Sedimentary Processes and Products (Lectures 36-41) Flashcards
Define sedimentology
Define sedimentary geology
Study of sediments
Study of sedimentary rock
Define lithification
The process of turning sediment into a sedimentary rock
What are the types of sediments?
Clastic
Carbonate
Organic
Chemogenic
What is clastic sediment formed from?
What is the most common type of clastic sediment?
Given that clastic sediment rocks are categorised by the dominant size of the individual grains, what are the group names?
What is a rarer type of clastic sediment?
Grains separated from a parent rock by erosion and moved by fluids
Siliciclastic
Mudstones, siltstones, sandstones and conglomerates
Volcaniclastic
What are the two most common ways carbonate sediment can form?
What can these carbonate sediments be converted to?
Shelly organisms in colonies, sedimentary accumulations of shelly material (reefs) OR shelly and skeletal material acts as grains
Limestones, oolites and chalk
What can form organic sediments?
What are these responsible for the creation of?
Mostly C-rich, photosynthetic organisms like plants and algae
Carbonaceous sediments, hydrocarbons (oil and gas) and peats (forms coal in burial)
How can chemical processes lead to chemogenic sediments? Give an example
How else can chemogenic sediments form?
Which chemogenic sediment can be inorganic or organic?
What are the two siliceous sediments?
Inorganic precipitation of different minerals
Ironstones
Interplay of chemical processes and biological products
Phosphorites
Chert and flint
How do evaporites form?
Salts which precipitate as masses of crystals out of natural bodies of water
Which processes cause early lithification?
Biological precipitation
Chemical precipitation
Cooling
What is cementation in the lithification process?
When does it occur?
Chemical processes solidifying sediment grains together
Water in the sediment precipitates out different minerals between sediment grains
How would a sediment lithify through compaction?
Enforced closer packing of grains under the weight of sediment deposited above
What is diagenesis?
Which two processes in lithification mark the first stages of diagenesis?
Processes that can result in mineralogical changes to a sediment after deposition
Cementation and compaction
What are the later diagenetic processes?
What induces them?
Recrystallisation, or dissolution and replacement
Increased p and T in burial
What is the primary technique for studying outcrops of sedimentary rock?
What are they?
Constructing sedimentary logs
Graphical representations of vertical rock sections
Define stratigraphy
The study of the relationships in time and space between different layers of rock
What is the most important aspect of stratigraphy?
The principle of superposition, read a succession of strata as a sequence of depositional events
What is the second important aspect of stratigraphy?
What does it recognise?
Give an example of this
Walther’s Law
If you see two sedimentary units superimposed on top of each other without an unconformity then the vertical transition reflects a lateral transition between two neighbouring environments
Modern sedimentary environments are diachronous - adjacent components of a sedimentary environment may be active at the same time
A river channel (sandy) and a nearby floodplain (muddy)
Sedimentary processes can be broadly grouped into what three categories?
Erosion - liberates new grains from previously deposited sediments or lithified rock
Transport - fluids or gravity move sedimentary grains around
Deposition - where fluids lose the capacity to keep moving the grains and deposit them in a temporary or final resting place
What does physical weathering involve?
The weakening and breaking up of rocks as cracks form and widen in the parent rock due to T changes, freeze-thaw action, salt growth or biological intrusion (roots)
What does chemical weathering involve?
Weakening the rock by altering the mechanical properties of original rock minerals - by solution in groundwater or brine, hydrolosis, or oxidation
What does weathered material produce?
What is it available for?
A loose regolith
Transport by fluids
Soil can form when what interacts with a regolith?
Biological processes
Erosion of weakened bedrock can occur by what means?
Through gravity alone - rockfall
Through gravity acting on water-saturated regolith - slumping
Incision from water
Abrasion by sedimentary grains already held in a fluid
Plucking by ice
Which fluids are mainly responsible for sediment erosion, transportation and deposition? (and what forms can they be in)
Water - rivers or marine currents
Air - wind
Ice - polar ice caps and glaciers
Sediment-water mixtures - debris flows and turbidites
What is laminar flow?
