Transcript Lecture 10- Sedimentation and Sedimentary Rocks

Sedimentation and
Sedimentary Rocks
GLY 2010 - Summer 2016 - Lecture 10
1
Sediment
• Unconsolidated material that accumulates
at the earth’s surface
• Minerals and organic remains of plants and
animals are the major components of soil
2
Sedimentary Rock
• As sediment accumulates, pressure,
and often temperature, increases
• Changes occur which convert the
sediment from unconsolidated form to
a consolidated form, sedimentary rock
• Sedimentary rock makes up 5% of the
crust of the earth, but accounts for 75%
of the rock exposed at the surface
3
Importance of Sedimentary Rock
• Provide clues to the earth’s past –
examples:
 Including erosion of mountain ranges
 Transgressions of the sea over the land
4
Fossils
• Often contain
fossils, which
provide clues to:
 Types of life
living in the past
 The environment
they lived in
5
Types of Sediment
• Detrital
• Chemical
• Biogenic
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Sediment Transport
• Water, glaciers, or wind moves the
sediment from high elevations to lower
elevations, where it may accumulate
• Annual transport of detrital sediments
to the oceans is about 10 billion tons
• During transportation, sediment size is
often reduced
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Transport of Sediment
By rivers
By glaciers
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Formation of Detrital Rock
• Deposition
• Sorting
• Shape
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Clastic
Rock
and
Matrix
10
Lithification
• Literally means creation of stone
• Involves three possible processes
 Compaction
 Cementation
 Recrystallization
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Compaction
• Fragments will be compacted by the
weight of accumulating sediment
• Air and water are expelled from spaces
between grains
12
Cementation
• Dissolved substances in water may
precipitate solids which act as cements
13
Recrystallization
• Unstable minerals may reorganize due
to heat, pressure, and fluid interaction
into more stable minerals
• Process must occur at low temperatures
 Ex: Aragonite  Calcite
14
Sedimentary Structures
• Bedding - sediments are ordinarily
deposited in horizontal units called beds
 Graded bedding
 Cross-bedding
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Graded
Bedding
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Graded Bedding Animation
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Cross Bedding Animation
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Cross-bedding,
Zion National
Park
Photos: Duncan Heron
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Principle of Original Horizontality
• Most sediments settle through bodies of
water
• They will be deposited in horizontal, or
very nearly horizontal, layers
• Beds which are not horizontal have often
had their position changed by postdepositional processes
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Right Side Up
• Beds may occasionally be completely
overturned, so we need ways to tell if beds are
right side up
• Selected indicators:




Ripple Marks
Mudcracks
Raindrop impressions
Salt crystals
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Aeolian Ripple Marks
• Aeolian (wind) ripples at White Sands, New Mexico
(Photo Yamato Sato)
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Fossilized Ripple Marks
• Parallel ripple marks
preserved on a slab of
sandstone
• The rocks contain the
fossilized remains of
marine animals - these
ripples were formed in
shallow sea water by
gentle currents
• Fossilized ripple marks. Capitol
Reef National Park, Utah
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Mudcracks
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Mudcracks
Mudcracks form by
desiccation of mud
or clay
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Raindrop Impressions
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Salt Crystals
• If the deposition
occurs in the ocean,
and the water is
quite saline, salt
crystals may
precipitate and
settle on the
sediment
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Size Range of Detrital Particles
•
•
•
•
•
•
•
Clay
Silt
Sand
Granule
Pebble
Cobble
Boulder
< 0.004 millimeters
0.004 to 0.063 millimeters
0.063 to 2 millimeters
2 to 4 millimeters
4 to 64 millimeters
64 to 256 millimeters
>256 millimeters
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Sediment Types
• Mud is composed of clay or silt
• Sand is composed exclusively of
sand sized particles
• Gravel includes granules, pebbles,
cobbles, and boulders
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Detrital Rock Types
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Shale
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Siltstone
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Sandstone with Silica Cement
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Sandstone with Hematite Cement
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Conglomerate
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Quartz Pebble Conglomerate
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Breccia
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Formation of Chemical
Sedimentary Rocks
• Precipitation is the process of converting
materials dissolved in a fluid (water or
air) to another form
• Water dissolved in the atmosphere may
precipitate as rain, or as some solid form
such as snow, sleet, hail, etc
• Solids dissolved in water precipitate as
solids
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Saturated Solution
• In order for precipitation to occur, the
fluid must be saturated
• A saturated solution holds as much as it
can of a particular substance
 A fluid may be saturated with respect to one
substance (e.g. lime) and undersaturated
with respect to another substance (e.g.
halite)
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Chemical Precipitation
• Solutions which are saturated, or slightly
supersaturated, may spontaneously form
crystals, which settle in the solution
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Biochemical Precipitation
• Some organisms
have the ability to
concentrate an
unsaturated solution
internally to the
point where
precipitation occurs
This fossiliferous limestone
contains carbonate shells,
produced by biochemical
precipitation
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Evaporites
• When saline solutions (sea-water) evaporate, a
series of substances precipitate in a definite
sequence, from the least to the most soluble
• Sequence is:




