GEOL3074: Geology for Engineers - Midterm 3

What does sediment consist of?

Matrix, grain, cement, and porosity.

What does the matrix represent?

Mud

What is the difference between allogenic and authigenic components?

Allogenic components are transported and deposited, usually end up in oceans. Authigenic components are old organisms made in place, think cements.

What is a way sediment can be characterized?

Whether it is matrix or grain supported.

What is the Udden-Wentworth scale?

Scale at which sediment size is classified.

What are the main categories of the Udden-Wentworth scale and which is largest/smallest?

1. Gravel - Largest

2. Sand

3. Mud - Smallest

Where is the middle of the Udden-Wentworth scale and explain the characteristics of it in terms of mm and phi size units.

The middle of the scale is 1 mm or 0 phi which is sand size. For mm units, it doubles every time. For phi size units, negative values equal larger sizes and positive values equal smaller sizes.

How is sand and sandstone classified?

They are classified based on grain size and relative amounts of grain types.

Explain the sand and sandstone classification chart.

Sand-dominated rocks and sediments carrying less than 15% matrix are called arenites; those with 15-75% matrix are wackes, and those with greater than 75% matrix are mudstones.

What are the sand and sandstone major grain types?

Quartz, feldspar ("arkose"), and rock fragments ("lithic").

How does grain size sorting affect maturity?

Increasing sorting means increasing maturity.

How does roundness affect maturity?

Increasing roundness means increasing maturity.

What role does sediment transport play?

Sediment transport results in grain size fining, mainly due to reduced topographic slope, and hence, reduced transport energy. Transport is very effective at sorting sediment into different size classes.

Explain sediment maturity and what are the types of maturity?

Maturity is a function of sediment transport. There are two types of maturity, textural and mineralogical. Textural maturities refer to the degree of roundness of the grains and the amount of sorting of the grain sizes. Mineralogical maturity refers to the percentage of quartz grains.

What do mineralogically mature sandstones have?

They have mostly quartz grains.

What do texturally mature sandstones have?

They have well-rounded and well-sorted grains.

Explain sandstone maturity.

Sandstone maturity is related to time, climate, and transport distance. Less maturity closer to the source-shorter transport and less sediment reworking.

What is the lithification processes?

1. Compaction

2. Cementation

3. Porosity & permeability

4. Mineralogical alteration

What is diagenesis?

Diagenesis includes the processes-physical and chemical-that change sediments to sedimentary rocks.

Explain compaction and grain re-orientation.

For coarser sediment, grains shift or rotate, filling in gaps; softer grains may become deformed. For finer sediment, grains rotate to horizontal alignment (perpendicular to overburden pressure), open spaces collapse.

Explain compaction and grain suturing.

If there is early cementation, there is minimal compaction and dissolution, cements form compromise boundaries. If there is no early cementation, there is dissolution at point contacts; sutured grain contacts; intraformational cementation.

What do stylolites formations represent?

Stylolites form in response to pressure solution and the amplitude of a stylolite may indicate how much material has been lost to dissolution.

How do cements form?

They form through precipitation of dissolved matter from sediment pore fluids.

What is porosity?

The volume of pore space in a material relative to the total volume of the material.

How is porosity expressed?

Porosity is expressed as a volume percentage.

What is permeability?

The ability of a material to transmit a fluid.

What are the types of porosity and how do they differ in the soil levels?

Intergranular porosity (pore space) is found in the upper levels of soil. Fracture porosity (fracture) is found in the bedrock levels.

What are the effects of natural cements on porosity-permeability characteristics of sediment?

- Reduced porosity

- Strongly reduced permeability (because narrow pore throats are blocked first by cement)

- Porewater chemistry controlled by local reactions

- Stabilization of sediment (further compaction becomes less likely)

Why is pore throat size important?

Small pore throats lead to strong frictional resistance to fluid flow.

What is consistent with porosity as they are buried to a greater depth?

All sediments show an 80-100% reduction in porosity during burial to great depth.

Can porosity be generated?

It can be generated in a burial environment through such events as grain dissolution and fracturing.

What does mercury prosimetry measure?

Pore throat size, which is measured with mercury injection tests, which yield graphs of injection pressure versus pre throat size.

How does small grain size and poor sorting affect permeability?

It sharply reduces permeability.

How is permeability related to grain size?

Higher permeabilities are associated with coarser grain sizes.

What is the relationship between pore throat diameter and permeability?

Generally, a linear relationship, where the smaller the pore throat diameter, the lower the permeability.

What are the effects of clays on porosity and permeability?

- Porosity and permeability are higher in well-sorted sediments and lower in poorly sorted sediments.

- Shales are relatively more porous than sands due to large number of small interparticle pores but sands are relatively more permeable than shales

What is a mineral?

A solid inorganic material that has both a specific chemical composition and a specific, spatially repeating crystalline structure.

How is a crystalline structure determined?

It is determined by the chemical composition and arrangement of atoms in a given mineral.

What is the basic building block of all silicate minerals?

Silicate tetrahedron which is a silicon and four oxygen atoms arranged in the shape of a tetrahedron with the silicon in the center.

