What is Diagenesis?4 points.

Sediments get buried (under other sediments) Physical and chemical changes Decrease in water content, porosity, increase in grain packing and cementation Leads to lithificationEAE2522 1ac

What are the 3 types of diagenesis?3 points.

EogenesisMesogenesisTelogenesisEAE2522 1ad

What are authigenic minerals?5 points.

During lithification/diagenesis, additional minerals can form/precipitateMinerals involved: Quartz Calcite Fe-oxides Salts Clay mineralsEAE2522 1ae

What is the Matrix?No Neo not that matrix ☺

Fine (small grained) sediment between larger grains. <0.03mm.EAE2522 1af

DescribeMatrix -Clay minerals6 points.

Different to the size group 'clay' Layered/sheet silicate minerals Four main groups: Kaolinite, Illite, Smectite, and Vermiculite Main difference is the spacing between the layers Make up bulk of matrix Identification hard under microscopeEAE2522 1ah

DescribeCement3 points.

Precipitation of minerals in pore space. Silica and carbonate cement. Cement can be around grains ,or filling up more or less whole pore space EAE2522 1ai

DescribeComposition3 points.

Sediments usually have a higher silica content (compared to parent rocks) → chemical stability, weathering In general (clastic sediments): High SiO₂ Moderately Al₂O₃ Low Fe, Mg, Ca, Na and K Chemical composition of clasts of higher interest → provenanceEAE2522 1aj

Classification Conglomerates4 points.

More than 30% gravel size particles (>2mm) Clasts in conglomerates are usually rock fragments Oligomict: mainly one type of clasts Polymictor petromict:: mixture of clasts Classification can follow process ('genetic'), or clast typeEAE2522 1ak

Classification Mudstones5 points.

More than 50% silt and clay size (<0.062mm) grainsAlso known as shales (if laminated)Main components are clay, quartz and feldsparUsually slightly lower in SiO₂Classification based on field characteristicsEAE2522 1al

Black Shales3 points.

Contain organic carbon (~5% or more)Anoxic conditionsUsed for RedOx studies of GOEEAE2522 1am

Classification based on chemical compositionChemical sediments

Carbonates: → limestone (Ca) and dolomite (Mg) Cherts: SiO₂ Evaporites: salts Fe rich (>15% Fe) Phosphorites (>15% P₂O₄, 6.5% P)EAE2522 1an

What is Provenance?5 points.

Where does the sediment come from?Why: By reconstructing the history of a sediment we can learn about: Source regions (rocks, climate) Transport mechanisms Erosion Deposition sites/environments Tectonic reconstructionsEAE2522 1ao

EAE2522 Week 3 Flashcards by Andrew crisp

What is Petrology?

Petrology looks at the composition, texture and structure of rocks, and how they form.

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What influences deposition?

Deposition is highly dependent on the form of transport environment.

<div>Glacial</div>

<div>Aeolian</div>

<div>Rivers and alluvial fans</div>

<div>Lakes</div>

<div>Oceans</div>

<div>Deltas</div>

<div>Estuaries</div>

<div>Shallow marine</div>

<div>Volcanic</div>

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What is Diagenesis?

4 points.

<ul><li>Sediments get buried (under other sediments) </li><li>Physical and chemical changes </li><li>Decrease in water content, porosity, increase in grain packing and cementation </li><li>Leads to lithification</li></ul>

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What are the 3 types of diagenesis?

3 points.

<ul><li>Eogenesis</li><li>Mesogenesis</li><li>Telogenesis</li></ul>

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What are authigenic minerals?

5 points.

During lithification/diagenesis, additional minerals can form/precipitate

<div>Minerals involved: </div>

<ul> <li>Quartz</li> <li>Calcite</li> <li>Fe-oxides</li> <li>Salts </li> <li>Clay minerals</li></ul>

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What is the Matrix?

<div>No Neo not that matrix ☺</div>

Fine (small grained) sediment between larger grains. <0.03mm.

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What is Cement?

Precipitation in pore space during diagenesis. Always secondary.

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Describe

Matrix -Clay minerals

6 points.

