Sedimentary rocks Flashcards
What is the texture of sedimentary rocks?
Clastic - broken/fragmented
WELL SORTED
- same sized clasts
POORLY SORTED
- diff sized clasts
What are the clast sizes?
COARSE: over 2mm
MEDIUM: 1/16th-2mm
FINE: under 1/16thmm
What are the clastic grain shapes?
Rounded
Sub rounded
Sub Angular
Angular
What is erosion?
Wearing away/breakdown of rock during transport or by movement
What factors erode?
Ice
Wind
Water
What happens to moving sediment in erosion?
- moving material will erode and be eroded
- erosion makes sediment smaller and rounder
What is weathering?
The breakdown of rocks in situ
What are the three important types of weathering?
Physical
Chemical
Biological (as a part of chemical)
What is physical weathering?
Give examples
rocks broken down in situ without being chemically changed
e.g. freeze-thaw (frost shattering)
insolation weathering
What is chemical weathering?
the in situ chemical breakdown of minerals, taking place in solution, resulting in formation of new minerals and ions in solution
e.g. reaction with:
water
acid
oxygen
organisms
What are the types of chemical weathering?
- carbonic acid
- solution
- hydrolysis
- oxidation
- biological (facilitates chem)
- spheroidal weathering
What is/how does carbonic acid contribute to chemical weathering?
- soil = high CO2 content
- water percolating the soil becomes acidic: CO2+H2O=H2CO3
(carbonic acid)
What is/how does solution contribute to chemical weathering?
- dissolving the rock
- rainwater = always slightly acidic, dissolves limestone = carbonation
- forms caves
What is/how does hydrolysis contribute to chemical weathering?
- breakdown of minerals by water or acidic water
What is/how does oxidation contribute to chemical weathering?
Give examples
- addition of oxygen, loss of hydrogen
OIL RIG - facilitated by water/rain/acidic conditions
e.g.
olivine+H2CO3=Iron (dissolved)
iron+oxygen=haematite
What is biological weathering?
How can this facilitate chemical weathering?
- weathering caused by the movement of plants and animals
- because by opening up the cracks, water and air can get in
Give examples of biological weathering?
e.g.
a rabbit burrows into a crack in the rock
- crack becomes bigger
- eventually splits rock
e.g.
a plant grows in a crack in a rock
- roots grow
- crack widens
e.g.
lichen: growth of some lichen species can accelerate the weathering of minerals
- they can physically burrow up to 12mm into rock
- they can excrete various acids which can effectively dissolve minerals
What is spheroidal weathering?
when chem weathering moves along joints in bedrock producing rounded features
How does spheroidal weathering work?
- water runs along joints/cracks in the rock
- reacts chemically with the rock, oxidising and causing hydrolysis
- DECOMPOSITION
- rock decomposes along joints/cracks, particularly on corners due to large SA
- leaves behind a spherical core of unweathered rock
What is decomposition in weathering?
when a rock has been broken down by chemical weathering
What is porosity?
- the volume of pore space in a rock or sediment
- usually as a % of a total rock volume
What is permeability?
A measure of how fast fluid moves through rock or sediment
What factors affect porosity?
Sorting
Secondary porosity
Diagenesis
Grain shape
Packing
What is sorting?
the variability of the grain size within a sedimentary rock
How does sorting affect porosity?
well sorted has more pore space as all the grains are similar sizes and so do not tesselate very well
- HIGHER POROSITY
if a rock or sediment is poorly sorted, smaller clasts can fill up the pores
- LOWER POROSITY
How does diagenesis affect porosity?
- compaction, pressure dissolution and cementation all reduce pore space
- BURIAL increases compaction therefore deeper burial reduces pore space: 40% porosity at the surface, 10% porosity at 10km down
- PRESSURE DISSOLUTION produces a solution that precipitates out as as a cement - reduces porosity
- CEMENTATION: from pore fluids fills in pore space - reduces porosity
How does grain shape affect porosity?
- rounded grain = more porosity
- angular grains fit together better than rounded grains = less porosity
- What is packing?
- How does it impact packing?
- Give examples
- The way the grains fit together
- increased depth will cause particles to pack closer together (pressure)
e.g. cubic packing = 48% porosity
e.g. rhombohedral packing = 26% porosity
How does secondary porosity affect porosity?
increases porosity through:
- joints or bedding planes
- faults/caves
- dissolved minerals or cements
Also increases permeability
What is diagenesis?
the combination of all the processes resulting in lithification, altering the chemical composition and physical form of sediments and rocks, transforming them into different sediments and rocks
- post depositional process
What processes does diagenesis include?
compaction
cementation
pressure dissolution
What is lithification?
the turning of unconsolidated (not stuck together) sediment into solid rock
GEOLOGICAL PRINCIPLE: what is the law of included fragments?
any fragment in a rock must have formed first
What are the sedimentary rock groups?
