Sedimentology Flashcards
Sedimentology =
Study of processes that create sediments and dynamic environments where they accumulate
~75% of rocks are sedimentary rocks
Useful for palaeoclimatology, palaeooceanorgaphy, economic geology, record of life, correlation and dating of rocks
Uniformatarian principles work best with sediments
MOUNTAINS: Benefit/threat
+ Source of sediment
- Extreme events deliver too much sediment
DRAINAGE BASINS: Benefit/threat
+Source of sediment
- Source of contaminants
FLOOD PLAINS: Benefit/threat
+ Forms berms to floods
+ Provides nutrients and soil
- Buries vegetation
BEACHES: Benefit/threat
+Forms beaches
- Lack of sediment leads to erosion
OFFSHORE/MARINE: Benefit/threat
+Provides nutrients to benthic habitats
- Excessive sediment buries habitat and fragments ecosystems
- Creates turbid water = loss of light for aquatic vegetation
DELTAS AND MARSHES: Benefit/threat
+Enables marsh accretion, buffers sea level rise
- Excessive sediment buries habitat
- Lack of sediment = inundation due to SL rise
RIVERS: Benefit/threat
+Creates spawning habitat
- Channel deposition increases flood risk
- Changes in sediment size affect spawning habitat
Stratigraphy =
Study of rock layers, usually sedimentary
Sediment yields =
Total sediment exported from a drainage basin in a given time (Km2/a)
“Liberation of sediment flux”
Due to weathering/erosion:
- chemical or physical weathering e.g. freeze thaw, onion skin weathering, dissolution etc
CAUSED BY:
- uplift
- climate
- lithology (certain rocks erode more than others)
- vegetation
CLASTIC ROCKS (define)
Clasts = grains/pebbles/boulders
Names: Conglomerates Gravels Sandstones Siltstones Mudstones Clays
Mass wasting =
Downslope movement of soil/rock under gravity
Creep =
Slow downslope movement of soil/rock under gravity, more confined than mass wasting
Landslides =
Rapid down and outward soil/rock movement
Types of mass flows (fast to slow)
Rock/debris fall
Snow/debris avalanche
Rock/debris slide
Mudflow
Slump
Creep/rock glacier
Thermohaline circulation
Bottom water currents
- water heated at equator
- travels to poles
- cools, sinks,
- travels back to equator
= governs many sedimentary processes
N.B. Global warming could affect this
For example; During the late Cretaceous, chalk (a pelagic sediment formed in shallow warm seas) was deposited in South England
This formed when the oceans were stagnating and the ocean temps were therefore warmer
Sedimentary basins =
Bowl-shaped depressions/topographic lows of the Earth’s crust where sediment can accumulate into successions hundreds to thousands of metres thick
Their sedimentary fill provides unique evidence for the environmental conditions during the basin’s lifetime
Main control producing sedimentary basins = TECTONICS
Subsidence =
Local/regional scale change in Earth’s crust in form of downward shift relative to e.g. SL
The 5 basin settings
RIFT BASINS
INTRACRATONIC BASINS
PASSIVE MARGIN BASINS
FORELAND BASINS
STRIKE-SLIP BASINS
Rift basins
Extensional
Crust thinned as stretched
As rift grows, blocks of crust on the border fault slips = low areas and narrow mountain ridges
Can be continental/marine
e.g. E African rift system has many rift basins along its length
Intracratonic basins
In the interiors of continents
Round/oval shapes
Long geological histories of slow subsidence
DUE TO THERMAL SAG
- over areas of previous rifting
- hot, stretched crust cools, contracts and sinks
Usually filled with continental sediments but flooding from adjacent oceans can cause large epicontinental seas
Epicontinental sea =
Shallow sea overlying a continent
Passive margin basins
Form along margins of continents, NOT tectonic plate boundaries
Usually underlain by former rift with oceanic crust
- long after rifting ceases, thermal relaxation/subsidence continues
Carbonate and clastic sediments 10-20km THICK
E.G. Gulf of Mexico margin along southern US
Foreland basins
Adjacent and parallel to mountain belts
Regions of compressional tectonics
- downward flexing of LITHOSPHERE in response to weight of adjacent mountain belt
- sediment eroded from mountain belt
- gradually decrease in thickness away
Sediments transition from deep marine to continental environments
Thicknesses greater than 10km
e.g. Persian Gulf, produced by Zagros Mountains of Iran
Strike slip basins
“A horizontal sense of movement along the fault plane”
Fault planes not straight = areas of localised compression/extension
“Pull-apart” basin
Can be filled with continental/marine sediments
E.g. San Andreas Fault, California
North Anatolian Fault, Turkey
Sediment to sedimentary rock, process
Lithification ~diagenesis~ Burial Compaction Cementation (new minerals ppt in between)
MUDROCKS
- lose about 60-70% water = shale
- can’t lose 100% because contains hydrous minerals which would have to be broken down/changed form
CLASTIC SEDIMENTS =
Cemented together fragments and grains derived from pre-existing rocks
- Conglomerates/breccia (>2mm)
- Sandstone (0.063mm-2mm)
- quartz arenite (95% quartz)
- arkose (25% feldspar)
- litharenite (rock fragment rich)
- greywacke (more than 15% matrix; n.b. turbidity currents) - Mudstone (<0.063mm)
BIOGENIC SEDIMENTS =
Derived from the skeletal remains and soft organic matter of pre-existing organisms
e.g. Great Barrier Reef, Australia = LIMESTONE
Allochems
- grains (skeletal/non-skeletal e.g. ooids)
Orthochems
- matrix (micrite)
- cement (sparite)
Ooid formation
Small fragment of sediment e.g. piece of shell
Strong currents wash fragment around sea bed = accumulate layers of chemically precipitated calcite from supersaturated seawater
Carbonate sediment composed of ooids = oolite
E.G. Mid Jurassic oolite; decorative stone in World Heritage City of Bath, England
Dolomite
Most formed from replacement of limestone
- chemical reaction of calcite with Mg-bearing groundwater
- occurs soon after burial
e.g. Dolomite Mountains, N Italy
?? primary precipitate or secondary replacement product??
