1.3 Sedimentary Rocks and their Fossil Content Flashcards
What are processes carried out by?
Agents
What categories can the geological processes taking place on earth be divided into?
- weathering
- erosion
- deposition
- transportation
What is weathering?
The breakdown of rocks in situ
What are the three main types of weathering?
- physical
- chemical
- biological
What is physical weathering?
The mechanical breakdown of rocks
What is chemical weathering?
The breakdown of rocks by chemical reactions
What is biological weathering?
The breakdown of rocks by organic processes
What type of weathering can form tors?
Chemical
What are tors?
Rock outcrops on the tops of hills
How are tors formed?
Chemical weathering:
- granite contains feldspar - reacts with water
- produces kaolin (clay)
- forms outcrops on top of hills
What does orthoclase feldspar react with water to produce?
Potassium carbonate, kaolin (china clay) and silica
What is kaolin also known as?
China clay
What is the process that takes place when the feldspar in granite reacts with water?
Hydrolysis
What is physical weathering also known as?
Mechanical weathering
What type of weathering is freeze thaw?
Physical/mechanical
How does freeze thaw weathering occur?
Water fills cracks in rock, freezes and expands at night and thaws in the day. The pressure on the rocks breaks angular pieces off
What is formed from freeze thaw weathering?
Scree fans - where angular pieces of rock broken off by freeze thaw collect
What are scree fans?
Where the angular pieces of rock broken off by freeze thaw collect
What is limestone pavement?
Where flat layers of limestone are weathered along joints to form blocks (clints) surrounded by cracks (grykes)
What is a scarp?
Steep rock face
How does chemical weathering occur?
- permeable rock (e.g. limestone) lets water pass along its joints and bedding planes
- calcium carbonate of limestone is dissolved away by weak carbonic acid in rainwater
- cave system dissolved out of limestone water
How does biological weathering occur?
Roots of trees grow down into cracks in rocks, widening them and breaking the rock up
Is clay impermeable?
Yes
Is shale impermeable?
Yes
How are limestone pavements formed?
- limestone laid down - tropical shallow marine environment
- layers of sedimentary rock above the marine environment removed by glaciers
- exposes limestone at surface - limestone has joints and bedding planes
- rain-stream water has slight acidity and flows down joints - widening them to grykes eventually → limestone pavement formed
- swallow holes formed due to widening of bedding plane into an underground cavern
What type of weathering is exfoliation?
Mechanical
How does exfoliation occur?
Rocks warm in day and cools at night - forming an onion skin texture
What rocks could exfoliation occur to?
Granite and sandstone
How are some dry valleys eroded?
- end of last ice age, ground was permanently frozen
- nearby glaciers were melting, so a lot of melt water
- melt water flows over surface as ground frozen
What is erosion?
The process where material is removed from the site of weathering by wearing it away and then transportation
What are the 5 main agents of erosion?
- wind
- rivers
- gravity
- sea
- glaciers
What are the ways that erosion can take place?
- corrasion
- attrition
- abrasion
- hydraulic action
- plucking
What is corrasion?
Where rock fragments erode the bedrock
What is attrition?
Where rock fragments hit against one another
What is abrasion?
A combination of corrasion and attrition
What is hydraulic action?
Where air is trapped and squeezed by water
What is plucking?
Where ice freezes to bedrock and plucks it away
What is transportation?
The process where eroded material is moved to another location
What is the eroded material called in transportation?
The load
What are the main processes of transportation?
- solution
- suspension
- saltation
- traction
What is solution?
Where the load is dissolved in water
What is suspension?
Where the load is carried in the main body of the water, or in the air by wind
What is saltation?
Where the load is bounced along the eroded surface
What is traction?
Where the load is rolled or dragged along the eroded surface by water or wind
How can clasts be described?
- size
- sorting
- roundness
- sphericity
How is the size of a clast found?
By measuring the length of its long axis, or if too small, by sieving
How are clasts named according to their size?
