SEDIMENTS

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SEDIMENTS
WINDOW TO THE PAST
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What is a sediment?
Bart, sediments are DIRT!
Actually, sediments are particles of rocks and
organisms that are eroded, transported, and
deposited by a fluid (wind or water).
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Marine sediments are deposited under the oceans
Note the snow or particles of organic matter
sinking to bottom.
Sloping muddy sediments in submarine canyon wall
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Some sediments are deposited at the beach as
mudflats
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or as sand.
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but most sediments are deposited in the
oceans near the continents.
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Rivers appear to be a major source of sediment.
WHY?
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Interaction of the atmosphere and hydrosphere
with the lithosphere results in erosion of the
lithosphere
Rain erodes and transports sediment to the ocean.
Energy for this process comes from gravity!
(Stuff moves downhill due to gravity)
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Which is easier to move, a boulder or a grain of
sand?
NO! NOT THE BOULDER!!
If the boulder is harder to move, then what
do you think gets left behind as the water slows
down?
In general, only the smallest particles make
it to the ocean (sand, mud)
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Another way to think about this is how long
does it take a certain grain size to fall out of
the water? This is the settling rate.
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Sediments are classified by composition
(what makes them) and by grain size (how big are
the particles)
Grain size is used as a measure of the energy of
the water depositing the sediment, and often
indicates distance from source.
Example, beaches are energetic places (waves!)
so sands get deposited but silts and clays remain
in the water (THINK what comes out of your swim
suit when you shower after a visit to the beach?)
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Composition is simple!
Continental rocks contain other minerals and
quartz.
weathering, erosion, transport
sand and silt sized particles of quartz
clay minerals
(aka silica, SiO2)
note that there are some special
compositions we will talk about later that are
important indicators of paleoenvironment (paleo
past)
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Terrigenous sediments (derived from erosion of
the terra, the Earth) are the most abundant by
volume in the ocean (gt87 of volume, 45 of ocean
floor).
That is why sediments are thickest near the
continents (and consist mostly of silt and sand.)
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While sediments are not as thick in the
deep ocean, there still ARE sediments there (13
by volume, 55 of ocean floor). Where did these
come from?
1. Some clays (terrigenous sediments) - very
long settling times.
2. Primarily biogenous sediments (sediments
formed from remnants of organisms)
Shells get broken up at beach and tiny fragments
may get transported to the deep ocean
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Biogenous sediments (cont)
or remains of organisms that live on the deep
ocean floor (brittle stars here)
5 cm
0.02 cm
0.0002 cm
but most come from the shells of microscopic
plankton that drift in the upper levels of the
open ocean and die.
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Plankton tests (another name for a
microscopic shell) are made of either silica
(SiO2) or calcite (CaCO3)
an ooze is a really goopy sediment composed
of at least 30 biogenous material
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Calcium carbonate compensation depth (CCD)
Calcareous ooze is NOT found everywhere
because deep seawater contains more carbon
dioxide (CO2) which makes the water slightly
acidic. Below the CCD, calcium carbonate
dissolves.
Do you think that the presence or absence of
calcareous ooze can be used as a measure of
water depth?
YES!
If most of the deep ocean floor is above 4500 m
depth, what do you think is the most common
sediment there?
calcareous ooze
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Siliceous ooze is not found everywhere either!
In polar regions, the CCD is shallower AND
siliceous plankton productivity is greater in
the oceans where upwelling brings nutrient-rich
water to the surface
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Hydrogenous sediments (produced by water)
are precipitated directly from seawater (lt1 by
volume).
Precipitation is a chemical process involving the
joining of ions in solution to form a
molecule that becomes a solid particle.
Example Ca2 CO3-2 --gt CaCO3
(calcite)
(Note this is also the reaction use by plankton
and other animals to form their shells)
Other examples manganese nodules, phosphorite
nodules
Hint this is a very important reaction so it
might appear on a test!
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Cosmogenous sediments - interplanetary dust
and micrometeorites (lt1 by volume)
Microtektites - tiny glassy particles resulting
from melting during a meteor impact.
RARE, but VERY important - evidence of big
meteorite collisions with Earth.
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Contin
Summary
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Turbidites
Debris flows off continental shelf into deep
water
Monterey Bay, CA
Example of turbidites from past times
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Demonstration
1. What colors did I add (what order? List first
to last)
2. List the colors you see from bottom to top.
3. What do you notice about the lists in 1. and
2?
4. Can you come up with a principle that
relates the age of deposition to the location in
a cross section?
They should be the same.
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Principle of Superposition
In a normally oriented section of sedimentary
rocks, the oldest rock is on the bottom and the
youngest one is on the top.
DUH!
Principle of Uniformitarianism
Processes in the past behaved like the
processes we see today (Laws of nature do not
change the present is the key to the past.)
With these two principles, we can reconstruct
the past history of the ocean (and the Earth).
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Combination of these two principles with
observations of modern day processes in nature or
in the laboratory allow us to conclude that these
sediments were deposited by turbidity currents.
