Post on 01-Apr-2020
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SEDIMENTARY
ROCKS
Processes, Environments, Structures and
Rocks
Sedimentary Processes and Rocks
Origins of Sedimentary Rocks
Sediment transport and texture
Sedimentary structures
Lithification
Classifying Sedimentary Rocks
Detrital/Clastic sedimentary rocks
Chemical/Biogenic sedimentary rocks
Interpreting Sedimentary Rocks
Depositional environments
Sedimentary facies
Sedimentary rocks and Plate Tectonics
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Most sediments are
deposited in the ocean
basins but they are also
deposited in other
environments such as
lakes, streams, desert
basins, etc.
Sediment is loose,
unconsolidated material
and is transformed into
sedimentary rock
through a process
known as lithification
(cementation and
compaction).
Sedimentary rocks
Preserve evidence of the surface processes
responsible for deposition
Contain fossils – the only record of past life
Can be interpreted for depositional
environment
Can be used to interpret past Plate Tectonic
History
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Sedimentary Rocks
Mechanical and chemical weathering produce the raw materials for
sedimentary rocks.
Sedimentary rocks only comprise ~5% of the Earth's crust — however,
they are much more important than this percentage would imply.
Sedimentary rocks occur in the portion of Earth where we live.
Important source of resources:
energy (coal, petroleum)
water (storage of groundwater)
minerals (gold, aluminum, lead/zinc,etc)
Origins of Sediment
Weathering results in two types of products:
1. solid particles (sand grains, clay minerals, etc.)
2. dissolved constituents in water
This results in two fundamentally different types of sedimentary rocks:
1. detrital (clastic) sedimentary rocks - formed from transported solid
particles (detritus or clasts).
2. chemical sedimentary rocks - formed by the precipitation of
dissolved substances by either inorganic or biologic processes
river rocks salt flat
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Detrital Sediment
Quartz and clay minerals are the dominant constituents of detrital
sediment.
quartz - very resistant to weathering.
clay - product of chemical weathering of silicate minerals - very
stable at Earth’s surface.
Feldspars, micas and other silicate minerals may occur in sediment that
has not traveled far.
Texture of Detrital Sediment
Sedimentary rocks are classified partly on
the basis of Texture :particle size and
shape.
Sizes of particles are related to mode of
transport and the environment of
deposition:
• Gravels are found in high energy
environments such as mountain streams
and some beaches.
• Sands are found in intermediate energy
environments such as beaches and river
deposits.
• Clays are found in quiet environments
such as lakes and the deep ocean.
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Transport and Texture
Type of transport, e.g., waves, stream flow, wind
(capacity, energy)
Distance traveled – angular or rounded (more or less
abrasion)
Tendency of medium to sort (well or poorly sorted)
Composition of original weathered rock (soft or hard)
Changes in the energy of medium in different
environments (fast/high energy to slow/low energy)
Sedimentary Rock Properties
Sedimentary Structures
Compaction and Cement (lithification)
Texture
Composition
Rock geometry (Sedimentary Facies)
Fossils
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Sedimentary Structures
Laminations and Bedding
Graded Bedding
Cross-Bedding
Ripple Marks
Mudcracks
Soft-sediment deformation
Biogenic structures
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Sedimentary structures
Laminations and Beds
Beds, Outcrop and Formations Death Valley
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Sedimentary Structures
Beds
Examining Recent Undersea Fan Deposits
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Sedimentary Structures Graded Bedding
Sedimentary Structures
Cross-bedding
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Dune Crossbeds
Valley of Fire State Park,
Nevada
Sedimentary Structures
Ripple Marks – Wind or water currents
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Sedimentary Structures
Mudcracks –Alternate wetting and drying
Mudcracks and Ripple Marks in a Vertical Wall
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Sedimentary Structures
Trace fossils and bioturbation
LITHIFICATION
Diagenesis
Compaction
Cementation
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CLASSIFICATION AND
IDENTIFICATION
Texture (Grain Size, Crystal
size)
Composition (Mineral
Composition)
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Detrital Sedimentary Rocks - Conglomerate and Breccia
These rocks consist of dominantly gravel-sized particles.