Using the Wentworth scale
In most cases, if a clast is smaller, what does this mean for the distance it has been transported?
Smaller clast, greater distance it has been transported
In most cases, if a clast is smaller, what does this mean for its hardness?
It is less hard
What factors can have an effect on the size of a clast?
- distance it has been transported
* the hardness of mineral or rock
What is sorting?
The nearness of the clasts in a sediment come to being the same size
How will the clasts be in a well sorted sediment?
Similar in size (may be described as mature)
How will the clasts be in a poorly sorted sediment?
Different sized clasts (may be described as immature)
How will the degree of sorting be represented?
Using a histogram or cumulative frequency curve
What does roundness describe?
How angular the edges of and corners of clasts are
What does it mean, in terms of erosion, when a clast is more rounded?
Greater degree of erosion and transportation
What factors will affect a clasts roundness?
- hardness of rock or mineral
* degree of erosion and transportation
How is roundness measured?
A visual roundness chart
What does sphericity describe?
How near clasts come to being spheres
What type of processes does deposition include?
Physical and chemical
When will clasts be carried by a current?
If it has a high enough velocity, and in turn enough energy
Why might the current of water fall?
- change in angle - alluvial fan - steep slope, fast moving but then at bottom its slow (still going fast at top)
- change in speed of movement - fast river to slow moving sea
What are clastic rocks?
Sedimentary rocks formed by the deposition of clasts
When are sediments called organic?
When they have a high percentage of animal and plant fossils that have been deposited like clasts
Example of a clastic sedimentary rock?
Conglomerate
How does precipitation occur?
If water is saturated or oversaturated with a certain compound, then these compounds can be precipitated out as a crystalline substance
When is precipitation more likely?
If the water is agitated (swirled around)
Example of precipitation?
Precipitation of calcite from warm tropical sea water, resulting in formation of limestones due to CO2 being released
What are chemical precipitates?
Rocks formed by precipitation
Why does a solution often become saturated?
Due to heat from the sun
How is halite formed?
When temperature rise evaporates away much of the water in seas or salt lakes
Example of a chemical precipitate?
Carboniferous limestone
What type of weathering is scree usually formed by?
Freeze thaw
What do sedimentary rocks form from?
Compaction of soft sediment into hard rock
What type of clasts are more easily compacted?
Fine clasts (as oppose to large ones)
If sediments have clasts, how will they stick together?
Cemented
What natural cements can stick together clasts in sedimentary rocks?
- silica
- calcite
- iron oxide
What will it mean if sediments are crystalline, in terms of how they are formed?
They will have precipitated directly from water
Do crystalline sediments need cement?
No
Types of sedimentary environments?
- warm shallow marine
- cold shallow marine
- deep marine
- rivers and deltas
- deserts
Types of depositional environment?
- screes
- rivers
- shallow/deep seas
- wind-formed dunes
What do porosity and permeablility of sedimentary rock depend upon?
- characteristics of original sediment
* degree of compaction and cementation
Examples of sedimentary rocks?
- breccia
- conglomerate
- shale
- sandstone
- evaporites
- limestone
Example of warm shallow marine environment?
the Bahamas
What environment does limestone form in?
Warm shallow marine
What are ooliths?
Spheres of calcite made of concentric layers
How are ooliths formed?
When calcite has been precipitated around a nucleus of a sand grain or piece of shell
What are cocoliths?
Remains of the minute skeletons of simple sea creatures
How is shale formed?
Flood plain of meandering river - deposited when river channel overflows
How is reef limestone formed?
Warm clear shallow oxygenated, turbulent tropical marine conditions by build up of organic matter on a reef
How is chalk formed?
Clear, warm, shallow tropical marine conditions (low energy) coccoliths create skeletons of calcite = very pure rock
How is chemical limestone formed?
Warm, clear, shallow, tropical sea (low energy) by the precipitation of calcite from water rich in CaCO3
What are turbidity currents?
