Sedimentary
Rocks
Sedimentary rocks are
those that are formed
from the collection of
sediments from preexisting rocks or through
organic materials.
Formation of Sedimentary
Rocks
Most sedimentary rocks are composed of
sediments (sometimes called “clasts”) associated
with the weathering of older rock.
The sediments that are weathered from older rock
are transported to new locations by an erosional
agent – like streams or glaciers.
Most of these sediments will eventually be
deposited as horizontal beds in large bodies of
water. Over time, these deposited sediments will
transform into sedimentary rock through a number
of different processes including:
-- COMPACTION
-- CEMENTATION
-- CHEMICAL ACTION
Compaction and
Cementation
Clastic
Sedimentary Rocks
Many sediments exposed to movements of the
Earth and/or the weight of overlying large bodies of
water over long periods of time are “compressed”
together – reducing pore space and volume
between sediments or clasts. This is known as
compaction.
Shale is formed
from the
compaction of
clay sediments.
Sandstone is
formed from the
compaction and
cementation of
sand sediments.
Most sediments that experience compaction also
experience cementation. Cementation occurs when
dissolved minerals in water precipitate in the pore
space between sediments “cementing” them
together.
Chemical
Non-Clastic
Sedimentary Rocks
Action
Water at Earth’s surface contains many soluble minerals
(minerals dissolved in water).
When water evaporates, the minerals in solution will
precipitate out as a solid, crystalline form known as an
evaporite. Evaporites are composed of one mineral and are
non-clastic. Non-clastic sedimentary rocks are rocks that
are not made up of “clasts” or sediments.
Evaporites are also called chemical sedimentary rock.
A
B
C
Examples of chemical sedimentary rocks (evaporites) include:
rock salt (A), rock gypsum (B), and dolostone (C).
Organic
Bioclastic
Sedimentary Rocks
Processes
Some sedimentary rocks are formed from the
organic processes of living things. These
rocks are often referred to as bioclastic
sedimentary rocks.
Bioclastic sedimentary rocks are those made
by living things or are composed of minerals
precipitated out of living things.
Bituminous Coal
is compacted
remains of
plants.
Fossiliferous
Limestone is
formed by the
compaction and
cementation of
shell fragments.
Distinguishing Features of
Sedimentary Rocks
1. Most are clastic.
2. Most are clasts are round.
3. Most clasts are sorted by size.
4. Fossils are found in sedimentary rock.
Distinguishing Features of
Sedimentary Rocks
5. Most sedimentary
rock is arranged in
layers.
6. Some sedimentary rock
preserves features that may
appear in a depositional
system (Ex: mud cracks or
ripple marks from a stream.).
Identifying Sedimentary Rocks
RT = Pg. 7
The clasts within a sedimentary rock
measure .002 cm in diameter. What is
the name of the rock?
SILTSTONE