a2 sedimentary processes & rocks. sedimentary rocks sedimentary processes weathering erosion...

A2 Sedimentary Processes & Rocks

SEDIMENTARY ROCKS

Sedimentary Processes

Weathering •

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Erosion

Transportation

Deposition

Diagenesis

Weathering

Types•

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Products

Rates

Mineral Susceptibility

Maturity

Textural maturity•

• Compositional maturity

Sedimentary Rocks

Conglomerate & Breccia•

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Sandstones

Shale/siltstone/mudstone

Limestone/chalk/coal/halite/gypsum

Sedimentary Structures

Bedding•

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Cross-bedding & graded bedding

Desiccation cracks

Ripple marks & sole marks

Fragmental

Chemical & Biogenic

Sedimentary

Environments•

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Marine – deep/shallow

Land – glacial/alluvial/desert

Transitional – delta/beach

How can sedimentary rocks be classified?

Grain Size Sediment Name Rock Name

Coarse>2 mm

>256 mm64 - 256 mm4 - 64 mm2 - 4 mm

BouldersCobblesPebblesGranules

Conglomerate (rounded fragments)Breccia (angular fragments)

Medium0.06 - 2 mm

1 – 2 mm0.5 – 1 mm0.25 – 0.5 mm0.125 – 0.25 mm0.06 – 0.125 mm

Very coarse sandCoarse sandMedium sandFine sandVery fine sand

Sandstone

Fine<0.06 mm(<63 µm)

4 µm – 63 µm<4 µm

SiltClay

SiltstoneMudstone/Shale

1 µm = micrometer = 0.001 mm

•Arkose

•Greywacke

•Orthoquartzite•Desert sst

Fragmental/Clastic Very Well Sorted Well SortedModerately Sorted Poorly Sorted

Rounded Sub-Angular Angular

Coarse

Boulder >256mm

Conglomerate Breccia-Conglomerate BrecciaCobble 64 - 256mm

Pebble 4 - 64mm

Granule 2 - 4mm

Medium

Very coarse 2 -1mm

Arkose

Coarse 1 - 500umOrthoquartzite

Medium 500 - 250um Greywacke

Fine 250 - 125umDesert Sandstone

Very fine 125 - 63um

FineSilt 63 - 4um Siltstone

Clay <4um Mudstones/Shales

Summary - Classifying clastic sedimentary rocks

Sediments to Rock

1. Define the following terms:

• Sediment• Particle• Clast• Clastic• Fragment• Grain

2. How long does it take to turn a sediment into a rock?

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.

- unconsolidated material deposited by water, ice or wind- in general language - a piece. Geological language, same as grain.

- a particle within a rock which has been broken off a pre-existing rock. - a group of sedimentary rocks composed of particles e.g. sandstone.- a large clast, usually of a rock (no definite size limit)

- a small clast, usually of a mineral (no definite size limit)

- millions of years

Sedimentary Processes

3 processes of weathering:

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5 processes of erosion:

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4 agents of transportation:

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3 processes of deposition:

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Weathering

Transportation

Erosion

Deposition

Lithification

2 processes of lithification:

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Physical

Chemical

Biological

Abrasion

Attrition

Hydraulic action

Cavitation

Plucking

Rivers

Sea

Wind

Ice

Loss of energy

Accumulation of dead animals

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?

• What are the specific products of this weathering?

• Explain how it works?

Carbonation

Frost shattering

Exfoliation

Hydrolysis

Oxidation

Salt crystallisation

Chemical Weathering

4FeSiO3 + O2 2Fe2O3 + 4SiO2

CO2 + H2O

H2CO3 + CaCO3 Ca + 2HCO3

Ca Na K ALSiO + H2O AlSiO(OH) + K + Ca + Na + 2HCO3

Oxidation

Carbonation

Hydrolysis

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?

Original Mineral

Chemical Weathering

Process

Solid Product Soluble Product

Feldspar

Ferromagnesian

Muscovite mica

Quartz

Calcite

KNaAlSiO

FeMgSiO

KAlSiO

SiO

CaCO3

Hydrolysis Clay K, Na, Ca

Oxidation Iron oxide Si0

Hydrolysis Clay K

- Quartz grains -

Carbonation - Ca

Products of Weathering

Quartz SiO2

Muscovite mica K Al SiO2

Orthoclase feldspar K Al SiO2

Biotite mica Fe Mg K Al SiO2

Hornblende Ca Mg SiO2

Plagioclase feldspar

Na Al SiO2

Augite Ca Mg Fe SiO2

Plagioclase feldspar

Ca Al SiO2Olivine

(Mg Fe) SiO2

~600°C

~1200°C

Susceptibility to Weathering

4FeSiO3 + O2 2Fe2O3 + 4SiO2

CO2 + H2O

H2CO3 + CaCO3 Ca + 2HCO3

Ca Na K ALSiO + H2O AlSiO(OH) + K + Ca + Na + 2HCO3

Rates of Weathering

Frost shattering/ freeze-thaw

Rates of Weathering

RockJoints

Faults

Fractures

Bedding planes

Pore spaces

Rates of Weathering

Feldspar

Biotite mica

Augite

CaCO3

Quartz

Feldspar

Rates of Weathering

Summary of Weathering

1. Why do rock outcrops disintegrate, leaving shattered fragments to accumulate on hill slopes?

2. How can quartz minerals be extricated from the rocks from which they are derived ?

3. Where does mud come from (i.e. the clay minerals that make up the muds we see in places like river estuaries)?

By physical weathering (frost shattering) – expansion of water on freezing in cracks & joints exerts pressure breaking rocks apart.