How do clay and clay minerals differ?

Clay refers to a particle size of less than 0.06 mm, where clay minerals refer to the structure.

How are different clay minerals distinguished?

1. Layering types

2. Major cations

3. Presence/absence of water in interlayer sites

4. Behavioral characteristics

What are the basic building blocks of clay minerals?

- Tetrahedron: central cation (Silicate or Aluminum) surrounded by 4 oxygens.

- Octahedron: central cation (Aluminum, Iron, or Magnesium) surrounded by 6 oxygens.

What are the two sheet structures and how do they differ?

Tetrahedral and Octahedral sheets. Tetrahedral sheets always have something else attached to it while octahedral sheets can exist by themselves.

What are the two silicate clay layer combinations?

- 1:1 (TO) where there is one tetrahedral sheet to one octahedral sheet.

- 2:1 (TOT) where there are two tetrahedral sheets to one octahedral sheet.

What is interlayer space and how does it impact clay layer combinations?

In 2:1 (TOT), T layers cannot bond together so TOT sandwiches are separated by an interlayer space in which water or absorbed cations may be present.

Explain the covalent bonding found in the 2:1 clay layer combination.

In T layers, there is an extra oxygen to share with adjacent sheets. In O layers, there is a conversion from minus 1 to minus 2 due to a loss of a Hydrogen atom which generates a charge imbalance that is satisfied through sharing of oxygens with adjacent T layers.

What is isomorphous substitution?

One cation replaces another cation. All substitutions involve replacement of a cation by another one with the same or a lower charge, leading toa net negative charge balance.

Which types of clay minerals were compared?

Kaolinite, smectite, vermiculite, illite, and chlorite.

Which clay minerals were nonexpanding and expanding?

- Kaolinite: nonexpanding (no swelling)

- Smectite: expanding (maximum swelling)

- Vermiculite: expanding (some swelling)

- Illite: nonexpanding (minimum swelling)

- Chlorite: nonexpanding (minimum swelling)

What are characteristics of Kaolinite?

- 1:1 (TO) structure

- Nonexpandable (always 7A)

- Few isomorphic substitutions

- Low CEC

- Soft, chalky rock

- Dominant clay mineral in highly weathered soils

What are characteristics of Smectite?

- 2:1 (TOT) structure

- Characterized by water and swelling

- Many isomorphic substitutions

- High CEC

- Layers weakly held together by weak O-O bonds or cation-O bonds

- Most expandable of all clays

- In dry soils, bonding force is strong and hard clods form; deep cracks, but in wet soils, water is drawn into interlayer space and clay swells.

What are characteristics of Illite?

- 2:1 (TOT) structure

- Potassium layer in interlayer space which strongly binds layers

- Non-expandable

What are characteristics of Vermiculite?

- 2:1 (TOT) structure

- Hydrated cations in interlayer space

- Similar to smectite except Aluminum for Silicate in tetrahedral layer and more organized interlayer substitutions

- Expandable, but water molecules and cations bridge layers, so it isn't as expandable as smectites (water-absorbing and water-retention)

- High CEC

What are characteristics of Chlorite?

- 2:2 (TOTO) structure

- Interlayer space occupied by a stable, positively charged octahedral sheet

- Non-expandable

- Central cations in octahedral sheet are magnesium or iron

What is cation exchange capacity?

The cation exchange capacity (CEC) describes the amount of exchangeable cations per unit soil mass.

How do clay-rich sediments and soils compare to clay-poor types with regards to cation exchange capactiy?

Clay-rich sediments and soils have higher CECs than clay-poor types.

How is cation exchange capacity influenced?

It is influenced by both clay-mineral type and specific surface area.

How can clay minerals be identified by x-ray diffraction?

X-rays reflected off of crystallographic planes inside minerals; the spacing between planes reveals the minerals' identities.

What do diffractograms show?

They show interlayer spacing in clay minerals.

What does soil represent?

It represents all the weathered material on the earth.

What is Saprolite?

Weathered rock that retains remnant rock structure.

What is soil?

A mixture of mineral and organic matter lacking any inherited rock structure.

What are the soil components?

Inorganic minerals, water and air, organic matter, and living organisms.

How are soil profiles developed?

They develop mainly through leaching and translocation of organic and mineral matter downward from the surface.

What is horizonation?

Formation of soil horizons.

What are the different soil horizons?

O - accumulation of organic material

A - zone of leaching of soluble salts (topsoil)

E - Eluviation

B - zone of accumulation of salts (subsoil)

C - weathered parent material (bedrock)

R - fresh parent material (bedrock)

What is soil horizonation closed linked to?

Soil-forming processes.

What are soil development factors?

1. Parent material

2. Climate

3. Physical and Chemical processes\

4. Organisms

5. Topography

6. Time

How does the parent material influence the soil properties?

- Soils developed on mafic parent rocks (basalt) tend to be more clay rich, less sandy, have higher pH, and higher nutrient content.

- Soils developed on felsic parent rocks (granite) tend to be sandier, less clay rich, more acidic (lower pH), and have a lower nutrient inventory.

