Sedimentology and sedimentary processes

• 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

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)

(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)

• 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

• 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

http://www.eos.ubc.ca/courses/eosc221/sed/sili/siligsize.html#size

Grain Size Distribution

Folk (1980)

COMPOSITION

• Gravel

• Sand

• Mud

– form is determined by:

• Grain shape (Bustin, 1995)

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

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

• 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

• 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

• 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

• 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

• 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 (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

• 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

• 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

• 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