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
Boulders
Cobbles
Pebbles
Granules
Conglomerate (rounded
fragments)
Breccia (angular
fragments)
1 – 2 mm
0.5 – 1 mm
0.25 – 0.5 mm
0.125 – 0.25 mm
0.06 – 0.125 mm
Very coarse sand
Coarse sand
Medium sand
Fine sand
Very fine sand
Sandstone
4 µm – 63 µm
<4 µm
Silt
Clay
Siltstone
Mudstone/Shale
1 µm = micrometer = 0.001 mm
•Arkose
•Greywacke
•Orthoquartzite
•Desert sst
Summary - Classifying clastic sedimentary rocks
Fragmental/Clastic
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
Coarse
Arkose
Orthoquartzite
Medium
Greywacke
Desert Sandstone
Fine
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
Maturity of Sedimentary Rocks
Minerals in
sedimentary
rocks
1. Extent of chemical
weathering
2. Type & amount of
transportation
Sandstone – 70% quartz & 30% orthoclase feldspar
What can you tell about the chemical weathering
experienced by the original rock?
Not much chemical weathering because feldspar unweathered.
Maturity of Sedimentary Rocks
Compositionally Immature
• undecomposed rock fragments
• feldspar
• ferromagnesian minerals
Compositionally Mature
• quartz
• clay minerals
End products of chemical
weathering
Compositional maturity describes ………
… the amount of weathering a sediment has suffered.
Maturity of Sedimentary Rocks
Examine Rock Specimen B by hand & hand lens:
• There is more than 1 mineral present
What minerals are they?
What are their relative proportions in the rock?
Explain how you made your identification
Examine picture of RS B through the microscope.
Draw & label a sketch of the rock to show the minerals.
How compositionally mature would you say this rock is?
• Repeat these stages for Rock Specimen G
Rock Specimen B
Rock Specimen G
Maturity of Sedimentary Rocks
What is Rock B?
Rock Specimen B:
Mineral 1 – quartz (~75%)
Arkose
• grey
• glassy
• not scratched by steel blade
Mineral 2 – feldspar (~25%) • rectangular
• white or pink
• white powder
Rock B is compositionally immature because there is a
high proportion of feldspar.
Maturity of Sedimentary Rocks
Rock Specimen G:
• Minerals too small to be seen by eye or a hand lens.
• Grey colour
• Scratched easily
Mineral 1 - clay
What is Rock G?
Mudstone
Rock G is compositionally mature because there is a high
proportion of clay.
Maturity of Sedimentary Rocks
Maturity of Sedimentary Rocks
1 mm
Maturity of Sedimentary Rocks
Texturally Immature
• sub-rounded
• poorly sorted
• large grains
Texturally Mature
• well rounded
• well sorted
• small grain size
Textural maturity describes ………
…. the time and distance a sediment has been transported.
Maturity of Sedimentary Rocks
Minerals in
sedimentary
rocks
1. Extent of chemical
weathering
2. Type & amount of
transportation
Medium-grained, moderately
sorted & sub-rounded
Texturally immature
70% quartz & 30% orthoclase
feldspar
Compositionally immature
What can you tell about the chemical weathering
experienced by the original rock and the following
sediment transport?
What can you tell about the chemical weathering
experienced by the original rock and the following sediment
transport?
Medium-grained, moderately
sorted & sub-rounded
Texturally immature
Arkose
70% quartz & 30% orthoclase
feldspar
Compositionally immature
The sediments forming this arkose have not undergone much
chemical weathering, probably due to a lack of water. The
sediments have also not been transported very far or for
very long (suggests desert conditions?).
Sediment Transportation
What factors affect how much sediment can be
transported?
• turbulence
• speed of flow
• viscosity
• density
• grain size
Predict how each of these
characteristics will affect how
much sediment is transported.
Describe the characteristics of
ice, wind & water in terms of
the first 3 bullet points.
Sediment Transportation
Sediment Entrainment & Deposition by
Water
Sediment Transportation
Sediment Transportation
Sediment Transportation
Sediment Transportation
Summary of Sediment Transportation
1.
Explain why water can transport larger particles than wind.
