Karst and Karst Terranes Flashcards
The word Karst stems from?
- German form of the Yugoslavian term “Kras”
- Meaning ‘bare stony ground’
In modern geomorphology, Karst refers to what?
- Landscape formed by dissolution of the underlying bedrock
- Characterized by distinctive landforms that don’t typically occur in any other circumstance
The term ‘pseudo-karst’ refers to?
- Karst-like development in non-carbonate lithology
- Exhibits characteristics similar to karst landscapes
- But lack dissolution as a primary means of landscape formation
Major known global karst regions
- Mexico, Mid US,
Canadian Rockies, SE Asia, Parts of Australia, Spain, Italy, etc. - Equatorial areas from high temps and are areas of both limestone formation and dissolution
- Exception is SA, not much there
BC carbonates
- Rockies
- Some on Van Isle and Haida Gwaii
Factors that affect Karst development
- Solubility of limestone (percent calcite)
- Climate (temp and moisture)
- Structure of limestone (joints, fractures, porosity)
- Mineralogy/Lithological content (percent classics, percent spar vs. micrite vs. skeletal)
- Vegetation/non-carbonate geology (acidity/pH of groundwater from humic acids)
- Atmospheric CO2 (affects solubility of carbonates)
What is the main thing karst topography requires?
- Limestone!
Overview of karst formation
- CO2 in air and soil combine w/ water to form carbonic acid, dissolves limestone, especially along fractures
- Subsurface dissolution/erosion forms water-filled caves
- Water table lowering drains caves, carbonates precipitate
- Cave roofs collapse, cause sinkholes/ other landforms
- Over time, erosion and dissolution lead to remnant rock towers (Asia)
Carbonate geochemistry
Rainfall is acidic (pH 5.6) b/c atmospheric CO2 and water produce carbonic acid
- Carbonic acid dissociates into hydrogen and bicarbonate ions
- Equilibrium equation
- Carbonic acid dissolves limestone to produce calcium and bicarbonate ions
Other controls on groundwater acidity and carbonate geochemistry
- Soil water gets highly enriched in CO2 from respiration of plant roots and microbe decomposition of organic matter
- Cold water can dissolve/hold more CO2 than warm
- More CO2 is dissolved in water as the partial pressure of the air increases (e.g. in soil pore space,), increases CO2 in oceans
- More limestone increases bicarbonate
What temperature of water can hold more CO2?
Cold water
Dissolution of carbonates
- Process by which rock is dissolved in water (assisted by carbonic acid)
What does increased CO2 in water do?
- Decreases pH
- Increases rate of dissolution
What does increased biological activity do?
- Enhances dissolution b/c it generates CO2
- CO2 generated by respiration of plant roots and decomposition of organic matter by microbes
What is the dominant ion in runoff?
- Bicarbonate, HCO3-
- Due to carbonate dissolution
Precipitation of carbonate is what?
- Opposite to dissolution
- Occurs when CO2 evaporates from water, e.g. in caves
Dissolution of Dolomite
- CaMg(CO3)2 breaks down to Ca2+, Mg2+, and 2C032-
- Rate of dissolution is higher if carbonic acid is involved, just like limestone
What are examples of other minerals that can dissolve in water?
- Dolomite, chalk, gypsum
- Break down into constituent ions
Karst landscapes are shaped through…?
- Dissolution of soluble bedrock
- Commonly limestone or dolomite
- Sinkholes develop and cavities are dissolved below ground
- Surface water may become limited
3 Cycles of karst terrain erosion
- Youthful (Normal surface drainage, to dry valley’s, swallow holes, underground streams)
- Mature (Roof collapse, surface hollows)
- Old age (Hums, and remnant rock towers, worn down to impermeable basement in areas)
Landforms that form above the water table
- Collapse sinkholes, karren, grikes, abandoned caves, speleothems
Grikes
- Kluftkarren
- Solution enhanced vertical fractures
- Divide surface into distinct pieces referred to as Clints
Speleothems
- Cave deposits
Landforms that form below the water table
- interconnected caves, tunnels, other cavities, solution sink-holes, springs
Karrens
- Large tracts of limestone karst terranes that are devoid of unconsolidated material
- Often solution enhanced vertical fracture/ joint sets
- Can be filled w/ debris from dissolution or devoid of residuum
- Not good for growing, sometimes loess may provide nutrients for plants to grow in cracks
- Openings known as grikes, kluftkarren or solution grooves
Ireland
- Karren, grikes and clints
- Limestone pavements, rugged and bare w/ flat areas of rock
- The Burren in county Clare formed during carboniferous (355-290Ma)
Formation of grikes and clints
- Water drainage and flow concentrates along joints, fractures and structural weaknesses
- Dissolution preferentially along the cracks enlarges them
- Joints/ fractures may connect at subsurface and underground drainage systems develop
Karst collapse landforms
- Sinkholes, Dolines, Cenotes, Cockpits
- Often funnel shaped
- From as material above cavity becomes too thin to support weight
- Sometimes filled w/ colluvium from edges or overlying strata
- Can form circular lakes
- Entrances to karst caves are often old sinkholes
What are the 2 categories of sinkholes?
