SEDIMENTS2003a

8/7/2019 SEDIMENTS2003a

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SEDIMENTS

A collection of transported fragmentsor precipitated materials that

accumulate, typically in loose layersas of sand or mud or Sediment iscomposed of loose particles that arenot stuck together. This is thematerial that is deposited and later ismade into sedimentary rock. In fieldconditions sediments will destroy inyour hands or in the water.


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Types of sediments:

Detrital sediments: is composed of solid

fragments or detritus of preexisting rocks.

Chemical sediments: forms from previously

dissolved minerals that have either

precipitated from solution in water or been

extracted from water by living organisms

shells, skeleton or organic substance aredeposited when the organism die.


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Sedimentary Media

Sediments are produced by weathering. A number ofmedia can erode/remove, transport, and deposit

sediments. The different means by whichsediments can be eroded, transported, and deposited

include:

- runoff by rivers and streams- wind- marine/ocean currents and waves

- glaciers- land sliding


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Environments of sedimentation


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Upland Streams: Mountainous streams that have high flow.Sediments are dominated

by large/coarse sizes; so the rocks thatcommonly form include breccias and

conglomerates. Because flow is so rapid, mostsedimentary structures do not form

very well. There may be very poorly developedbedding/layering, but other features

such as cross-bedding, ripple marks, etc. are notlikely to be present. Colors usually

contain some rusty color because of theoxidation of Iron.


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Alluvial Fans: Deposits can be dumped along the baseof mountain ranges where valleys are present. The fan-shaped deposits are known as alluvial fans. Thematerials that are present are large/coarse grained

cobbles, gravels, and boulders; so breccias andconglomerates are common sediments in alluvial fans.Like mountainous streams, some poorly developedbedding is present, but other sedimentary

structures are usually absent. Colors usually containsome rusty color because of the oxidation of Iron.


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River channel environments:- sediments dominated by sands; so sandstones

are the common rock type- bedding may be very thick = massive but

bedding can range down to layersthat are only a few inches thick

- sedimentary structures present include smallto medium-sized ripple marks and

cross-beds; colors usually contain somerusty color because of the oxidation

of Iron; fossil casts of tree trunks, roots, andbark may also be found.


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Floodplain environments:- sediments dominated by silts and

clays; so siltstones & shales- sedimentary structures present

include well developed layering, ripplemarks, mudcracks, fossil plant

materials usually in the form of carbonizedremains or impressions; colors

usually range from rusty red to black dependingupon the amount of carbon inplant remains


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Freshwater Lakes: Lake deposits are much like floodplain deposits because lakesare

dominated by very fine/small sediments such as clay. Usually lakes arequite still

so the layering is very very thin and continuous and not marked byripple marks ormany other sedimentary structures. Plant fossils that settle into lakes

are oftenwhole or complete and very very well preserved. Seasonal die-off of

algae coatthe bottom with carbon residues. During spring and summer months

there is littledie-off; so not much carbon residue is added to the sediments

accumulating at thebottom of the lake; therefore alternating dark and light layers develop -

- very thin,but alternating light and dark. Natural fresh water lakes are associated

withfloodplains and with glaciated landscapes (in this class glaciated

terranes and rocksare rare in the record; so if a rock is interpreted as being from a lake, it

is most likelyfrom a lake associated with lowlands and rivers.


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Desert Lakes: Contain thick salt deposits; from time to timesediment may wash into

a desert basin and separate salt deposits from eachother. The sediments are

likely to have Iron which stains the sediments and often

times the salts a rustycolor.

Desert Windblown Dunes: Dunes are dominated by sand. Massive= large scale cross

beds and massive ripples develop from the migration ofdunes as the wind blows.

Again, often times ancient dune deposits are stained arusty color.

Glacial Deposits: not common as sedimentary rocks


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Deltas: Deltas are deposits along coastlines

that have been made by streams entering the

oceans. The Mississippi Delta is a classic

example. Because flow of the rivers that are

forming the delta fluctuate with years and

seasons.


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Beaches/Nearshore Settings: Beaches andnearshore shallow marine environments: Thewater in active so the finer/smaller materials arewashed offshore leaving sand-sized grains along

the shoreline Nearshore Settings: Some nearshore areas are

protected by sandbars or headlands; so they arequiet environments. Small/fine grained materials

deposit. These quiet nearshore environments areusually "muddy", and only a few organisms cantolerate the muddy conditions.


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Deep marine: Conditions are quiet; so verysmall/fine grained materials deposit.Thematerials may be clays or small powder-sizedpulverized shell fragments; so rocks aredominated by shales or fine-grained limestonescalled micrites. The water may be so deep thereare not fossils. The setting is still so the only

features are usually very very well developedlayers. If fossils are present, they are likely to bewhole.


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Reefs: Reefs are organic buildup in the marine

setting. Most have been formed by the

intergrowth of corals, sponges, algae and

other animals such as worms and clams. If the rock is solid fossils that are whole and

have corals, then the chances are, is that the

rock formed in an ancient reef.


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Continental sedimentary rocks

Shale

43%

Carbonates16%

Silica

2%

Evaporate

1%

Volcanic

14%Sand and Sandstone

24%


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Oceanic sedimentary rocks

Sand andSandstone

20%

Shale37%

Carbonates32%

Silica7%

Evaporate1%

Volcanic3%


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Abundance of sedimentary rocks


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SEDIMENTS2003a

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