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)