Earth - Sedimentary Rocks

What is the sedimentary cover?

Earth is covered by a ‘veneer’ of sediment, which can range in thickness from 0-20 km.

How are sedimentary rocks formed?

Either by deposition and consolidation of (in)organic materials, or from precipitation of minerals from solutions. They result from processes at the surface of the Earth and are usually deposited horizontally on top of each other.

What are the classes of sedimentary rocks?

1. Clastic: Fragments (clasts) cemented together

2. Biochemical: Cemented shells of organisms

3. Organic: Carbon-rich remains of deceased organisms

4. Chemical: Minerals that crystallize directly from water

What are the processes involved in the formation of clastic rocks?

1. Weathering

2. Erosion

3. Transportation

4. Deposition

5. Lithification

Discuss the processes of weathering

This is when rocks are broken down into smaller particles. It can be through physical weathering, where they are mechanically broken down, or chemical weathering, where the mineral composition is altered and creates sediments as a byproduct of reactions.

Explain the processes involved in physical weathering

1. Jointing: Joints are natural cracks formed in rocks due to expansion of another rock and form sediments

2. Frost wedging: Water enters cracks, freezes and expands, forcing the rock to widen fractures and break

3. Root wedging: Growing roots penetrate fractures, exerting force that widens cracks and breaks surrounding rock

4. Salt wedging: Dissolved salt crystallizes are water evaporates and creates pressure on the rock, repeated salts widen fractures and weaken rock structure

Explain the processes involved in chemical weathering

1. Dissolution: Minerals dissolve in water

2. Oxidation: Loss of electrons when minerals react with oxygen and form oxides, common in iron-bearing minerals

3. Hydrolysis: Breaks down silicate minerals by reacting with water, transforms feldspar into clay minerals, releases ions

Discuss the rates of chemical weathering

• Mafic minerals weather by oxidation

• Felsic minerals weather through hydrolysis

• Carbonates and salts weather by dissolution

• Minerals that crystallize early in Bowen’s series are least stable, and weathering products like hematite are most stable

What is differential weathering?

The rate of weathering depends on geometry, it is faster at the edges and corners than on a flat surface

What are the kinds of erosion?

1. Water erosion: Streams and rivers carry sediments downstream

2. Ice erosion: Glaciers drag clasts, scour and reshape landscape

3. Wind erosion: Airborne particles abrade surfaces and transport fine sediments

4. Gravity-driven erosion: Mass wasting (ex. landslides) move clasts downhill under gravity’s force

What are some mediums of transportation of sediments?

• Ice (can carry sediments of any size)

• Water (medium viscosity, fast and slow velocity, carry coarse and thin)

• Wind (low viscosity, fast, carry sand and dust)

What is erosion?

The process of transporting weathered materials from their sources through natural agents

What is transportation and how does it differ from erosion?

Transportation is dispersal by wind, water and ice of sediments. The rate at which they carry depends on their viscosity and velocity.

What is deposition and what are the mechanisms?

The transition from transport to the formation of sedimentary deposits. The kinetic energy of the medium decreases and the transporting medium can no longer carry particles. In water and air, when the velocity drops, the sediments begin to settle. In ice, sediment accumulates as ice melts, forming frontal moraines.

What is lithification?

The process of transforming loose sediments to rocks

What are the processes involved in lithification?

• Burial: Sediment accumulates layer by layer, increasing pressure on underlying material

• Compaction: Overburden weight compresses sediment, reducing pore space and expelling water

• Cementation: Fluids with dissolved ions flush through pore spaces, and minerals slowly crystallize, binding sediment grains

What are the different clast sizes and what are their measurements?

• Boulder, cobble, pebble, gravel ( >2mm)

• Coarse, medium and fine sand (2-0.06mm)

• Coarse, medium and fine silt (0.06-0.004mm)

• Coarse and fine clay (<0.004mm)

What does angularity indicate?

Angularity indicates the transport distance. If something is more angular, it indicates less transport and that it is closer to the source, since it hasn’t had the opportunity to round itself out.

What does sorting indicate?

Sorting is an indicator of the transport distance. If sorting is poor, the rock is closer to the source, since at a further distance, grains would have only traveled as far as the viscosity and velocity would allow that specific size to go.

What is maturity in terms of sedimentary rocks?

The degree of ‘processing’. The textural maturity is the degree of roundness and sorting, the mineral maturity is the degree of unstable mineral removal.

What are the characteristics used to classify sedimentary rocks?

Clast size, clast texture, and clast composition

How might breccia have been formed?

Breccia has angular rock fragments, which suggests little to no transport, so it would have been deposited close to the clast source.

How might conglomerate have been formed?

Conglomerate has rounded clasts, suggesting that they may have been deposited away from the clast source, such as in a river

How might arkose have been formed?

Arkose has coarse clasts, but also sand and gravel sized clasts. The feldspar present within it suggest short transport in arid conditions.