ORGANIC SEDIMENTS =
Organic carbon compounded from relics of plant or animal material that has not completely decayed e.g. leaves/roots/planktonic organisms/algae/spores/pollen
E.g. coal, oil shales, shale gas
CHEMOGENIC SEDIMENTS =
Formed from direct precipitation of minerals from a saturated solution e.g. evaporites
- gypsum
- halite
- anhydrite
Crystalline texture
Crystal size varies due to concentration of salts in sea/lagoon/lake depositional setting
Prone to diagenetic modification following burial; dissolution/recrystallisation/deformation
E.g. Zechstein Basin in NW Europe
VOLCANICLASTIC SEDIMENTS =
Composed of grains/fragments derived from volcanic activity
Ash (<2mm)
Lapilli (2-64mm)
Bombs/blocks (>64mm, round/angular)
Pyroclastic fall deposits
Pyroclastic flow deposits (ignimbrite)
Epiclastic sediments (erosion of volcanic rocks)
Pyroclastic flow =
Fast moving currents of hot gas and rock fragments that move rapidly down volcano slopes in response to gravity
Lahar =
Type of volcanic mudflow/debris flow composed of a slurry of pyroclastic material, rocky debris and water
Diagenesis =
All physical/chemical/biological processes that occur during burial, prior to metamorphism
What do flow hydraulics depend on?
Grain size/density Grain drag Grain roughness Flow velocity Flow viscosity Slope
Pebble clustering
Peak discharge when the very large grain is deposited in a 1/100 or 1000 year event
Larger grains then clustered on upstream side of large grain
Palaeocurrent indicators
Presence indicative of perennial gravel-bed rivers
Beds vs laminae
Beds = >1cm Laminae = <1cm
What to consider within beds/laminae
Attitude (i.e. strike/dip)
Thickness of units
Lateral variations
Sedimentary structures =
Marks/traces/sediment disturbances preserved in sedimentary strata
Not many are preserved - often cancelled out by the same sedimentary process e.g. rain drops = rare, flute casts = common
- must know preservation potential and interpretation limitations
- can only preserve once sediment is buried
Types of sedimentary structures
BP DICE
Biogenic
Post depositional
Depositional
Internal
Chemogenic
External
Geopetal structures
= “palaeospirit level”
- original CaCO3 shell of living animal
- mud infilled empty shell after animal died
- remaining space filled with crystals during burying
Mud side = lower side
DEPOSITIONAL STRUCTURES
Bedforms
= develops at the interface of fluid and a moveable sediment-dominated bed as a result of the bed being moved by fluid flow
Ripples/dunes
Can be used to infer flow depth and velocity (and Froude number)
Current ripples vs wave ripples
Current = unidirectional flow
- asymmetric cross section
- down-current (lee) slope steeper than up-current (stoss) slope
Wave = bidirectional flow
- truncating top
- tapering base
Current ripple development
Current ripples 2-5cm high
Up to 40cm wavelength
Migrate along bed in current direction
Sediment eroded from stops side, carried to crest and avalanches down lee side
Series of laminae build up // to lee slope
Successive laminae = cross stratification
Flow current increases = more sinuous ripple crests, eventually break up into linguoid ripples
Increasing current further = dunes
- up to 1m high
- 0.5-10m wavelength
Increased even further = dunes washed out
Sediment sweeps over flat surface = planar stratification
Higher speeds = flat bed»_space;> low relief, undulating mounds (ANTIDUNES) - rarely preserved as reworked
Types of cross-stratification
PLANAR
- 2D bedforms (straight crests)
TROUGH
- 3D bedforms (curved/sinuous crests)
Flaser bedding =
Alternating periods of moving and slack water e.g. tide-dominated marine settings
= alternating rippled sand and mud layers
Wave ripple development
Produced by oscillatory motion of waves in shallow marine/intertidal settings
Storms/large storm waves = large-scale undulatory bedroom; HUMMOCKY CROSS STRATIFICATION
- circular to elliptical hummocks and swales
- low height (10s of cams)
- long wavelengths (1-5cm)
Wind ripples vs current ripples
Better sorting
