Sediments Sedimentary Rocks Sedimentary Environments

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Transcript of Sediments Sedimentary Rocks Sedimentary Environments

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Sediments

Sedimentary Rocks

Sedimentary Environments

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Sedimentary Stages

1. weathering 2. Erosion

5. Burial and compaction and diagenesis

6. Diagenesis

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Sedimentary Stages

1. weathering 2. Erosion

5. Burial and compaction and diagenesis

6. Diagenesis

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Mechanical Weathering Disintegration

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Mechanical Weathering

• Mechanical/physical weathering - physical disintegration of a rock into smaller fragments, each with the same properties as the original. Occurs mainly by temperature and pressure changes.

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• Frost (ice) wedging – Mountain tops

• Abrasion – Wind, water, glaciers

• Exfoliation – Release of pressure; parallel to surface

• Plant and animal action – Roots; burrows; anthropogenic activity

• Humidity changes – Wetting/drying; shale into clays

• Temperature changes – Heating/cooling; expansion/contraction

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Types of Mechanical Weathering

• Frost Wedging - water expands when it freezes.

• individual layers within the sedimentary rock break apart through repeated cycles of freeze-thaw.

• A similar process happens when the rock is repeatedly wetted and dried as salt crystals dissolve from the rock then grow when it is dried.

• Both processes can result in the rocks being heaved - so what was once a nice regular pattern of bricks set in a pavement will eventually become a chaotic jumble of bricks oriented every which way.

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New Hampshire's symbolic Old Man In The Mountain surrenders to weathering and rock fall. In 2001 the profile collapsed in response to frost wedging and chemical weathering along horizontal and vertical dilation joints that weakened the face. Modified from Russel and Demarco,2003, N.H. says "farewell, Old Man" Boston Globe Online.

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Abrasion

• Abrasion - physical grinding of rock fragments.

• The next photo shows some pits that have been eroded into the rock by sandblasting.

• Along with the physical weathering (the sandblasting), chemical weathering has taken place as the rock shows some signs of solution weathering as well.

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Abrasion

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Exfoliation

• Mechanical Exfoliation - rock breaks apart in layers that are parallel to the earth's surface;

• as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation.

• Exfoliation is very common whenever plutonic igneous rocks are exposed. Since the plutonic rocks cool at depth under great pressure, they essentially de-pressurizes once the overburden is removed. This causes sheets of rock to peel off subparallel to the earth's surface, or whatever is the least pressurized surface.

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Thermal Expansion and Contraction

• heating causes rock to expand

• cooling results in contraction

• different minerals expand and contract at different rates.

• looks very similar to frost wedging and salt crystal growth, but will typically happen in climates that undergo extreme diurnal temperature changes.

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Thermal shattering of sandstone by forest fire. Here's an unequivocal example of thermal weathering. Matagamon Sandstone, near Harrington Lake, north-central Maine..

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Organic activity

• growth of roots and rootlets, bioturbation, hominidation, etc

• Burrowing and drilling organisms, plants, and people are also responsible for mechanical weathering.

• Tree roots are probably the most significant agents of biological weathering as they are capable of prising apart rocks by growing into cracks and joints. Plants also give off organic acids that help to break down rocks chemically.

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Even the tiniest bacteria, algae

and lichens produce chemicals

that help break down the rock on

which they live, so they can get the

nutrients they need.

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•Precipitation of salts (CaSO4.2H2O, Na2SO4, Na2CO3.H2O, NaCl, KCl, NaF, KF) •Elements from rocks, water, and the atmosphere combine to produce soluble minerals that crystallize in rock pores, along grain boundaries and in fine fractures, causing disintegration. •Coastal regions from salt spray •Salt weathering also attacks urban monuments and buildings. •Sulfur dioxide released by the burning of fossil fuels dissolves in rainwater to produce dilute sulfuric acid. The gypsum crystallizes in cracks and pours causing the rock to mechanically peel and flack.

Salt Weathering

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Rock particles released by salt weathering of granite in the surf zone.

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Even the tiniest bacteria, algae

and lichens produce chemicals

that help break down the rock on

which they live, so they can get the

nutrients they need.

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Chemical Weathering Dissolution

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Chemical Weathering • Related to the

– stability/instability of minerals – solubility of minerals – rate of dissolution of minerals

• Reverse of Bowen’s Reaction Series • Which mineral most stable on Earth’s

surface? – quartz

• Which mineral(s) most UNstable on Earth’s surface?

• Feldspar’s break down into clays • Mafic minerals alter to oxides

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• Oxidation – Oxygen, in the air we breathe, reacts with iron in

minerals to form iron oxide minerals, e.g. hematite (rust). Many minerals contain iron, so it is not unusual to see red-colored rocks.

– Iron (Fe) + Oxygen (O) Iron Oxide (rust) (crumbles)

• Hydrolysis – chemical reaction between the minerals in the rock

and hydrogen in rain water. For example, during hydrolysis, the feldspar in granite changes to clay minerals which crumble easily, weakening the rock and causing it to break down.

• Solution – process by which minerals in the rocks dissolve

directly in water.

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• Hydration

– process where minerals in the rock absorb water and expand, creating stress which causes the disintegration of rocks.

• Acid Dissolution

• Carbonation

– process by which dissolved carbon dioxide in rainwater or moisture in surrounding air forms carbonic acid and reacts with the minerals in the rock. This process weakens the rock thus breaking it down in the process.

