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Terrigenous Sediments

Terrigenous Sediments. Weathering. Sediment Production and Weathering. Sedimentary Cycle Components of the Sedimentary Cycle Weathering Physical Types Chemical Types Products clays. Sedimentary Cycle. Rock Cycle Sedimentary Cycle Mass movement (non- sedimentary)

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Terrigenous Sediments

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  1. Terrigenous Sediments Weathering

  2. Sediment Production and Weathering • Sedimentary Cycle • Components of the Sedimentary Cycle • Weathering • Physical • Types • Chemical • Types • Products • clays

  3. Sedimentary Cycle • Rock Cycle • Sedimentary Cycle • Mass movement (non- sedimentary) • Components of Sediment Cycle • Weathering • Erosion • Transportation • Deposition • Lithification • Uplift • Weathering again

  4. Components of Sediment Cycle • Weathering • Processes which break down rock at the E’s surface to form discrete particles • Erosion • Processes which remove newly formed sediment from bedrock • Transportation • Gravity driven (creep, mass flow, glaciers, rivers) • Segregates/ sorts the weathering products • Deposition • Energy is exhausted • Lithification • Compaction, cementation

  5. Physical Weathering • Mechanical fraction of the rock • Aids in Chemical weathering • RETAINS CHARACTERISTICS OF ORIGINAL ROCK • Works best in cold, dry, high relief • Producesmineralogically immature particulate material

  6. Physical Weathering and Sediment Production • Physical weathering is a function of: • Climate • Temperature • Precipitation • Vegetation • Slope Angle (gravity) • Area

  7. Area

  8. Physical Weathering Mechanisms • Freeze-thaw/ frost- wedging (ice expands) • Daily heating/ cooling (deserts, maybe) • Plant Roots (expand cracks) • Crystallization of salts (salts expand) • Release of overburden pressure • Erosion or melting of thick glaciers • Volume changes as primary (original minerals) are converted to clay minerals (secondary)

  9. PhysicalWeathering • Insolation • Large diurnal temperature variations • Hot arid climates: Mohave • Spring 48°F; to 92°F; Summer 71°F to 108°F • Fall 59°F to 100°F, Winter temperature 41°F to 68°F • Expansion/ contraction due to temperature change • Minerals respond differently, aids in generating stress • If it’s rapid, can crack the rock • Rocks can pop and crack after sun sets (cooling)

  10. Physical Weathering • Volume changes from hydration/ dehydration • Alternating wet and dry seasons • Clays, lightly indurated shales expand with water • Upon dehydration, shrinkage cracks develop • Increases permeability to aid in chemical weathering • Reduces rock strength

  11. Physical Weathering • Stress Release of overburden • At depth, rocks are compressed by overburden • Elastic-- returns to original size after compression • With weathering, erosion of overburden, rock expands • Can fracture • Creep can aid fracturing • Fractures impacted by other weathering processes • Sheeting • Exfoliation domes

  12. Stress Release Steven Marshak

  13. Orange River, South Africa Christensen

  14. Orange River, South Africa Christensen

  15. Chemical Weathering • Meachanical weathering produces sediments • Quartz: 25 - 50% of igneous rock • Beach sands: 50 - 99% quartz • Limestones and evaporites

  16. Chemical Weathering • Destruction of rock by solution • Therefore dependent upon water (not frozen) • Water itself only really dissolves evaporites • Needs acid! • Groundwater is acidic • Carbonic acid (CO2 from atmosphere) • Humic acids (from soils) • Usually accompanies mechanical weathering

  17. Chemical Weathering • Rock broken down into three main constituents • Residua • Often quartz rich • Feldspar and mica dependent upon weathering • Solutes (end up in ocean!) • Na, K (other alkali metals- base soluble in water) • REE, Ca, Mg, Sr • Newly formed minerals • Clays (hydrated aluminosilicates) • Classification on basis of combination with Ca, K, Mg, Fe

