Sedimentology and Palaeontology Lecture 16: Sandstone Petrology Flashcards
Describe quartz grains in thin section, PPL and XPL
PPL
Translucent, no cleavage, no alteration
XPL
White/grey/black interference colours
Mono- vs. polycrystalline
Unit vs. undulose extinction
Describe Plagioclase feldspar grains in thin section
XPL
Lamellar twins
Looks like normal plagiocase thin section however is more grainy/ looks more like static
Describe Microcline Feldspar grains in thin section
XPL
Cross-hatch twins
Looks the same as normal Microcline thin section
Describe Orthoclase feldspar grains in thin section
PPL
Dusty appearance, white
Describe lithic grains in thin section
PPL
Rock fragments yet to disaggregate into component crystals
Sedimentary: siltstone, mudstone, chert
Metamorphic: slate, quartzite, schist, phyllite
Igneous: porphyry
Describe how cement looks in thin section
PPL
Translucent crystalline precipitate after deposition
Pale white beige (but does not look brown) colour
Describe how matrix looks in thin section
PPL
Opaque/pasty fine-grained material between grains
Describe porosity in thin section
PPL
Void space between grains (blue resin)`
Describe the sandstone composition interpretations (+pettijohn classification groups)
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
Define Lithics
fragments of other rocks that have been eroded into sand-sized grains.
Describe mineralogical maturity
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
Define Provenance
Provenance = parent rock from
which sediment is derived
Describe mechanical compaction of grains
Mechanical compaction
* Grain re-arrangement
* Fracture brittle grains
* Squash ductile grains
Describe chemical compaction of grains
Chemical compaction
* Dissolution at contacts
between grains
* Product re-precipitated
as cement
Describe compaction examples in thin section
Point contacts
Long contacts
Bent Mica
Sutured contacts
Describe dissolution and replacement examples in thin section
Corroded felspar
Oversized pores
Feldspar slightly replaced by calcite