What is turbulent flow?
How is it determined how a fluid will flow?
All fluid molecules move parallel to transport direction
Individual molecules move randomly, with only net movement in transport direction
Re = u.l/v (Re is Reynolds Number, u is flow velocity, l is flow diameter, v is kinematic viscosity)
Laminar flow: Re < 500
Turbulent flow: Re > 2000
Sediment grains can be composed of what?
Complete or partial individual mineral crystals
Agglomerations of multiple crystals (polycrystalline)
Once minerals become sediment grains, what are they referred to as?
What does this mean?
Detrital minerals
Physical and chemical forces can act on them that may change them in some way
Why is quartz the most abundant mineral in clastic rocks?
Physically durable
Can survive multiple generations of recycling through weathering and deposition
Can be liberated from many rock types
Feldspars are the most common what?
Why are they only the second most abundant grains in sedimentary rocks after quartz?
Rock forming minerals on Earth
Far less resistant to chemical and physical destruction
Often altered or removed during sediment formation
What are lithic fragments?
What brings them about?
Why are they prone to destruction?
Entrained polycrystalline fragments of pre-existing rock
Weathering
Internal weaknesses
What kind of abundance is expected of accessory minerlas in any clastic sediment?
Less than 1%
What are clay minerals?
How can they form?
What are the properties of clay minerals?
Sheet silicates with layers of silicon tetrahedra and aluminium tetrahedra
Within a sedimentary rock during diagenesis, or as detrital minerals due to alteration of other silicates during weathering
Cohesive (due to surface tension and electrostatic forces) and may flocculate together as larger aggregates
What are bioclasts?
Which kind of rocks are they important in?
Grains formed by shell, plant, bone, algal or bacterial debris
Carbonate rocks
What can sandstones and carbonates be classified using?
Q-F-L plots
Folk or Dunham classifications
What are the physical properties of grains?
Grain size
Grain shape
Porosity
How is grain size numerically indexed?
How is grain size classified?
For sedimentary rock, the dominant grain size is used to determine what?
Logarithmic phi scale
Uden-Wentworth scale
Rock type
What is grain shape determined by?
Roundness - the surface roughness of a grain
Sphericity - how close a grain is to a perfect sphere
What is porosity?
What is pore space dependent on?
What does pore space provide?
The measure of the small gaps that exist between grains piled on top of each other
Grain shape and grain size
The locus for the precipitation of mineral cements and the migration of pore-water fluids after deposition
Define entrainment
The moment when a fluid begins to exert enough force on a sedimentary grain to begin moving it
Once sediment is moving in a fluid, how can it do so?
What explains the lift force involved in saltation or suspension?
Bedload - rolling (parallel drag force) or saltation (jumping of individual grains)
Suspended load
The Bernoulli effect
What is Stoke’s Law?
The settling velocity of a particle out of a fluid V = g.D^2 .(ρs - ρf)/18μ V = terminal settling velocity D = grain diameter ρs = grain density ρf = fluid density μ = fluid viscosity
What is grading?
Why does it occur?
The grain-size at the bottom of the sediment pile is coarser than that at the top
As critical flow velocity decreases, the fluid deposits the larger grains first
What are beds of sediments?
The small scale change in grain size between beds means what?
What are the two kinds of bedding series that can be exhibited?
Individually deposited packages of deposited sediments when removed from the influence of erosion/deposition
They are separated by a bedding plane - a plane of weakness that can be highlighted by weathering or exposure
Fining-upwards or coarsening-upwards
What is unidirectional flow?
Give some examples
What is bidirectional flow?
Give an example
Fluid moves from one location to another
Rivers, glaciers, ocean currents
One unidirectional flow, then a later flow in the opposite direction
Tides
What is the product of unidirectional flow?
Distinct sedimentary bedforms
Different to the graded bedding from the stillstand of water or bedforms from oscillatory flow
What happens to grains that settle in a moving fluid?
Sculpts physical patterns onto the surface of the deposited bed such as ripples or dunes
Define bedform
The surface features and their internal stratification
For aqueous sedimentary bedforms, a flow-regime can be applied to determine what?
