Sedimentary Rocks Flashcards
What is weathering?
The in-situ chemical alteration, and mechanical and biological breakdown of rocks by exposure to the atmosphere, water, or organic matter
What is chemical weathering?
The process of chemicals in the rainwater making changes to the minerals in a rock
What is mechanical weathering?
Process of rocks crumbling due to rain, wind, or other atmospheric conditions (e.g. freeze thaw)
What is biological weathering?
Occurs when plants or animals break up rocks. Caused by the movement of animals or plants
What are the two types of chemical weathering?
Carbonation and hydrolysis
What are the three types of mechanical weathering?
Exfoliation, Frost shattering, Pressure release
What are the two types of biological weathering?
Root action and burrowing
What is carbonation in rocks?
Carbon dioxide reacts with water to make carbonic acid. CO2 is in the atmosphere and water in soil pore spaces. Soil has more carbon (decomposition) than the atmosphere. Water in soils is more acidic and groundwater is more acidic than rainwater. Limestone particularly susceptible, leaving clay particles
What is hydrolysis in rocks?
Water reacts with silicate minerals, especially feldspars with a residual material of clay. Hydrolysis speeds up with carbonic acid
What is exfoliation?
Outer layers of a rock are broken down first. Occurs when there is a significant difference between daytime and night (desert). Rock expands and contracts and the outer layers more so
What is frost shattering?
A.K.A freeze-thaw.
Water enters cracks, joints and bedding planes. Only occurs in climates that regularly fluctuate above and below freezing temps. When water freezes, it expands by 9%. This causes pressure on the rock, leading to failure and fracturing. This produces fragments called scree
What is pressure release?
Rocks can be under pressure if they are buried between underlying rocks, or under glaciers. Pressure can reduce by melting glaciers or weathering of overlying rocks. The rock that was under pressure expands the direction of pressure release (usually up)
What is root action?
Roots can grow along bedding planes, joints and crack/fractures. Roots can force the rocks apart mechanically. Trees that sway in the wind can pry cracks open in rocks. Root action makes rocks more susceptible to other types of weathering
What is burrowing?
Burrowing animals include worms, mammals and reptiles. Their activity bring rocks from depth to the surface. Those rock fragments can now be weathered further (other types). The burrow also allows atmospheric gases to penetrate deeper into the soil and increase the chance of carbonation
What types of weathering and climate are in Arctic zones?
Average temperatures of -4 maximum 10. Precipitation is low <250 mm.
Frost shattering here. No biological weathering
What types of weathering and climate are in Temperate (e.g. UK) zones?
Moderate precipitation 750-1500 mm. Temperatures between 0-20 usually.
All three weathering types. Chemical water weathering, sometimes cold enough for frost shattering. And biological weathering
What types of weathering and climate are in desert zones?
Around 100 mm. Huge temperature variation from -5 to 40 at different times in the day.
Exfoliation weathering
What types of weathering and climate are in tropical rainforest zones?
Huge precipitation 1500-3000 mm. 21-30 average temperatures. Very humid.
Burrowing - High biodiversity. Lots of water allows chemical weathering. No mechanical weathering
What are the 5 ways sediment is transported?
Rivers, wind, sea, ice, gravity
What is the difference between weathering and erosion?
Weathering is the in-situ breakup of rocks. Erosion is the transport of rocks and their fragments
What are the 4 types of erosion through transport?
Abrasion, attrition, traction, saltation
What are the 2 ways ions can be carried in solution?
Solution and suspension
What is abrasion?
The process of friction, wearing away buy dragging or hurling
What is attrition?
Material such as rocks and stones carried by waves hit and knock against each other (collisions)
What is traction?
The rolling or sliding of large grains along a river bed or shore, aided by the push of smaller grains
What is saltation?
Bouncing of sand grains as they are picked up, carried alone, and dropped repeatedly by flowing water
What is solution?
When seawater dissolves certain types of rocks. Ions are transported.
What is suspension?
Method of transporting very fine sediment in a river without touching the world’s surface. This can be water or wind
What is sphericity?
How close to a spherical shape a grain or particle is
Why would we use a logarithmic scale?
When we have a large range of data so graphing us easier
What are some examples of logarithmic scales?
Phi, richtor, pH, decibels
What is the phi scale?
Logarithmic scale to measure grain size. Grain size can vary massively (hence log). Diameter of grain measured in mm
How does the grain size influence the phi scale reading?
As grain size increases, the phi number decreases.
High phi = small grain
Low phi = Big grain
What is the phi for a diameter >2mm?
-2 to -8.
