sediments and sedimentary rocks - science at...
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Sediments and sedimentary rocks
Assembled rocks
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1. Precipitants and water-lain
fragments, low T and P,
Sedimentary.
2. Re-equilibrated materials, wide
range of T and P, Metamorphic.
3. Melted materials, high T,
Igneous. Me
ch
an
ica
l
Th
erm
alAssembling minerals
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Surface (map or plan view) - cover much of the earth.
Depth (cross section) - thin veneer.
Easy to observe, and contain economic materials (including
fossil fuels).
Abundance
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New York Bedrock
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Extensive Paleozoic sediments
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Map modified by T. Wayne Furr, after Branson and Johnson; WWW version by Jim Anderson.)
Thick sediments
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Fragments of pre-existing rocks. The fragments
are produced by weathering and erosion.
Weathering - mechanical and chemical breakdown
of rocks and minerals.
Erosion - fragments are moved away from source
(downhill).
May operate together or separately.
Clastic
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Physical: hardness, fracture, cleavage
Chemical: Resistance of bonds to chemical
attack
Si-O bonds very strong. Increased
polymerization means more resistant.
Tectosilicates are among the more resistant
components. Solubility of silica in water at STP
aids stability.
Weathering
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Fast paths for water-rock interactions
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Corner areas
Elephant rocks, Saint François Mountains, MO. Tor type
weathering of widely-spaced jointed granites.
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Variable resistance
Resistant sandstone remnant on shale, Green River, WY.
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Weathering
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Press and Sevier, 1986
Weathering
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Frost wedging
Water ice is more
voluminous than
liquid water.
Trapped water that
freezes splits the
rock. Or in the
case of Troy in
winter, the roads.
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34
.7 m
23
.3 m
27
.5 m
20
.5 m
Mount Scott Granite Oxides
Diffusive alteration
Pristine igneous
rock oxides
become more
oxidized near
exposed surface.
Note alteration at
27.5m along
fractures in grains.
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Dissolution
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If a mineral is abundant in the crust
and resistant to chemical attack, it
is likely to be a major constituent of
clastic sedimentary rock.
Good clastic materials
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Small and less dense
Phyllosilicates (cleavage)
These can be transported with lower
amounts of energy
Fast versus slow moving streams
Wind (loess)
Clay minerals
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Chemical and mechanical breakdown of rocks results in
particles of increasingly smaller size.
Earth scientists have formal names for size ranges
Cobble > 10 mm
Gravel 1 mm – 10 mm
Size matters
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0.001
0.01
0.1
1
10
100
1000
10000
Bould
er
Cobble
Peb
ble
Gra
nule
V. c
oarse
san
d
Coar
se s
and
Med
ium
san
d
Fine
sand
V. f
ine
sand
Coar
se s
ilt
Med
ium
silt
Fine
silt
V. f
ine
silt
Cla
y
Gra
in s
ize (
mm
)
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Press and Sevier, 1986
Gravity driven
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Clastic particles
transported by water
movement
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Stream deposits
Unconsolidated sediments
reveal the clastic
processes at work in cut
bank adjacent to a small
stream. Channel
movement layers
conglomertic sediments on
top of bank sands. Sands
at top reworked by wind.
Big Bend NP, TX.
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Streams over time
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Peter Mozley,
NM Tech Website
Wind and water
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Stratigraphic layering
The sediments are always products of
surface environments. These are a
snapshot into Earth’s past.
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Dunes
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Large particles of eroded rock, typically embedded in
finer particles (typically silicate)
Origin: High energy fluid transport
James Madison Univ. Sedimentology
Conglomerate
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Sand sized particles (typically quartz, feldspar, or rock
fragments – typically silicate) from eroded rock
Origin: Moderate energy fluid transport
Sandstone
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Silt sized particles (typically quartz and feldspar
framework silicate) from eroded rock
Origin: Low energy fluid transport
James Madison Univ. Sedimentology
Siltstone
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Silt sized particles (typically clay – sheet silicate) from
eroded and highly weathered rock
Origin: Low energy fluid transport
James Madison Univ. Sedimentology
Q: What is the difference between
siltstone and shale?
Shale
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Press and Sevier, 1986
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(Compaction, Limited re-equilibration, and Cementation)
Continuing deposition, sediments buried (increase in P < 5
kbar and T < 100 oC).
Sediments are frequently porous (grainsize dependent), lots
of fluids.
Sedimentary minerals grow
Minerals grow between grains
Diagenesis
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10% GROWTH
Pores
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Preservation
Modern desiccation
cracks in mud
300 Ma desiccation
cracks in shale
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Gastropoda and Pelecypoda,Cretaceous, New Jersey
Trilobite Ameura ,
Upper Pennsylvanian,
Kansas
KGS Website
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Corals
Corals are great limestone
builders. They put a lot of time
and energy into building
skeletons made from
aragonite
The coral polyp only inhabits the
outer 1-2 cm of the structure.
The rest is just a framework to
increase reproduction and
feeding
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Coccoliths
Tiny plankton with
calcareous shells.