Lime – a carbonate
Gypsum, a sulfate, precipitates second
Halite, common table salt, is next
Potassium and magnesium salts are last
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Bonneville Salt Flats
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Layered Gypsum
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Evaporite Nodules
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Chemical
Sedimentary
Rocks
• Limestone is
composed of
calcium carbonate,
CaCO3
• Dolostone is
composed of
dolomite,
CaMg(CO3)2
46
Oolitic Limestone
• In warm, saturated, and highly
energetic environments, lime may
precipitate around tiny bits of
suspended matter
• They stick together to form an oolitic
limestone, such as the Miami oolite
formation
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Oolitic Limestone
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Biochemical Sedimentary Rock
• Coral reefs
• Coquina
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Ancient Marine Reef
• El Capitan reef, Guadalupe
Mountains National Park,
formed as part of a very
large reef complex during
the Permian period
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Florida Coquina
• Castillo de San
Marcos, St.
Augustine, Fl
• Anastasia Formation
coquina, used to
construct the fort
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Organic Sedimentary Rocks
• Coal always forms from fresh-water
environments
• Petroleum, liquid organic mater, is typically
formed in salt-water environments
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Peat to
Lignite
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Bituminous
Coal
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Anthracite
• Anthracite is actually a metamorphic rock
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Organic chert
• Chert is a
form of
silica,
SiO2
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Fossiliferous chert
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Sedimentary Environments
• Careful study of sedimentary rocks can often
reveal information about the type of
environment in which the sediment was
deposited
• Useful in studying the earth’s history
• May reveal information about past climate
conditions (paleoclimatology)
58
Continental
Deposits
• Lake
deposits
• River
deposits
• Glacial
sediments
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Sandstone and Shale
Western
California
• Interbedded layers of sandstone and shale
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Turbulent Stream Channel
• Only coarse particles
are deposited
• Fines move further
downstream before
deposition
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Coastal Sediments
• Shallow marine
environments
consist of:
 Detrital material
 Carbonate-rich
deposits
 Abundant fossils
of plants and
animals, because
light penetrates
shallow water
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Coastal Sediments
• In a marine environment, different types of sediment are
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associated with varying depths of ocean water
Shallow Marine Environments
• Wave-action pulverizes soft minerals and fossils
• Pulverized material swept out to sea
• Remaining material is well-sorted, rounded, sandsized deposits
• Most of these deposits will be durable minerals,
such as quartz
• Dominant rock in Florida is limestone, so we
have soft carbonate grains on our beaches as well
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Deep Marine Deposits
• Mainly remains of carbonate and
silica microorganisms which die and
settle to the sea-floor
• Submarine landslides may carry
material off the continental shelf, and
sub-marine volcanoes may contribute
• Landslide deposits are poorly sorted
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Unraveling Sedimentary Sequences
• Large exposures of sedimentary
rocks often contain rocks with more
than one depositional environment
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Geologic History
• Geologists study entire sequences of
rocks to attempt to unravel geologic
history
• Sedimentary rocks provide evidence for
ancient mountains, long since eroded
away
• These mountains are often evidence of
plate collisions
67
Marine Rocks in Mountains
• Many mountains contain sedimentary strata
deposited in marine environments
• The earth has not lost enough water so that these
mountains would have once been flooded
• Therefore, we believe that plate tectonic forces
have lifted these former sea-beds miles high!
68
Bokkeveld Shales
• Fossiliferous shales from the Groot River area,
South Africa
• Shales were deposited in a quiet marine
environment
69
Marine Fossils in Mountains
• Fossil: Stephanoceras
sp. with different shells
and one small snail
• Location: Sommerau,
Swiss Jura Mountains
• Geologic age:
Humphriesian-layer,
Bajocian, middle
Jurassic
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Marine Rocks in New Mexico Video
• Dr. Spencer Lucas, Curator of Paleontology at New Mexico Museum of Natural
History and Science, discusses marine fossils in Prehistoric Trackways National
Monument
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