Karst terrains and associated environmental concerns

Content:


Figure to left is portion of the sacred cenote (a circular 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 significant rivers or other surface water bodies, except the cenotes. There are so many fissures and cave openings in the rock that rain water just disappears into them. The level of the water in the cenote basically marks the top of the groundwater table in the limestone.

Introduction

Karst terrane: landscape underlain by soluble bedrock, typically limestone, with a suite of distinctive features, such as caves, that are related to the solubility of the bedrock.

What are features that characterize karst terranes?

Note the scale. The dark green areas are lakes, and one can see the hallmark urban signature in between them. The pattern looks a bit like moldy swiss cheese. The lakes are sinkholes in Florida, where the limestone bedrock has been dissolved away, which has abundant karst terrane. Photo source from http://water.usgs.gov/ogw/karst/aquifers/floridan/features

This is a close-up image of limestone. Note the round circular pieces, which are fossils known as crinoid stems. Limestones are primarily made up of the skeletal remains of past coral, shells, echinoderms (such as sand dollars and sea urchins) and a large host of other marine critters that made hard parts out of CaCO3 (calcite and/or aragonite) - i.e. limestones are typically biogenic.

UNO students climbing out of a partially plugged sinkhole in Buffalo Gap area in NW Arkansas during the 2014 UNO field trip. The layers of limestone can be seen exposed in the far walls of the sinkhole. The students are climbing up some of the reddish sediment that is plugging the sinhole.

Looking upwards from an underlying cave passage through a sinkhole in the Buffalo Gap area near the Buffalo River. This one is obviously not plugged/filled with sediment, in part because the cave passage has been breached by surface erosion and so sediment easily moves out of it.

Smaller vertical solution tubes in the grey limestones in the Buffalo Gap area. These are like mini-sinkholes. Continued solution along the walls will enlargen them.

The pit in the center is a sinkhole collapse associated with another type of soluble bedrock - gypsum. The view is from the French Alps, with Italy in the far distance.

This is a close up of the gypsum, and one can see the pitted and holey nature of the rock surface that has been caused by solution of the gypsum. Gypsum is even more soluble than limestone. However, it doesn't tend to form caves because the gypsum is relatively weak and collapses readily.

Where does significan karst development exist in the U.S. ? Some examples of significant karst development in the U.S. include:

USGS map of karst aquifers from http://water.usgs.gov/ogw/karst/karstphotos.html. For some of these aquifers the limestone is buried and karst features may be buried.


Figure to the left 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.

UNO student diving into sinkhole and blue spring entrance to Blue Hole in Ichtucknee State Park in Florida. This is also the entrance to a submierged cave popular with cave divers.


Factors influencing karst processes:

Valley floor near Fitton Cave. Note the grey, layered limestone cliffs, the cave opening to the right, and how portions of the channel are dry. All of these are typical of karst terranes.

Small cave room in Fitton Cave. Note the abundance of speleothems, calcium carbonate forms that grow layer by layer and take a great variety of forms. Where they are wet they may still be actively forming as calcium carbonate mineral matter precipitates out of the water on to the outside of the form. The small hanging forms are known as soda straws. Note how some stalagmites and stalactites have recently connected to form the beginning of a column.

A particularly tall and thin column in a large chamber in Fitton cave. Layers within these types of formations are a bit like tree rings and carry a record of past water chemistry and temperatures.

Fitton and a host of other caves are home to unique species adapted for life in the cave. Since coloration is not useful in their dark world they often are white or pink. Eyes are often not well or fully developed. They live off of what washes in. Here is a pink salamander in a pool on the clay mud floor.

The motley caving crew, 4 of which were UNO students. Note the mud. Caving can be dangerous - make sure you go with someone who is experienced.

 


Environmental concerns associated with karst terranes?

Thge below are low-grade chronic environmental problems typically affecting karst localities.

Image showing one of the more dangerous types of sinkhole collapse. Note that the sinkhole, as a solution passage, was initially there. These do not form suddenly, but slowly through the dissolution of the hard limestone bedrock. It is the overburden, sediment that covers and plugs the sinkhole that is crucial, and it is the plug material that collapses. Sinkhole collpase is often associated with the lowering of the groundwater table. Such lowering can promote erosion of the plug material from below (either as running water that carries it further down the cave passage, or from compaction and shrinkage of the plug material as it dries. Image source USGS site on sinkholes: https://water.usgs.gov/edu/sinkholes.html .

Localization of cavernous and sinkhole development:

Picture showing maps of cave passages and the rose diagrams that show direction frequencies of the passages and also the regularly oriented fractures in the adjacent rock (known as joints). One can see the similarity. Joints influence water access to the rock, localizing dissolution, and thus passage directions in caves. Diagram from USGS site: http://water.usgs.gov/ogw/karst/kigconference/rco_geologicozarks.htm


Caves as valuable scientific resources

Many scientists have a deep affection for caves. Why?

Related web sites:


Image of cave painting in Lascaux Cave (a UNESCO World Heritage site) from roughly 17,000 years ago, France. Image from Prof saxx obtained from https://en.wikipedia.org/wiki/File:Lascaux_painting.jpg . More information on the Lascaux site can be found at: http://www.lascaux.culture.fr/#/fr/00.xml .

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.

This is a mammoth skull left within the fine-grained lake sediments that filled up the sinkhole with time. Other skeletons have been removed for study, but some were left so that people could see them in place.

UNO students in one of the museum prep rooms in the back, with Larry Agenbroad, the museum director and chief scientist explaining the type of research they do. A replica of a lower jaw of a mammoth sits on the table.

Above is a photograph taken on the same UNO field trip to the Black Hills 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 on the passage walls, forming these cave formations. Most cave formations form above the water table, but arguably these form above. The last phase of precipitation covered the walls with crystals forms, giving Jewel Cave its semi-unique character and name. Thus, captured 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.

Longitudinal cross sections of a speleothem showing the layers of accretion. In this case the oldest layers are to the left, and the younger ones to the right, recording how the speloethem grew in length. If there is strong seasonality of rain fall in the area then each layer can represent a year. There is information in the layer thickness changes with time. Thinner packets represent draught times. There is also more information about the precipitation temperatures in the detailed chemistry of the layers (especially the isotope geochemistry). This photo is from the USGS site on Paleoclimate Research - Archives: https://www2.usgs.gov/climate_landuse/clu_rd/paleoclimate/archives.asp.


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