A2 sedimentary rocks student copy

A2 Sedimentary Processes
& Rocks


Sedimentary Processes

Weathering

•

Weathering

•

Types

•

Erosion

•

Products

•

Transportation

•

Rates

•

Deposition

•

Mineral Susceptibility

•

Diagenesis

Sedimentary
Environments
•

Marine – deep/shallow

•

Land – glacial/alluvial/desert

•

Transitional – delta/beach

Maturity

SEDIMENTARY ROCKS

Sedimentary Structures
•

Bedding

•

Cross-bedding & graded bedding

•

Desiccation cracks

•

Ripple marks & sole marks

•

Textural maturity

•

Compositional maturity

Sedimentary Rocks
•

Conglomerate & Breccia

•

Sandstones

•

Shale/siltstone/mudstone

•

Limestone/chalk/coal/halite/gypsum

Fragmental

Chemical & Biogenic


How can sedimentary rocks be classified?

Coarse
>2 mm

Medium
0.06 - 2 mm
Fine
<0.06 mm
(<63 µm)

Grain Size

Sediment Name

Rock Name

>256 mm
64 - 256 mm
4 - 64 mm
2 - 4 mm
1 – 2 mm
0.5 – 1 mm
0.25 – 0.5 mm
0.125 – 0.25 mm
0.06 – 0.125 mm
4 µm – 63 µm
<4 µm

Boulders
Cobbles
Pebbles
Granules
Very coarse sand
Coarse sand
Medium sand
Fine sand
Very fine sand
Silt
Clay

Conglomerate (rounded

fragments)
Breccia (angular
fragments)

1 µm = micrometer = 0.001 mm

•Arkose

Sandstone

•Greywacke
•Orthoquartzite
•Desert sst

Siltstone
Mudstone/Shale


Summary - Classifying clastic sedimentary rocks

Fragmental/Clastic

Coarse

Medium

Fine

Very Well Sorted

Boulder

>256mm

Cobble

64 - 256mm

Pebble

4 - 64mm

Granule

2 - 4mm

Very coarse

2 -1mm

Coarse

1 - 500um

Medium

500 - 250um

Fine

250 - 125um

Very fine

125 - 63um

Silt

63 - 4um

Siltstone

Clay

<4um

Mudstones/Shales

Well Sorted

Moderately
Sorted

Arkose
Orthoquartzite

Desert Sandstone

Greywacke

Poorly Sorted
Rounded

Sub-Angular

Angular

Conglomerate

Breccia-Conglomerate

Breccia


Sediments to Rock
1.

Define the following terms:

•
•
•
•
•
•

- unconsolidated material deposited by water, ice or wind
Sediment
Particle - in general language - a piece. Geological language, same as grain.
Clast - a particle within a rock which has been broken off a pre-existing rock.

Clastic - a group of sedimentary rocks composed of particles e.g. sandstone.
Fragment - a large clast, usually of a rock (no definite size limit)
Grain - a small clast, usually of a mineral (no definite size limit)

2. How long does it take to turn a sediment into a rock? - millions of years
3. The major assumption which underlies sedimentary rocks is
the Principle of Uniformitarianism. What does this mean and
why is an understanding of this principle crucial to making
sense of sedimentary rocks
4.

Draw the rock cycle.



Sedimentary Processes
Weathering

4 agents of transportation:
• Rivers
• Sea

Erosion

• Wind
• Ice
Deposition

Transportation
3 processes of weathering:

•

Physical

•

Chemical

•

Biological
5 processes of erosion:
•

Abrasion

•

Attrition

•

Hydraulic action

•

Cavitation

•

Plucking

3 processes of
deposition:

Lithification

•

Loss of energy

•

Accumulation
of dead animals

2 processes of
lithification:

Precipitation

•

Compaction

•

Cementation

•

Weathering

Two main types of weathering were covered at AS.
What are these, and what are the main products?

1. Physical weathering which produces rock and mineral fragments.
2. Chemical weathering which leads to the production of new minerals and
products in solution.


