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. 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 Types of Sedimentary Material

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/particulate 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. Tephra 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. 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 Production of TC sediment

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. 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 ---> 2H2O + CO2 + Energy 2)   H2O + CO2 ---> 2H2CO3 (carbonic acid, only weakly dissociated) 3) H2CO3 ---> H+ +HCO3- (bicarbonate ion) HCO3-  ---->H+ + CO3– (carbonate ion) Weathering: Source of Sedimentary "Raw Materials" Chemical Weathering

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. Least stable (high temperature minerals) Olivine Ca plagioclase feldspar Pyroxene Amphibole Biotite Na plagioclase feldspar Potassium feldspar Muscovite Quartz Most stable (low temperature minerals) 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 ---> 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++--->Fe+++, Mn++---> Mn+4, S--->S+6 (SO4--).

17. Main Processes of Chemical Weathering • Common sequential reactions in the surface weathering environment • Hydrolysis + Oxidation Hydrolysis: liberates metal cations: Fe2SiO4 + 4H2CO3(aq) ---> 2Fe++  + 4HCO3- +H4SiO4 (olivine, fayalite) Oxidation: reprecipitates oxides: 2Fe++  + 4HCO3- + 1/2O2 +2H2O --> 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 --> 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 • congruentsolution: 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 --> KAlSi3O10(OH)2 + 6H4SiO4 + K+ (K-feldspar)          (mica/illite) (silicic acid) • Temperate Humid Climates: 2KAlSi3O8 + 2H+ + 3H2O --> 3Al2Si2O5(OH)4 + K+ (K-feldspar)                          (kaolinite) • Humid Tropical Climate: Al2Si2O5(OH)4 + 6H2O --> 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. 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 Chemical Weathering Products