Post on 27-Dec-2015
transcript
A2 Sedimentary Processes & Rocks
SEDIMENTARY ROCKS
Sedimentary Processes
Weathering •
•
•
•
•
Erosion
Transportation
Deposition
Diagenesis
Weathering
Types•
•
•
•
Products
Rates
Mineral Susceptibility
Maturity
Textural maturity•
• Compositional maturity
Sedimentary Rocks
Conglomerate & Breccia•
•
•
•
Sandstones
Shale/siltstone/mudstone
Limestone/chalk/coal/halite/gypsum
Sedimentary Structures
Bedding•
•
•
•
Cross-bedding & graded bedding
Desiccation cracks
Ripple marks & sole marks
Fragmental
Chemical & Biogenic
Sedimentary
Environments•
•
•
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:
•
•
•
5 processes of erosion:
•
•
•
•
•
4 agents of transportation:
•
•
•
•
3 processes of deposition:
•
•
•
Weathering
Transportation
Erosion
Deposition
Lithification
2 processes of lithification:
•
•
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?
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.
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
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
Maturity of Sedimentary Rocks
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
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.
70% quartz & 30% orthoclase feldspar
Maturity of Sedimentary Rocks
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 Transportation
Sediment Entrainment & Deposition by Water
Sediment Transportation
Sediment Transportation
Sediment Transportation
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.
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
• Type of transport
• Time in transport
• Type of 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.
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:
FluvialEnvironment
Deltaic Environment
Shallow MarineEnvironment
DeepMarine 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
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
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
Point barChannel
Aeolian/Desert
Transitional Environments
Marine Environments