Gravels, pebbles, cobbles, boulders
What is the phi for a diameter 2mm?
-1.
Very coarse grains
What is the phi for a diameter 1mm?
0
Coarse sand
What is the phi for a diameter 0.5mm?
1
Medium sand
What is the phi for a diameter 0.25mm?
2
Fine sand
What is the phi for a diameter 0.125mm? (1/8)
3
Fine sand
What is the phi for a diameter 0.0625mm? (1/16)
4.
Silt
What is the phi for a diameter 0.0039mm? (1/32)
8
Clay
What equipment do you need for a grain size analysis?
A sieve stack
A scale of some sort, something to weigh with
What is step 1 of grain size analysis?
Arrange the sieve stack in order, with the largest sieve at the top, and the solid pan at the bottom.
Smallest phi at top
What is step 2 of grain size analysis?
Weigh the original mass of the sediment and record this
What is something you should check before a grain size analysis?
You have all the sieves.
They are in the right order.
The sediment is dry
What is step 3 of grain size analysis?
Place dry sediment in the top pan
What is step 4 of grain size analysis?
Agitate the sieve stack for 1 minute
What is step 5 of grain size analysis?
Separate the sieves and record the mass in each pan
How can the data recorded in a grain size analysis be represented?
On a cumulative frequency curve. Used to find the degree of sorting
What does a well sorted sediment sample look like on a histogram?
One bar is dominating. A high percentage is recorded at one phi point
What does a poorly sorted sediment sample look like on a histogram?
Bars of similar size
What is the equation to find the coefficient of sorting?
(ɸ84 - ɸ16) / 2
What coefficent of sorting (p) represents a well sorted sediment?
<0.5
What coefficent of sorting (p) represents a moderately sorted sediment?
0.5 - 1.00
What coefficent of sorting (p) represents a poorly sorted sediment?
> 1
How does the coefficient of sorting link to the sorting of sediment?
The lower the coefficient of sorting, the better sorted the sediment
What is sediment like in wind blown sands?
High energy environment.
Coarse, medium, fine sand. Very high quartz %, softer material wear away. Texturally and mineralogically mature. Very well sorted
What is sediment like in glacial till?
Low energy environment.
Mixture of grain sizes and minerals. Not much erosion. Angular grains and very poorly sorted. Texturally and mineralogically immature
What is sediment like in river deposits?
Usually high energy.
Fine, medium or coarse - decrease with distance. Quartz, but not as mineralogically mature. More angular near source. Sorting increases downstream
What is sediment like in beaches or offshore bars?
High energy environment.
Coarse sand containing pebbles and shells. Lots of quartz and shell fragments of CaCO3. Sub-rounded to rounded. Moderately sorted
What is sediment like in gravity-moved sediment (mass movement)?
Low energy
Any grain size, very poorly sorted, angular. Composition varies, can contain any minerals present in the rocks
What is rudaceous?
Grain size >2mm
What is arenaceous?
Grain size 0.0625-2mm
What is argillaceous?
Grain size <0.0625mm
What observations can be made to identify Breccia?
Grain size >2mm - Rudaceous.
Angular clasts
What observations can be made to identify Conglomerate?
Grain size >2mm - Rudaceous.
Rounded clasts
What observations can be made to identify Greywacke?
Grain size 0.0625-2mm - Arenaceous.
Rock does not consist mainly of quartz or feldspar.
Contains many rock fragments
What observations can be made to identify Arkose?
Grain size 0.0625-2mm - arenaceous.
Clasts mainly quartz with >25% feldspar
What observations can be made to identify Desert sandstone?
Grain size 0.0625-2mm - arenaceous.
Rock consists mainly of quartz.
Very well sorted, rounded and high sphericity
What observations can be made to identify Orthoquartzite?
Grain size 0.0625-2mm - arenaceous.
Rock consists mainly of quartz.
Not very well sorted, rounded and lower sphericity
What observations can be made to identify Shale?
Grain size <0.0625mm - argillaceous.
Layering visible
What observations can be made to identify Clay?
Grain size <0.0625mm - argillaceous.
No layering visible and a plastic texture
What observations can be made to identify Mudstone?
Grain size <0.0625mm - argillaceous.
No layering visible. not a plastic texture
What is the definition of diagenesis?
Turning clastic sediment into rock. Involves compaction (burial), cementation and sometimes mineralogic changes and loss of water too
What is the first thing that happens to sediment grains when they are buried?
They are subject to the pressure of overlying sediment.
This pressure causes them to lose pore spaces due to compaction. Resulting in a loss of porosity
How does sediment burial differ between sand grains and clay particles?
