Transcript Sedimentology and Sedimentary Processes

Sedimentology and
Sedimentary Processes
Virginia T. McLemore
Sedimentology
• "The scientific study of sedimentary rocks and of
the processes by which they were formed; the
description, classification, origin and
interpretation of sediments" (Glossary of
Geology, AGI, 1974)
• Study of modern sediments such as sand, mud
(silt),and clay
• Understanding the processes that deposit them
• Studies of ancient sedimentary rocks
http://www.geo.umass.edu/faculty/cooke/geo101/GeologicTime.htm
http://www.bbc.co.uk/schools/gcsebitesize/chemistry/changestoearthandatmosphere/0rocksrev5.shtml
http://csmres.jmu.edu/geollab/fichter/SedRx/SimpModl.html
Sedimentary rock types
• Clastic rocks
– particles derived from the weathering and erosion of
precursor rocks and consist primarily of fragmental
material
– classified by grain size and composition
• Carbonates
– precipitated by a variety of organic and inorganic
processes
• Evaporites
– evaporation of water at the Earth's surface
• Chemical sedimentary rocks (chert, jasperiod)
Principles
• Uniformitarianism, which states that the
sediments within ancient sedimentary rocks
were deposited in the same way as sediments
which are being deposited at the Earth's surface
today
• Principle of superposition Sedimentary layers
are deposited in a time sequence, with the
oldest on the bottom and the youngest on the
top.
• Principle of original horizontality sediments are
deposited at their angle of repose which, for
most types of sediment, is essentially horizontal
Principles
• Principle of lateral continuity states that
layers of sediment initially extend laterally
in all directions unless obstructed by a
physical object or topography
• Principle of cross-cutting relationships
states that whatever cuts across or
intrudes into the layers of strata is younger
than the layers of strata
http://en.wikipedia.org/wiki/Image:Taum_Sauk_precambrian-cambrian_unconformity.jpg
http://en.wikipedia.org/wiki/Image:Vallisvale.jpg
Methodology
• Measuring and describing the outcrop and
distribution of the rock unit
• Descriptions of rock core
• Sequence stratigraphy (Describes the
progression of rock units within a basin )
• Describing the lithology of the rock
• Analysing the geochemistry of the rock
Sediment characteristics
• Porosity is the volume of voids within a rock
which can contain liquids.
• Permeability is the ability of water or other
liquids (e.g. oil) to pass freely through a rock.
• Roundness refers to the roughness of the
surface of the sedimentary grain.
• Sorting refers to the range of particle sizes in a
sediment or sedimentary rock.
• Matrix is the fine-grained material (usually clays
or silt) that is deposited originally with the
coarser-grained material
http://www.eos.ubc.ca/courses/eosc221/sed/sili/silpet.html
Classification by GRAIN SIZE
• Gravel > 2mm
• Sand >1/16 mm < 2 mm
• Mud <1/16 mm
• Decide what the relative proportions of
each size
http://www.eos.ubc.ca/courses/eosc221/sed/sili/siligsize.html#size
Grain Size Distribution
Folk (1980)
COMPOSITION
• Gravel
• Sand
• Mud
TEXTURE
• Grain shape (attributes which refer to the external
morphology of particles)
– surface texture,
– roundness
– form. is determined by:
• Grain shape (Bustin, 1995)
– internal structure
– mineral cleavage
• characteristics of source rock such as jointing and
bedding
• lithology
• hardness
• fracture
• transport
COLOR
• Munsell color chart
Any other features?
• Is there anything outstanding about this
rock?
• Does it have any important minerals or
clasts?
• Anything worth making note of?
• Any important clasts?
• Are there any fossils?
• Any visible sedimentary structures (ie
cross bedding)?
Give the rock a NAME
• [colour][texture][cement][important
minerals or clasts] ROOT NAME
[with...]
http://www.eos.ubc.ca/resources/slidesets/clastic/clastic.html
http://darkwing.uoregon.edu/~millerm/depenvsmap.html
Sediment transported by
• Suspension load is when sediments are carried
in suspension (usually fine-grained sediments
that can be carried along easily by the flow)
• Bed load is when the forward force of the
moving current acts more directly on the larger
particles at the bottom as it pushes, rolls, and
slides them along
• Saltation is more complex and usually affects
sand-sized particles. Here, the particles are
sucked up by eddies into the flow, travel with the
flow for a while, and then fall back to the bottom
Sediment structures
• Asymmetrical ripples are ripples that
have a gentle slope upstream and a steep
slope downstream.
• Cross-bedding is inclined bedding and
commonly forms in alluvial environments.
• Potholes are rounded depressions
caused by swirling currents and eddies.
• Mud cracks are formed by evaporation on
mudflats or in shallow lakes.
Sedimentary environments
Alluvial Fans
• Alluvial fans are sedimentary deposits that
typically form at the margins of a dry basin.
• They typically contain coarse boulders and
gravels and are poorly sorted.
• Fine-grained sand and silt may be deposited
near the margin of the fan in the valley,
commonly in shallow lakes.
• These lakes may periodically dry, and evaporite
deposits may result.
http://www.uoregon.edu/~millerm/fan.html
Eolian
• Wind is an effective sorting agent and will
selectively transport sand.
• Gravel is left behind and dust-sized
particles are lifted high into the
atmosphere and transported great
distances.
• Windblown sand forms dunes that are
characterized by well-sorted grains
showing large-scale crossbedding.
http://www.uoregon.edu/~millerm/sanddunes.html
Glacial
• Glaciers do not effectively sort the
materials that they transport.
• Common type of resulting deposit is an
unstratified accumulation of boulders,
gravel, sand, and fine silt for which the
term "till" is usually applied.
Rivers
• Fluvial environments include braided and
meandering river and stream systems.