• e.g.: Calcium Carbonate + Water + Carbon Dioxide ---> Calcium Carbonate (soluble)

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Types of Chemical Weathering

• Dissolution • H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3- • water + carbon dioxide + calcite dissolve

into calcium ion and bicarbonate ion • Dissolution is very common in areas that

have a great deal of limestone. Acidic waters (from pollution or natural) dissolve limestone allowing for additional water to gain entrance. Can cause sinkholes and karst features as well as dissolution of statutes and grave stones.

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Types of Chemical Weathering

• Oxidation (rust)

• 4Fe+2 +3O2 --> 2Fe2O3

• ferrous iron + oxygen combine to form ferric iron oxide (hematite)

• Will happen to all iron-bearing silicates to varying degrees. Common reaction minerals are hematite, limonite, and goethite.

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Types of Chemical Weathering

• Hydrolysis

• 2KAlSi3O8 + 3H20 --> Al2Si2O5(OH)4 + 4SiO2 + 2K(OH)

• potassium feldspar in acidic water hydrolyses to kaolinite + quartz + potassium hydroxide

• Silicate minerals (unstable at the earth's surface) weather to form clay minerals such as kaolinite (stable at the earth's surface). Feldspars typically weather to produce clay minerals.

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Erosion Transportation

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Even the tiniest bacteria, algae

and lichens produce chemicals

that help break down the rock on

which they live, so they can get the

nutrients they need.

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Chemical Weathering Dissolution

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Chemical Weathering • Related to the

– stability/instability of minerals – solubility of minerals – rate of dissolution of minerals

• Reverse of Bowen’s Reaction Series • Which mineral most stable on Earth’s

surface? – quartz

• Which mineral(s) most UNstable on Earth’s surface?

• Feldspar’s break down into clays • Mafic minerals alter to oxides

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• Oxidation – Oxygen, in the air we breathe, reacts with iron in

minerals to form iron oxide minerals, e.g. hematite (rust). Many minerals contain iron, so it is not unusual to see red-colored rocks.

– Iron (Fe) + Oxygen (O) Iron Oxide (rust) (crumbles)

• Hydrolysis – chemical reaction between the minerals in the rock

and hydrogen in rain water. For example, during hydrolysis, the feldspar in granite changes to clay minerals which crumble easily, weakening the rock and causing it to break down.

• Solution – process by which minerals in the rocks dissolve

directly in water.

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• Hydration

– process where minerals in the rock absorb water and expand, creating stress which causes the disintegration of rocks.

• Acid Dissolution

• Carbonation

– process by which dissolved carbon dioxide in rainwater or moisture in surrounding air forms carbonic acid and reacts with the minerals in the rock. This process weakens the rock thus breaking it down in the process.

• e.g.: Calcium Carbonate + Water + Carbon Dioxide ---> Calcium Carbonate (soluble)

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Types of Chemical Weathering

• Dissolution • H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3- • water + carbon dioxide + calcite dissolve

into calcium ion and bicarbonate ion • Dissolution is very common in areas that

have a great deal of limestone. Acidic waters (from pollution or natural) dissolve limestone allowing for additional water to gain entrance. Can cause sinkholes and karst features as well as dissolution of statutes and grave stones.

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Types of Chemical Weathering

• Oxidation (rust)

• 4Fe+2 +3O2 --> 2Fe2O3

• ferrous iron + oxygen combine to form ferric iron oxide (hematite)

• Will happen to all iron-bearing silicates to varying degrees. Common reaction minerals are hematite, limonite, and goethite.

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Types of Chemical Weathering

• Hydrolysis

• 2KAlSi3O8 + 3H20 --> Al2Si2O5(OH)4 + 4SiO2 + 2K(OH)

• potassium feldspar in acidic water hydrolyses to kaolinite + quartz + potassium hydroxide

• Silicate minerals (unstable at the earth's surface) weather to form clay minerals such as kaolinite (stable at the earth's surface). Feldspars typically weather to produce clay minerals.

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Erosion Transportation

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Erosion

•Erosion is the displacement of solids (soil, mud, rock and other particles) by the agents of wind, water or ice, by downward or down-slope movement in response to gravity or by living organisms (in the case of bioerosion). •Erosion is a natural process but in many places it is increased by human land use.

•Poor land use practices include deforestation, overgrazing, unmanaged construction activity and road or trail building. •Improved land use practices can limit erosion, using techniques like terrace-building and tree planting.

•A certain amount of erosion is healthy for the ecosystem. •Gravels continually move downstream in watercourses.

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The physical weathering and erosion of a limestone landscape. The freezing

and thawing of rain and its mild acidic properties cause cracks and joints to

enlarge, forming limestone pavements, potholes, caves, and caverns.

(Image © RM)

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Montauk Point in 1838 (vertical section, west to east). The lighthouse is 220' from the bluff.Copied from the 1838 United States Coast Survey.

Montauk Point in 1990 (vertical section, west to east). The lighthouse is 100' from the bluff.

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Sediment

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Sediment Transport

• Movement by wind, water or ice

– Mode of transport produces distinct deposits

• Provenance

• Most sediments are carried by rivers – More than 25 billion tons a year

– Strong currents – faster than 50 cm/sec carries gravel and finer material

– Moderately strong currents – 20-50 cm/sec carries sand

– Weak currents – slower than 20 cm/sec carries mud and silt

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Degree of sorting is a value of textural maturity

Sediment Sorting and Shape

Sorting and shape tell stories about environment of deposition

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