EAE2522 Week 3 Flashcards

Question 1

Q

What is Petrology?

Answer

A

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

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Question 2

Q

What influences deposition?

Answer

A

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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Question 3

Q

What is Diagenesis?

4 points.

Answer

A

<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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Question 4

Q

What are the 3 types of diagenesis?

3 points.

Answer

A

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

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Question 5

Q

What are authigenic minerals?

5 points.

Answer

A

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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Question 6

Q

What is the Matrix?

<div>No Neo not that matrix ☺</div>

Answer

A

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

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Question 7

Q

What is Cement?

Answer

A

Precipitation in pore space during diagenesis. Always secondary.

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Question 8

Q

Describe

Matrix -Clay minerals

6 points.

Answer

A

<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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Question 9

Q

Describe

Cement

3 points.

Answer

A

<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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Question 10

Q

Describe

Composition

3 points.

Answer

A

<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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Question 11

Q

Classification Conglomerates

4 points.

Answer

A

<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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Question 12

Q

Classification Mudstones

5 points.

Answer

A

<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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Question 13

Q

Black Shales

3 points.

Answer

A

<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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Question 14

Q

Classification based on chemical composition

Chemical sediments

Answer

A

<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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Question 15

Q

What is Provenance?

5 points.

Answer

A

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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Question 16

Q

How do we determine provenance?

4 points.

Answer

A

<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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Question 17

Q

What do we learn from

Structures and textures

5 points.

Answer

A

<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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Question 18

Q

Describe

Clast types and mineralogy

4 points.

Answer

A

<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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Question 19

Q

Describe impact of:

Tectonic settings

3 points.

Answer

A

<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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Question 20

Q

What is the Gazzi Dickson method?

4 points.

Answer

A

<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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Question 21

Q

Describe

Quartz

5 points.

Answer

A

<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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Question 22

Q

Describe

Fekdspar

4 points.

Answer

A

<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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Question 23

Q

Describe

Zircon

3 points.

Answer

A

<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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Question 24

Q

What are the problems and limitations of provenance investigation?

4 points.

Answer

A

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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Question 25

Q

Thin Section Petrology

Crystal Habit

5 points.

Answer

A

<div>How well-developed crystal faces are: e.g.</div>

<ul> <li>Euhedral</li> <li>Subhedral</li> <li>Anhedral </li></ul>

<div>Note: Habit will vary depending on the sample </div>

<ul> <li>It depends mainly on the conditions of mineral formation</li> <li>In sediments we rarely deal with crustal habit!</li></ul>

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Question 26

Q

Thin Section Petrology

Crystal/Grain Shape

Answer

A

<div>E.g., prismatic, equant, fibrous, polygonal, tabular, hexagonal, etc.</div>

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

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Question 27

Q

Thin Section Petrology

Relief

2 points.

Answer

A

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

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

<div>The boundary between topaz and quartz is highlighted by a thick black line. </div>

<div>That between quartz and K-feldspar has no line (low relief).</div>

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Question 28

Q

Thin Section Petrology

Colour

4 points.

Answer

A

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

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Question 29

Q

Thin Section Petrology

Pleochroism

Answer

A

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._{EAE2522 1bc}

Question 30

Q

Minerals in Cross Polarised Light (XPL)

Birefrigence ("double refraction")

3 points.

Answer

A

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

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

Isotropic minerals (e.g., glass, garnet) are black in XPL!_{EAE2522 1bd}

Question 31

Q

Minerals in Cross Polarised Light (XPL)

Twinning

3 points.

Answer

A

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

<div>Diagnostic of feldspars </div>

<ul> <li>K-feldpars - simple or Carlsbad twins </li> <li>Microcline - cross hatch (tartan) twins </li> <li>Plagioclases - multiple or polysynthetic twins</li></ul>

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Question 32

Q

Minerals in Cross Polarised Light (XPL)

Twinning

3 points.

Answer

A

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

<div>For minerals with cleavage or twins, the angle between the cleavage or twins and the extinction position is the extinction angle </div>

<div>This is measured by</div>

<ol> <li>lining the cleavage up the the N-S microscope axis</li> <li>rotate to the extinction position</li> <li>measure the angle of the microscope stage</li></ol>

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EAE2522 Week 3 Flashcards

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