Sedimentology and Palaeontology Lecture 16: Sandstone Petrology Flashcards

1
Q

Describe quartz grains in thin section, PPL and XPL

A

PPL
Translucent, no cleavage, no alteration
XPL
White/grey/black interference colours
Mono- vs. polycrystalline
Unit vs. undulose extinction

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2
Q

Describe Plagioclase feldspar grains in thin section

A

XPL
Lamellar twins

Looks like normal plagiocase thin section however is more grainy/ looks more like static

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3
Q

Describe Microcline Feldspar grains in thin section

A

XPL
Cross-hatch twins

Looks the same as normal Microcline thin section

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4
Q

Describe Orthoclase feldspar grains in thin section

A

PPL
Dusty appearance, white

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5
Q

Describe lithic grains in thin section

A

PPL
Rock fragments yet to disaggregate into component crystals
Sedimentary: siltstone, mudstone, chert
Metamorphic: slate, quartzite, schist, phyllite
Igneous: porphyry

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6
Q

Describe how cement looks in thin section

A

PPL
Translucent crystalline precipitate after deposition

Pale white beige (but does not look brown) colour

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7
Q

Describe how matrix looks in thin section

A

PPL
Opaque/pasty fine-grained material between grains

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8
Q

Describe porosity in thin section

A

PPL
Void space between grains (blue resin)`

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9
Q

Describe the sandstone composition interpretations (+pettijohn classification groups)

A

Quartz arenites: Over 95% Quartz;
polycyclic; warm and humid climates; beach and desert deposits, cream/beige colour

  • Arkoses: Over 25% Feldspar;
    common K-feldspar; continental in semi-arid settings or proximal to granitoids/gneisses in humid settings; red/yellow colour
  • Lithic arenites: Over 25% Lithics;
    proximal and commonly first cycle; green/brown colour
  • Greywackes: Over 15% matrix;
    proximal and common as turbidite deposits close to volcanic arcs
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10
Q

Define Lithics

A

fragments of other rocks that have been eroded into sand-sized grains.

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11
Q

Describe mineralogical maturity

A

Mineralogical maturity = extent to which the unstable minerals have been altered of lost

  • Supermature = quartz-rich sand
  • Mature = quartz sand with rare and badly altered
    feldspars and lithics
  • Submature = quartz sand with fresh K-feldspar and mica
  • Immature = quartz-feldspar-mica sand with plagioclase, pyroxenes, or unaltered volcanic lithics
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12
Q

Define Provenance

A

Provenance = parent rock from
which sediment is derived

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13
Q

Describe mechanical compaction of grains

A

Mechanical compaction
* Grain re-arrangement
* Fracture brittle grains
* Squash ductile grains

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14
Q

Describe chemical compaction of grains

A

Chemical compaction
* Dissolution at contacts
between grains
* Product re-precipitated
as cement

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15
Q

Describe compaction examples in thin section

A

Point contacts
Long contacts
Bent Mica
Sutured contacts

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16
Q

Describe dissolution and replacement examples in thin section

A

Corroded felspar
Oversized pores
Feldspar slightly replaced by calcite