Conglomerate - composed of rounded particles
indicating some degree of transport (e.g., mountain
stream, beaches).
Breccia - composed of angular fragments
indicating that they did not travel far from their
source (e.g., landslide, alluvial fan).
Gravel Bar to Conglomerate
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From Alluvial Fans to Breccia
Conglomerate and Sandstone in
Outcrop
Ancient Stream Deposits, Split
Mountain, California
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Detrital Sedimentary Rocks — Sandstone
Arenite (Quartz Sandstone) is composed of
sand-sized quartz particles. Quartz is the
dominant mineral in sandstone - due to its
durability at the Earth’s surface. A well-
sorted pure quartz sandstone with rounded
grains indicates that it may be the result of a
great deal of transport - maybe over several
cycles.
Arkose is a type of sandstone that contains
considerable amounts of feldspar. It
probably was deposited close to the source
of sediment. Likely source was weathering
of granitic rocks.
Detrital Sedimentary Rocks – Sandstone (cont.)
Greywacke is a sandstone composed of
sand sized grains of quartz and lithic
fragments, usually of volcanic rock, with
a matrix of clay. It was probably
deposited on an active marine margin as
a part of a deep sea fan or turbidite
sequence. Likely source of sediment is
volcanic mountains.
photomicrograph
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Air Photo of Underwater Sand Patterns
Gulf of California
Sandstone in Outcrop, Domengine Formation, California
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Sandstone in Outcrop, Coldwater Sandstone Formation,
Santa Ynez Mountain Range, Santa Barbara, CA
Detrital Sedimentary Rocks - Siltstone and Shale
• Mudstone is a general term for fine-grained
sedimentary rocks, usually without fine layering.
• Siltstone is composed of predominantly silt-sized
particles and is often difficult to distinguish from
shale.
• Shale is a sedimentary rock consisting of silt- and
clay-sized particles that commonly consists of
fine layers of clay minerals and can split into thin
layers
• Shale and siltstone commonly contain fossils of
plant remains and other organisms.
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Shale is formed from silt and clay deposited in relatively still
waters of lakes and the ocean.
Flathead Lake, Montana Russian River entering Pacific
Sandstone and Shale
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Silty Shale in Outcrop
Ancient Playa Lake
Bed Shales
Zabriske Point
Death Valley National Park
California
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Ancient Lake Bed Mudstone and Shale, Bryce Canyon Natl. Park
Chemical Sedimentary Rocks
— derived from material that is carried in solution to lakes and seas.
Precipitation from water occurs in two ways:
1. Inorganic processes such as evaporation can produce chemical
sediments. (ex. evaporites, dripstone, travertine, etc.)
2. Biologic processes (forming shells) results in chemical sediment.
Organisms die and collect on the seafloor (ex. coral, chalk, etc.).
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There is a wide variety of
chemical sedimentary rocks.
Note that there is a mineral
composition that is characteristic
for each type of chemical
sedimentary rock.
Chemical Sedimentary Rocks – Inorganic
Precipitates and Evaporites
Inorganic Limestone composed of calcite
precipitated from water without the influence of
living organisms, e.g., lithographic limestone,
travertine
Rock Salt and Rock Gypsum – composed of Halite
and Gypsum crystalized during evaporation of
water in arid conditions.
Travertine
Rock salt
Rock gypsum
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Bedded Limestone, Arkansas
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Rock Salt, an Evaporite
Owen’s Lake, California
Interior of Salt Mine
Chemical Sedimentary Rocks –
Inorganic Precipitates and Evaporites
Dolostone – similar to calcitic
limestone but made of calcium
magnesium carbonate
Inorganic chert –
microcrystaline quartz
precipitated from
supersaturated solutions
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Chemical Sedimentary Rocks –
Chert Chert is a chemical sedimentary rock
composed of microcrystalline silica (SiO2).
chert nodules (flint) are found especially in
limestones (chalk) - inorganic precipitation of
biogenic silica
Flint Nodules on a
Beach in Southern
England
dark gray due to
inclusions of organic
matter
Chemical-Biogenic
Sedimentary Rocks —
Biogenic Limestone
Coquina - composed of poorly cemented
coarse shell fragments
Chalk - made up of the shells of microscopic
marine organisms (foraminifera, calcareous
nannoplankton)
Chalk cliffs of Normandy
Foraminifera
coquina
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Source of Coquina
Massive Biogenic Limestone, Carlsbad
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Layered or bedded cherts are derived
from accumulation of silica-rich shells of
radiolaria (microscopic sea critters).