1. Similar particle sizes. 2. Similar
layering. 3. Similar particle composition.
This does not PROVE that these sediments
were deposited by turbitity currents it only
allows us to conclude that it is the MOST
REASONABLE explanation.
Things are only disproved in science.
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Armed with these principles, let us examine
some special kinds of sediments that form in
specialized environments
Super- position
Uniformitarianism
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Past climates
Greenland
Maine
Deserts- sand dunes, dry lake deposits
(or shallow seas)
Tropical climates-reefs, coal
Glacial deposits
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Dry lake deposits
As seawater evaporates, the dissolved
minerals get more concentrated and begin
precipitating.
First, calcium carbonate, then calcium sulfate.
(limestone)
(gypsum--gt wallboard)
Potassium chloride, sodium chloride next.
(salts)
Dead Sea, Israel
Badwater, Death Valley
Layers of evaporites in the geologic record
indicate hot, dry climates where water is drying
out.
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Example
Gypsum layers at the bottom of the
Mediterranean Sea indicate that at one point, the
sea was almost closed off from the Atlantic Ocean
and dried up!
(Salters, Florida State University)
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Example
Coral reefs need warm, shallow clear water
(formed from organisms that filter feed and
photosynthesize)
Reefs in the geologic record indicate warm,
shallow clear water at that time in that location!
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Example
The Monterey Formation in central California is
made of siliceous muds that contain plankton
shells.
This formation is a major reservoir for
offshore oil in California!
The rock is assumed to have formed on a
continental slope in water up to 2000 m deep.
(Oil from plankton!!)
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Geologists can determine sequences of events and
relative ages from
Cores
Cross Sections
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E
Which came first?
D
C
B
A
E (because it cuts across A-D)
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These get us ONLY relative ages (who came
first). How might we get absolute ages?
Some sediments show annual cycling (varves).
Simply count the dark and light bands.
Works OK in specialized depositional environs.
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Counting will not work in older sections.
First part is pretty easy. A--gtM--gtE--gtL--gtS
old ---gt
young
D cuts across all but S, so it is older than S
and younger than L.
H cuts across all but L, S, so it is older than L
and younger than E.
So, A--gtM--gtE--gtH--gtL--gtD--gtS
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L is therefore older than D and younger than H.
H and D are types of rocks that form from cooling
magma, and the magmas contain radioactive
elements like U, K.
Once the magma solidifies, the radioactive
elements are (mostly) locked in!
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Half lives 4.5 billion years 8.4 billion
years 5,700 years
Radioactive elements are unstable and
decay Uranium 238 --gt Lead 206 Potassium 40
--gt Argon 40 Carbon 14 --gt Nitrogen 14
The half-life is the amount of time for 1/2 of
the radioactive element to decay to its daughter
product.
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If we measure the amount of uranium remaining and
the amount of lead present, we can determine when
the magmas solidified to form H and D.
A--gtM--gtE--gtH--gtL--gtD--gtS
95 Ma 90 Ma
We can say that L is between 90-95 My (million
years) old.
Radioactive dating is the ONLY way to establish
absolute ages (years since present day to time of
formation) for rocks.
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A trick question What are sediments made of?
Particles of other rocks!!
Will radiometric dating of those particles
give us the age of formation of the sediment?
NO!!! It will give us only the age of the
formation of the rocks from which the particles
came.
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Absolute ages of sediments (such as in the
ocean) can come only from dating igneous rocks
(rocks from cooling magmas) that lie above and
below them.
However, volcanic ash falls to the bottom of
the ocean all the time.
We date these ash layers to get absolute ages for
sediments.
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We also can get the absolute ages of the basalt
forming the ocean floor.
Hint where does basalt come from?
WHY??
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Dating of sediments and basalts actually
provides evidence for plate tectonics
IMAGE OF DATES of seds/basalts
The age of the sediments lying directly on
basalt is the age of the basalt.
Because sediments are raining down
constantly, the basalt lacking the older
sediments did NOT exist at the time the older
sediment was deposited.
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Summary
1. Sediments contain a history of the oceans
but we must be able to date the sediments.
2. Relative ages (which is older and which is
younger) are established by comparing different
rocks and can be done with sediments.
3. Absolute ages (how many years before present
was the rock formed) can only be determined with
radiometric dating and this does not work on
sediments because these rocks are composed of
pieces of other rocks.
4. A range of absolute ages for sediments CAN be
determined if the relative age of the sediment
is bracketed by two rocks with absolute ages.
(Hint do you think you might have to use these
on a test?)
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Two lectures ago, I said that the ocean was 4 By
old. What principle did I use when I stated
that the sediments in the 4 By old rocks were
formed at the ocean bottom because they look
like sediments we see at the ocean bottom today?
Uniformitarianism
The present is the key to the past.
Can we prove this?
NO!
but we can certainly evaluate all
new observations in light of this assumption and
question it when needed.