, karst processes are active in the limestone-dominated terrain of northeast Iowa. Geologic investigations, however, show that karst conditions also occurred elsewhere during the state’s geologic past. These ancient karst conditions, or "paleokarst" as it is known, include similar features that developed in much older limestone landscapes, then were buried and preserved beneath younger geologic formations.

 

Paleokarst in mine  
A shale-filled chamber is exposed in the River Products Company limestone mine in Louisa County. Weak shale in these pockets of paleokarst causes safety hazards for mine workers and adds impurities that must be removed to maintain product quality.
Photo by Michael Bounk


Karst conditions result when water and carbon-dioxide from the atmosphere and soil combine to form mild carbonic acid which then mixes with groundwater. As the water descends along fracture openings and through thin separations between rock layers, the weakly acidic groundwater slowly dissolves portions of the limestone, eventually forming subterranean cavern systems that can have complex links back to the land surface. Karst development eventually ceases as climate, vegetation, groundwater infiltration and local relief change, and the subsurface openings gradually fill with various earth materials.

In Iowa and other parts of North America, conditions conducive to karst formation recurred periodically between 470 and 315 million years ago, during the Ordovician, Devonian, Mississippian and Pennsylvanian periods of geologic time. These ancient episodes of karst development are known to Iowa geologists from the paleokarst settings seen in the state’s mines and quarries. Natural outcroppings of paleokarst are rare, though a fine example is found below Bridal Veil Falls at Pikes Peak State Park in Clayton County, where Ordovician sandstone fills a paleokarst basin in the underlying dolomite. More frequently, paleokarst is exposed during road construction or the quarrying and underground mining of limestone for road and concrete aggregate. Typically, the material filling the paleokarst basin is composed of soft, interbedded shale and sandstone, which is unsuitable for aggregate use and must be segregated from the marketable stone. Operators may find that significant areas of their property, previously thought to contain quality stone reserves, are riddled with paleokarst features. In underground mining, the working-face and ceiling may intersect paleokarst fillings, thus mixing impurities with the limestone. In addition, these weaker materials can cause safety hazards, as the softer paleokarst filling can collapse, endangering mine workers.

 

Quarry face  
The recessed opening in this old quarry face at the Linwood Stone Company property in Scott County was excavated for access to additional limestone resources underground. Subsurface features encountered here during mining include open paleokarst caverns lined with beautiful mineral crystals.
Photo by Robert McKay


In an unusual geologic setting seen at the Linwood Stone Company mine near Davenport, a portion of a paleokarst cavern complex was preserved without a sediment filling, and the cavern openings contain exquisite crystals, which were precipitated by mineralized groundwater flowing through the buried paleokarst system over long periods of time (photos, below).

The presence of paleokarst features in subsurface rock formations may also adversely affect the drilling of water wells in local areas. In some cases, the pore system of the formation, normally filled with groundwater, is "plugged" by fine-grained paleokarst fill, a situation which usually necessitates redrilling of the well nearby.

Despite problems associated with this geological condition, paleokarst provides a valuable and unique rock record of past earth processes, landscapes and environments, and frequently reveals new insights into Iowa’s earth history.


Museum-quality varieties of the mineral calcite edge the paleokarst chambers encountered underground at the Linwood Stone Co. mine in Scott County.
Photos by T. Scott Krenz
Calcite crystals  
Brilliant prisms of flat-topped or "nailhead" calcite are accented with pyrite ("fool’s gold") and bronze-colored chalcopyrite.

 

Calcite crystals  
Sharply pointed crystals of "dog-tooth" calcite are laced with the gold luster of pyrite.

 

Calcite crystals  
Gray, fine-grained limestone grades to a band of white crystalline calcite, then to tiny points of "dog-tooth" calcite tipped with iron, giving the crystals their brownish color.
Calcite crystals    

 

Adapted from Iowa Geology 1996, No. 21, Iowa Department of Natural Resources