  18. Chemical Weathering • Volumetrically, most significant process in the production of sediments • Chemical alteration (reaction) under at surface Conditions: • low temperature (slow reaction rates) • abundant water • high Eh (oxidizing conditions) • generally low pH (acidic conditions; especially in the presence of decaying vegetation)

  19. Chemical Weathering • Sequence of Rock Weathering • Relative mobility of main rock- forming elements • decreases from Ca and Na, to Mg, Si, Fe and Al. • Rocks undergoing weathering • Depleted in Ca, Na, Mg • Enriched in Fe- oxides, Al, Si • Particulates produced in reverse of Bowen’s reaction series

  20. Chemical weathering • Sequence • Early: particulates are produced and altered • Mafic minerals (olivine, amphibole, pyroxene) form chlorite clays (Fe-, Mg- rich) • Feldspars produce smectites, illites, kaolins • Clays are flushed out as colloidal clay particles • Some stay to form residuum • Mg-, Ca- bearing minerals removed if weathering continues • Ultimately, rock residuum is just Q (if present in parent) + kaolin, bauxite, and limonite • requires warm humid climate, slow erosion

  21. Types of Chemical Weathering • Hydrolysis • Oxidation • Solution

  22. Types 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

  23. Types 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--).

  24. Types 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

  25. Types 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

  26. Types of Chemical Weathering • 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

  27. Types of Chemical Weathering • Limiting Factors: • Water • facilitates most weathering reactions • Sufficient Activation Energy (Temperature) • initiates chemical reactions • Long residence time in the soil horizon • access to checmial weathering • minimal physical weathering

  28. Products of Chemical Weathering • Insitu Minerals (minerals formed in place) • Clay Minerals : hydrous Alumino-silicate minerals (phylosilicates;) • 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)

  29. Generalized 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 + 5H2O --> 2Al(OH)3 + 2K+ + 4H4SiO4 (kaolinite) (gibbsite)

  30. Clays: Important Chemical Weathering Products • Clay Mineral Species are a function of • environmental conditions at the site of weathering • available cations produced by chemical degradation

  31. Mica and clay minerals are Phyllosilicates Sheet or layered silicates with Two dimensional polymerization of silica tetrahedra Common structure is a Si205 layer Sheet Silicates: the Mica's and Clay Minerals Phyllosilicates Si2O5 sheets of silica tetrahedra

  32. Structure of Phyllosilicates • Octahedral layer • Layer of octahedral coordinated • magnesium (brucite layer) or • Aluminum (gibbsite layer) • Makes up the other basic structural unit Kaolinite: Al2Si2O5(OH)4 1:1 tetrahedral – octahedral sheets

  33. The Major Clay Mineral Groups • Kaolinite group: • 1:1 TO clay minerals • Mica (illite) group: • 2:1 TOT clay minerals • Expandible clays: • Smectite- montmorillonite complex 2:1 clay minerals • Chlorite • Fe- and Mg-rich TOT clays

  34. 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

  35. Biological Weathering • Breakdown of rock by organic processes • Biochemical solution • bacteria • humic acids (rotting organic matter) • Physical fracturing • Tree roots • Burrowing (promotes chemical weathering) • Worms ingest up to 1 mm diameter, can reduce size • Up to 107 earthworms/km2 ; around since Precambrian • Bring 104 km (0.5 cm) of soil to surface • Product • soil

  36. Weathering • Products • Solute • Soluble fraction of rocks which are carried in water • Residua • Insoluble products of weathering • Boulder to colloidal clay • Colloid • Substance made up of very small, insoluble nondiffusable particles that remain in suspension

  37. Erosion • Water • ? environments • Wind • Sandblasting

  38. Transportation • Agents vary in effectiveness at sorting • Gravity, ice (avalanches, glaciers) • Competent to transport ALL weathering products • Inefficient at segregation • Water • Competent to carry material in solution • Less efficient transport residua (?boulders) • Wind • Highly selective (< 0.35 mm) • Medium- fine sands (saltation) • Silty loess (suspension)

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