Sources and Origins of Sediments Flashcards
(16 cards)
what is physical weathering and what are some examples
- rocks are broken into smaller fragments, minimal impact on chemical or mineral composition
- produces regolith and rock fragments
-rock fragments can be broken down further into smaller mineral particles by subsequent erosion and transport processes
-Freeze-thaw (frost) weathering
-Stress-release weathering
-Wetting and drying
-Biologic factors
-composition of sediments from dominantly physically weathered&mechanically eroded rocks usually reflect their source rocks
explain freeze thaw weathering
what is the role of preexisting joints?
explain stress release weathering
freeze-thaw
-water infiltrates a rock through existing joints and fractures, water freezes and expands, pressure causes rock to crack
-slow process, repeated cycles are necessary to break a rock
-mechanically weak rocks disintegrate more readily (eg shale)
- Produces large, angular blocks of rocks
-> shape and size depends on the internal fracture network
rocks dont move far, all the same lithos, not very eroded
-works better with preexisting vertical joints -> water infiltrates more easily
stress release
sheet joints
typically influences horizontal joints, contributes to creating blocks of rocks
a body of (commonly intrusive igneous) rock is under the surface (eg a batholith), under pressure from weight of overlying rock, being held together
-overlying rock is eroded, stress is released, rock expands and breaks off in sheets (outer part is under the least pressure, will break first)
compositional maturity
physically weathered/mechanically eroded rocks that are proximal to source contain abundant rock fragments and particles whose mineralogy closely reflect their source rocks
* A mix of lithic fragments, quartz and feldspar, micas, heavy mx, etc.
* These sediments are said to have a low compositional maturity
Chemical Weathering
-changes to the chemical and mineralogical compositions of rocks
Chemical decomposition, produces dissolved chemical substances
produce a loose residue of resistant grains & secondary minerals (constituents may dissolve and recombine to form new mineral phases)
Water is the principal agent of chemical weathering
H+ and OH- ions in water react with minerals
-Low temperature and very slow process
-at the surface or near surface
-simple solution
->CO2 from vegetation
-Carbonation
In igneous rocks, mineral stability at Earth’s surface tends to be inversely proportional to the temperature at which they formed
- (lower t/p conditions = higher resistance to chemical weathering)
Simple solution (congruent dissolution*)
-> CO2 dissolves in water to form carbonic acid then becomes H+ and bicarbonate
-> H+ reacts with minerals
CO2 (aq) + H2O → H2CO3 (carbonic acid) → H+ + HCO3- (bicarbonate)
carbonation
-common form of chemical weathering (particularly limestone)
-calcite(caco3), h2o and co2 combine to make ca2 and hco3
-> Results in karst topography
–> characteristic features like sinkholes (hydrolysis eats calcite, leaves holes), caves, underground drainage systems, and springs. common in humid regions with carbonate rocks
Most of the carbonate dissolution takes place near the surface (exokarst and epikarst)
-carbonation happens primarily at the surface as that is where co2 production happens and in areas of:
-decomposition of organics in soil
-vegetation
-high effective precipitation
-temperate climate and medium rainfall
Hydrolysis & incongruent dissolution
What do you think it means to find a kaolinite deposit in a place like Mojave County, Arizona?
reactions between H + and OH- ions with minerals
eg weathering of granite containing feldspar, yielding soluble cations, silicic acid and clay minerals (by-products)
- H + ions supplied by the dissociation of carbonic acid in water
- Clay minerals such as kaolinite, illite, and smectite may form as a by-product of hydrolysis if Al is present (eg orthoclase feldspar can breakdown to yield kaolinite or illite)
favorable natural conditions
-> formation of kaolinite favored by tropical conditions
->reaction rates double for each 10 degree C increase
rocks weathered by hydrolysis:
-residual blocks
(rounded blocks sticking out of side of outcrop)
kaolinite needs water and tropical conditions. finding kaolinite in arizona means that at one point, the now arid region was tropical
how is Ferricrust and Bauxite formed
bauxite and ferricrust form bc aluminum is left behind after chemical weathering
what are the 4 climates the Neo-mineral groups prefer
neominerals:
May be eroded at the site of weathering and transported away
Part of the matrix of a sediment
- Temperate to sub-tropical climates
- Montmorillonite/smectite types
- Humid temperate climate
- Illite/muscovite type
- Tropical climates
- Kaolinite and gibbsite
- Semi-arid to sub-tropical
- Iron oxides (goethite, hematite)
what does tds tell us about weathering processes?
dissolved constituents:
-derived from chemical weathering
-transported in groundwater and surface water
- Total Dissolved Solids (TDS) in water provide useful information about weathering processes and source rocks in the drainage basin
- organic and inorganic precipitation processes in lakes and oceans generate solids that can become sediment particles (e.g. bioclasts, calcareous mud).
Precipitation also takes place in the subsurface leading to lithification of sedimentary rocks (e.g. diagenesis)
Types of particles found in sediments
Framework grains - biogenic and chemical components
* Fossil fragments, ooids and peloids intraclasts
* Skeletal phosphate
* Detrital glauconite
* Disseminated organic matter
* Hydrogenous sediment (e.g. evaporites)
- Biogenic and chemical (e.g. carbonate) particles
-> Often mixed with siliciclastic sediments (e.g. shallow marine environments)
-> Lithic carbonates (from carbonate bedrock regions) - Human-made particles (in modern/recent deposits)
->Plastic and glass beads, metallic particles
siliciclastic, terrigenous sediments
-derived from terrestrial environments (continental)
- detrital rocks (rock/mineral grain resulting from physical weathering/mechanical erosion & found in a sediment)/clastic sediments
* Quartz, feldspar, lithic fragments, heavy minerals, etc.
->(from igneous and/or metamorphic and/or sedimentary rock regions)
* Detrital clay minerals (e.g. kaolinite)
->From the erosion of continental areas that experienced chemical weathering
-composition controlled by the climate, substrate geology and topography of their source region
weathering and denudation
Land surface slowly lowered due to weathering and erosion
Involves the transport of sediments downstream into continental basins or into the oceans
Large quantities of siliciclastic sediment deposited in sedimentary basins are derived from land areas exposed to subaerial weathering
* Weathering and erosion rates are controlled by climate and topography - Precipitation, slope, vegetation, geology, etc. play an important role
Involves the transport of sediments downstream into continental basins or into the oceans
The processes are controlled by a variety of physical, chemical and biological processes
- Surface run-off, river flow, wind, ice flow, groundwater flow…
Which type of weathering would be favored by a steep slope, young jointed rocks, high precipitation, and lack of vegetation?
physical
what characteristics would provide supporting evidence for a pile of large rocks being a felsenmeer or block field, i.e., resulting from long-term (1000+ yrs) physical weathering of a bedrock outcrop
lithologies of the block field correspond to expected bedrock in the area (e.g., from geological maps and geophysics)
The rock fragments have angular edges
Crystalline rocks, like granites, subjected to high physical weathering and high erosion rates and after some transport downstream, will produce clastic (terrigenous) sediments containing mostly
Quartz, feldspars, lithic fragments, and heavy minerals
what are the most favorable conditions for weathering of limestone
Soil with organic matter overlying fractured limestone, avg. precipitation = 1000mm/yr, avg. temp. ranging -8 to +27oC annually
-temperate climate, medium rainfall
