Sedimentary%20rocks%20are%20derived%20from%20sediment%20and%20chemical%20precipitates%20that%20result%20from%20weathering%20and%20erosion%20processes.%20Sediment%20and%20chemical%20precipitates%20accumulate%20in%20sedimentary%20basins,%20such%20as%20lakes%20or%20oceans,%20and%20are%20converted%20to%20sedimentary%20rock

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Sedimentary rocks are derived from sediment and
chemical precipitates that result from weathering
and erosion processes. Sediment and chemical
precipitates accumulate in sedimentary basins,
such as lakes or oceans, and are converted to
sedimentary rock over time.
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Sediment and chemical precipitates can accumulate
in a variety of sedimentary environments.
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The type of sediment or chemical precipitate can
tell you a lot about the environment of
deposition. For example think about where coal
or limestone forms? What information can you
infer from these rocks.
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Sediment is converted to sedimentary rock
following three processes. 1. Accumulation of
sediment in a sedimentary basin, such as this
deltaic environment. 2. Burial and compaction of
sediment as it accumulates. 3. Cementation of
the sediment by cementing agents dissolved in
groundwater..
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• Cementing Agents
• Silica, 2. Fe-oxide
• Calcite, 4. Clays
• 5. Tar (organics)

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Sediment may composed of various grain sizes that
range from the smallest clays to boulder-sized
clasts. The grain size of sediment provides
information regarding the energy regime of
transport. In the case of these stream cobbles
we can infer that the stream has a relatively
high energy regime during certain times of the
year.
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This conglomerate formed from sediment deposited
in a river channel, similar to the previous
slide. The clast lithology provides information
regarding the eroded source rock and imbrication
tells you the direction of paleo-stream flow.
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Clastic sedimentary rocks are subdivided based on
grain size. Conglomerate, shown above, can have
grain sizes that range between gravels and
boulders.
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Sedimentary breccia has grain-sizes similar to
conglomerate, except the clasts are angular and
have not been subjected significant transport and
erosion. Can you think of a natural environment
of deposition where large clasts can be
transported without significant erosion or
abrasion. What does the degree of rounding or
angularity tell you about distance of transport?
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Sandstone is clastic sedimentary rock comprised
of cemented sand-sized clasts. The sandstone
pictured above is predominantly quartz clasts
cemented with silica. How could you determine
the cementing agent of given sandstone rock?
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What cementing agent holds the sand grains
together in the above sandstone? What is the
diagnostic property that helps you identify the
cementing agent?
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Mineralogically mature sandstones are
predominantly composed of quartz. Immature
sandstones (shown above) will contain minerals,
such as feldspar and micas, which have not been
weathered. What does the maturity of a sandstone
tell you about distance of transport?
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Siltstones form in low energy environments, such
as lake of marine basins. They tend to be
laminated.
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Shale or mudstone is deposited in the lowest
energy environments and is typical of marine
settings distal from the continental margin
(i.e., terrerstrial sediment source).
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Oolitic limestone forms from oscillation (by wave
action) of sand grains or shell fragments and the
precipitation of concentric rings of calcite
around the nucleating particle. The oolites are
cemented together by calcite.
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Most limestones are biogenic and derived from
calcite formed from biological processes, such as
this fossiliferous limestone shown above.
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Coal forms from terrestrial organics that
accumulate in sedimentary basins where organic
production exceeds organic decomposition. As
organics accumulate peat form and is later
converted to lignite and then coal with
additional compaction and pressure.
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Inorganic limestone precipitates in warm waters
because the solubility of calcite (calcium
carbonate) is directly proportional to CO2
content in water. Warm water typically holds
less CO2 than cold water, which causes the
solubility of calcite to decrease.
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Chert is composed of almost pure silica (SiO2)
that tends to precipitate in deep ocean
basins. Chert, like obsidian and quartz,
fractures conchoidally, which makes it an ideal
material to construct arrowheads and axe blades
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Mudcracks will form when shrink-swell clays dry
out during a period of dessication, such as an
ephemeral lake.
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Deltas form when rivers flow into a still body of
water (i.e., a lake or marine embayment) and
deposit sediment as the streams energy is
reduced. Deltas are subdivided into three
distinct layers 1. Topset, 2. Foreset beds, and
3. Bottomset beds.
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This post-glacial delta formed when melt water
streams flowed from the Cascades into a
proglacial lake dammed behind by the Puget Ice
Sheet. Can you see the contact between foreset
and topset beds?
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Cross-beds can also develop in wind-blown
sediment such as these barchan sand dunes. Sand
grains saltate (bounce) along the stoss-face and
then avalanche down the steep, lee face, where
wind velocities are reduced.
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The Navajo Formation in Arizona and Utah is
comprised of sandstone that formed from migrating
dune fields. Can you determine the paleo-wind
direction based on the cross-bedding shown in the
two images?
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Ripples can from from currents (wind and water),
such as those shown on the left image or from
wave oscillation, such as those shown on the
image on the right. Which of these ripples
patterns is comprised of asymmetric ripples and
why do think asymmetric ripples form versus
symmetric ripples?
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Graded beds form because large sediment grains
settle faster in a water column than smaller
sediment grain. In an event such as a submarine
landslide, a turbidity current may be created
with a chaotic mixture of sediment suspended in
the water. It will settle out onto the bottom of
the ocean floor and be graded with the largest
grains on the bottom of the turbidite layer.
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Multiple turbidity currents can result in
turbidite sequences that contain many units of
graded beds.
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This turbidite sequence was deposited in a marine
setting similar to the Puget Sound, which
occupied the Central Valley of California. The
turbidite sequence has been deformed by
compressive stresses and compised the Vaca
Mountains lying between the Central and Napa
Valleys.
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Horizontal layers of chalk deposits comprise the
white cliffs of Dover, England.
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Another important attribute of sedimentary rocks
is the presence of fossils, most commonly found
in fine-grained clastic sedimentary rocks or
biogenic sedimentary rocks.