<ul><li>Different to the size group 'clay' </li><li>Layered/sheet silicate minerals </li><li>Four main groups: Kaolinite, Illite, Smectite, and Vermiculite </li><li>Main difference is the spacing between the layers </li><li>Make up bulk of matrix </li><li>Identification hard under microscope</li></ul>

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Describe

Cement

3 points.

<ul><li>Precipitation of minerals in pore space. </li><li>Silica and carbonate cement. </li><li>Cement can be around grains ,or filling up more or less whole pore space </li></ul>

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Describe

Composition

3 points.

<div>Sediments usually have a higher silica content (compared to parent rocks) → chemical stability, weathering </div>

<div>In general (clastic sediments): </div>

<ul> <li>High SiO₂</li> <li>Moderately Al₂O₃</li> <li>Low Fe, Mg, Ca, Na and K </li></ul>

<div>Chemical composition of clasts of higher interest → provenance</div>

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Classification Conglomerates

4 points.

<ul> <li>More than 30% gravel size particles (>2mm) </li> <li>Clasts in conglomerates are usually rock fragments </li> <li>Oligomict: mainly one type of clasts </li> <li>Polymictor petromict:: mixture of clasts </li></ul>

<div>Classification can follow process ('genetic'), or clast type</div>

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Classification Mudstones

5 points.

<ul><li>More than 50% silt and clay size (<0.062mm) grains</li><li>Also known as shales (if laminated)</li><li>Main components are clay, quartz and feldspar</li><li>Usually slightly lower in SiO₂</li><li>Classification based on field characteristics</li></ul>

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Black Shales

3 points.

<ul><li>Contain organic carbon (~5% or more)</li><li>Anoxic conditions</li><li>Used for RedOx studies of GOE</li></ul>

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Classification based on chemical composition

Chemical sediments

<div>Carbonates: → limestone (Ca) and dolomite (Mg) </div>

<div>Cherts: SiO₂ </div>

<div>Evaporites: salts </div>

<div>Phosphorites (>15% P₂O₄, 6.5% P)</div>

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What is Provenance?

5 points.

Where does the sediment come from?

<div>Why: By reconstructing the history of a sediment we can learn about:</div>

<ul> <li>Source regions (rocks, climate)</li> <li>Transport mechanisms</li> <li>Erosion</li> <li>Deposition sites/environments</li> <li>Tectonic reconstructions</li></ul>

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How do we determine provenance?

4 points.

<ul> <li>Sedimentary structures and textures</li> <li>Clasts/components and mineralogy</li> <li>Geochemistry (whole rock and minerals separates)</li> <ul> <li>Minerals used for this have to 'detrital', meaning they are derived from somewhere else (ie, not the minerals that precipitate during diagenesis)</li> </ul></ul>

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What do we learn from

Structures and textures

5 points.

<ul> <li>Clast shape, size and sorting: transport length, energy and duration</li> <li>Current/directional indicators</li> <li>Grain size: relief of source area</li> <li>Shape of clasts: transport medium</li> <li>Volume of sediments/thickness of beds/units</li></ul>

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Describe

Clast types and mineralogy

4 points.

<ul><li>Which clasts types are found in the sediment depend not only on transport, but also on weathering (and therefore climate, relief etc) </li><li>More robust minerals, like quartz, zircon and some feldspars, as well as other heavy minerals might be 'artificially' enriched in sediments </li><li>Rock fragments </li><li>Minerals can indicate rock type too (assemblage of minerals)</li></ul>

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Describe impact of:

Tectonic settings

3 points.

<div>Continental block</div>

<ul> <li>Continent, only few volcanic rocks, quartz, feldspar and rock fragments</li></ul>

<div>Magmatic arc </div>

<ul> <li>Convergent plate margin ('subduction zones'), volcanic rock, plagioclase, less quartz</li></ul>

<div>Recycled orogen </div>

<ul> <li>Convergent plate margin between two continental masses, sediments and metamorphic rock fragments</li></ul>

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What is the Gazzi Dickson method?

4 points.