The flow conditions under which the bedform developed
What are the two bedform groups in bedform stability diagrams?
Lower flow-regime forms - sculpted bedform is out of phase with the patterns seen on surface water
Upper flow-regime forms - bedforms and surface water patterns are in phase
Why do bedforms develop?
Unusual flow conditions near the base of moving water
What is the layer called that has the distinct flow conditions that forms a bedform?
What is the lowest part of it?
What allows ripple marks to develop?
Boundary layer
Viscous sub-layer - flow is more laminar
If grains are sufficiently small to rest within the viscous sub-layer (< 0.7 mm) (bed is hydraulically smooth)
What interrupts flow in the viscous sub-layer in fluids?
How?
The repetition of this leads to what?
Erratic eddies
Sweeps grains into local clusters
Grain cluster develops into a current ripple
What happens in a current ripple in a fluid?
What is preserved in this process, what is not?
What is produced from this process?
Grains roll up the upstream stoss side Until they reach the ripple crest Then avalanche down the lee side Reworks the stoss side, so successive lee slopes are preserved as sets of downstream-dipping laminae Cross-lamination
What can be gained from the presence of cross-lamination?
Common feature of sediment deposited in unidirectional flowing water
Always dips downwards in the flow direction, so can determine palaeocurrents
What are the three types of cross-lamination that can be seen, and how does each develop?
Planar cross-lamination: ripples are straight-crested
Trough cross-lamination: ripples are sinuous or linguoid
Climbing ripple lamination: sediment supply is increasing, successive generations of ripples will climb up the back of previous ones
If grain size is greater than 0.7 mm such that current ripples cannot form, what is found instead?
Featureless sediment surface called lower plane bed
As flow strength over current ripples and lower plane beds is increased, what can they be reworked into?
Larger bedforms: dunes
What do dunes resemble?
How are they different?
Large-scale ripples by shape and internal organisation
Lack of overlap between the dimensions
A positive relationship between dune dimensions and flow depth
Absence of dunes in grain sizes < 0.1 mm
Ripples can occur on top of dunes
What do dunes create instead of cross-lamination?
What are the two types?
Cross-bedding
Planar cross-bedding
Trough cross-bedding
In what kind of flow-regime do ripples and dunes form in?
Lower flow-regime
What is formed in an intermediate flow-regime?
An upper plane bed
Internal structure presents as planar lamination
Bed surface is marked with parting lineation - streaks of different grain size parallel to flow
Define supercritical flow
When does it develop?
What is the equation?
The flowing water can flow faster than waves on its surface
Froude Number > 1
Fr = u/sqrt(gh)
u is flow velocity, h is flow depth
Bedforms in phase with the water surface are called?
What is the structure?
When can they preserve?
Antidunes
A concave upwards erosional base
Filled in with upstream-dipping cross-lamination
A large flood
Define wavebase
The depth to which wave effects are felt
Define substrate
The surface of the temporarily dormant sediment (when there’s no active sedimentation)
What are common sedimentary structures found on substrates?
Raindrop impressions
Desiccation cracks
Trackways of animals
What is facies analysis?
A method employed to make sense of the highly variable sedimentary record and the changes represented within it
Define sedimentary facies
A body of rock characterised by a particular combination of lithology and physical and biological structures different from the adjacent rock bodies
Define facies associations
Different physical facies that share things in common in terms of their depositional environment
Define sedimentary environments
What are the two types, and how are they grouped?
Any environment in which sediment is being produced or deposited
Depositional and erosional, the dominant process
Define sedimentary basin
Relative topographic lows in the crust that are generated by subsidence relative to surrounding areas
How do extensional basins develop?
What is a rift basin?
If extension continues on a rift basin, what happens?
If extension stops on a rift basin, what happens?
What can also be generated if extension stops?
Strike-slip faulting can generate which kind of basin?
