Weathering and the Production of Sediment

1
Weathering and the Production of Sediment

• Surface Portion of the
• Geological Cycle

2
Types of Sedimentary Material

• The most important (volumetric) sedimentary
  materials
• Quartz/polyquartz
• Clay minerals
• Illite Kaolinite Mixed layer/expandable
  Chlorite
• Feldspar
• Carbonate minerals
• Calcite
• Dolomite
• Rock fragments

3
Types of Sedimentary Material

• Terrigenous Clastics (TC)
• Detrital Particles
• Derived from pre-existing rocks
• Derived external to the depositional basin

• Transported by surface processes to the site of
  deposition
• Particulate Residues quartz, feldspar, rock
  fragments, etc (unaltered rock forming
  mineral/rock grains)
• Secondary Minerals minerals new-formed in the
  surface weathering environment clay minerals,
  oxides, amorphous silica, etc

4
Types of Sedimentary Material

• Allochemical Particles formed in situ (at the
  site of deposition) of chemical/ biochemical
  origin
• Carbonates ooids, fossil fragments, pellets,
  lithoclasts
• Glauconite, phosphate insitu authigenic/particula
  te minerals
• Biogenic sediments pelagic tests, siliceous and
  calcareous

5
Types of Sedimentary Material

• Orthochemical Components
• Chemical Precipitates
• Secondary cement
• Primary chemical sediments halite, etc
• Organic Particulate Material (detrital organic
  matter )
• terrestrial and particulate
• marine pelagic
• 95 found in mudrocks and indicative of low Eh
  and low current strength

Laminated Castile Formation basinal evaporites.
Dark laminae are calcite plus organic matter
light laminae are gypsum (Peter Scholle)
Coal
6
Types of Sedimentary Material

• Pyroclasts
• particles fragmented and transported by volcanic
  processes
• Tephra tuff deposits
• Volcanic mudflows lahar and volcanic breccia
  deposits

Volcanic Ash
7
Production of TC sediment

• Provenance
• All aspects of sediment genesis
• Protolith (Source Rock type)
• Climate/Weathering
• Topography (bedrock slope angle)
• Transport History
• Reworking
• Modification at the depositional site

• Primary Composition and Texture of TC sediment is
  a function of
• Parent Rock Composition
• Surface Geological Processes
• The mineralogic maturity of a clastic sediment
  is the extent to which it approaches the ultimate
  end product to which it is driven by the
  formative processes that operate on it.
• Francis J. Pettijohn

8
Climate Belts

• Control
• Precipitation
• Temperature
• Dependant on
• Latitude
• Geography
• Oceanography

9
Weathering Source of Sedimentary "Raw
Materials" Physical Weathering

• Physical Weathering is most significant in
• Cold
• dry
• high relief areas

10
Physical Weathering

• Produces
• mineralogically immature (chemically unstable, if
  available) particulate material
• frost wedging/insolation
• exfoliation
• mass wasting (gravity induced down slope
  movement)

11
Weathering Source of Sedimentary "Raw
Materials" Physical Weathering

• Rate of production of sediment by physical
  weathering at the site of bedrock exposure is
  proportional to
• Inverse the rate of chemical degradation
  (climate temperature and precipitation
  controlled)
• slope angle
• Inverse of the degree of vegetation (climate
  influenced)

12
Weathering Source of Sedimentary "Raw
Materials" Chemical Weathering

• Volumetrically most significant process in the
  production of sedimentary raw materials
• Chemical alteration (reaction) under
• Surface Conditions (typical)
• low temperature (slow reaction rates)
• abundant water
• high Eh (oxidizing conditions)
• generally low pH (acidic conditions especially
  in the presence of decaying vegetation)

• Carbonate Equilibia (simplified)
• 1) CHx 2O2 ---gt 2H2O CO2 Energy
• 2)   H2O CO2 ---gt 2H2CO3
• (carbonic acid, only weakly dissociated)
• 3) H2CO3 ---gt H HCO3-
• (bicarbonate ion)
• HCO3-  ----gtH CO3
• (carbonate ion)

13
Chemical Weathering Products

• Resistant Particulate Residues
• Stable (with respect to surface conditions)
  primary mineral grains quartz, feldspar, rock
  fragments
• Ions in Solution
• Ions introduced into the surface and ground water
  by chemical degradation of primary mineral grains
  
• Congruent solution
• Produces only ions in solution (NaCl? Na Cl-)
• Incongruent solution
• Ions in solution new mineral phase (see Insitu
  minerals, below)
• Elements with preference to ionic bonding are
  generally most soluble
• Insitu Minerals
• Minerals formed in place through incongruent
  solution of unstable primary mineral grains clay
  minerals, Fe oxides, silica minerals

14
Chemical Weathering Products

• Goldich Weathering Stability Series
• Predicts relative abundance of particulate
  residues produced in a (typical) weathering
  environment from rock-forming, protolith minerals
• What are the Assumptions?

