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Sedimentary Rocks
Sedimentary Rocks
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Sediments = pieces of solid material
deposited on the Earth’s surface.
Sedimentary Rock
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Rocks that are composed of the
weathered remains of preexisting
rock, or plant and animal remains.
Sedimentary rocks commonly
originate from sediments laid down in
horizontal strata by water or wind.
Horizontal layers called “Beds”
separated by “Bedding planes”, are a
common feature in sedimentary rocks.
Bedding planes
Beds
Horizontal Beds of Sedimentary Rock
Sedimentary Rocks
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How is a sedimentary rock formed?
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Sediments get compacted and cemented
together.
Sedimentary Rocks
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Detrial – made from fragments of other
rocks, that have been transported,
deposited, then compacted and cemented
together.
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Shale, sand, conglomerate, siltstone, breccia
Classified by the size of the fragments in the
rocks
Think about it: What environments would
create these types of rock?
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High energy fast moving water (carrying fragments),
then water slows down (lake or ocean) and deposits
fragments.
Cemented Rocks
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Detrial sedimentary rock – rocks
composed of weathered sediments:
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held together by a natural cement
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Pebbles or gravel – usually quartz
Sand – usually quartz
Clay and silt – weathered feldspars and
mica :
Silica – commonly gray-white in color
Calcite – commonly gray-white in color
Iron oxide – commonly reddish to yellow
brown in color,
or by compaction of clay and silt.
Conglomerate – cemented
sand, silt, and pebble
sediments. If large
fragments are angular this
rock is called a breccia.
Sandstone –
cemented quartz
sand grains. Feels
gritty. Unfilled
spaces between
grains make most
sandstones
porous and
permeable to
water.
Bedding planes
Shale – clay and silt sized particles
lithified by dehydration and compaction.
Note the cleavage at bedding planes.
Thumps when you tap it with a nail and,
moistened, it smells like damp earth.
Sandstone in the Pinnacle Desert, Australia
Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL
Sedimentary Rocks
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Chemical – formed when minerals
fall out of solution. From
evaporation of salt water or from
chemical reactions.
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Rock salt, rock gypsum, some
limestones
Think about it: Where would these rocks
form?
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Sea, lake, swamps, or underground waters
that contain dissolved minerals
Chemical Sedimentary Rock
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Rocks formed either as precipitates or
as evaporites of dissolved chemical
sediments.
Mineral salts that accumulate in water
become concentrated by evaporation
until they precipitate from solution, or
Mineral salts are deposited as a result
of the total evaporation of the
solution.
Rock salt, the mineral
halite (NaCl), left as
an evaporite as a
shallow sea
evaporated.
Alabaster, the mineral
gypsum (CaSO4), also an
evaporite.
Compact (or precipitate) limestone, the mineral
calcite (CaCO3), precipitated from sea water as
evaporation increased concentration. Many
cavern systems are formed in this type of
limestone.
Sedimentary Rocks
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Organic Rocks – formed from the
remains of plants and animals.
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Shells of marine animals pile up,
compact and cement to create
fossiliferous limestone (coquina).
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Where would these form?
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Oceans
Plants pile up and compress over time to
form coal.
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Where would these form?
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Swamps – large amount of build up of organic
material.
Organic Sedimentary Rock
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Rocks formed from the altered remains
of plant and animal material
An exception to the definition of rocks
as a mixture of minerals (Remember:
Minerals are inorganic substances).
Organic material exists in sequences of
sedimentary rock to the extent that it
cannot be ignored as a true rock type.
Organic Sedimentary Rock (Cont)
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Molds, casts, and other traces of
ancient plant and animal life,
called fossils, are a common
feature of sedimentary rock.
Some layers of rock, coal and
limestone in particular, are formed,
almost, entirely of the remains of
living things.