China Flashcards
(27 cards)
Who was Xu Xiake?
- Father of modern speleology (study of caves) and karstology.
- Coined term Fenglin: ‘Peal Forest’.
What is the difference between Chinese geomorphologists and Western geomorphologists?
Chinese distinguish karst types by presence or absence of karst plain between hills, Western use shapes of hills.
What is a Fenglin Karst?
- Isolated limestone hills rise up above flat alluvial plain.
- Western geomorphologists call this ‘tower karst’.
- Fenglin is a very mature form of karst terrain, requires very thick limestone sequences.
What is a Fengcong Karst?
- Conical hills separated by deep, closed depressions, forms continuous terrain of steep slopes and significant relief.
- Western geomorphologists call this ‘Cone Karst’.
How are carbonate rocks (e.g. limestone) formed?
- Many marine organisms use ions dissolved in seawater to precipitate calcium carbonate as mineral calcite/aragonite.
- Minerals can form rocks called limestone.
- Ions product of chemical weathering reactions on land, transported to oceans via rivers.
- Solubility of limestone means it can produce karstic landscapes.
How are carbonates different to siliciclastic sediments?
Carbonate: ions produced by chemical weathering, used in inorganic/biogenic precipitation of calcite (CaCO3).
Clastic: physical weathering + transport + deposition of pre-existing grains. Roundness and sorting of grains reflects transport and depositional environment.
What are some properties of calcite?
- More soluble in low temperatures, high pressures, more acidic water.
- Components of limestone: non-skeletal grains (inorganic), skeletal grains (biogenic), matrix and cements.
What are non-skeletal grains?
- Ooids ( <2mm diameter) and pisoids (>2mm diameter).
- Spherical-subspherical grains, consist of 1+ regular concentric lamellae around nucleus.
- Tiny particles of CaCO3 precipitated from seawater.
- Particles accreted onto ooid during agitation.
What are peloids?
- Spherical, ellipsoidal, angular grains.
- Microcrystalline carbonate.
- No internal structure.
- Mostly faecal pellets.
What are skeletal grains?
Bivalves, cephalopods, corals, echinoids, sponges, algae, cyanobacteria etc.
What are matrix?
Grains in many limestones sit within matrix of calcareous mud (micrite).
What is micrite?
Generally <4 microns.
- Disaggregation of calcareous algae.
- Bioerosion of carbonates.
- Mechanical disaggregation of carbonates.
- Disaggregation of peloids.
- Inorganic/biochemical precipitates.
What is cementation?
Major process producing hard limestone from loose sediment.
What are Karstic Landscapes?
- Karst is topography formed from dissolution of soluble rocks like limestone.
- Same chemical reaction that causes dissolution of limestone statues.
- Initial karstification process is formation of sinkholes or dolines.
- Doline karst field often contains caves and subterranean drainage systems.
Why Guilin landscape is so different to our own: climate
- Guilin is a tropical climate influenced by East Asian Monsoon.
- Fengcong and Fenglin karst requires hot and wet climate, with significant rates of carbonate dissolution in regime of abundant biogenic carbon dioxide.
Why Guilin landscape is so different to our own: thick limestone sequence
- Allows massive lowering of surface without reaching base of limestone.
- Area along Li River contains limestone 2.6km thick, continuously deposited in Devonian and Carboniferous.
- During Carboniferous, China was at low latitude and largely covered by shallow seaway.
Why Guilin landscape is so different to ours: tectonic uplift
- Enables doline karst to evolve into fengcong karst.
- Will continue to evolve towards higher relief until base of dolines reach base of limestone sequence or regional base level.
Why Guilin landscape is so different to our own: lateral
- ‘Egg-box’ Fengcong karst can evolve into ‘tower’ Fenglin karst.
- Depends on balance between: rate of tectonic uplift, rate of surface lowering due to chemical weathering, rate of lateral planation.
What is lateral planation?
- Requires enhanced dissolution by corrosive water at level of water table, close to alluvial plane.
- Leads to under-cutting of rounded hills, facilitating formation of tower-like structures.
- Input from clastic sediment from neighbouring regions important to maintain flat, alluvial plain between towers.
What is limestone dissolution?
Water passing through limestone dissolves limestone = ions in solution.
What happens when water re-enters air?
1) Releases dissolved CO2 so can hold less dissolved carbonate.
2) Evaporate, concentrating dissolved ions. CaCO3 precipitated.
What are speleothems?
- Good archives of past climate change.
- Help to understand different components of climate system interact.
- Dated using radiometric dating, analysed for oxygen isotope composition.
- Reflects change in hydrological cycle.
How are speleothems dated?
- CaCO3 contain tiny amounts of uranium.
- Part of 238U-260Pb decay series often used to date speleothems.
- Over time, uranium-234 decays to thorium-230. Ratio of U to Th decreases.
- Ratio can be analysed using mass spectrometer to calculate age of sample.
Is the half life of 234U longer or 14C?
234U is longer (245,000 yrs), 14C is 5700 yrs.
