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Sedimentary Rocks
• Sedimentary rocks form when weathered
sediment is deposited and is lithified.
– Review. What is:
• Sediment
• Weathering
• Deposition
• Lithification
Formation of Sedimentary Rocks
• Lithification?
– The process of cementing and/or compacting
sediment together.
• In the lab we did the other day (sand and
food), which one represented compaction and
which one represented cementation and
why?
• What is an evaporite?
– Rocks that form after water has evaporated
Subclasses of Sedimentary Rocks
• What’s a subclass?
• Sedimentary rocks are sub-classified by the
processes by which the rocks form and by
the composition of the rocks.
• The three main sub-classes of sedimentary
rocks are:
– Clastic
– Chemical
– Organic
Clastic Sedimentary Rock
• Clastic is Latin for “broken.”
• Clastic sedimentary rocks form when fragments
of preexisting rocks are compacted or cemented
together
• Clastic sedimentary rocks are classified by the
size and shape of the sediments they contain.
Classification of Clastic Sedimentary Rocks
Sediment Size
Large
Sediment
Shape
Rounded
Rock Name
Conglomerate
Small – Large
Angular
Breccia
Small
Rounded
Sandstone
Very Small (clay
or Mud Sized)
Rounded and/or Shale
Angular
Characteristics of Clastic Sediments
• The physical characteristics of sediments are
determined by the how much weathering took
place before lithification.
– Lots of weathering = smaller and rounded sediment
– Little weathering = larger and more angular sediment
Characteristics of Clastic Sediments
• The tendency for currents of air or water to separate
sediments according to size is called sorting.
– Happens during weathering and erosion
– Well-sorted sediments, all of the grains are roughly the same
size and shape.
– Poorly sorted sediment consists of grains that are many
different sizes.
Chemical Sedimentary Rocks
• chemical sedimentary rocks form when minerals
precipitate (solid out of liquid) when water
evaporates.
• When water evaporates, the minerals that were
dissolved in the water are left behind.
– What lab was this like?
• Rocks that form through evaporation are called
evaporites. Rock gypsum and rock salt are
common evaporites.
Organic Sedimentary Rocks
• organic sedimentary rocks form from the
remains of plants or animals
• Coal and some limestones are examples of
organic rocks (shell limestone, coquina, and
chemical limestone).
• Organisms such as clams and coral make their
shells from minerals. When the organism dies,
their shells settle to the bottom of the ocean,
accumulate, and are compacted and/or
cemented to form limestone.
Organic Sedimentary Rocks
The diagram below shows the formation of organic limestone.
Sedimentary Rock Summary
Parent Material
Rock
Minerals
Class
Round pebbles
Conglomerate
Any
Clastic
Angular sediment
Breccia
Any
Clastic
Sand
Sandstone
Mostly quartz
Clastic
Clay
Shale
Mostly feldspar clastic
Dissolved salt
Rock salt
Halite
Chemical
Dissolved gypsum
Rock gypsum
Gypsum
Chemical
Dead plants loosely compacted Peat
Organic
Dead plants tightly compacted
Bituminous Coal
Organic
Cemented shells
Shell limestone
Calcite
Organic
Compacted shells
Coquina
Calcite
Organic
Dissolved shells
Chemical limestone
Calcite
chemical