Sand is equidimensional, meaning the potential for compaction and loss of pore space is less. Compaction is done with bending, rotating and fracturing of grains.
Clay particles are compacted more because they’re flat, resulting in a greater loss of porosity
What is pressure dissolution/solution?
The contact point where 3 clasts/grains meet is where there is greatest pressure. Increased pressure can cause some mineral to go into solution. This can then resolidify and hold the grains together or precipitate out by entering pore spaces
What can happen to plant materials during diagenesis?
Breaks down anaerobically. There is a loss of hydrogen and oxygen, and the carbon contact of that material increases, e.g. coal
How does cementation happen in sandstone?
Formed mainly from quartz. Has a high silica content. Sandstone tends to (not always) have a silica cement.
Silica that has dissolved (pressure solution) may come out of solution and fill pore spaces and set
How does cementation happen in limestones?
Made up of CaCO3. During pressure solution, the calcium goes into solution and so limestone tends to have a calcite cement (not all)
What does the suffix Micrite mean?
Micro-calcite cement. Very fine
What does the suffix Sparite mean?
Coarser calcite cement
What does the prefix Oo mean?
Ooliths
What does the prefix Intra mean?
Intraclasts
What does the prefix bio mean?
Fossils or shells
What does the prefix Pel mean?
Pellets (excretion remains)
What are oolites?
Tiny CaCO3 spheres. Form from a tiny grain of sand that rolls in a CaCO3 mud on the seafloor
What are the 4 rocks involved in the Dunham classification?
Mudstone, Wackestone, Packstone, Grainstone
Using the Dunham classification, what is mudstone?
Contains mud (clay and fine silt).
Mud supported with less that 10% grains
Using the Dunham classification, what is Wackestone?
Contains mud (clay and fine silt).
Mud supported with more than 10% grains
Using the Dunham classification, what is Packstone?
Contains mud (clay and fine silt).
Grain supported
Using the Dunham classification, what is Grainstone?
Lacks mud and grain supported
What are the 9 siliciclastic rocks we study?
Breccia, conglomerate, orthoquartzite, desert sandstone, arkose, greywacke, clay, mudstone, shale
What are the 3 carbonate rocks we study?
Oolitic limestone
Fossiliferous limestone
Chalk
What are the characteristics of breccia?
Coarse grained clasts often mixed with finer grained matrix. Angular clasts. Poorly sorted.
Commonly form as scree, alluvial fans or wadis. Medium energy
What are the characteristics of Conglomerate?
Coarse grained rounded clasts in a finer matrix. Poorly sorted. In some, clasts are held in place by mineral cement. Form as beach and river deposits
What are the characteristics of Orthoquartzite?
Quartz grains held by quartz cement. >90% quartz. Well sorted, well rounded, high sphericity. Long transport, extensive weathering (less stable minerals not present). Form on beaches and shallow marine deposits
What are the characteristics of desert sandstone?
Very well sorted, rounded and high sphericity. Medium-coarse grain size. Made of quartz and iron-oxide (gives red colour). Wind blown sands
What are the characteristics of Arkose?
Medium-coarse grained sandstone with >25% feldspar (other main mineral is quartz). Rock fragments and mica are also present. Low energy. Transported for very long. Formed in alluvial fan in arid areas
What are the characteristics of Greywacke?
Fine-coarse grain sizes. Poorly sorted, angular to sub angular clasts. Rock is mainly made of fragments. >15% of the rock is clay matrix. Often show graded bedding. Form as turbidite
What are the characteristics of Clay?
Clay size grains with a variety of colours depending on carbon or iron content. Low energy
What are the characteristics of mudstone?
Very fine grained rocks. Clay minerals, mica and quartz. No preferred alignment. Well sorted. Low energy
What are the characteristics of Shale?
Fine grained with distinctive layers (alignment). Described as fissile. Clay minerals, quartz and mica. Not plastic, but hard, brittle and impermeable
What are the characteristics of Oolitic limestone?
Sub-spherical sand sized oolites. In cross-section, these show concentric layers of CaCO3 surrounding a sand grain or shell fragment. Oolites surrounded either by fine grained calcite mud matrix or a crystalline calcite cement. Form with high energy waves on the ocean floor
What are the characteristics of Fossiliferous limestone?
Fossils and fossil fragments. May have sparite or micrite cement. Whole fossils means low energy, fragments mean high energy. Fossil rich limestone for in range from low energy fresh water or lagoons, to high energy marine beds
What are the characteristics of Chalk?
Low energy. Coccolithophores, calcareous disc or oval-shaped platelets (single-celled algae skeletons). White, pure calcium carbonate
