Environmental Geology Lecture - Karst terrains and associated environmental concerns:

Environmental Geology Lecture - Karst terrains and associated environmental concerns

Features that characterize karst terranes:

Figure to left is of the sacred cenote (sinkhole) at Chichen Itza, a Mayan site on the Yucatan peninsula of Mexico. Offerings and sacrificial victims were thrown into the sinkhole which was seen as a portal to the gods of the underworld. Mayans that live in this area today continue to have their lives affected by the karst setting they live in. It provides the unique situation of a jungle without rivers or other surface water bodies, except the cenotes.

caves, and cavernous development, subsurface drainage.
sinkholes.
large springs.
abandoned surface drainage (good surface-subsurface communication).
rock spires, enlargened joints, dolines.
terra rosa soil.

Areas in U.S. with extensive karst development:

Figure to the right is of a blue hole, a large spring, in northern Florida. This single spring creates enough flow to feed a fair sized stream, and is the surface exit of part of a large submerged cave system. The groundwater table surface is very shallow and this is a good example of a cave network under phreatic conditions.

Florida ( a huge carbonate platform).
Paleozoic limestones flanking the Ozark dome in Missouri.
Valley and Ridge province, from Virginia to Alabama.
Paleozoic limestones flanking the Black Hills uplift.

Factors influencing bedrock solubility:

bedrock solubility: limestone most common, dolomite to lesser degree, evaporites also a problem. Salt and gypsum are also soluble, but they are not as common a rock type, and do not have the strength to support openings for prolonged periods of time.
water chemistry and limestone dissolution:
- role of carbon dioxide in producing carbonic acid, and acidity
- carbonate reverse solubility.
- tufa deposits and the importance of organic activity (biologically added precipitation).
importance of position of water table:
- phreatic development.
- vadose development:
  - infilling by speleothems, collapse and sedimentation.

Environmental concerns associated with karst terranes:

sinkhole collapse:
- case history of Winter Park, Florida 1981: 100 m depression, 13 m deep, $2 million in damage.
- case history of Hershey, SE Pennsylvania in 1949: over 26 km2 area over 100 sinkholes appeared, 2-7 m in diameter, 8 m deep. Nearby quarry had lowered groundwater table by 15 m.
- case history near Johannesburg, S. Africa: 1960 lowered groundwater table up to 450 m to dewater gold mine. In 1962 a 55 m diameter and 32 meter deep hole took lives when it destroyed crusher station. In 1964 four houses went down another hole.
- role of groundwater? It provides for plug stability. When water removed from plug material, it drys, the organics can decay, and there is loss of support.
karst terranes, because of the large surface-underground connections, are easily polluted aquifers.
hard water: amount of Ca++ and Mg ++ in water, difficulties with plumbing (scale), health considerations.
thin residual soils, terra rosas. This makes for considerable difficulty in waste burial or disposal.

Localization of cavernous and sinkhole development:

utility of this knowledge? You can avoid building on it.
importance of fractures, joints.
importance of groundwater table position.
geophysical exploration for plugged sinkholes using electrical resistivity and other techniques.

Caves as valuable scientific resources:

as paleontologic traps, sites (see below).
as archeologic sites.
speleothems, cave sediments and paleoclimate history.

Related web sites:

This is a photo of an excavation into sediments that filled a sinkhole near Hot Springs, S. Dakota taken on the 1997 Dept. of Geography and Geology field trip to the Black Hills. The sinkhole trapped a number of young male mammoths over time among other organisms, and represents a paleontologic treasure trove of information on the environment in this area during the last Ice Age. Distinct thin laminations of sediments may represent annual deposits or storm events. This filled sinkhole is part of karst terrain that encircles the interior of the Black Hills and which is due to relatively soluble limestone formations. Jewel and Wind caves occur in these same limestone formations.

Above is a photograph taken on the same field trip of crystal studded speleothems in Jewel Cave. After 'aggressive' waters dissolved an extensive cavernous void, then saturated waters started filling in the void by precipitating calcite and other minerals and forming these cave formations. The last phase of precipitation covered the walls with crystals forms, giving Jewel Cave its semi-unique character and name. Thus, capture in cave sediments and speleothems is a history of the groundwater table levels and chemistry in the area, and this in turn is a function of climate.

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