Extension of the lithosphere, it thins by ductile thinning in the mantle and lower crust, and by brittle faulting in the upper crust
Combination of down-faulted blocks and up-faulted areas
Rift breaks apart, new ocean crust, forms passive margins of continental crust
Lithosphere beneath cools and contracts, thermal subsidence occurs, sag basin forms on top of the rift basin
Intracratonic basin
Pull-apart basin
How do flexural basins develop?
In ocean-continent collision what may form, why?
In continent-continent collisions, what may form, why?
Lithosphere flexes when loaded, flexural bending generates flexural basins either side of the load
Retroarc foreland basins, loading by volcanic arc and associated orogeny
Foreland basins, load generated by the collisional orogen
What does basin type control?
Thickness and type of sedimentary rock deposites
Glacial sedimentary environments are restricted to?
High altitude or high latitude
What is the stratigraphic bias to glacial deposits in the rock record?
Record of icehouse intervals has a greater abundance of glacial sedimentary rock than greenhouse intervals
What are the zones that active glaciers can be divided into?
Accumulation zone - snowfall > melting
Ablation zone - melting > snowfall
How are cold-based and warm-based glaciers different?
What is the intermediate?
Cold-based: permanently frozen to the ground, move due to internal shear in the ice
Warm-based: slide over their bed, so can erode and entrain material at their base
Polythermal glaciers: move during sporadic surges despite normally being frozen to the bed
Sediment grains can become entrained into a glacier as what kinds of debris, and in which settings are they possible?
Basal debris: entrained by abrasion and plucking, warm/polythermal
Supraglacial debris: rock falls onto the top of the glacier, or wind-blown sediment buried by snow
Once the sediment is deposited from a glacier, what is it called?
What is it called when lithified?
How sorted is till, what is it called?
How are the conglomerates supported?
Till
Tillite
Very poorly sorted conglomerate called a diamictite (mixed throughout)
Matrix supported
If glacial sediment is transported to the front of the glacier, what can it be reworked as?
If a glacier terminates in a body of water, what develops?
What are dropstones?
Loess: very fine glacial outwash sediment, reworked by the wind and carried far
Glacio-fluvial sediment: transported by rivers from melting glaciers
Varves - laminated sediments
Icebergs produced by glaciers, melt and deposit isolated very large clasts in normal marine deposits
What are aeolian processes?
What do these processes dominate?
Sedimentary processes about the movement of grains in the air
Transport and deposition of sediment
What are aeolian ripples?
How do they differ to ripples formed in water?
Ballistic ripples: they migrate because saltating grains move a regular distance at a certain windspeed, forming straight-crested ripples transverse to flow direction
Avalanching of lee-side grains and cross-lamination formation is less common than water
How do aeolian dunes differ from those formed in water?
How does aeolian cross-bedding develop?
Grow much larger because flow depth is large
Avalanching down lee-slopes
What are the different types of aeolian dunes?
Outline their properties
Transverse dunes: unidirectional flow, high supply, sheet of sand with dune crests on top
Barchan dunes: unidirectional flow, limited supply, discrete cuspate mounds of sand
Linear dunes: two directions of wind, isolated, elongated and steep sand-ridges
Star dunes: complicated multi-directed wind
Why do most desert sediments have a red colour?
Iron oxide develops as a weathering coating on grains and doesn’t get reduced during burial
What are the two layers in a lake? How do they differ?
What separates them?
Epilimnion: upper warm layer, agitated by waves so is oxygenated
Hypolimnion: lower cold layer, can become anaerobic
A density-stable thermocline horizon
What happens in the hypolimnion of a lake?
How is sediment supplied to a lake?
Organic-rich sediments accumulate, undisturbed by bioturbation
Sediment supplied by feeder rivers
How do saline lakes and ephemeral lakes differ from temperate, clastic lakes?
Deposit couplets of mudstone and evaporites
Precipitated as the lake dries up
Mud enters as the lake refills
What is the common feature of lacustrine deposits?
They are highly climate-controlled
Define alluvial
Define fluvial
Alluvial: any process that leads to the deposition of sediment by running water on the land surface
Fluvial: anything directly and specifically pertaining to a river, including deposition, geomorphology etc
(all fluvial are alluvial, not all alluvial are fluvial)
What is an alluvial fan?