How does the climate influence the soil properties?

The climate (precipitation, temperature) exerts a major control on soil development.

- Tropical zone: deeply weathered

- Subtropical zone: weakly weathered soils

- Temperate zone: strongly but thinly weathered smectite-rich soils

- Tundra zone: frozen soils (permafrost) and histosols

How does precipitation influence clay-mineral production?

Chemical weathering proceeds more rapidly at higher precipitation levels.

What is dissolution of carbonates?

It is where carbon dioxide is released from decaying organic matter and rainwater from the atmosphere form weak carbonic acid. This acid attacks limestone, dissolving carbonates which may create extensive underground cavern systems.

What is oxidation and hydration?

Oxidation produces iron oxide minerals in well aerated soils, usually in the presence of water. Iron contained in minerals combines with oxygen and water to form hydrated iron oxides which corrodes.

What is hydrolysis?

This reaction is responsible for the formation of clays. It occurs when orthoclase feldspar reacts with slightly acidic water to form clay minerals, potassium ions, and silica. The ions released form silicate minerals in the weather process which are carried away by rain and river water or become important soil nutrients.

What controls the rate of chemical weathering?

The chemical stability of minerals, surface area, and climate.

How does topography influence soil development?

It influences how much time soils have to form, whether soils are gaining or losing material from the external environment, and whether water is retained and whether it ponds or flows through the soil.

What is the difference between weathering-limited versus transported-limited soils?

Weathering-limited soils are controlled by the rate of weathering of material, which is quickly removed (steep slopes). Transport-limited soils are controlled by rate of removal of weathered material, which is slow (flat surfaces).

Where does soil organic matter come from?

Living organisms, mostly abudantly from leaf and root litter, although microbial biomass can become dominant after degradation of primary organic materials.

What is important about soil texture and what can it provide information about?

Soil texture is the single most important physical property of the soil. Knowing the soil texture alone will provide information about:

1. water flow potential

2. water holding capacity

3. fertility potential

4. sustainability for urban uses

What 3 percentages matter for soil texture?

Clay, silt, and sand percentages.

Sand goes from right to left, clay goes from bottom left to top, and silt goes from top to bottom right.

What are the criteria for recognizing fossil soils?

Peds, cutans, mottled fabrics, and soil nodules.

What are peds?

Aggregates of soil particles that give a soil visible structure.

What are cutans?

Coatings on soil particles and are produced by weathering and translocation of material within a soil profile.

What is mottled fabric?

Splotchy discoloration of the soil due to localized redox reactions.

What are soil nodules?

Localized deposits of material within a soil profile which represents translocated material within the soil profile. The most common composition being calcium carbonate.

What role do plants play in weathering?

Plants play a dominant role in weathering processes. With regard to physical weathering, plant roots serve to break up substrates, although the binding effect of roots can hold the weathered material together transiently. With regard to chemical weathering, plants extract minerals which causes soil minerals to disintegrate.

What are the 3 soil order categories?

Soil orders in well-weathered humid climates, lightly weathered conditions, and special environmental conditions.

Which soil orders are found within well-weathered humid climates?

Alfisols, spodosols, ultisols, and oxisols.

Which soil orders are found within lightly weathered conditions?

Inceptisols, ardisols, and entisols.

Which soil orders are found within special environmental conditions?

Gellisols, histosols, mollisols, vertisols, and andisols.

What are the characteristics of Alfisols?

- Found in semi-arid to moist areas.

- Have clay-enriched subsoil and relatively high fertility

Which soil orders is one of the most important for food and fiber production?

Alfisols.

What are the characteristics of Spodosols?

- Similarly found to Alfisols

- Tend to be more acidic and relatively infertile owing to low nutrients

- Produces a highly leached A layer

What are the characteristics of Ultisols?

- "Georgia Red Earth"

- Represents more intense chemical weathering

- Results in a concentration of aluminum and iron

- Relatively thick soil profile

- Generally low in nutrients and high in soil acidity

What are the characteristics of Oxisols?

- Represents highly weathered soils of tropical and subtropical regions

- Develop on surfaces that have been stable for a long time

- Have indistinct layering

What are the characteristics of Inceptisols?

- Soil development limited by insufficient time

- Poorly developed soils in semi-arid to humid regions

- Form quickly through alteration of parent material

- Common in river flood planes

What are the characteristics of Aridisols?

- Soil development limited by climate aridity

- Soils that are too dry for growth of mesophytic plants

- Weathering rates are low

- Have a very low concentration of organic matter

- Associated with nodules

What are the characteristics of Entisols?

- Soil development limited by unweatherable parent materials

- Show very little development of pedogenic horizons

- Found on sand dunes, steep hillslopes, and river floodplains

What are the characteristics of Gelisols?

- Permafrost greatly restricts the engineering use of gelisols

- Soils of very cold climates with permafrost near the surface

- Have no A or B horizons

What are characteristics of Histosols?

- Primarily consist of organic materials

- Mostly water-saturated year-round

- Exposure to air will cause histosols to rapidly break down