Weathering

• Describe what each type of weathering is?
• Explain how it works?
• What are the specific products of this weathering?

Carbonation

Hydrolysis

Frost shattering

Oxidation

Exfoliation

Salt crystallisation

Chemical Weathering
Oxidation

4FeSiO3 + O2

2Fe2O3 + 4SiO2

Carbonation

CO2 + H2O

H2CO3 + CaCO3

Ca + 2HCO3

Hydrolysis

Ca Na K ALSiO + H2O

AlSiO(OH) + K + Ca + Na
+ 2HCO3


Weathering

If exposures of basalt, granite and orthoquartzite
each underwent chemical weathering, explain which of
the rock types would be likely to show a surface
coating of iron oxide.

Basalt & granite – they contain mafic minerals
(augite, hornblende & biotite mica).
Chemical weathering of these minerals releases
Fe 2+ ions which are immediately oxidized to Fe
3+ and deposited as ferric oxide (rust).
Quartzite – composed entirely of quartz & so has
no mafic minerals.


Products of Weathering
What sedimentary rocks
would be formed?


Products of Weathering
What sedimentary rocks
would be formed?


Products of Weathering
Original Mineral

Feldspar
KNaAlSiO

Ferromagnesian
FeMgSiO

Muscovite mica
KAlSiO

Quartz
SiO

Calcite
CaCO3

Chemical
Weathering
Process

Hydrolysis
Oxidation
Hydrolysis

Solid Product

Clay
Iron oxide
Clay

Soluble Product

K, Na, Ca
Si0
K

-

Quartz grains

-

Carbonation

-

Ca


Susceptibility to Weathering

~1200°C
Olivine
Augite

Plagioclase
feldspar

(Mg Fe) SiO2

Ca Mg Fe SiO2

Hornblende

Plagioclase
feldspar

Ca Mg SiO2

Biotite mica

Fe Mg K Al SiO2

Orthoclase feldspar
Muscovite mica
~600°C

Ca Al SiO2

Quartz

SiO2

K Al SiO2

K Al SiO2

Na Al SiO2


Rates of Weathering

4FeSiO3 + O2
CO2 + H2O

Ca Na K ALSiO + H2O

2Fe2O3 + 4SiO2
H2CO3 + CaCO3

Ca + 2HCO3

AlSiO(OH) + K + Ca + Na
+ 2HCO3

Frost shattering/ freeze-thaw


Rates of Weathering

Joints
Faults
Fractures
Bedding planes
Pore spaces

Rock


Rates of Weathering

Feldspar

CaCO3

Biotite mica
Augite

Quartz

Feldspar


Rates of Weathering


Summary of Weathering
1. Why do rock outcrops disintegrate,
leaving shattered fragments to
accumulate on hill slopes?
By physical weathering (frost shattering) –
expansion of water on freezing in cracks &
joints exerts pressure breaking rocks apart.

2. How can quartz minerals be extricated
from the rocks from which they are
derived ?
Physical weathering (frost shattering) may have shattered rock into smaller
fragments which were then attacked by chemical weathering. Then chemically
less resistant minerals would have been decomposed, leaving a residue of
chemically resistant quartz grains..

3. Where does mud come from (i.e. the clay minerals that make up the
muds we see in places like river estuaries)?
Mud consists predominantly of clay minerals. These are the products of the
chemical weathering of various Al-bearing minerals such as feldspar & mica.


The photograph below is a side view of an igneous body.

Explain the features shown in the photograph [4]
• Dolerite igneous body, with cooling joints and pressure release joints
• Clay from the hydrolysis of plagioclase feldspar.
• Red-brown staining due to oxidation of augite.
• Spheroidal shapes due to preferential weathering along joints.


Triangular Graphs

Q – 50%
RF – 20%
F – 30%

20%

30%


Triangular Graphs
3%

Q – 68%
RF – 3%
F – 29%

68%

29%

Triangular Graphs

Arkose
Q – 40%
F – 55%
RF – 5%


Triangular Graphs

ARKOSE