8. sedimentary rocks Flashcards
What is sediment?
What are sedimentary rocks?
Solid fragmental material that originates from weathering of rocks and is transported or deposited by air, water, or ice, or that accumulates by other natural agents, such as chemical precipitation from solution or secretion by organisms and that forms in layers on the Earth’s surface at ordinary temperatures in a loose, unconsolidated form, such as, sand, gravel, silt, mud, till, loess, alluvium.
Sedimentary rock is the compaction and cementation of sediments.
What are the main classes of sedimentary rocks?
how are they form?
- (Silici)clastic rocks - Inorganic rocks formed by the compaction and cementation of sediments. They are classified in breccia, conglomerate, sandstone, siltstone and shale.
- Carbonate rocks - Rocks formed by the accumulaton and lithification of carbonate minerals that are directly or indirectly precipitated by organisms. Limestone and dolostone are examples of carbonates.
- Evaporites - Evaporite rocks are chemically precipitated from evaporating seawater or, in some cases, lake water.
- Organic rocks - Rocks formed by the compacted organic matter.
What are the sedimentary processes?
Weathering - Physical or chemical breakdown of rocks.
Erosion - The transport of weathered material away from the weathering site.
Deposition - Deposition is the laying down of sediment carried by wind, water, or ice.
Diagenesis (lithification) - all changes in a sediment that occur from its formation (deposition) until it is metamorphosed or destroyed after uplift (weathered).
How do rocks weather?
Mechanically - Physical disintegration of rock without changing in the chemical composition. It causes decrease in grain size.
Chemically - Chemical disintegration of rock. It causes alteration and dissolution of minerals in the rock. Mainly formation of clay minerals, oxides & solutions
What are the types of mechanical weathering processes? (6)
Thermal cycling
Frost wedging
Salt wedging
Biological disintegration
Unloading
Abrasion
What are the types of chemical weathering processes? (6)
Hydrolysis - Hydrogen ion replaces other positive ions.
Hydration and dehydration - Water is added to a mineral creating new mineral without addition of other ions.
Oxidation - It is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
Carbonation (dissolution of CO2 in a liquid)
Ion exchange
Dissolution - Mineral + acid or water dissolves ions.
What are the controlling factors for weathering? (2)
- Rock type
Unstable minerals with weak bonding types; small crystals/glass having higher reaction surface and weak rock texture (layering, high porosity…) leads to fast weathering.
-Climate
There are different types of weathering in different climate.
Warm, humid climate = fast weathering (chemical weathering dominate)
Cold, dry climate = slow weathering (mechanical weathering dominate)
How is soil formed?
Soil is the end result of weathering and biologic activity. It is unconsolidated mineral or organic material on the Earth’s surface in which land plants grow.
What is erosion?
What affects erosion?
The transpThe elements affecting erosion and transport are gravitation, water, wind and ice.ort of weathered material away from the weathering site.
What are the different types of gravitational transport?What are its effect on erosion? (7),(3)
Transport of unconsolidated material: Creep ,Earth flow, Debris flow, Mud flow, Slump, Debris slide and Debris avalanche
Transport of consolidated material: Rockslide, Rockfall, Rock avalanche
The gravitational transport’s effect in erosion is to help moving rocks that have been broken apart by weathering farther along by mass-wasting processes.
What are the reasons for gravitational transport?(3)
Slope instability caused by:
1. Water saturation:
- Rain, snowmelt -> Adds more weight
- Lubricating effect -> less friction
2. Oversteepening of slope (empinar)
-Weathering (water may be important here as well)
- Removal of vegetation (fires, clearing for construction) causes stabilisation by roots to disappear.
- Constructions, roads (remove or fill up with material)
3. Earthquakes
including man-made vibrations (ex: trucks)
What are the different transport media?How do they transport sediment?
The transport media are:
- Water (stream environment) - Rivers and streams carry sediment in their flows.
- Wind (aeolian environment) - Particles are transported by winds through suspension, saltation and creeping (rolling or sliding) along the ground.
- Ice in glaciers (glacial environment) - Glaciers erode terrain through abrasion and plucking.
How different material from the same rock can be transported?
Most sediments are transported by currents of air or water.
Strong currents will carry gravel ( boulders, cobbles and pebbles).
Moderately strong currents will lay down sand beds.
Weak currents carry muds composed by the finest clastic particles (silt and clay).
And surplus ions will add to the salt content of the water.
How transport affects sediment composition?
Large particles become smaller particles.
Angular particles become rounded particles.
Polymictic sediment (many mineral types) become monomictic sediment (only one mineral type).
Detrital components become new minerals.