2. How do water and wind transport different sized particles?
3. Explain how sorting of sediments occurs when transported by
water or wind.
4. Describe the shape & surface structure of material transported by
water and wind.
5. Brainstorm a list of factors which can determine the transport
history of a sedimentary rock.
1mm = 1000 microns
Transport History & Environments of Deposition of Sedimentary Rocks
1. Lithology
Texture
Grain size
•
Energy levels
•
Distance transported
Gravel or coarser =
High & short
Medium to coarse sands = Medium
Fine sands, silts & clays = Low & long
Coarse & fine = Fluctuating energy levels
Grain shape
•
Type of transport
•
Distance transported
Grain sorting
•
Type of transport
•
Time in transport
Grain surface •
Type of transport
Fabric
(relationship between the grains & the matrix)
Grain-supported Matrix-supported
Intensive reworking Common in tills,
By waves/currents
debris flows deposits
Transport History & Environments of Deposition of Sedimentary Rocks
1. Lithology
Mineralogy
Immature minerals
Mature minerals
Iron oxide
Sediments from solution
Why is the sea salty?
Why does the composition of seawater in the open oceans stay
constant, instead of becoming ever more salty?
Classifying limestones by their grain types
Examine Specimens H - K
1.
Describe colour of grains & matrix
2. Relationship between grains & matrix (fabric)
3. Shapes of grains & estimate the size
4. Origin of grains
5. Sketch grains from microscope view on board
6. Biogenic limestone or chemical limestone
Specimen H
Specimen I
Specimen J
Specimen K
H
Colour – grains Dark grey
I
J
K
Creamy-white
Grey
White
Creamy-white
Grey
White
Grain-supported
Grain-supported
Grain-supported Matrix-supported
Shape
Rounded/coiled
Rounded
Too varied
Rounded
Size
2 – 5mm
0.5 -1 mm
1 – 2mm
<0.05mm
Fossils
Ooids
Fossils
Coccoliths
- matrix Pale grey
Fabric
Origin
Sketch
Quartz centre
Concentrically
layered
Coccoliths
Biogenic or
chemical
Biogenic
Rock Name
Shelly Limestone Oolitic Limestone Shelly Limestone Chalk
Chemical
Biogenic
Biogenic
Diagenesis
Research the changes that occur when sediment is changed to rock
p.56- 57.
1. Draw a flow diagram to show the processes involved in diagenesis.
2. Make notes on these processes.
Pressure
dissolution
is a process of compaction when quartz grains are progressively
buried, the pressure at the grain contacts increases until the quartz
begins to melt slightly and dissolve.
Compaction
is an important first step in lithification where the pressure of the
overlying sediments packs the grains closer together and more
efficiently, reducing the volume of pore space and squeezing out the
pore water.
Lithification
is a diagenetic process in which loose, unconsolidated sediments in is
converted into sedimentary rocks by compaction & cementation.
Cementation
is the second stage of lithification, and involves the gluing together
of compacted grains to form a rock. Often compaction alone will not
produce a lithified rock.
Diagenesis
the group of processes which change sediment into a sedimentary
rock after deposition has occurred, because of this they are
referred to as post-depositional processes.
Mineral changes
Mineral
alignment
Diagenesis often results in the formation of new minerals which grow
in the sediment or sedimentary rock .
is a process during compaction, where any elongated or flaky grains
such as clay or mica will become aligned parallel to the bedding plain.
This alignment of clay minerals may lead to mudstone and shales
splitting easily into layers and being known as fissile.
Diagenesis
the group of processes which change sediment into a sedimentary
rock after deposition has occurred, because of this they are
referred to as post-depositional processes.
Lithification
is a diagenetic process in which loose, unconsolidated sediments in is
converted into sedimentary rocks by compaction & cementation.
Compaction
is an important first step in lithification where the pressure of the
overlying sediments packs the grains closer together and more
efficiently, reducing the volume of pore space and squeezing out the
pore water.
Cementation
is the second stage of lithification, and involves the gluing together
of compacted grains to form a rock. Often compaction alone will not
produce a lithified rock.