15
Main Processes of Chemical Weathering

• Hydrolysis
• hydrogen ion (H) combines with silicate group
• Mg2SiO4 4H20 ---gt 2Mg  4OH- H4SiO4
• (olivine, unstable protolith mineral)
  (hydroxyl) (silicic acid)
• reaction raises pH, and
• releases silicic acid (a weak acid)
• In the presence of dissolved CO2 ( increased
  conc. by 10x to 100x) of biogenic origin
• production of carbonic acid (2H2CO3) drives
  reaction to the right

16
Main Processes of Chemical Weathering

• Oxidation
• Loss of an electron with positive increase in
  valence (charge).
• Due to the presence of an oxidant which is
  Reduced (gain of an electron) with negative
  increase of valence.
• Most metals immediately oxidize in the presence
  of Oxygen (the most famous surface oxidant)
  especially
• Fe---gtFe, Mn---gt Mn4, S---gtS6 (SO4--).

17
Main Processes of Chemical Weathering

• Common sequential reactions in the surface
  weathering environment
• Hydrolysis Oxidation
• Hydrolysis liberates metal cations
• Fe2SiO4 4H2CO3(aq) ---gt 2Fe  4HCO3- H4SiO4
• (olivine, fayalite)
• Oxidation reprecipitates oxides
• 2Fe  4HCO3- 1/2O2 2H2O --gt Fe2O3 4H2CO3
•                                   hematite or
  amorphous iron oxide

18
Main Processes of Chemical Weathering

• Solution
• ionization of ionically bonded metal cations
  (Ca, Na, Mg, K) by dipolar water molecule.
• H2O CaCO3 --gt Ca CO3 H2O
• Produces the metal cations common in natural
  waters

19
Chemical Weathering Products

• Ions in Solution
• Ions introduced into the surface and ground water
  by chemical degradation of surface exposed
  rock-forming minerals
• congruent solution only ions in solution
• incongruent ions in solution new mineral phase
• Elements with preference to ionic bonding are
  generally most soluble

20
Main Processes of Chemical Weathering

• Limiting Factors for chemical Weathering
• Presence of water
• facilitate most weathering reactions
• Sufficient Activation Energy (Temperature)
• to initiate chemical reactions
• Sufficiently long residence time in the soil
  horizon
• minimal physical weathering

21
Chemical Weathering Products

• Insitu Minerals (minerals formed in place)
• Clay Minerals hydrous Alumino-silicate minerals
  (phylosilicates)
• Illite
• Chlorite
• Kaolinite
• Expandable/mixed layer
• Oxides
• Hemitie - iron oxide
• goetite/limonite - iron hydroxide
• pyrolusite - mangenese oxide
• gibbsite - aluminum hydroxide
• Amorphous Silica
• product of hydrolysis reactions of silicate
  minerals (see above)

22
Main Processes of Chemical Weathering

• Temperate Climates
• 3KAlSi3O8 2H 12H2O --gt KAlSi3O10(OH)2
  6H4SiO4 K
• (K-feldspar)          (mica/illite) (silicic
  acid)
• Temperate Humid Climates
• 2KAlSi3O8 2H 3H2O --gt 3Al2Si2O5(OH)4 K
• (K-feldspar)                          (kaolinite)
  
• Humid Tropical Climate
• Al2Si2O5(OH)4 6H2O --gt 2Al(OH)3 4H4SiO4
• (kaolinite) (gibbsite)

23
Chemical Weathering Products

• Clay Mineral Species are a function of
• environmental conditions at the site of
  weathering
• available cations produced by chemical
  degradation

24
Chemical Weathering Products

• As the age of sedimentary rocks increases clay
  mineral assemblages in the subsurface transform
  through diagenesis to illite chlorite
• Clay mineral assemblages in the subsurface
  provide an indication of the time/temperature
  conditions experienced (enjoyed???) during burial