Transcript 6.3 2005

SEDIMENTARY ROCK
Section 6.3
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Objectives
Explain the processes of compaction and
cementation.
 Describe how chemical and organic
sedimentary rocks form.
 Describe how clastic sedimentary rock
forms.
 Identify seven sedimentary rock features.
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Sediment
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Loose fragments of
rock, minerals, and
organic material
that result from
natural processes.
Sedimentary Rock Characteristics
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Determined by the
source of the
sediment, the way the
sediment was moved,
and depositional
conditions.
2 Main Processes Convert Sediments
Into Sedimentary Rock
Compaction
Cementation
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Compaction
 The
weight of
overlying sediments
causes pressure,
squeezing the
sediment, reducing the
size of the pore
(empty) space between
sediment grains.
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Cementation
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Water carrying dissolved minerals
passes through the sediments and
then forms small crystals between
the rock fragments to hold the
fragments together.
Three Main Classes of Sedimentary Rocks
 Chemical
 Organic
 Clastic
Based upon how the rocks form and
their composition.
Each class is further subdivided based
on the shape, size, and composition of
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the sediments that form the rocks.
Chemical Sedimentary Rocks
 Rocks
that form
from minerals
that were once
dissolved in
water.
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Precipitation (Settling)
Some minerals settle out of water as a
result of a change in temperature.
Some chemical limestones form when
cool ocean currents lower the
temperature of ocean water and calcite
(calcium carbonate; CaCO3)
precipitates and eventually solidifies
on the ocean floor.
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An example: The Redwall Limestone of
the Grand Canyon area
Great thicknesses of
limestone can result.
The topmost cliff is the
Redwall Limestone. It
averages 500 feet in
thickness and is about
335 million years old.
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Evaporites
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Dissolved minerals
can be left behind
when water
evaporates. These
minerals then form
rocks called
evaporites.
Gypsum and halite
(rock salt) are two
examples.
Organic Sedimentary Rocks
 Rock
formed from the remains
of living things.
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Limestone
Organic limestones formed from shells made up
of calcite from clams, oysters, snails, corals,
and plankton.
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Coal
Formed from
plants that are
buried and
compacted into
matter that is
mostly carbon.
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Chalk
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White Cliffs of Dover
A type of limestone
made up of the
shells of tiny,
one-celled
marine organisms
that then formed
a mud on the
bottom of an
ancient sea.
Clastic Sedimentary Rocks
 Made
up of accumulations of rock
fragments carried away from their source
by gravity, water, wind, and/or ice.
 Clastic sedimentary rocks are classified
according to the kind and size of
sediments that form them.
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Size Classifications
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Conglomerate
 Formed
from rounded rock fragments
greater than 2 mm, held together by a
matrix of sand, clay, and cement.
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Breccia (BRECH ee uh)
 Like
a
conglomerate but
formed from
fragments which
are angular and
have sharp
corners.
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Sandstone
 Coarse-grained
rock formed from
sand 2 mm to 1/16
mm in diameter.
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Shale
 Fine-grained
rock, some small rock
particles may be seen
 Usually clays, shows thin layering.
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Mudstone
 Made
from mud, rock particles
too small to see, thick layers
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Clastic
sedimentary
rock formations
can reach great
thicknesses.
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Navaho Sandstone, Utah
Characteristics of Clastic Sediments
Determined by the way
sediments were
transported.
 Four main agents: water,
wind, ice, and gravity.
 Both the distance the
sediment is moved and
the agent that moves the
sediment determine the
25characteristics.

Sorting
The tendency for air or water to separate
sediments according to size.
 Due to the change in the speed of the agent
that is moving the sediment.
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Angularity
As sediment particles are
transported they collide
with each other and
objects in their path.
 In general, the farther
sediments travel the finer
(smaller) and smoother
the particles of sediment
become.
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Sedimentary Rock Features
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These allow scientists to identify the depositional
environment, the setting in which sediment is
deposited.
Includes rivers, deltas, beaches, and oceans.
Stratification
 Occurs
when there is a change in the kind of
sediment being deposited. The layers, or beds,
vary in thickness depending on how long each
type of sediment was being laid down. Most
beds are horizontal.
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Cross-beds
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Slanting layers that
form within beds.
Usually form in
sand dunes or river
beds.
Graded Bedding

Different sizes and
shapes of sediment
settle out in
different levels in a
bed (largest and
most rounded on
the bottom to
smallest on top).
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Reverse Bedding
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Happens in
mudflows, in which
the smallest grains
are on the bottom
and the largest
grains are on top.
Ripple Marks
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Formed from the
action of wind or
water on sand or
mud.
Mud Cracks
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Formed when
muddy deposits dry
and shrink to form
cracks. Later
sediments may fill
in the cracks and
preserve them in
sedimentary rock.
Fossils
The remains (hard
parts) or traces
(impressions) of
ancient plants and
animals.
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Concretions
Lumps, or
nodules, of
rock with a
composition
different from
that of the main
rock body.
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Geodes
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Dissolved
minerals can
crystallize
inside cavities
(holes) in
sedimentary
rocks (igneous
also).
Assignment – Due Friday
 Directed
Reading
 6.3 Key Terms
 Ternary Diagram &
Mineral Composition
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