Ex: Breccia -> Conglomerate -> Sandstone -> Siltstone -> Claystone
How do we classify clastic sediment and sedimentary rocks?
We classify clastic rocks according to grain size:
- Coarse-grained: Conglomerate and Breccia - Formed by gravel (boulder, cobble and pebble).
- Medium-grained: Sandstone - Formed by sand
- Fine-grained: Formed by mud
Siltstone- formed by silt
Claystone, shale and mudstone - formed by clay
How do we classify sandstone?
- Quartz sandstone - are made up almost entirely of quartz grains, usually well sorted and rounded. Low permeability after diagenesis but not as low as the others.
- Arkose - are more than 25% feldspar. The grains tend to be poorly rounded and less well sorted than those of pure quartz sandstones. Intermediary permeability after diagenesis.
- Lithic sandstone - contain many fragments derived from fine-grained rocks. Lowest permeable after diagenesis because it is composed of fine grained sediments.
What are the factors affecting sediment input into the oceans?
Climate - Warm and humid - chemical weathering
Cold and arid - mechanical weathering
Vegetation cover - Protection against erosion
Lithology - Bowen’s series
Relief - High relief - high erosion rate
Agriculture and forestry - Land use cause increased erosion
What are the main fluvial processes?
The rivers move sediments downhill from the source (mountains) to the mouth (oceans-delta or alluvial fan).
Erosion: particularly where the current hits the wall of the river channel (outer).
Sediment transport: particularly in the main current.
Sediment deposition: particularly where the current slows down (inner).
All processes occur at the same time in different parts of the river.
What is the difference between the main river types? -
Braided rivers
Network with connected stream channels.
Numerous islands (gravel bars) that change position due to erosion.
High flow velocity
Often gravel-dominated
High relief
Often seasonal discharge
- Meandering rivers
One main, curved channel
Meanders migrate laterally due to erosion. They may form oxbow lakes
Low flow velocity
Often dominated by suspended load
Low relief
What is the effect of fluvial processes on the landscape?
The fluvial processes have external land-shaping agents that work by erosion and deposition:
Running water is the most important external agent. It contributes more to shaping landforms than all other external agents combined. It forms:
V-Shaped Valleys are formed by braided rivers
Floodplain is formed by the erosion of v-shaped valleys by meandering rivers.
What are the factors affecting delta formation?
It forms when a river enters a lake or the ocean:
Decrease in velocity - bedload and suspended load is deposited.
Channels form - they change position depending on topography.
There are 3 main types of delta:
Sediment-dominated
Wave-dominated
Tide-dominated
What are waves? How do they transport material?
Waves are the main aeolian agent. The wind blowing over the surface of the water creates waves by transferring the energy of motion from air to water.
Waves move sediment, cause erosion and form sediment structures above the wave base.
- Waves transport material in the same ways as rivers transport material e.g. traction, saltation, suspension and solution. The energy of the waves dictates the type of material carried. The load is the total amount of material carried by a wave. T
Why does tsunami waves turn so high?
Tsunamis are large ocean waves caused by undersea events such as earthquakes, landslides, and the explosion of oceanic volcanoes.
When a megathrust ruptures, it can push the seafloor landward of the oceanic trench upward by as much as 10 m, displacing a large mass of the overlying ocean water. In the deep ocean, a tsunami is hardly noticeable, but when it approaches shallow coastal waters, the waves slow down and pile up, inundating the shoreline in walls of water that can reach heights of tens of meters.
A tsunami carries with it the energy of an earthquake and not the energy of the wind. While the wind can move only the upper layer of the water, the earthquake deforms the sea floor, displaces the whole wather columns, even the water deep down in the sea and generates tsunami.
What are the most common sedimentary structures?
Cross bedding, Graded bedding, Flat bedding, Ripples (current ripples or wave ripples), Bioturbation structures, Bedding sequences
How do ripples and dunes are formed?
When sand grains on a streambed are transported by saltation, they tend to form cross-bedded dunes and ripples. Dunes are elongated ridges of sand that can be many meters high in large rivers. Dunes form when wind blows sand into a sheltered area behind an obstacle. Ripples are very small dunes—with heights ranging from less than a centimeter to several centimeters—whose long dimension is formed at right angles to the current.
What are the different types of subaquatic mass transport?
Slide
Slump
Debris flow
Tubidity current
What is a turbidite current?
A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density than the fluid through which it flows—the driving force of a turbidity current derives from its sediment, which renders the turbid water denser than the clear water above. The deposit of a turbidity current is called a turbidite.
Smallest grain size at top. Largest grain size at bottom
Where most carbonate rocks form? Why?