Rounded grains
Straight crested
Asymmetric
EROSIONAL STRUCTURES
Bed surface:
- rain drops
- prods
- downcutting
Bed base:
- sole marks
- flute casts
- grooves
Flute casts =
Heel shape with bulbous upstream end that flares downstream and merges with bedding
Localised erosion of sand-laden currents passing over cohesive muds
Common in submarine environments (sediment laden flows common)
BIOGENIC STRUCTURES
= action of plants/animals
Bioturbation
- irregular disruption of sediment
Discrete organised markings
- trace fossils/ichnofacies
- MORE RELIABLE INDICATOR OF ANCIENT SEDIMENTARY ENVIRONMENTS THAN BODY FOSSILS AS PRESERVED IN SITU
Direct biogenic growth structures
- stromalolites
INTERNAL STRUCTURES
Graded bedding = grain size distribution in a regular fashion
Normal/inverse
Inverse is less common and occurs in debris flows/aeolian ripples
POST DEPOSITIONAL STRUCTURES
Seismically active/sudden addition of sediment = instabilities and internal deformation…
- Convoluted bedding = tectonic forces cause unstable wet sediment to move; beds above and below not folded; = “seismite bed”
- Flame structure = wet mud sediment and rapid sedimentation on top = compress and break into overlying sediment for fluid to escape; always point UP
Also desiccation = evaporation of water/drying up = shrinkage and cracks develop in polygon shapes
CHEMOGENIC STRUCTURES
A type of post depositional structure
Concretions = secondary diagenetic features found as globular mass which stand out from the outcrop and are poorly cemented, formed by the precipitation of mineral cement between particle spaces
Septarian nodules = concretions containing cracks
- mystery
Source area =
Locality where rocks have been eroded to provide the sediment
How can sediment composition indicate source type?
Conglomerate - pebbles of certain substances indicate that these were present in source area
Arkose - likely from granitic-rich area
Well rounded quartz - erosion of pre-existing sandstones
- quartz resistant to erosion to more rounded = more cycles of erosion/transportation/deposition
Types of depositional environments
CONTINENTAL
- glacial
- alluvial
- aeolian
- lacustrine
- fluvial
MARGINAL MARINE
- deltas
MARINE
- continental shelves
- reefs
- deep sea
Glacial environments
= ice transports and deposits sediment, shapes the landscape as rocks and sediment carried by the glacier smooths and polishes the underlying rock OR large boulders cause STRIATIONS
Ice melts and deposits TILL = poorly sorted and undifferentiated material
Presently 10% of land surface is covered with glacial ice
Earth has been glaciated to different degrees at different times
Alluvial environments
Alluvial fan = broad, fan/cone-haped sedimentary deposit when streams emerge from a point source of higher topographic relief
- gradient decreases = drops coarse
- reduces channel capacity = changes direction
2-15km radius
Poorly sorted with coarsest sediments closest to mountain front (proximal) Finest further (distal)
Ancient alluvial debris flow - characteristics
Poorly sorted
Angular
Often matrix supported
Conglomerates
Breccia
Sandstones
(Less) mudstones
Desert =
Area of intense aridity less than 250mm rain per year and vegetation less than 15% of surface
Aeolian environment
Desert
Well rounded grains and frosted surface produced by frequent grain collisions during wind movement
Red due to iron oxide coatings
Large scale cross bedding
Absence of finger-grained sediment
Highly porous = :) water/hydrocarbon reservoirs
Ventifact =
Pebble whose surface has been polished/etched/grooved/faceted by wind-driven sand in arid environments
Can be used to determine paleo-wind directions
Lacustrine environment
Can be fresh/saline, shallow/deep
Coarse sediment near margins
Fine towards lake centre - finely laminated mudstones/varves
Ephemeral lakes = semi arid climate; evaporitic sediments; mud cracks; fossil fish
Varve =
Shows annual cycle of deposition, layer of sandstone/mudstone
Fluvial environment
Controls sediment supply to all others!
