Sedimentary Rocks And Fossils

Front 1

Weathering to Form Sediment

Back 1

1. Weathering Changes
2. Mechanical Weathering
3. Chemical Weathering

Front 2

Weathering Changes in Conditions

Back 2

1. Different conditions at earth surface than at depth
A. Low temperature
B. Low pressure
C. Presence of significant organic activity in recent geologic past

Front 3

Weathering Changes in Abundance: Minerals that form

Back 3

1. Changes % of minerals in sediment compared to % in rock
A. Average GRANITE (CONTINENTAL CRUST)
- 80% feldspar
- 15% QUARTZ
- 10% hornblende, mica, and others
B. Average SANDSTONE (detrital rock)
- 85% QUARTZ
- 15% chert, feldspars, rock fragments, and others
2. Destroys labile minerals
3. Makes new stabile minerals
A. Most abundant sedimentary rock is Mudrock with abundant clay minerals

Front 4

Weathering Changes in Abundance

Back 4

Front 5

Minerals that Form in Soils

Back 5

1. Not common in Granite
2. Very common in Soils
3. Some very common in
A. Sedimentary Rocks
- Clay (hydrous Al silicates)
- Aluminum oxides
- Iron oxides

Front 6

Minerals that MAY Persist in Soils

Back 6

1.Uncommon in Soils and Sedimentary rocks
A. Sulfide
- Pyrite
B. Silicates
- Olivine
- Ca-plag
- Pyroxene
- Amphibole
- Biotite
- Na-plag
2. Common in Soils and Sedimentary Rocks
A. Silicates
- K-spar
- Muscovite
- Quartz

Front 7

Non-clastic

Back 7

1. Not common in detrital sedimentary rocks, are common in Chemical or Biochemical sedimentary rocks
A. Carbonates
- Dolomite
- Calcite
B. Sulfate
- Gypsum
C. Evaporite
- Halite

Front 8

Weathering Processes

Back 8

1. Physical
2. Chemical

Front 9

Physical and Chemical Weathering along Joints and Bedding Planes

Back 9

Front 10

Physical Weathering

Back 10

1. Reduces grain size
2. Does NOT change chemistry or mineralogy

Front 11

Three Processes

Back 11

1. Expansion/Contraction
A. Insolation
- Crystals expand with heat
B. Hydration/dehydration
- Some crystals expand with water
C. Freeze Thaw
- Ice increases 9-10% by volume compared to water
2. Stress release (unloading)
3. Organic Activity

Front 12

Thermal Expansion and Contraction:Fire-cracked Rock

Back 12

Front 13

Salt Crystal Hydration

Back 13

Front 14

Salt Hydration??

Back 14

Front 15

Freeze/Thaw

Back 15

Front 16

Frost Wedging forms Talus

Back 16

Front 17

Stress Release - Exfoliation

Back 17

Front 18

Exfoliation Dome: Independence Rock

Back 18

Front 19

Organic Activity - Root Wedging

Back 19

Front 20

Properties of Water that Contribute to CHEMICAL WEATHERING

Back 20

1. Polar
2. Dissociates
3. Surface tension and capillarity

Front 21

Polarity

Back 21

Front 22

Water Dissociates

Back 22

1. pH is the hydrogen ion concentration
2. pH neutral = 7
A. H+1 and OH-1 = 10-7 at 25o C
3. pH acid = 1-6
4. pH alkaline 8-14
5. Normal range = 4-9

Front 23

Acidity = abundance of H ion
pH 1-14

Back 23

Front 24

Water dissociates to form acids in soils

Back 24

1. H2O + CO2 ---> H2CO3
2. H2CO3 ---> H+ + HCO3-
3. CO2 is present as a soil gas derived from plant respiration
4. H2CO3 is carbonic acid
5. HCO3- is bicarbonate

Front 25

Processes of Chemical Weathering

Back 25

1. Dissolution
2. Hydrolysis – solid products
3. Hydration – solid products
4. Oxidation – solid products

Front 26

Dissolution by dipolar H2O

Back 26

Front 27

Dissolution

Back 27

1. NaCl + H2O ---> Na+1 + Cl -1
2. All products are in solution

Front 28

Hydrolysis – new mineral

Back 28

1. K Feldspar + H2O ---> Clay mineral + K +1

Silicate + water ---> hydrous aluminum silicate (solid) + K +1 (solution)

Front 29

Hydration – “same” mineral

Back 29

1. Anhydrite + H2O ---> Gypsum

2. CaSO4 ---> CaSO4 . H2O

Front 30

Oxidation – new mineral

Back 30

1. 2Fe+2O-2 + O2 ---> Fe2+3O3-2

Magnetite ---> Hematite

2. 2Fe(+2)S2(-4) + O2 ---> Fe2(+3)O3(+2) + 2S(-4)

Pyrite ---> Hematite

Front 31

Oxidation/Reduction: Redox in the Weathering Environment

Back 31

1. OXIDATION
A. Atom or ion loses electrons (becomes more positive)
B. Best oxidizing agent is atmospheric oxygen, O2
C. Normally occurs above the water table where water is present and O dissolved in water is abundant.
D. Fe++ ---> Fe +++
E.Metallic ions (Fe, Mn) are usually immobile in oxidizing environments

Front 32

Oxidation/Reduction: Redox in the Weathering Environment

Back 32

1. REDUCTION occurs when an ion in a mineral structure gains an electron.
2. This normally occurs below the water table where water is present but dissolved O in the water is absent.
3. Fe+++ ---> Fe ++
4. Metallic ions (Fe, Mn) are usually more mobile in reducing environments

Front 33

Eh

Back 33

1. The tendency for oxidation or reduction to occur is the REDOX POTENTIAL
A. Measured in millivolts (electricity)
B. Positive = oxidation
C. Negative = reduction

Front 34

Eh/pH Conditions of Water

Back 34

Front 35

Ion Exchange

Back 35

1. Substitution of an ion in a mineral structure for an ion in solution
2. Cation Exchange Capacity (CEC) in milliequivalents per 100 g of clay
3. Important for clay mineral weathering
4. Ca++, Na+, Mg++, K+, and Fe++, are mobile
V. Si+++, Ti++++, Fe+++, Al+++ are immobile

Front 36

Ion Mobility: Processes

Back 36

1. Temperature
2. pH
3. Eh
4. Through Flow

Front 37

Ion mobility: Chart

Back 37

Ca++ > Mg++ > Na+ > K+ > Fe++ > Si++++ > Fe+++ > Al+++

Front 38

Silica Solubility due to pH depends on crystallinity

Back 38

Front 39

Weathering Products

Back 39

1. Silicates
A. Clay minerals – hydrous Al silicates formed by hydrolysis
B. Quartz – concentrated because immobile

2. Oxides
A. Fe and Mn – concentrated because immobile
B. Al – concentrated because immobile

Front 40

Weathering Susceptibility

Back 40

1. Bonding
2. Rate
3. Cation mobility
4. Mineral mobility
5. Grain size
6. Voids
7. Si-O bonds become stronger as more O ions shared between tetrahedra
8. Other cation-O bonds weaker

Front 41

Organic Preservation

Back 41

1. No free oxygen
A. Stagnant fresh water
- Swamps
- Bogs
B. Below groundwater table
C. Stagnant sea water
- Euxinic marine environment
- Organic-rich saline water