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Sedimentary Rocks
Weathering
Erosion
Transport
Deposition
Sediments
Lithification
http://www.teachnet-lab.org/ps101/bglasgold/rocks/EFCycleP2.gif
WeatheringThe sediments that make up sedimentary rocks are produced by:
Mechanical & Chemical Weathering
Mechanical Weathering
Types of mechanical aka physical
weathering
• unloading – rx expand when overburden lessened
• frost wedging – H2O expands 9% to ice
• insolation – solar heating � expansion
• salt crystals
• root wedging
• seismic deformation
Chemical Weathering
hydrolysis, oxidation,
dissolution, dehydration,
solutioning, bio-chemical
weathering
Mineral
Stability
big point: some
minerals are more
stable than others…
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Products of Weathering
• Lithic (Rock) Fragments(granite, basalt,schist, etc.)
• Dissolved Ions(Calcium, Potassium, Sodium, etc.)
• Rust Minerals (Hematite, Goertite, etc.)
• Clay Minerals(Bentonite, Montmorillonite, etc.)
• Residual Minerals(Quartz, Orthoclase, Muscovite, etc.)
Erosion & Transport
Water
Wind
Ice
Gravity
Common Depositional Environments Marine shoreline and near-shore
environments
Lithification
Sediment
Rock
Cementation
Typical Cements:
•Calcite
•Quartz
•Iron oxide
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Sedimentary Rocks
Identification
Note: Use many of the same terms to describe
sediments & sedimentary rocks.
Only difference is lithification!
Types of Sedimentary Rocks
Detrital or clastic Rocks Chemical Rocks
vs.
Difference: Texture
Detrital (Clastic Texture)
vs.
Chemical (Crystalline Texture)
Detrital Rock Composition
Detrital Rocks
• Lithic Fragments
• Quartz
• Clay Minerals
• Fossil Fragments
• Rust Minerals
• Orthoclase
• Muscovite / Biotite
Detrital Rock Texture
Clastic Texture
• Grain Size
• Grain Sorting
• Grain Rounding
• Sediment Maturity
Grain Size
•Gravel
2mm
•Sand
1/16 mm
•Mud - Silt
1/256 mm
•Mud - Clay
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Detrital Rock Names(Based Primarily on Grain Size)
Sand Sized: Sandstone
Gravel -Sized:
Conglomerate
Siltstone Shale
Mud-Sized:
Mudstone
Sorting
Gravel Sized Detrital Rocks(Subdivided Based on Grain Roundness)
Breccia
Conglomerate
Maturity
Mature
•Well Sorted
•Well Rounded
•All Quartz
Immature
•Poorly Sorted
•Angular grains
•Mixed Composition
including clay
Sandstones(Subdivided based
on maturity)
Quartz SandstoneArkose (Sandstone)
Lithic Sandstone Greywacke
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Sandstones Under a Scope
Detrital Sedimentary Rock ID
Chemical Rock Texture
Crystalline Texture
• Coarse
• Fine
Chemical Sedimentary Rocks(Names based primarily on composition)
Basic Rock Name
• Limestone
• Chert
• Rock Salt
• Rock Gypsum
• Peat or Coal
Composition
• Calcite
• Quartz (silica)
• Halite
• Gypsum
• “Carbon” / Plant
Remains
Chemical Rocks(Names based mainly on composition)
LimestonesCrystalline
Limestone
Chalk
Fossiliferous Limestone
Chemical Rocks (cont.)Evaporites
Rock
Gypsum
Rock Salt
Coal
Chert
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Chemical Sedimentary Rock ID
Sedimentary Rocks
Interpretations
What do Sedimentary Rocks Record?