- Conglomerate/breccia
- sandstones
- limestones
- mudstones (pelitic rocks)
- coals
What is the difference between a conglomerate and a breccia?
CONGLOMERATE:
- rounded
- high energy required to make coarse sediment round
- transported a lot
- found on a beach/in a river
BRECCIA:
- angular
- low energy required, little rounding
- only transported a tiny amount
- found: fault, volcanic neck, alluvial fan (v near source), scree
BOTH:
- coarse (>2mm)
What is the equation for total porosity?
pore space volume/ bulk volume
What is primary porosity?
porosity at the time that sediments were deposited
What is secondary porosity?
porosity created through alteration of the rock, faults, joints, dissolved minerals
What is effective porosity?
the amount of pore space that is interconnected and therefore able to transmit fluids
How is porosity influenced by pore interconnection?
Give examples
porosity is only effective if the pores are interconnected
e.g. sandstone
- 15% porosity, but most are interconnected
- HIGH EFFECTIVE POROSITY
e.g. clay
- 50% porosity, but most pores are not interconnected
- LOW EFFECTIVE POROSITY
What factors affect permeability?
Grain size
Temperature
How does grain size affect permeability?
GS doesn’t affect porosity, but does affect permeability, as good effective porosity = high permeability
- small pores/fine grains = more resistance to flow around smaller grains = low permeability
- large pores/coarse grains: less resistance to flow around larger grains
NOT SORTING: a well sorted rock can have low permeability if the grain size is small, and vice versa
How does temperature affect permeability?
- affects the viscosity of a fluid
- more viscous = less permeable
e.g. oil
- oil = less viscous the hotter it is
- this means permeability of oil through a rock will be greater if the rock is hotter i.e. buried at depth
How do we measure how round a sediment is?
Power’s scale of roundness
How does Power’s scale of roundness work?
All to do with how rounded the sediment is, all to do with corners
How many are there?
Have they been knocked off?
What are the measures on Power’s scale of roundness?
Very angular
Angular
Sub-angular
Sub-rounded
Rounded
Well rounded
How can deposited sediments be classed?
Texturally immature
Texturally mature
What are the measures of how well sorted a sediment is?
Very well sorted
Well sorted
Moderately sorted
Poorly sorted
Very poorly sorted
What do we use to measure the size of sediment?
Wentworth size class
What is a Hjulström curve?
a graph used to determine whether a river will erode, transport, or deposit sediment
What are the sedimentary structures?
- bedding
- graded bedding
- ripple bedding
- cross bedding
- desiccation marks
- load and flame
- flute casts
- tool marks
- imbrication
What is the difference between a cast and a mold?
CAST:
- sticks out/wraps around
MOLD:
- indent/filled in
What is a sandstone?
- sedimentary rock composed of sand sized grains (1/16th- 2mm)
How is a sandstone colour determined? What colours could it be?
By what cemented it
red, black, green, grey, pink white
What is a cement?
a post-depositional secondary material that fills the voids in the matrix, binding sediment into rock
What is a matrix?
the fine grained stuff (silt and clay) that was in the sediment along with the sand
What are the two different classifications of sandstone?
Marine
Desert sands
What are marine sandstones?
- sandstones that used to be beaches and if often texturally mature, with well-rounded quartz grains
- purity is created with the constant reworking of sediment by tides and longshore drift
What are desert sands sandstones?
- sandstones that contain evidence of sand dunes and are deposited by wind
- often well sorted
- consist mainly of quartz
- they are cross bedded on a very large scale
What evidence do we look for to determine if something is a marine sandstone?
- symmetrical oscillation ripples (created by tides)
- marine gastropod, bivalve and brachiopod fossils
What evidence do we look for to determine if something is a desert sands sandstone?
- absence of marine/freshwater fossils
- red staining (due to oxidation of sediment)
- large scale cross bedding
Where are minerals stable?
at the temps at which they form
What does:
a) oxidation
b) hydrolysis
c) carbonation
form?
a) haematite
b) clay minerals
c) solution
What temps are minerals least stable?
Hotter temperatures
- 1200 degrees
What temps are minerals most stable?
Cooler temperatures
- 600 degrees
How fast does
a) water
b) wind
need to be flowing to carry 1mm sand grains?
Why is this important for desert sands?
a) 0.2m/s
b) 10m/s
Desert sands = wind deposited, so a LOT of energy is needed to move them, they form in VERY HIGH ENERGY ENV
How are desert sands carried?
- very high energy env
- grains not cushioned by water, leads to very high levels of attrition
- grains are well rounded and often frosted
What does it mean if a desert sands grain is frosted?
a lustreless ground-glass or mat surface on rounded mineral grains, resulting from the innumerable impacts of other grains during wind action/deposition of many microscopic crystals in high energy environments
What are the characteristics of classic sand grains?
- rounded/well rounded
- sorted/well sorted
- all smoothed and frosted by the process of sand blasting with a coating of haematite