Abundant enough to live
and die to make massive
limestone deposits like
Santa Elena Canyon, TX
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Carbonate producing organisms die, but their
shells will only be preserved if they are deposited
above 4 km depth
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Ice cores trap gas and dust. Temperature from
concentration of deuterium in ice.
375 Current
Atmosphere
CO2
Vostok, Antarctica Ice Core
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Feely et al., 2001
Increased atmospheric CO2 means big changes in ocean
chemistry. This could be detrimental to much of marine
life.
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Feely et al., 2001
Carbon isotopes show
the location of industrial
CO2 in the worlds
largest oceans. As we
know ocean mixing is a
slow process - the most
recent excess CO2
remains in the first 1 km
of the ocean.
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By now you know that minerals can be recorders of the processes that
create and sustain them.
Carbonates - some powerful information
•Trace element incorporation (divalent cations, such as Sr in calcite)
•Isotopes of C and O (13C and 18O)
•These are influenced by temperature (directly or indirectly)
Because carbonate materials are incorporated into marine organisms,
carbonate minerals may record ocean climate history.
Details
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18O is a stable isotope
influenced by global ice
content.
!
"18 =
18O16O
#
$ %
&
' ( sample
)18O16O
#
$ %
&
' ( s tandard
18O16O
#
$ %
&
' ( s tandard
*
+
, , , ,
-
.
/ / / /
01000
Standard for carbonates is a
belemenite fossil from the K
Peedee formation, SC
18O/16O = 0.0020672
H2O16 vs. H2O
18
For a volume of water, more 16O
water evaporates relative than
water with 18O.
Evaporated water from near the
equator is eventually transported
toward the poles through repeated
evaporation and precipitation.
The 18O/16O ratio will be lower in the
snow that falls on a glacier than it is
in the ocean from which the water
evaporated.
If global ice volume increase, the18O values of seawater become
larger as more 16O stored is locked
away in ice.
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Analyzing the 18O/16O ratio of
a dated carbonate mineral
from a fossil or marine
precipitate gives you an
estimate of global ice - a
proxy for two parameters
1.Global climate trends
2.Sea level
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Carbonate originsCarbonate is abundant at the earth’s surface, but may be
produced deeper in the earth where the activity of CO2 is
elevated.
Igneous: Carbonatites - rare magmatic systems that
produce carbonate materials
Hydrothermal: Alteration of mantle rocks and dissolution
in aqueous fluids
Ground/surface water: Chemogenic precipitation, karst
Metamorphic: Recrystallization and metasomatism
Sedimentary: Biogenic and reworking of biogenic
materials, clastic continental materials
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Critters make minerals
Aragonite and calcite
In general the oceans are nearly saturated in CaCO3
Ca2+ + HCO3- = CaCO3 + H+
Temperature is important (latitude, depth)
Biogenic Sediments
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Diagenesis
In sedimentary rocks,
the individual particles
need to be cemented
together. Carbonate
minerals precipitate out
of water to cement
grains.
Thin section under XP
shows calcite
overgrowth on micrite
grains.
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0.3mm©Ryan Hanson
Fragmented or whole aquatic invertebrate hard parts
Origin: Bodies of water with suitable environments
Limestone
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Fossiliferous limestone - pore space colored blue
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Oolitic limestone
Ooids form through growth and/or
accumulation in dynamic carbonate
environments.
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Gypsum CaSO4 2H2O
Anhydrite CaSO4
Barite BaSO4
Epsomite MgSO4 7H2O
Image from mineral.galleries.com
Sulfates
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Geology!Volume 35, Number 4
Cave of Crystals in
the Naica mine,
Chihuahua, Mexico.
The giant faceted and
transparent single
crystals of gypsum
measure up to 11 m
in length. Garcia-Ruiz
et al. propose that
these crystals derived
from a self-feeding
mechanism driven by
a solution-mediated,
anhydrite-gypsum
phase transition.
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Image from Klein and Hurlbut, 1985
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Minerals precipitate due to
oversaturation of an evaporating fluid.
Some form in closed bodies of water,
with significant evaporation.
Gypsum, Anhydrite, Halite, Sylvite
Chemogenic Sediments
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Sea water - crystallization
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Sabkha
Arid near-marine
environments
may host
anhydrite and
gypsum deposits.
The mineral
precipitated is
largely a function
of proximity to
water
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Salt Flat
Shallow seas, lakes
in closed basins
where evaporation
outpaces input.
Shallow mesozoic seas
covered modern-day
Colorado, leaving thick
deposits.
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Lansing mine,
6 miles of
room and pillar
2,300 feet
below Lake
Cayuga.
300 Ma
deposit made
from a shallow
sea, now
buried deep.Daily Ithican Online
NY Salt
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Salt domes
Salt is less dense
and more fluid than
surrounding rocks.
May move upwards
as diapirs.
Classic examples
are found along
Gulf Coast -
Louann SaltAmerican Scientist, Sept.-Oct. 1991, p.426
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Gypsum expansion
Anhydrite
becomes
rehydrated -
forming
gypsum. The
expansion
produces
bowing of layers
in Triassic rocks
in Caprock
Canyon, Texas