Intro to Petrology: Understanding Sandstones and Conglomerates - 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

Sandstones and ConglomeratesSandstones

and Conglomerates

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• Detrital or clastic sedimentary rocks – made

of solid products (detritus) from weathering

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of pre-existing rocks

• They make up 20-25% of the stratigraphic

record, but receive much more attentionfrom sedimentary petrologists than 25%

What clues are present in Sandstones?What

clues are present in Sandstones?

• Grain size• Grain shape• Grain sorting• Grain sorting• Grain mineralogy• Sedimentary structures

Grain size

• Detrital or clastic rocks have a huge range in

grain size

• We need a log scale to represent this wide size

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p

range

• The Phi (

φ

) Scale:

φ

= -log

2

(mm)

The Phi (

φ

) Scale:

φ

log

2

(mm)

mm = 2

-^

φ^

(memorize)

E

h

φ

t^

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d

bli

(^

ll

φ

• Each

φ

step represents a doubling (smaller

φ

or

more neg.) or halving (larger

φ

#) in size

Size ranges are

given namesgiven names

Gravel > -

φ

(>2mm)

Sand: 4

φ

to -

φ

Sand: 4

φ

to -

φ

(0.0625mm to 2mm) Mud < 4

φ <0.0625mm<62 5

m

<62.

μ

m

Clay < 8

φ <

(^1) / <

(^1) /^256 <0.004mm<

μ

m

Loose sediments can be separated by sieving

φ

(4mm)

φ

(2mm) φ

(^

φ

(0.5mm)

φ

(0 25mm)

φ

(0.25mm)

φ

(0.125mm)

φ

(0.062mm)

• closed

Sorting = range in grain size

• Usually the size range (in

φ

) that includes

3

of the grainsf grains

f grains

% o

% o

φ^

size

φ^

size

poorly-sorted

well-sorted

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Sorting by comparison

MudrocksMudrocks

• Composed mostly of detrital material

Composed mostly of detrital materialsmaller than 4

φ

, i.e., smaller than 0.062 mm

or 62.

μ

m (mud=silt+clay)

or 62.

μ

m (mud silt clay)

• Non-fissile (-stone) or fissile (-shale)

N

d b

ti

f Silt

d Cl

• Named by proportion of Silt and Clay

> 2/3 silt

Siltstone

Silt-shale

1/3 to 2/3 silt

Mudstone

Mud-shale

>2/3 clay

Claystone

Clay-shale

Conglomerates

• No agreement about the % of gravel sized

material required to make a sediment a

q

GRAVEL or a sedimentary rock aCONGLOMERATE

• We’ll say >30% gravel size material for a

conglomerateconglomerate

• Small amount of pebbles or cobbles is very

noticeable so bewarenoticeable, so beware

Accessory Minerals (other minerals)• Micas - may float in water; muscovite,

especially is very stable

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• Heavy minerals - higher density than quartz

and feldspar, some are very stable: zircon,

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tourmaline, rutile, hornblende, garnet,ilmenite, magnetite, apatite, pyroxene, etc.f l

d

di

d

d

i

If loose sand or disaggregated sandstone isput in a heavy liquid (s.g. 2.8-3.0) “heavies”sink to the bottomsink to the bottom

• These minerals can be quite informative

Detrital Grain Types: QuartzDetrital

Grain Types: Quartz

Feldspars distinguished

from quartz byfrom quartz by

alteration, twinning

and perthiteand perthite

What’s the large grain?