Intro to Petrology: Sedimentary Rocks and Weathering - Prof. Helen Lang, Study notes of Geology

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Dr. Helen LangDept. of Geology & Geography

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West Virginia University

SPRING 2008

GEOLOGY 285:

INTRO. PETROLOGYINTRO. PETROLOGY

Sedimentary Rocks

Rocks resulting from the consolidation

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di

of loose sediment

or chemical precipitation from solution

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E

h’

f

at or near the Earth’s surface; or organic rocks consisting of the

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f^

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d

secretions or remains of plants and

animals

Weathering

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• Clastic rocks - conglomerates, sandstones,

mudrocks are composed of fragments andsolid weathering products of pre-existingrocks

• Even carbonate rocks and cements, whose

constituents are precipitated from seawaterby biological organisms, are made from

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g

g

ions that come from the weathering process

Two aspects to Weathering

• Mechanical Weathering - physical breakup of

rocks (esp frost wedging: water

ice causes

rocks (esp. frost-wedging: water

ice causes

9% volume increase)Ch

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l W

th

i^

h

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l b

kd

f

• Chemical Weathering - chemical breakdown of

minerals in the presence of water

+δ

• δ

• δ

• δ

• Chemical Weathering is MUCH more important

than mechanical weathering, due to the

g

extraordinary dissolving power of H

O 2

Weathering Reactions of Orthoclase

Step 1:

3 KAlSi

O 3

8

• 2 H

+^

• 12 H

O --> 2

KAl

Si 3

O 3

10

(OH)

2

• 6 H

SiO 4

4

• 2 K

orthoclase

illite (~muscovite)

soluble silica

Step 2:

2 KAl

Si 3

O 3

10

(OH)

2

• 2 H

+^

• 3 H2O -->

illite

3 Al

Si 2

O 2

(OH) 5

4

• 2 K

illite

kaolinite

All feldspars weather similarly

• React with H

O and H 2

• Release silica in solution and cations• Produce clay minerals (sheet silicates)• Albite + H

O + H 2

+^

Sodium montmorillonite + H

SiO 4

4

• Na

Sodium montmorillonite + H

SiO 4

4

• Na

• Anorthite + H

O + H 2

+^

C l i

ill

i^

SiO

C

2+

Calcium montmorillonite + H

SiO 4

4

• Ca

2+

Iron in minerals weathers differently

• Fe in most ferromagnesian minerals is

red ced (Fe

2+

) beca se the ’re formed in

reduced (Fe

2+

), because they’re formed in

reducing conditions (low oxygen)

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i h i

• Surface waters are very oxygen-rich, i.e.,

oxidizing

• Fe

2+

released during weathering immediately

oxidizes to Fe

3+

• Fe

3+

precipitates rapidly as EXTREMELY

INSOLUBLE Fe(OH)

3

and other hydroxides

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Weathering of pyroxene for exampleWeathering

of pyroxene, for example

C F Si O (F

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f^

it ) + H O + H

CaFeSi

O 2

6

(Fe part of augite) + H

O + H 2

+^

=

Calci m montmorillonite + H SiO + Ca

2+

Fe(OH)

Calcium montmorillonite + H

SiO 4

4

• Ca

2+

Fe(OH)

3

Fe(OH)

3

and other Ferric oxyhydroxides precipitate

(orange or rusty) and eventually dehydrate to(^

g

y)

y

y

Hematite, which gives subaerial soils and sediments(red beds) their red color

Which will weather more rapidlyWhich

will weather more rapidly,

basalt or granite? Why?

Weathering of BasaltWeathering

of Basalt

• What will be the most common sand-sized

fragments?

• What will be the most common mineral

What will be the most common mineralweathering products?

• Weathered ash deposits form bentonite - a

i t

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i^

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tl

mixture of clay minerals, mostlymontmorillonite - expand when wet, veryli

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k f

d ti

slippery! can crack foundations!

Grus:

disaggregated

grains ofgrains of

quartz and

feldspar

Grus is a commonfirst step in granitefirst step in graniteweathering

Sedimentary Structures

• Sedimentary structures can tell a lot about

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d

iti

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t

the depositional environment

• Some sedimentary structures tell which way

was UP during deposition

• Some indicate current direction• You’ll hear about lots more in Strat-Sed

Graded bedding is typical of

bidi

turbidites*

*Turbulent mixtures ofTurbulent mixtures ofsand, gravel, mud andwater that are producedb^

b^

i^

l^

d lid

by submarine landslides,deposits fine upward andbasinward, submarine mudflows/debris flows

Bouma Sequence

Distal=far from source

Proximal=close to source

Graded Bedding in Turbidite

Flame Structures

G

d d UP Graded-UP

Scour surface

Graded-UP