<ul> <li>Point counting method</li> <li>Thin section</li> <li>300 to 500 randomly selected points</li> <li>Mineral at each point</li></ul>

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Describe

Quartz

5 points.

<ul><li>Most common particle espin sandstones </li><li>Liquid inclusions (point to volcanic origin) </li><li>Mineral inclusions </li><li>Under the microscope (extinction and polycrystallinity) can point to volcanic, igneous or metamorphic source </li><li>Cathodoluminescense</li></ul>

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Describe

Fekdspar

4 points.

<ul><li>Presence of feldspar points to crystalline, 'first generation' source rocks (weathering!) </li><li>Indicator of weathering and erosion in source area </li><li>Some feldspars are indictors for certain rock types: alkali feldspars igneous and metamorphic, plagioclase volcanic origin </li><li>Under the microscope: zoning (igneous)</li></ul>

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Describe

Zircon

3 points.

<ul> <li>Heavy mineral</li> <li>Extremely robust</li> <li>ZrSiO₄</li></ul>

<div>Hafnium: Hf isotopic composition of source is preserved</div>

<div>Uranium: U-Pd dating reveals age of source</div>

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What are the problems and limitations of provenance investigation?

4 points.

Selective! Occurrence of minerals and rock fragments is highly depended on their resistance against weathering and transport

<ul><li>Tracing the provenance of reworked sediments is very difficult </li><li>Diagenesis (and other late stage processes) can alter the minerals and change the chemical and isotopic composition. </li><li>You are never 100 % sure you captured every source region. </li><li>You need something to compare your data to (geochemical)</li></ul>

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Thin Section Petrology

Crystal Habit

5 points.

How well-developed crystal faces are: e.g.

Euhedral
Subhedral
Anhedral

Note: Habit will vary depending on the sample

It depends mainly on the conditions of mineral formation
In sediments we rarely deal with crustal habit!

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Thin Section Petrology

Crystal/Grain Shape

E.g., prismatic, equant, fibrous, polygonal, tabular, hexagonal, etc.

In sediments we are usually more concerned about grain shapes (angular, rounded, etc.)

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Thin Section Petrology

Relief

2 points.

High relief = sharp black line around mineral = large difference in refractive indices (RI).

RI is hard to measure - we use high, medium and low relief
You can describe relative mineral relief or compare it to epoxy glue (RI = 1.54).

The boundary between topaz and quartz is highlighted by a thick black line.

That between quartz and K-feldspar has no line (low relief).

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Thin Section Petrology

Colour

4 points.

Colour of the mineral in PPL
The large biotite grain at the centre is brown to cream in colour. It contains dark spherical spots = radiation damage from tiny zircons.
The other minerals are colourless except for opaque oxides.
Some minerals do not transmit PPL - they appear black and are called OPAQUE minerals. Iron oxides and sulfide minerals are opaque.

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Thin Section Petrology

Pleochroism

Pleochroism is present if the colour of a mineral changes upon rotation of the sample. For pleochroic minerals, you should describe the change in colour upon rotation of the stage in PPL.

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Minerals in Cross Polarised Light (XPL)

Birefrigence ("double refraction")

3 points.

Anisotropic minerals have different refractive indices in different directions (fast and slow)

Light is polarized into two different directions
Light is split into two different speeds (rays)
In cross polars these light waves interfere to make interference colours

Isotropic minerals (e.g., glass, garnet) are black in XPL!

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Minerals in Cross Polarised Light (XPL)

Twinning

3 points.

Some crystals are made up of twins = differently oriented parts or layers

Diagnostic of feldspars

K-feldpars - simple or Carlsbad twins
Microcline - cross hatch (tartan) twins
Plagioclases - multiple or polysynthetic twins

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Minerals in Cross Polarised Light (XPL)

Twinning

3 points.

When anisotropic minerals are rotated on XPL there will be a position where they become black

For minerals with cleavage or twins, the angle between the cleavage or twins and the extinction position is the extinction angle

This is measured by

lining the cleavage up the the N-S microscope axis
rotate to the extinction position
measure the angle of the microscope stage

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