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..

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

20%

30%

Q – 50%

RF – 20%

F – 30%

Triangular Graphs

Q – 68%

RF – 3%

F – 29%68%

3%

29%

Arkose

Q – 40%

F – 55%

RF – 5%

Triangular Graphs

ARKOSE

Triangular Graphs

Sandstone – 70% quartz & 30% orthoclase feldspar

Maturity of Sedimentary Rocks

Minerals in sedimentary rocks

1. Extent of chemical weathering

2. Type & amount of transportation

What can you tell about the chemical weathering experienced by the original rock?

Not much chemical weathering because feldspar unweathered.


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.


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


Rock Specimen B:

Mineral 1 – quartz (~75%) • 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.

What is Rock B?

Arkose


Rock Specimen G:

• Minerals too small to be seen by eye or a hand lens.

• Grey colour

• Scratched easily

Rock G is compositionally mature because there is a high proportion of clay.

Mineral 1 - clay

What is Rock G?

Mudstone


1 mm


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.

70% quartz & 30% orthoclase feldspar


Minerals in sedimentary rocks

1. Extent of chemical weathering

What can you tell about the chemical weathering experienced by the original rock and the following sediment transport?

2. Type & amount of transportation

Medium-grained, moderately sorted & sub-rounded

Texturally immature

Compositionally immature

Arkose 70% quartz & 30% orthoclase feldspar

Medium-grained, moderately sorted & sub-rounded

Texturally immature

Compositionally immature

What can you tell about the chemical weathering experienced by the original rock and the following sediment transport?

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

• grain size

• viscosity

• density

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 Entrainment & Deposition by Water

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.

Summary of Sediment Transportation

1mm = 1000 microns

Transport History & Environments of Deposition of Sedimentary Rocks

1. Lithology

Texture

Grain size

Grain shape

Grain sorting

Grain surface

• Energy levels

• Distance transported

Gravel or coarser =

Medium to coarse sands =

Fine sands, silts & clays =

High & short

Medium

Low & long

• Type of transport

• Distance transported

Coarse & fine =Fluctuating energy levels


• Time in transport


Fabric (relationship between the grains & the

matrix)

Grain-supported Matrix-supportedIntensive

reworking

By waves/currents

Common in tills,

debris flows deposits

1. Lithology

Mineralogy

Transport History & Environments of Deposition of Sedimentary Rocks

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 I J K

Colour – grains

- matrix

Fabric

Shape

Size

Origin

Sketch

Biogenic or chemical

Rock Name

Dark grey

Pale grey

Grain-supportedRounded/coiled2 – 5mm

Fossils

Biogenic

Shelly Limestone

Creamy-white

Creamy-white

Grain-supportedRounded

0.5 -1 mm

Ooids

Quartz centre

Concentrically layered

Chemical

Oolitic Limestone

Grey

Grey

Grain-supportedToo varied

1 – 2mm

Fossils

Biogenic

Shelly Limestone

White

White

Matrix-supported

Rounded

<0.05mm

Coccoliths

Coccoliths

Biogenic

Chalk

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.

Diagenesis

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.

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.

is a diagenetic process in which loose, unconsolidated sediments in is converted into sedimentary rocks by compaction & 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.

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.

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.

Pressure dissolution

Compaction

Lithification

Cementation

Diagenesis

Mineral changes

Mineral alignment

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.


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 .

Indicators of particular climatic zones:

Desert Environment

Glacial Environment

Tropical Shallow Marine

Lithology, fossils & sedimentary structures describe a sedimentary facies.

Indicators of particular environments:

FluvialEnvironment

Deltaic Environment

Shallow MarineEnvironment

DeepMarine Environment


Sedimentary environments:

• Alluvial

• Deltaic

• Desert

• Glacial

• Deep marine

• Shallow marine

Lithology Fossils Sedimentary Structures


Backshore Zone

Foreshore Zone

Shoreface Zone

Offshore Transition Zone

Offshore Zone

Mean high tide

Mean low tide

Fairweather wave-base

Storm wave-base

TEX

TU

RE

MIN

ER

ALO

GY

FO

SS

ILS

STR

UC

TU

RES

PR

OC

ES

SES

• very well sorted• very well rounded• fine-medium sand• frosted grains

• well sorted• well rounded• medium-coarse sand or• coarse pebbles• glassy grains

• well sorted• well rounded• fine-medium sand• glassy grains

• well sorted• well rounded• fine silt

• very well sorted• fine silt & clay

• quartz • quartz• rock fragments

• quartz• silt

• silt• silt• clay

• rootlets

• trace fossils (worm burrows)• fragments of shells

• trace fossils (worm burrows)• some fragments of shells (b,b,g)

• trace fossils (worm burrows)• no fragments• bivalves, brachiopods, gastropods

• asymmetrical dunes• cross-stratification

• some wave-formed ripples• planar stratification• chevron cross-stratification

• wave ripples• chevron cross-stratification

• hummock & basins• hummocky cross-stratification

• lamination

• aeolian (wind)• medium energy

• breaking waves (water)• high energy

• fairweather waves• medium energy

• storm waves only• low energy

• flocculation• very low energy

Dune

Beach

Backshore

Foreshore

Shoreface

Offshore transition zone

Offshore

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

Black anaerobic/lack of oxygen - deep water/ocean floor could be shallow(er) – just lack of oxygen

Graptolitic pelagic/fragile/pyritisation -float into deeper waters/lack scavengers/weathering/erosion found in deposits of all depthsextinct – 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


• 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

Point barChannel

Aeolian/Desert

Transitional Environments

Marine Environments