SEDIMENTARY ROCK COMPOSITION

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SEDIMENTARY ROCK COMPOSITION
Sedimentology 340
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The story behind sedimentary rock composition is
in the Rock Cycle
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• 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
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Source Area (provenance) Rock type Weathering
environment Transport direction
Provenance is the original source rockoften
mountain ranges that no longer exist
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• Weathering varies with
• - mineralogy
• climate
• amount and type of
• cement

Source Area (provenance) Rock type Weathering
environment Clast transport
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agent
energy
Clast Transport Energy Agent Distance Depositiona
l Environment Marine vs. non-marine Physical
environment (beach, river, delta, desert, etc.)
distance
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Rock exposed to weathering is the lithosphere
(continental and oceanic crust)
Crustal rocks vary igneous rocks, sedimentary
rocks and metamorphic rocks
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• The Earth's crust is dominated by seven mineral
  or groups of minerals. In decreasing order of
  abundance, these are
• feldspar 51
• 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
• TOTAL 91.6

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Physical and chemical weathering break down rocks
producing rock fragments and minerals
Stablity is related to the mineral structure
usually silicate structure
Bowens Reaction Series
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Source Rocks
granite
basalt
Sedimentary rock (conglomerate)
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Grain composition mineralogy proportions are
different from source (bedrock) to sink (site
of deposition)
Igneous Metamorphic
Weathering Transport process
Average in sed rocks shale sandstone
feldspar 60
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10-15
quartz 15-20
30
65
rock fragments
0
10-15
clay minerals 0
60
5
fine micas
heavy minerals 20
3
8
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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
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Major minerals quartz
Quartz structure varies with origin Monocrystall
ine quartz is igneous Polycrystalline quartz is
metamorphic
Si O2
framework silicate
Single-crystal grains monocrystalline ( in
igneous rocks)
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Major minerals quartz
Quartz structure varies with origin Monocrystall
ine quartz is igneous Polycrystalline quartz is
metamorphic
Si O2
framework silicate
Multiple interlocking quartz grains
polycrystalline (in metamorphic rocks)
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Major minerals feldspar
plagioclase
framework silicate
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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.
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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.
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Major minerals Mica and Clay minerals
Most clay minerals are too small to identify
without x-ray difraction
mica
Examples kaolinite, illite, chlorite
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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 gt 2.9 gm/cm3
garnet
stable, non opaque, Z,R,Ts Z zircon R
rutile T tourmaline Last to chemically weather
quartz
hornblende
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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.
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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