Pressure
dissolution
is a process of compaction when quartz grains are progressively
buried, the pressure at the grain contacts increases until the quartz
begins to melt slightly and dissolve.
Mineral
alignment
is a process during compaction, where any elongated or flaky grains
such as clay or mica will become aligned parallel to the bedding plain.
This alignment of clay minerals may lead to mudstone and shales
splitting easily into layers and being known as fissile.
Mineral changes
Diagenesis often results in the formation of new minerals which grow
in the sediment or sedimentary rock .
Pressure dissolution
Indicators of particular climatic zones:
Desert
Environment
Glacial
Environment
Tropical
Shallow Marine
Lithology, fossils & sedimentary structures describe a sedimentary facies.
Indicators of particular environments:
Fluvial
Environment
Deltaic
Environment
Shallow
Marine
Environment
Deep
Marine
Environment
Lithology, fossils & sedimentary structures describe a sedimentary facies.
Sedimentary environments:
• Alluvial
• Deltaic
• Desert
• Glacial
• Deep marine
• Shallow marine
Lithology
Fossils
Sedimentary Structures
Lithology, fossils & sedimentary structures describe a sedimentary facies.
Backshore
Zone
Foreshore
Zone
Shoreface
Zone
Offshore
Transition Zone
Offshore
Zone
ROCESSES STRUCTURES
FOSSILS
MINERALOGY
TEXTURE
Mean high tide
• very well sorted
• very well rounded
• fine-medium sand
• frosted grains
• well sorted
• well rounded
• medium-coarse sand
• well sorted
or
• well rounded
• coarse pebbles
• fine-medium
• glassy grains
sand
• glassy grains
• quartz
• quartz
• rock fragments
• rootlets
• trace fossils (worm
burrows)
• fragments of shells
• asymmetrical dunes
• cross-stratification
• aeolian (wind)
• medium energy
Mean low tide
Fairweather wave-base
• well sorted
• well rounded
• fine silt
Storm wave-base
• very well sorted
• fine silt & clay
• silt
• clay
• quartz
• silt
• silt
• trace fossils
(worm burrows)
• some fragments
of shells (b,b,g)
• trace fossils (worm burrows)
• no fragments
• bivalves, brachiopods, gastropods
• some wave-formed ripples • wave ripples
• hummock & basins
• planar stratification
• chevron cross- • hummocky cross• chevron cross-stratification stratification
stratification
• breaking waves (water) • fairweather waves • storm waves only
• high energy
• medium energy
• low energy
• lamination
• flocculation
• very low energy
Backshore
Foreshore
Shoreface
Offshore
transition
zone
Offshore
Dune
Beach
Discuss the extent to which the occurrence of greywackes and their sedimentary
structures, interbedded with black graptolitic shales, indicates that parts of
Britain once experienced deep-water marine conditions.
(25 marks)
Greywackes
description: texture/mineralogy turbidites/bottom of continental slope "any" environment/
rapid deposition
Blackanaerobic/lack of oxygen
-deep water/ocean floor could be shallow(er) – just lack of oxygen
Graptoliticpelagic/fragile/pyritisation -float into deeper waters/lack scavengers/weathering/erosion found
in deposits of all depths
extinct – problematic preservation/shale key feature
Shales fine-grained / travel distance/sorting -no current any depth
Sedimentary structures grading description: fining upwards/rapid deposition from turbidity currents
(Any valid sedimentary structure with context e.g. current bedding; bottom structures etc.)
Early Palaeozoic age for deep water as indicated by graptolites = zone fossils
Allow "negatives" e.g. lack of brachiopods, corals, trilobites, limestones etc, etc.
Total 25
Sedimentary Structures
Sedimentary Structures
Sedimentary Structures
Sedimentary Structures
Sedimentary Structures
Sedimentary Structures
Sedimentary Structures
Sedimentary Structures
• Laminations
• Bedding
• Graded Bedding
• Cross Bedding
• Desiccation Cracks
• Sole Marks
• Ripple Marks
Sedimentary Environments
1. Continental
Environments
2.Transitional
Environments
3. Marine Environments
Continental Environments
Channel
Aeolian/Desert
Point bar
Transitional Environments
Marine Environments