In the continental shelf because of the large amounts of calcium and carbonate present in seawater, which organisms can directly convert into shells. Calcium is supplied by weathering of feldspars and other minerals in igneous and metamorphic rocks.
How carbonate rocks form?
Carbonate sediments and carbonate rocks form mostly from the accumulation of carbonate minerals that are directly secreted by organisms or precipitated in the organism’s external environment. Also during burial and diagenesis—deposited carbonate sediments react with water to form a new suite of carbonate minerals. In all these processes, the minerals precipitated are either calcium carbonates (calcite or aragonite) or calcium-magnesium carbonate (dolomite).
What are the names of the most common carbonate types?
Limestone
Chalk
Dolomite
Stromatolites
How do we classify the most common limestones?
-Mud-suported = matrix suports and separates the grains: mudstone, wackestone
-Grain-suported = little matrix is present but the grains suport each other: Packstone, grainstone(has no matrix)
Both mud-suported and grain-suported limestones weren’t bound together during deposition
- Boundstone =bound together during deposition
What is a glacier?
A mass of ice which is formed on land by the compaction and recrystallisation of snow to ice. It moves or creeps downwards due to its own weight.
How does rock material is eroded and transported glacially?
- Frost wedging: Freezing and thawing (ice expansion) in cold, wet climate.
Water enters cracks in rocks. Water freezes as temperature drops; expands against walls of rocks. Cracks are enlarged; intervening rocks are dislodged. - Abrasion: Rocks within the ice smooth and polish the surface bello.
- Plucking - lifiting of rocks. The icebergs carry dropstones around and deposit it in places far from the origin of these rocks.
How do we (in the landscape) can see that glacial processes have taken place?
There will be rock flour and glacial striation caused by abrasion.
U-shaped valleys
Roche moutonnée - small hill of bedrock, smoothed by glacial ice on the upstrem side and plucked to a steep rough face on the downstream side as the moving ice pulls rock fragments from joints and cracks.
- Fiord
-Moraine
-Dropstone
What is the concept of porosity and permeability?
Porosity is the portion of interstices (pores) in the bulk volume of a rock (may be filled with fluids). Empty spaces.
Permeability is the connectivity of the pores (= the capacity to transport fluids)
How is ground water transported?
Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this recharge reaches the water table. Groundwater is transported through connected pores of rocks.
What is deposition?
What is diagenesis?
Deposition is the processes by which sediment settles out of the water or wind carrying it.
Diagenesis encompasses all changes in a sediment that occur from its formation (deposition) until it is metamorphosed or destroyed after uplift (weathered).
What are different diagenetic processes?
- Compaction = The weight of new layers can squeeze older sediments tight together. Compaction is the process that presses sediments together. When thick layers of sediment build up and new layers squeeze together older layers.(porosity loss)
- Cementation = while compactation is taking place, some minerals in the rock slowly dissolve in the water. Cementation is the process in which dissolved minerals crystallize and glue particles of sediment together. Common minerals that act as glue are calcite and quartz. (porosity loss)
- Dissolution: Mineral grains (or cement) disappear (porosity win).
- Recrystallisation: New size and shape, but the same composition (porosity win or loss)
- Replacement: New composition, but same size and shape (no porosity change)
- Autigenesis: New size, shape and composition (porosity win or loss)
What is the relationship between cement mineral and its effect on porosity and permeability?
Cementation decreases porosity and permeability, the percentage of a rock’s volume since it fill empty pores up and then crystallizes.
What is the difference between diagenesis and metamorphism? Metamorphism happens at higher temperatures and pressures and diagenesis happens at relatively low temperatures and pressures.
How do we identify some common sedimentary rock types?
Clastic sedimentary rocks contain clasts. These are fragments or pieces of rock or minerals. The composition of clastic sedimentary rocks is divided into three types - clay/silt, sand and gravel.
Carbonate rocks - They react with HCl. Most types of limestone are formed from the calcareous skeletons of organisms such as corals
Evaporites - Evaporite rocks are chemically precipitated from evaporating seawater or, in some cases, lake water.
Organic sedimentary rocks are which form as the result of the accumulation of organic material or biologic activity. Coal is the most commom. The dark brown to black color is the most obvious charateristic.
Conglomerate - gravel, rounded fragments
Breccia - gravel, angular fragments
Sandstone - sand
siltstone, shale, mudstone, claystone - mud
Crystalline Limestone - coarse to fine, crystalline, react with HCl
Fossiliferous Limestone - visible fragments of shells, react with HCl
Chalk - microscopic shells and clay, react with HCl
Chert - very fine crystalline
Gypsum - fine to coarse, crystalline
Rock Salt - fine to course, crystalline, tastes salt
Bituinous Coal - organic matter, fine