Where are they common?
How do they form?
A geomorphic landform that develops when a confined supply of sediment becomes unconfined at a break in a slope
At the margins of basins, where sediment/water exits confined mountain valleys (hot and cold deserts_
Sediment/water exits at a point source, it spills outwards radially, forming an alluvial fan on an alluvial plain
What is the sediment like in an alluvial fan?
When do they experience active deposition?
Very coarse-grained, gravels, cobbles and boulders
During discrete events
How is sediment supplied to an alluvial fan for it to be built up?
Debris flows: sediment-water mixture, no permanence and they don’t travel far
Sheet floods: more water in the system, flash floods remobilise deposited sediment as bedload, supercritical, short-lived, transport further than debris flows
Braided channels: small perennial or ephemeral braided channels develop on top of the fan if the rain is right, even further deposition
How can the ways that an alluvial fan is built up, be distinguished?
Debris flow: poorly sorted matrix-supported conglomerates
Sheet-flood deposits: internal horizontal stratification, may be clast-supported, can contain imbrication (water stacks conglomerates against each other, they dip upstream)
Braided channels: may be lenticular (lentil shaped) in cross-section, contain finer material than the debris flows
How are braided rivers typified?
What is the grain size of the average deposit?
What are they dominated by?
Lots of mid-channel bars that are exposed above the water level in normal conditions
Coarser grained
Bedforms that reflect the movement of the coarse sediment by bedload (rolling and saltation)
What is found in the deepest parts of the channel of a braided river?
Where is the bulk of the sediment?
A lag deposit of the coarsest sediment above a scoured base
Concentrated in bedforms - composed of multiple dunes and other bedforms, characterised by cross-bedding
Braided river deposits are often typified by what?
What may they be capped with?
Tabular beds
Extensive basal scours
Lag deposits
Complex arrangement of bar bedforms on top
Mudrocks, reflecting floodplains after the active river moved
When might a braided river develop?
Fluvial flow struggles to transport all the sediment that is provided
When might a meandering river planform develop?
When is this typical?
Fluvial flow is more efficient at moving sediment
Downstream reaches, as there are mostly fine grains
Why can meandering rivers be termed mixed-load rivers?
Sediments can be transported as bedload and suspended load
How are meandering rivers typified?
Why does this develop?
Sinuous form
Flow instabilities in a channel: different parts of the water move at different speeds, fastest flow moves to the outer bend
What does helical flow lead to within a channel?
What does the cross-section eventually look like?
Maximum velocities at the outer bend Minimum velocities at the inner bend Erosion at the outer bend Deposition at the inner bend Asymmetric
What does the depositing inner bend of a channel form?
What is the internal structure?
Why does the internal structure alternate, and what between?
A point bar
Lateral accretion: bedding dips into the channel, laterally to a flow direction
Mixed-load and seasonal variation, thin beds of sand and mud
What are meandering channels often bounded by?
What are they from?
Levees
Raised mounds of sand from past floods into the floodplain
What are crevasse splays?
What is found in the floodplain sediments?
When levees are breached and the sand spills out onto the floodplain
Heterolithic sediment, alternating thin layers of mudrock and sandstone
What are soils termed when they fossilise?
What is a very common type?
Where do they occur?
How do they develop?
Palaeosols
Calcretes
Semi-arid climates
Evaporation draws floodplain groundwater up through the soil, carbonate precipitates out, forms discrete layers of CaCO3 in clastic floodplain mudrocks
What are floodplain mudrocks, palaeosols and crevasse splays grouped as?
Overbank deposits
When does a delta form?
What do they mark?
A sediment-laden river enters the sea and the radially distributed sediment builds up at the river mouth due to the dissipation of the flow
A transition between continental and marine environments
What are the parts to a delta?
Delta top
Delta front
Delta slope
Prodelta
What is a delta top?
What can form with heavy vegetation?
The subaerially exposed part of the delta
Consists of fluvial and floodplain deposits
Deposition of peat
What does the delta front mark?