• River channels, bars, levees, and floodplains are
parts (or subenvironments) of the fluvial
environment.
• Channel deposits consist of coarse, rounded
gravel, and sand.
• Bars are made of sand or gravel.
• Levees are made of fine sand or silt.
• Floodplains are covered by silt and clay.
Flood Plains
• Rivers commonly meander across a flat
flood plain before reaching the sea and
depositing a considerable amount of
sediment.
• Rocks formed in a flood plain environment
are commonly lenses of "fluvial"
sandstone deposited in the meander
channel enclosed in a shale deposited on
the flood plain.
http://www.uoregon.edu/~millerm/braided.html
Lakes
• Lacustrine environments (or lakes) are
diverse; they may be large or small,
shallow or deep, and filled with
terrigenous, carbonate, or evaporitic
sediments.
• Fine sediment and organic matter settling
in some lakes produced laminated oil
shales.
http://www.gpc.edu/~pgore/geology/historical_lab/sedenvirons.htm
Delta
• Deltas are large accumulations of sediment that are
deposited where a river empties into a standing body of
water.
• They are one of the most significant environments of
sedimentation and include a number of subenvironments
such as stream channels, flood plain beaches, bars, and
tidal flats.
• The deposit as a whole consists of a thick accumulation
of sand, silt, and mud.
• Because of the abundance of vegetation in geologically
young deltaic environments, coals of various ranks
commonly are associated with these clastic sediments.
http://www.uoregon.edu/~millerm/delta.html
Swamps
• Swamps (Paludal environments)
Standing water with trees. Coal is
deposited.
http://www.gpc.edu/~pgore/geology/historical_lab/sedenvirons.htm
http://www.gpc.edu/~pgore/geology/historical_lab/sedenvirons.htm
Shoreline
• Beaches, bars, and spits commonly develop
along low coasts and partly enclose quiet-water
lagoons.
• Such sediments are well washed by wave action
and is typically clean, well-sorted quartz sand.
• Behind the bars and adjacent to the beaches,
tidal flats may occur where fine silt and mud are
deposited; evaporites may be present locally.
• Barrier islands
Shoreline
• Lagoons are bodies of water on the landward side of
barrier islands. They are protected from the pounding of
the ocean waves by the barrier islands, and contain finer
sediment than the beaches (usually silt and mud).
Lagoons are also present behind reefs, or in the center
of atolls.
• Tidal flats border lagoons. They are periodically flooded
and drained by tides (usually twice each day). Tidal flats
are areas of low relief, cut by meandering tidal channels.
Laminated or rippled clay, silt, and fine sand (either
terrigenous or carbonate) may be deposited. Intense
burrowing is common. Stromatolites may be present if
conditions are appropriate.
http://www.uoregon.edu/~millerm/beaches.html
• The continental shelf is the flooded edge of the
continent.
• The continental slope and continental rise
are located seaward of the continental shelf.
• The abyssal plain is the deep ocean floor.
Shallow Marine
• Shallow seas are widespread along continental margins
and were even more extensive during many periods of
the geologic past.
• Sediments deposited in these shallow marine waters
from extensive layers of well-sorted sand, shale,
limestone, and dolomite, that commonly occur in a cyclic
sequence as a result of shifting depositional
environments related to changes in sea level.
• When the rate of evaporation exceeds the rate of water
supply, chemicals dissolved in the water may be
concentrated and precipitated as beds of gypsum, halite,
and more complex salts.
http://www.uoregon.edu/~millerm/spits.html
Organic Reef
• An organic reef is a structure built of the shells and
secretions of marine organisms.
• The framework of geologically young reefs typically is
built by corals and algae, but the reef community
includes many types of organisms.
• A highly fossiliferous limestone commonly is the result of
these organisms in the rock record.
• Reworking of reef-derived sediments by wave and
biological activities commonly results in a complex group
of sedimentary facies that may be referred to as the reef
tract.
• Reefs are wave-resistant, mound-like structures made of
the calcareous skeletons of organisms such as corals
and certain types of algae.
http://www.uoregon.edu/~millerm/reefs.html
Deep Ocean
• The deep oceans contain a variety of sediment types. Adjacent to
the continents, a considerable amount of sediment is transported
from the continental margins by turbidity currents.
• As the current moves across the deep-ocean floor its velocity
gradually decreases, and sediment carried in suspension settles out.
• The resulting deposit is a widespread layer of sediment in which the
size of grains grade from coarse at the base to fine at the top.
• Such deep-sea deposits are characterized by sequences of graded
beds of these "turbidites".
• Distant to the continents, dust transported by eolian processes may
accumulate as muds.
• In sediment-starved parts of oceans away from the continents,
siliceous ooze formed of the tests of microorganisms called
radiolaria accumulate.
• These sediments form the radiolarian cherts of the rock record.
Facies and depositional environments
• The facies concept refers to the sum of characteristics of a
sedimentary unit, commonly at a fairly small (cm-m) scale
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Lithology
Grain size
Sedimentary structures
Color
Composition
Biogenic content
• Lithofacies (physical and chemical characteristics)
• Biofacies (macrofossil content)
• Ichnofacies (trace fossils)
Facies and depositional environments
• Facies analysis is the interpretation of strata in terms of
depositional environments (or depositional systems), commonly
based on a wide variety of observations
• Facies associations constitute several facies that occur in
combination, and typically represent one depositional
environment (note that very few individual facies are diagnostic
for one specific setting!)
• Facies successions (or facies sequences) are facies
associations with a characteristic vertical order
• Walther’s Law (1894) states that two different facies found
superimposed on one another and not separated by an
unconformity, must have been deposited adjacent to each other
at a given point in time
http://www.gpc.edu/~pgore/geology/historical_lab/environmentchart.htm