How might sandstone have been formed?

Sandstone can form in a lot of depositional settings. It has sand-sized clasts which suggests a good amount of travel.

How might silt and clay have been formed?

They have extremely small clasts which suggests a lot of transport.

How might biochemical sedimentary rocks have been formed?

These sediments are derived from the shells of living organisms. The organisms extract dissolved ions from seawater to build mineral skeletons, and these skeletons can accumulate after death to become part of the sediment.

How might organic sedimentary rocks have been formed?

Organic sedimentary rocks are formed from the altered remains of fossil vegetation within layers. This vegetation accumulates and burial helps to remove volatiles and concentrate the carbon.

What are the kinds of chemical sedimentary rocks?

Evaporites, travertine, dolostone, replacement chert

What are evaporites?

Rocks comprised of minerals precipitated from water solutions when water evaporates. This happens to create salt. They can occur in various aquatic areas

What is travertine?

A chemical sedimentary rock composed of calcium carbonate precipitated from groundwater at the surface.

What is dolostone?

Chemically modified limestone, where calcium carbonate is recrystallized into dolomite by magnesium-rich fluids. It resembles limestone, but it does not react with HCl.

What is replacement chert?

A chemical sedimentary rock formed when silica-rich fluids replace calcite crystals in limestone with cryptocrystalline quartz.

What are some other kinds of replacement chert?

Flint, jasper, petrified wood, agate

What are beds and bedding planes?

Beds are sedimentary rocks organized in horizontal layers, and bedding planes are the separations between these layers.

What are strata?

Several beds stacked one on top of the other.

What are ripple marks and what is their importance?

Water that flows over loose sediment creates bedforms. Bedform character is tied to flow velocity and the grain size. They are preserved in sandy sediments. Asymmetrical flows develop in one direction and symmetric ripples develop from wave oscillation.

Discuss dunes.

They are similar to ripples but much larger, and form from sand transported in the wind. They can range from tens of cm to hundreds of meters tall. Cross-bedding in dunes is created by ripple and dune migration.

Discuss mudcracks.

Mudcracks are a form of bed-surface marking from deposition while sediment is still soft. It forms polygonal desiccations, and indicate alternating wet and dry conditions. The mud dries up and retracts to form the polygons.

What are the terrestrial sedimentary depositional environments?

Glacial environments, alluvial fans, mountain streams, desert environments, floodplain environments, lake environments

Discuss glacial environments as depositional environments

Their transforming mediums are ice, the sediment is glacial till and the rock often formed is diamictite. Glaciers act as large conveyer belts and transport the sediment downstream, and leave sediment at the foot of the glacier to form moraines. Glacial till is sediment dropped down by ice, and it is unsorted and unstratified. It accumulates beneath the ice, at the toe of the glacier and along the glacial flanks.

Discuss alluvial fans as depositional environments.

Alluvial fans are sediments that pile up at a mountain front. The transport medium is water, the rocks are breccia and arkose.

Discuss mountain streams as depositional environments

The transport medium is water, and due to the long transport, the common rock is conglomerates

Discuss deserts as depositional environments

The transport medium is wind, the rock is sandstone with cross-laminations

Discuss floodplains as depositional environments

The transport medium is water, the rocks are often sandstones, siltstones and claystones due to the transport

Discuss lake environments as depositional environments

Lakes have gravels and sands trapped near the shore and sorted muds deposited deeper in the water, and laminated shales that may contain fish fossils!

What are the types of depositional marine environments?

Coastal beach environments, shallow marine environments, shallow water carbonate environments, deep marine environments

Discuss sedimentary depositions in coastal beach environments

These are often similar to riverbeds in which they have the sand ripples. May have clay or silt.

Discuss sedimentary depositions in shallow marine environments.

Water allows the sediments to settle on the bottom of the ocean floor. The most common rocks are siltstones and mudstones

Discuss sedimentary depositions in shallow water carbonate environments.

These are the environments where a small part of ocean near the island is separated by a reef barrier. There are more calcite sands and muds built up within that lagoon, and outside there are broken fragments of reefs and sands that sort finer as they run deeper.

Discuss sedimentary deposition in deep marine environments

The only sources of sediment in these areas are fine clay and plankton shells. The clay ends up forming finely laminated mudstone and shales, while the plankton shells form chalk (calcerous) and chert (siliceous).

What are sedimentary basins?

Sediment-filled basins. The type of basin depends on the geodynamic setting:

• Foreland basin: Mountain weight pushes down crust surface

• Rift basin: Downward slip on faults produces narrow troughs

• Intracontinental basin: Basin forms in interior of continent, possibly over an old rift

• Passive-margin basin: Subsidence occurs over thinned crust at the edge of an ocean basin

What is diagenesis?

Physical, chemical and biological processes that transform sediments into a sedimentary rock AND alter this rock after its formation. Occurs at temperatures between the surface temperature and 300, which is where metamorphism begins