How are the channels separated?
An area where levee-bounded channels may extend into the sea
By shallow marine interdistributary bays
What environment is the delta slope?
Where does it go?
What is it dominated by?
Fully shallow marine
Slopes downwards from shallower to deeper water
Sands from the river mouth
What is the prodelta?
What does it receive?
The offshore part of the delta
Fine-grained sediment that settles through suspension
Deltas can be seen to do what due to their sediment supply filling accommodation space?
What is the net effect of this?
What does this result in from the grain size of delta parts?
Prograde
Prodeltas are often overlain by other parts of the delta
Coarsening upwards succession
Deltas are prone to lobe switching, what does this mean?
What is this termed?
Why does it occur?
The process by which the river mouth suddenly moves laterally
Avulsion
So much sediment gets deposited at the delta top
How do fluvial/wave/tide-dominated deltas differ?
Fluvial: typified by channels that build out from the delta front
Wave: waves operate along coasts so they’re more smoothed
Tides: move sediment back and forth so terminate in elongate islands
What is longshore drift?
Wave crests approach a coast obliquely
Moving water and sediment diagonally alongshore
Drain directly out to sea
A net movement of sediment in one direction
Beaches with longshore drift are dominated by what?
Where might a coastal plain develop?
What do coastal plains resemble and what’s the difference?
Parallel-laminated sand
The gradient of the land nearest the sea is shallow and at risk of occasional flooding
Floodplains, but with marine signals (shells)
What coastal environment develops with quiet-water?
What coastal environment develops where a river mouth is flooded?
Lagoons
Estuaries
In warm environments away from a terrigenous sediment source, what kind of coast can develop?
What is deposited here?
What might the deposits form?
Carbonate coasts
Skeletal material or precipitated carbonate
Carbonate sands or mud
Where can thick evaporite succession occur?
Very arid climates with saline seawater
Hypersaline lagoons
Evaporitic coastal plains (sabkhas)
What happens in sabkhas?
Where are these often seen?
Evaporation draws saline groundwater up through the sediment
Precipitates gypsum and anhydrite
Close association with aeolian deposits associated with neighbouring deserts
How do carbonate sediments differ from clastic sediments?
The bulk of carbonate sediments have a biological origin
They also tend to form in the location they are preserved
What is the productivity of the “carbonate factory” dependent on?
Factors that influence biological productivity:
Water T - 20 to 25 C promotes CaCO3 ppt
Normal salinity and oxygen, more sediment-producer species adapted to it
Light availability - photic zone extends to ~70m
Terrigenous sediment supply - high = cloudy water = less light = lower productivity
Where do carbonate sediment environments tend to develop on modern Earth?
Equatorial regions
What are microbialites?
What are stromatolites?
Carbonates deposited due to microbiogeochemical ppt of direct calcification of microbe cells
A type of microbialite, layers of microbial mats calcified on top of each other forming layered domes
Define bioclasts
Fragments of skeletons mechanically broken down after the organism died
Define ooids
Why are they well rounded?
Carbonate grains that form due to the precipitation of CaCO3 around a nucleus
Constant agitation by waves and tides as they form
Define pellets w.r.t carbonates
Define peloids
Faecal carbonate grains produced by animals
Look like pellets, produced by diagenetic alteration of other grains
What is carbonate mud called?
How can it form?
Micrite
Direct ppt of carbonate microcrystals in the water column, then settle out of suspension
Direct ppt on the seafloor
Disintegration of bioclasts
Byproducts of bioerosion
Settling of calcareous plankton skeletons
What will be seen at the different parts of a carbonate ramp?
Higher energy: grainstones and packstones
Lower energy: wackestones and carbonate mudstones
What is an epicontinental sea?
A very shallow, very extensive sea
What are carbonate shelves typified by?
What do they provide in temperate settings?
What do they provide in tropical settings?
A distinct shelf-edge break
A connection with the open ocean
Sufficiently deep water to be undisturbed by physical processes such that carbonate reefs can form
What is a carbonate reef?
