SEDIMENTATION and SEDIMENTARY ROCKS - FAU

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Sedimentation and
Sedimentary Rocks
GLY 2010 - Summer 2012 - 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
6
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
7
Transport of Sediment
By rivers
By glaciers
8
Formation of Detrital Rock
• Deposition
• Sorting
• Shape
9
Clastic
Rock
and
Matrix
10
Lithification
• Literally means creation of stone
• Involves three possible processes
 Compaction
 Cementation
 Recrystallization
11
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
15
Graded
Bedding
16
Graded Bedding Animation
17
Cross Bedding Animation
18
Cross-bedding,
Zion National
Park
Photos: Duncan Heron
19
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
20
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
21
Aeolian Ripple Marks
• Aeolian (wind) ripples at White Sands, New Mexico
(Photo Yamato Sato)
22
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
23
Mudcracks
24
Mudcracks
Mudcracks form by
desiccation of mud
or clay
25
Raindrop Impressions
26
Salt Crystals
• If the deposition
occurs in the ocean,
and the water is
quite saline, salt
crystals may
precipitate and
settle on the
sediment
27
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
28
Sediment Types
• Mud is composed of clay or silt
• Sand is composed exclusively of
sand sized particles
• Gravel includes granules, pebbles,
cobbles, and boulders
29
Detrital Rock Types
30
Shale
31
Siltstone
32
Sandstone with Silica Cement
33
Sandstone with Hematite Cement
34
Conglomerate
35
Quartz Pebble Conglomerate
36
Breccia
37
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
38
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)
39
Chemical Precipitation
• Solutions which are saturated, or slightly
supersaturated, may spontaneously form
crystals, which settle in the solution
40
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
41
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
42
Bonneville Salt Flats
43
Layered Gypsum
44
Evaporite Nodules
45
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
47
Oolitic Limestone
48
Biochemical Sedimentary Rock
• Coral reefs
• Coquina
49
Ancient Marine Reef
• El Capitan reef, Guadalupe
Mountains National Park,
formed as part of a very
large reef complex during
the Permian period
50
Florida Coquina
• Castillo de San
Marcos, St.
Augustine, Fl
• Anastasia Formation
coquina, used to
construct the fort
51
Organic Sedimentary Rocks
• Coal always forms from fresh-water
environments
• Petroleum, liquid organic mater, is typically
formed in salt-water environments
52
Peat to
Lignite
53
Bituminous
Coal
54
Anthracite
• Anthracite is actually a metamorphic rock
55
Organic chert
• Chert is a
form of
silica,
SiO2
56
Fossiliferous chert
57
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
59
Sandstone and Shale
Western
California
• Interbedded layers of sandstone and shale
60
Turbulent Stream Channel
• Only coarse particles
are deposited
• Fines move further
downstream before
deposition
61
Coastal Sediments
• Shallow marine
environments
consist of:
 Detrital material
 Carbonate-rich
deposits
 Abundant fossils
of plants and
animals, because
light penetrates
shallow water
62
Coastal Sediments
• In a marine environment, different types of sediment are
63
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
64
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
65
Unraveling Sedimentary Sequences
• Large exposures of sedimentary
rocks often contain rocks with more
than one depositional environment
66
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
70
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