•Source of sediment
•Erosion and Transport Agent
•Distance of Transport
•Depositional Environment
•Paleogeography/Tectonic Setting
Interpretation of Composition
Detrital Rocks:
• Source of sediment
• Exposure to Weathering
(Distance of Transport)
(Type of Climate)
Note About Detrital Rock
Color & Composition
• Grain SizeSmaller is often darker
• CompositionCarbon - black or brown
Quartz - tan, clear, white
Orthoclase – orange or pink
• Depositional EnvironmentIron on land - red
Iron in deep, oxygen poor water - green
Detrital Rock Names(Based Primarily on Grain Size)
Sand Sized: Sandstone
Gravel -Sized:
Conglomerate
Siltstone Shale
Mud-Sized:
Mudstone
Interpretation: Grain Size
• Gravel
• Sand
• Silt
• Clay
• River, Beach
• River, Beach, Desert
• Delta, Shallow Ocean
• Deep Ocean, Lake,
Swamp
• High Energy
• Low Energy
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Interpretation: Sorting
Interpretation:
Poorly Sorted
Well Sorted
Transport Agent
Gravity and Glaciers (and Rivers)
Water and Wind
Clast Supported Conglomerate
(River Deposits)
Matrix Supported Conglomerate
(Glacial/Landslide Deposits)
“Modern”
Glacial Sediments
2.2 Gyr Conglomerate
Interpretation:
Distance of TransportShort Far
Interpretation: Grain Roundness
Limestone
Crystalline
Limestone
Chalk
Fossiliferous Limestone
Formation of Limestone
Calcite Precipitates in
Warm Water
Water Under Low Pressure (Shallow)
Or
It is secreted by biological organisms
Clams, Mussels, Scallops, etc.
Corals
Foraminifera
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Limestone Represents:
Shallow Tropical Ocean
Chalk
(Foraminifera)
Black Sea
Black Sea
Travertine
(Limestone)
Chert (Flint, Jasper, Agate…)
Chemical sed. rk formed
via silicious ooze in the
deep sea. Source of silica
= tests (tiny shells) of
radiolarians and diatoms
(which are made of silica)
Diatom Skeletons
Radiolarians
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Chert
Represents
Deep Ocean
Evaporites:Bonneville Salt Flats, Utah
Rock
GypsumRock Salt
Isolated,
Arid,
Salty
Sea/Lake
Peat and Coal
Swamp, Bog
Coal beds in Puget Group sedimentary rocks near Ashford
Sedimentary features and
structures
• beds
• crossbeds
• ripple marks
• graded beds
• fossils
• mud cracks
• raindrop imprints
• etc
Channel fill Sharon Conglomerate, Cuyahoga River valley,
Ohio
~315 Ma
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Crossbedded sandstone
Zion National Park. Festooned cross beds deposited by wind.
dune deposition
by wind!
Planar cross bedsSharon Conglomerate,
lower Pennsylvanian
Age (~315 Ma),
Cuyahoga River valley,
Ohio; interpreted as a
braided stream
sedimentary
environment (lens cap
~52 mm) – dune
deposition by water!
How crossbeds form—migrating dunes!
Turbidity currents
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Modern mud
cracks
modern mud cracks,
Ceres Road 2007
flood-deposited muds
Ancient mud
cracks in shale
Current ripples in wet sediment,
Baja CA
Ripple marks in sandstone, Capitol
Reef National Park, Utah
Ripple marks in tilted Puget Group sedimentary rocks
of Eocene age near Wilkeson WA
Pat Pringle, photo
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Fossil fish from Eocene Green River Shale in western
Wyoming
Fossil shells in sandstone, CA
Fossil metasequoia leaf from rocks
several miles east of Packwood,
Oligocene age
Lithologic Indicators of Climatehttp://www.scotese.com/
legend.htm
dipping beds in Centralia
Coal Mine, view to south
fossils from Skookumchuck
Formation
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http://orerockon.com/For_sale.htm
<= Portunites triangulum Crab,
Eocene, Wahiakum Co., Washingtonhttp://www.geo-tools.com/fossils.htm
More WA fossils!
Dinosaur footprint, Tuba City, AZ
Misc. Sedimentary Features
• Sorting
• Roundness
• Orientation: random vs. preferred
• Color
Sorting
Preferred orientationImbrication = strong current
Current direction is
right to left
Color
• Black = reducing conditions (ex: pyrite,
MnO2, organic matter)
• Green = near oxidation/reduction margin
(ex: glauconite, chlorite)
• Red = oxidation (ex: hematite, “red beds”—
most of these are subaerial)
Color is a function of size and composition
Finer grained = truer colors
Weathering & matrix cement also influence
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http://www.ci.tenino.wa.us/TeninoQuarry1_small.jpg
Eocene McIntosh Formation:
Tenino Sandstone
Photo donated by Larry McMIllan
outcrop along rr tracks near Chehalis R., Galvin WA
Rock Units
• FORMATION-
– Extensive enough to show on a map
– Distinctive from neighboring rock units
– Named locally
• CONTACTS
– Sedimentary contacts – bounding surfaces
between two sedimentary units
Grand Canyon – an awesome place to see strata! Monument Creek
Grand Canyon
Bob Webb, photo, 1986
Track of 1984 debris flow
The great unconformity