Chemical Sedimentary Rocks – Biogenic Chert
radiolaria
Coal
— very different
from other rocks
because it is
composed of organic
matter.
Stages in coal
formation:
1. accumulation of
plant remains in
swamp.
2. material does not
decay because of
lack of oxygen
(reducing
environment).
3. as material is
buried by
successive layers it
begins to transform
under increased
temperature and
pressure
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peat — soft brown
material
(recognizable plant
matter)
lignite — soft
brown coal
bituminous coal —
common low-grade
coal (90% reduction
in volume)
anthracite — rare
high grade coal
(metamorphic?)
Coal in Outcrop
Lignite beds in Arkansas Bituminous Coal, Colorado
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Interpreting Sedimentary Rocks
Depositional Environments
Sedimentary Facies
Sedimentary Rocks and Plate Tectonics
Depositional Environments
Continental Rivers and streams
Lakes
Deserts
Caves
Glaciers
Transitional Coastal deposits
Turbidity Currents
Marine Shallow Marine (Shelf)
Deep Marine
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Depositional Environments
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Depositional Environments
Continental – Fluvial (Streams and Rivers)
Ancient Stream Deposits, Split
Mountain, California
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Continental Environments
Desert – Sand dunes
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Continental Environment – Desert Dunes
Dune sandstone
Continental Environments - Desert
Alluvial fan
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From Alluvial Fans to Breccia
Depositional Environments
Glacial
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Glacial Till
Glacial till consists of rounded to angular fragments that are not
sorted by size and not laid down in horizontal beds.
South African Tillite, source where Indian Ocean is now
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Glacial Lakes - Varves
Transitional Environments
Where the land meets the sea
Deltas
Continental margins
Tidal flats
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Transitional Environments - Deltas
Colorado River Delta
Transitional Environments - Delta
Flathead Lake, Montana
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Sediments in Deltas
Delta topset and Foreset beds
Glacial Lake Deposit
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Barrier Islands
Barrier Island
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Depositional Environments
Marine
Detrital Marine Environments
Carbonate Marine Environments
Evaporite Environments
Marine depositional environments
Detrital Marine Environments
Continental Shelf
Continental Slope
Continental rise
Deep Ocean
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Detrital Marine Environments
Turbidites on Continental Slope, Rise
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Deep Marine Environment
Many forms of Planktonic life fill the deep marine sediments.
Distribution of deep marine sediments
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Lateral Relationships – Sedimentary
Rocks
Principle of Lateral Continuity – Sedimentary rock
layers extend outward in all directions until they
terminate.
Terminations
Edge of depositional basin
Later erosion of portion of basin
Truncation by faults
Pinching out
Lateral gradation
Sedimentary Facies
Different adjacent environments produce
different rocks and fossil assemblages
Marine Transgressions and Regressions
Transgression – rise in sea level or sinking of
land
Regression- lowering of sea level or rise of
land
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Walther’s Law
The facies seen in a conformable vertical
sequence will also replace one another
laterally
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Transgression / Regression
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Reasons for Sea Level Change
Amount of water frozen in glaciers
Rates of Sea Floor Spreading
Rapid spreading, high heat flow causes ridges to rise and displace water onto the continents causing universal transgression
Slower spreading, lower heat flow, causes ridges to subside, increased volume of ocean basins causes universal regression.
Sedimentary Rocks and Plate
Boundaries
Convergent-Subduction
Greywacke sandstone
Turbidite conglomerates
Limestone
Shale
Convergent-Collision/ Transform Continental/ Divergent Continental
Arkose sandstone
Continental conglomerates
Evaporites
Fluvial shales
Divergent Marine
Chert
Limestone
Shale
Passive Margins
Quartz sandstone
Shale
Limestone