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SEDIMENTARY ROCK COMPOSITION PowerPoint PPT Presentation


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SEDIMENTARY ROCK COMPOSITION. Sedimentology 340. The story behind sedimentary rock composition is in the Rock Cycle. Mineral (and rock) composition of sedimentary rocks is a fundamental property that is used for: interpretation of the history of the deposit

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SEDIMENTARY ROCK COMPOSITION

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SEDIMENTARY ROCK COMPOSITION

Sedimentology 340


The story behind sedimentary rock composition is in the Rock Cycle


  • Mineral (and rock) composition of sedimentary rocks is a fundamental property that is used for:

  • interpretation of the history of the deposit

  • determining source rocks (provenance)

  • classifying the rock

Composition is analyzed using a petrographic microscope and thin sections

Only crude description and preliminary identification can be done in the field or from hand specimens with eye or hand lens


  • Source Area(provenance)

    • Rock type

    • Weathering environment

    • Transport direction

Provenance is the original source rock……often mountain ranges that no longer exist


  • Weathering varies with

  • - mineralogy

  • climate

  • amount and type of

  • cement

  • Source Area (provenance)

    • Rock type

    • Weathering environment

    • Clast transport


energy

& agent

  • Clast Transport

    • Energy

    • Agent

    • Distance

  • Depositional Environment

    • Marine vs. non-marine

    • Physical environment

    • (beach, river, delta, desert, etc.)

distance


Rock exposed to weathering is the lithosphere (continental and oceanic crust)

Crustal rocks vary: igneous rocks, sedimentary rocks

and metamorphic rocks


The Earth's crust is dominated by seven mineral or groups of minerals. In decreasing order of abundance, these are:

feldspar51%

with two common varieties:

plagioclase [(CaAl2Si2O8 or NaAlSi3O8]39%

orthoclase [KAlSi3O8]12%

quartz [SiO2]12%

pyroxene [(Ca,Na)(Mg,Fe,Al)(Si,Al)2O6]11%

mica (muscovite, biotite) 5%

amphibole 5%

clay 4.6%

olivine [(Mg,Fe)2SiO4] 3%

TOTAL91.6%


Physical and chemical weathering break down rocks producing rock fragments and minerals

Stablity is related to the mineral structure; usually silicate structure

Bowen’s Reaction Series


Source Rocks

granite

basalt

Sedimentary rock (conglomerate)


Grain composition mineralogy proportions are different from source (bedrock) to “sink” (site of deposition)

Igneous &

Metamorphic

Weathering &

Transport process

Average in sed rocks

shalesandstone

feldspar 60% 5% 10-15%

quartz 15-20% 30% 65%

rock fragments 0% 10-15%

clay minerals 0% 60% 5%

fine micas

heavy minerals 20% 3% 8%


framework grains

Framework Grains

major minerals

quartz

feldspar

clay minerals

accessory minerals

mafic, heavy minerals

stable, non opaque, Z,R,Ts

rock fragments

igneous (IRF & VRF)

sedimentary (SRF)

metamorphic (MRF)

chemical cements

silicate, carbonate, iron oxide, sulfate

matrix

cement

See TABLE 5.1


Major minerals

quartz

Quartz structure varies with origin:

Monocrystalline quartz is igneous

Polycrystalline quartz is metamorphic

Si O2

framework silicate

Single-crystal grains = monocrystalline ( in igneous rocks)


Major minerals

quartz

Quartz structure varies with origin:

Monocrystalline quartz is igneous

Polycrystalline quartz is metamorphic

Si O2

framework silicate

Multiple interlocking quartz grains =

polycrystalline (in metamorphic rocks)


Orthoclase KAlSi3O8

Plagioclase NaAlSi3O8 - CaAl2Si2O8

Orthoclase

Plagioclase

Major minerals

feldspar

plagioclase

framework silicate


Petrography – thin sections

Quartz, orthoclase feldspar and plagioclase feldspar are main minerals.

Feldspar shows the alteration.

Mica and calcite are common as accessory minerals.

Grain boundaries and fractures on quartz and feldspar reveals evidence of intense weathering.

Q, Quartz, F, Feldspar, C, Calcite, M, Mica, B, Biotite.


Petrography – thin sections

Quartz, orthoclase feldspar and plagioclase feldspar are main minerals.

Feldspar shows the alteration.

Mica and calcite are common as accessory minerals.

Grain boundaries and fractures on quartz and feldspar reveals evidence of intense weathering.

Q, Quartz, F, Feldspar, C, Calcite, M, Mica, B, Biotite.


Sheet silicate

Major minerals

Mica and Clay minerals

Most clay minerals are too small

to identify without x-ray difraction

mica

Examples: kaolinite, illite, chlorite


Accessory minerals (1-2%)

mafic, heavy minerals

stable, non opaque, Z,R,Ts

Mafic = magnesium (mg2+) and iron (fe3+) = dark black/green in color

examples = hornblende, amphibole, magnetite

Heavy minerals = mineral density > 2.9 gm/cm3

garnet

stable, non opaque, Z,R,Ts

Z = zircon

R = rutile

T = tourmaline

Last to chemically weather

quartz

hornblende


rock fragments

igneous (IRF & VRF))

sedimentary (SRF)

metamorphic (MRF)

Pieces of ancient source rocks that were NOT physically or

chemically weathered

Rock fragments make up 15-20% of framework grains in sandstones; are most common in gravel-sized particles of conglomerates;

are rare in shales.


framework grains

chemical cements

silicate, carbonate, iron oxide, sulfate

matrix

Cements are precipitated from solutes (cations, anions, and complex molecules that are produced during chemical weathering.

cement

silicate cement – chert, opal, feldspar, zeolites

carbonate – calcite, dolomite (mg), siderite (fe)

iron oxide – hematite, limonite, geothite

sulfate – anhydrite, gypsum, barite

Table 5.1


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