Powerful, sediment charged underwater currents that flow down the continental slope
How is sandstone (greywacke) formed?
Turbidity currents flowing off the shelf (high energy) down the continental slope and settling rapidly on the abyssal plain
What is graded bedding?
Decreasing grain size upward through the bed - indicating deposition from a waning current
When does graded bedding form?
As a result of a slowing turbidity flow
How is sandstone (subgreywacke) formed?
As the point bar on the inside of a meandering river channel - sediment is deposited as the meander migrates laterally
What are deltas?
Areas of deposition where rivers enter the sea or a lake
How are canyons formed?
Upland areas are often cut by flash floods
What environment do canyons form?
Hot desert
What is a flash flood?
In hot desert environments - heavy rain produces rapid run-off
How are desiccation cracks formed?
Hot deserts - water in lakes dry out - and then mud in bottom dries out also - contracting
What are salt pans?
When water collected in a desert evaporates - dissolved salts precipitated out in layers
How are barchan dunes formed?
When wind blows sand over an obstruction - dune grows and takes on a crescent shape
What are moraines?
The deposits left by glaciers
What do moraines consist of?
Poorly sorted rock debris that has fallen onto or into the ice
How is desert sandstone formed?
As wind blows across a desert (high energy), with corrasion and attrition producing very well rounded and sorted grains deposited as dunes
How is halite formed?
Intense evaporation of a shallow lake or sea in a desert environment - with mineral content of the waters precipitated out as salts
How is quartz conglomerate formed?
On a beach (high energy) by continuous reworking of sediment - removing almost all minerals bar quartz
Cement in quartz conglomerate?
Silica
Examples of rocks that form in shallow marine environments?
- limestone
- sandstone
- conglomerate
Examples of rocks that form in deep marine environments?
- turbidites
* black shale
Examples of rocks that form from deposition in rivers and deltas?
- shale
- sandstone
- conglomerate
- coal
Examples of rocks that form from deposition by wind and water in deserts?
- breccia
* desert sandstone
Examples of rocks that form from precipitation from saline water during evaporation?
- evaporites
- halite
- gypsum
Examples of rocks that form from deposition by ice?
Glacial till
What is a bed?
A layer of rock separated from the layer above and below by a bedding plane
What is a flag/flagstone?
Bed 2-5mm thick
What is a bedding plane?
Defines the top or bottom of a bed - represents a change in the nature of sedimentation
Difference between a bed and lamination?
- lamination - layer of sediment < 1 cm thick
* bed - > 1 cm
In graded bedding, where is the finer sediment?
At the top
How might the fining upward sequence in graded bedding be produced?
Turbidity currents
How do turbidity currents form the fining upward sequence in graded bedding?
In comparatively calm bodies of water - larger denser fragments sink first, followed by small silt and clay particles
How is a graded bed with an erosional base formed?
When the layer above was laid down, the layer underneath was damaged
What does a graded bed with an erosional base represent?
An abrupt change from the much finer grained sediment underneath
What can cross bedding be left by?
Wind and rivers
How is cross bedding formed?
By uni-directional current of wind or water moving sediment as a series of asymmetrical ripples of dunes
If cross bedding is on a very large scale, what is it called?
Dune bedding
If cross bedding is on a very small scale, what is it called?
Cross lamination
How are rain pits formed?
By impact of rain drops on an exposed sediment surface
What does it show, if ripple marks are symmetrical?
The beach used to be horizontal
What does it show, if ripple marks are unsymmetrical?
Formed by uni-directional current - downstream/downwind side will have the steeper face
What do reef building corals indicate the past environment was like?
Marine, shallow, warm
What do trilobites indicate the past environment was like?
Marine
What do ammonites indicate the past environment was like?
Marine
What do plants indicate the past environment was like?
Terrestrial, indicating past climate
What do trace fossils indicate the past environment was like?
Tracks indicating terrestrial, burrows indicating shallow water