Range of grain sizes; conglomerate/sandstone/mudstone
Can have meandering/braided/straight channels
Braided river characteristics
Multi-thread channels High energy Steep valley gradients <0.5% Large/variable discharges Non cohesive banks
Meandering stream characteristics
Single channel High sinuosity Migrate Point bar deposition - FINING UP POINT BARS
Fining up point bars/point bar succession
In a meander a helical overturn flow develops
Causes coarse material at bottom and finer at top
= finding up sequence in lateral accretion depositions
Sea level and continental shelf relationship
Low sea level = wide continental shelf
High sea level = diminishing continental shelf
Delta =
Shoreline proturbance formed at the point where a river flows into an ocean basin or large standing body of water
Transports sediment laden water from a channel to an unconfined environment (unique)
Deltaic environment
Delta top/plain = where river meets ocean
- sub environments e.g. distributary channels, floodplains, swamps, lakes
- sandstones/mudstones/coals with abundant plant material preserved
Delta front = where sediment carried by distributary channels is deposited
- sands with cross bedding/ripples/bioturbation
- region of rapid deposition = seaward migration
Prodelta = farther offshore
- organic-rich, laminated and bioturbated mudstones
Sands of mouth bar/delta front migrate over finer sediments of pro delta = thick (20-40m) coarsening upwards units
Continental shelf environment
Continental shelf = staging post between erosion of the continents and final deposition in deep ocean
Water depth varies 5-500m
Complex mix of tides/waves/currents
Widespread sands/muds/CARBONATES e.g. oolites in agitated waters
Carbonate shelves - requirements
Warm temperature >15 degrees C
Low pressure
Agitation
Absence of silt/clay
Autochtonous =
Forms where deposited
Allochthonous =
Material exported to adjacent environemnts
Ooid =
Spherical grain with a nucleus (a mineral grain or biogenic fragement) and mineral cortex accreted around it <2mm in diameter
Reef environment
Reef = Intergrown organismal skeletons and sediment
- Fringing reef
- Smaller patch reefs in open lagoons
- Atoll reefs in open ocean on submerged volcanic islands
- Barrier reef
FOREREEF
FRAMEWORK AND CREST
BACKREEF
Components: Coral (10%) Carbonate mud (micrite) Skeletal elements Cement
FOREREEF =
Steep/talus slope
Agitated waters where waves are breaking
REEF FRAMEWORK AND CREST =
Main site of coral growth
Highest range of organisms
Highest wave energy
BACKREEF =
Sheltered from wave energy by reef front
Low diversity
Likely to be hyper saline
Deep sea environments
Greater than 500m
TURBIDITY CURRENTS
Further offshore dominated by pelagic sediments: Lithogenous Biogenous Hydrogenous Cosmogenous
Turbidity currents =
Main agent for transporting shallow water sediment to deep waters
High density, sediment laden fluids
Submarine canyon =
Turbidity currents trigger flow and erodes surfaces forming a canyon with a fan at the bottom
Bouma sequence
SANDS/LARGER GRAINS
- slow energy drop = graded
PARALLEL LAMINATED SANDS
- upper flow regime
- traction = flute casts
- CONVOLUTE LAMINATIONS AND FLAME STRUCTURES/SEISMITE BEDS
CROSS LAMINATED SANDS
- lower flow regime
- enough energy for saltation
PARALLEL LAMINATED SILTS
- slight current
MUD OFTEN BIOTURBATED
- suspension settling with no current
Lithogenous pelagic sediments
Terrigenous muds
RED CLAY
GREY MUD
From rivers and deserts
Biogenous pelagic sediments
Calcareous oozes
- coccolithophore
- foraminifers
- “periplatform ooze”
Siliceous oozes
- radiolarians
- diatoms
N.B. Unlike carbonate, surface waters not supersaturated wrt silica so dissolution occurs more rapidly
Fecal pellets bring down faster than dissolution so they survive
Cosmogenous pelagic sediments
Cosmic dust found in red clay
Common in South Pacific
Iron nickel and magnetite
50-200microns diameter
~300x10^3 tonnes fall on earth’s surface each year
Hydrogenous pelagic sediments
Formed directly from seawater in the pelagic zone (an oxygen environment)
Ion exchange and precipitation
E.g. ferromanganese nodules
Determining the environment e.g. ripples/cross bedded
Organism lived in moving water
Sediment moved by currents
Determining the environment e.g. broken shelves
High energy/storm conditions with waves pounding on beach
Determining the environment e.g. absence of corals
Murky water with high % suspended particles and low levels of light
Determining the environment e.g. organic material well preserved
Low levels of oxygen
Organism unlikely to have lived in these conditions but well preserved here
Types of sediments
Clastic
Chemogenic
Biogenic
Organic
Volcaniclastic