- Collapse, usually form when water level drops
- Solution, form due to dissolution at surface
Cenotes
- Sinkholes (Yucatan, Florida)
- Flood and dissolve during high SL interglacials
- Collapse during low SL glacials
Cockpit
- Sinkhole surrounded by hemispheroidal residual hills
- Ex. Arecibo Radio Astronomy observatory, Puerto Rico
Compound sinkholes
- Uvalas
- Several sinkholes coalesce, form larger structure (up to km scale)
- Sometimes floored w/ alluvium from subterranean streams, called karst gulf or polje
- Can develop quickly to >100m depth, can cause destruction/ death
Polje
- Large (25km^2) elongated basin in karst terrane
- Flat floor, steep walls, structurally controlled
- Formed by coalescence of many sinkholes (Uvulas)
- Often w/ disappearing streams
Drainage in European karst region
- Polje basin
- Flooded and backed up into overflow channels
Drainage characteristics of Karst terrain
- Chaotic or deranged pattern
- Few, if any, through flowing streams
- Blind valleys, lost rivers, tributaries and other surface streams
- Streams disappear into ‘swallow hole’ (called ponor, swallet or a sink)
- Often reappear as karst spring or gw rise
- Surface drainage replaced by underground
Australian karst terrain
- Karren plains w/ blind valleys and numerous sinkholes
Swallow holes
- AKA Ponor, swallet, sluggas
- River reaches permeable rock, Disappears down grikes
- Grikes enlarge through dissolution
- Forms swallow hole
Dry valley
- Remains of river valley downstream from sinkhole
- If river reappears at surface, its called ‘River of Resurgence’
River of Resurgence
- Reappearance of a stream downstream of a dry valley/ sinkhole
Mitchell Plain and Chester Escarpment
- Karst terrain w/ sinkholes, hummocky
- Equivalent in Kentucky
Karst surface water features
- Karst regions noted for ‘lack of well-established surface drainage’
- Surface streams tend not to flow very far
- Become ‘Disappearing streams’
- Can reappear at surface as springs, usually from a cave
Underground landforms, cave formation
- Surface water enters through swallow holes, grikes, etc.
- Passages enlarge via carbonation and dissolution, leads to large caverns
- Water erodes rock by abrasion and hydraulic action
- Caves develop at or below zone of saturation
- Water table lowers/ tectonic uplift drains cave
- Cave may continue to enlarge via rockfall
Cave development
- Dissolution along joints/ bedding planes
- Cave erosion and expansion
- Caves surface at artesian or gravity springs
- Drainage and abandonment, inactive
2 examples of large N. American caves
- Mammoth cave, KY
- Hall of Giants, Carlsbad Caverns, NM
Speleothems
- Cave deposits
- Soda straws, stalactites/ stalagmites, drapes, columns, flow stones, pool spar and shelfstones
Dripstones, speleothem
- Water drops containing dissolved limestone seep through cracks and fissures in cave roof
- Water enriched in CO2 from soil
- When exposed to cave air, water loses CO2 and deposits calcite
Stalactite
- Common interior cave formation
- C = ceiling, ceiling icicles
- Water dripping from cave top loses CO2 and deposits calcite, initially as delicate soda straws
- Over time deposition forms large hanging features
Stalagmite
- Common interior cave formation
- G = ground
- Water droplets fall to cave floor and lose more CO2, deposit more calcite
- Over time forms pillars growing upward from cave floor
- Directly below stalactites, usually thicker
Column
- Cave formation
- When stalactite and stalagmite combine/ grow together
Pool spar
- Common interior cave formation
- Crystallization of dissolved limestone in water
- Looks like spiny starbursts
Shelfstone
- Common interior cave formation
- Develops when spar attach to side of cave pool
Growth of stalctites
- Soda straws initially hollow, allow for dissolved limestone to travel through tube
- Dissolved solid can plug tube
- Forces dissolved limestone to ‘back-up’ and flow on outside of straw
- Thickens and becomes stalactite
Largest free-hanging stalactite in N. hemisphere
- Doolin Cave Ireland
- 7.3m long
Soda Straws
- Delicate, hollow tubes of calcite deposited and hanging from cave ceiling
- Initiation of stalactite formation
Pillars
- Dripstone features
- Stalactites and stalagmites grow toward each other
- Eventually join in a pillar or column
Curtains
- Water drips from long cracks in cave roof
- Form continuous strips of calcite called curtains/ drapery
Flowstones
- Sheets of calcite deposited on the floor or walls by flowing water
How are mini-terraces formed in caves?
- Increased turbulence at flowstone margins release more CO2
- Encourages precipitation and development of terraces and calcite ‘waterfalls’
Mature karst terrains
- Humid regions (Puerto Rico, Jamaica, SE Asia/China)
- Form towers, mounds, cockpit karst
- Erosional remnants of thick sequences of limestone that have been greatly degraded
Tower Karst
- Formation due to combo of tectonic uplift and tropical erosion
- China, Vietnam
Importance of Karst
- Covers approx. 10 percent of earth
- 1/4 of world population depends on water from karst areas
- Highly vulnerable to gw pollution, rapid rate of water flow into aquifer and lack of natural filtration
- Tropical karst w/ extensive forest canopy hosts 100’s of unique plants and animals
- Coastal wetlands rely on complex hydrological dynamics of karst to function properly
Why is much of the 10 percent of karst earth surface w/o surface features?
- Due to mantling of non-soluble strata
Engineering solutions, Applied science building at UCSC example
- Foundation cement kept disappearing into subsurface karst
- Solution was structural pillars sunk deep into solid rock
- Large cement pillars part of building foundation all the way to the top floor
Van Isle cave examples
- Horne Lake Caves Provincial Park
- Sinkholes (funnel shaped) beside trails
- Relict carbonate flowstones in Central Van Isle
Stratification in caves…?
- Records depositional history
- Paleoclimate reconstruction
University of California, Santa Cruz
- Built on dolines (sinkholes) formed by dissolution of bedrock or collapse of shallow caves
- Home to Empire cave, formed by uplift of Santa Cruz mnts. and marine terraces, erosion of Cave Gulch
- N-S orientation of passage way reflects other canyons and joints in quarries that are N-S
- Much construction has to be modified to withstand dangers of landforms