Reefs can buffer against storms so back-reef deposits consist of what?
Colonies of organisms that bind together and build up a framework of cemented skeletons
Less physically-reworked carbonate sediment than a carbonate shelf that lacks a reef rim
Where do these places lie regarding bathymetry, continental shelf, continental slope and rise, and abyssal plain?
Continental shelf: shallow water over normal-thickness continental crust
Continental slope and rise: lie over transitional continental-oceanic crust
Abyssal plain: in the deepest water on thin oceanic crust
The gravitational attraction of the Moon on the ocean surface causes what to develop?
How often are these per day?
As the ocean flows to accommodate the water level change, what are generated?
Due to the orbits, what are generated twice a month?
Two tidal bulges, a larger and a smaller
Diurnal (twice-daily) rises and falls
Tidal currents
Spring tides: tidal bulge is higher when the Moon and Sun are aligned
Neap tides: tidal bulge is lower when the Moon is normal to the Earth-Sun
In the shallowest water closest to the coast, what ebbs and flows twice daily?
What sedimentary structures are then seen in shallow marine sediments?
A tidal prism
Herringbone cross-stratification (cross-beds dip in two directions to reflect bidirectional flow)
Mud drapes
Flaser bedding (mud deposited at high and low tide, sand on the falling and rising tide)
Reactivation surfaces (dune crests are shaved off by the reverse flow of the tide
Wave size depends on which two things?
Fetch - the distance over which waves move
Wind strength
What are storm waves able to do regarding sedimentation?
Why do shallow-marine storm-dominated settings have a bias for storm sands?
Move large amounts of sand and larger sediment
Erode the seabed
Storms erode the normal sedimentation that happens in between them
What are the bedforms like where there are storm flows?
When storms are common, how do these bedforms change?
Hummocky cross-stratification: combined flows sculpt mounds (hummocks) into the sediment, separated by hollows (swales)
Hummocks are shaved off, only swales preserved: swaley cross-stratification
Sedimentation in the deep ocean is influenced by which two current forms?
Thermo-haline - driven by the sinking of cold seawater in polar oceans, return flow of warmer water at shallower levels
Geostrophic - surface water driven by global wind patterns
Define pelagic sediment
What are the sources?
As these sediments settle out of suspension, what is the grain size and are there any features in the bedform?
Fine material held in suspension in the open ocean and which settles slowly onto the deep seafloor
Dust blown in by winds from continents, volcanic dust and skeletons of calcareous plankton and siliceous plankton
Very fine-grained, no evidence for bedload deposition
What is the CCD?
Above the CCD, calcareous sediment settles out as what?
If the plankton is mostly siliceous, what forms instead and what can this lithify as?
Calcite compensation depth: calcite more easily dissolved due to low T and high p
Calcareous ooze
Siliceous ooze, chert
What causes turbidity currents?
Unstable deposits on the shelf edge
They catastrophically slump as mass-flow movements into deeper water
How are turbidity currents typified?
Low concentration of grains in water
Enough to create a density contrast between the turbidite and the ambient water though
In what form do turbidites deposit sediment?
How is it arranged from top to bottom?
Where do you see more a and b?
Where do you see more d and e?
Bouma sequence a) Massive sands b) parallel laminated sands c) cross-laminated sands d) laminated silt e) pelagic mud Proximal regions Distal regions
Turbidite movement can result in two types of sole marks on the base of the bed, what are they?
Scour marks caused by flow turbulence
Tool marks formed by objects that have been dragged along (pebbles, shells etc)
When can contourites form from turbidites?
What are contourites?
What do contourites reflect?
If sediment supplied by mass flow and is reworked by deep ocean currents
Very long wavelength shallow-dipping muddy bedforms
Movement of deep ocean currents along shelves
When deep marine mass-flow sediment is supplied from a point source, what is formed?
How do the sedimentary characteristics vary with distance from the point source?
A submarine fan
Proximal fan: submarine channels that cut through slope mudstones
Medial fan: dominated by sand-rich turbidites
Distal fan: forms a fringe of mud-rich turbidites around the fan edge
