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)

Physical Weathering –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 AfricaChristensen

Chemical Weathering Meachanical weathering produces sediments –Quartz: % of igneous rock Beach sands: % 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 Mg 2 SiO 4 + 4H > 2Mg OH - + H 4 SiO 4 ( olivine, unstable protolith mineral ) (hydroxyl) + (silicic acid) reaction raises pH, and releases silicic acid (a weak acid) –In the presence of dissolved CO 2 ( increased conc. by 10x to 100x) of biogenic origin – production of carbonic acid (2H 2 CO 3 ) 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 (SO 4 -- ).

Types of Chemical Weathering Common sequential reactions in the surface weathering environment –Hydrolysis + Oxidation Hydrolysis:liberates metal cations: Fe 2 SiO 4 + 4H 2 CO 3 (aq) ---> 2Fe HCO 3 - +H 4 SiO 4 (olivine, fayalite) Oxidation: reprecipitates oxides: 2Fe HCO /2O 2 +2H 2 O --> Fe 2 O 3 + 4H 2 CO 3 hematite or amorphous iron oxide

Types of Chemical Weathering Solution –ionization of ionically bonded metal cations (Ca ++, Na +, Mg ++, K + ) by dipolar water molecule. H 2 O + CaCO 3 --> Ca ++ + CO 3 = + H 2 O –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 Climates 3KAlSi 3 O 8 + 2H H 2 O --> KAlSi 3 O 10 (OH) 2 + 6H 4 SiO 4 + K + (K-feldspar) (mica/illite) (silicic acid) Temperate Humid Climates: 2KAlSi 3 O 8 + 2H + + 3H 2 O --> 3Al 2 Si 2 O 5 (OH) 4 + K + (K-feldspar) (kaolinite) Humid Tropical Climate: Al 2 Si 2 O 5 (OH) 4 + 5H 2 O --> 2Al(OH) 3 + 2K + + 4H 4 SiO 4 (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 Si layer sheets of silica tetrahedra Si 2 O 5 Phyllosilicates

Structure of Phyllosilicates Octahedral layer –Layer of octahedral coordinated magnesium (brucite layer) or Aluminum (gibbsite layer) –Makes up the other basic structural unit Kaolinite: Al 2 Si 2 O 5 (OH) 4 1:1 tetrahedral – octahedral sheets

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 10 7 earthworms/km 2 ; around since Precambrian –Bring 10 4 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)