terrigenous sediments weathering. sediment production and weathering sedimentary cycle –components...
TRANSCRIPT
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)
• Components of Sediment Cycle– Weathering– Erosion– Transportation– Deposition– Lithification– Uplift– Weathering again
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
Physical Weathering
– Mechanical fraction of the rock– Aids in Chemical weathering
• RETAINS CHARACTERISTICS OF ORIGINAL ROCK– Works best in cold, dry, high relief– Produces mineralogically immature particulate
material
Physical Weathering and Sediment Production
• Physical weathering is a function of: – Climate
• Temperature• Precipitation• Vegetation
– Slope Angle (gravity)– Area
Area
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)
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)
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
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
Stress Release
Steven Marshak
Orange River, South Africa Christensen
Orange River, South Africa Christensen
Chemical Weathering
• Meachanical weathering produces sediments– Quartz: 25 - 50% of igneous rock
• Beach sands: 50 - 99% quartz
• Limestones and evaporites
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
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
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)
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
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
Types of Chemical Weathering
• Hydrolysis
• Oxidation
• Solution
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
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
--).
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
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
Types of Chemical Weathering• 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
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
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)
Generalized Chemical Weathering
• Temperate Climates3KAlSi3O8 + 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)
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
Sheet Silicates: the Mica's and Clay Minerals
• Mica and clay minerals are Phyllosilicates– Sheet or layered
silicates with
– Two dimensional polymerization of silica tetrahedra
– Common structure is a Si205 layer
sheets of silica tetrahedraSi2O5Phyllosilicates
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 – octahedralsheets
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
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
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
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
Erosion
• Water– ? environments
• Wind– Sandblasting
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)