Transcript Sedimentary Rocks

Sedimentary Rocks
Land sedimentary environments
• Mountains – Formed on bedrock. Steep
slopes allow sediment to travel far.
• Desert – wind picks us fine particles
• Glaciers – snow compacts to ice and the
ice moves sediment below it.
• Beach – stretches of coastline where
sediments has accumulated.
Clasts and weathering
• Clasts – pieces of rock
• 2 types of weathering
• Physical – breaking apart of rocks
that are exposed to the environment.
• 4 ways
• Near-Surface Fracturing – Processes
near the surface cause rocks to
fracture
Physical weathering cont’d
• Frost and Mineral Wedging –
Rocks are pried apart when water
freezes
• Thermal Expansion – Heating
during the day may cause rocks
to break
• Biological Activity – Roots can
grow into fractures
Making Sediment: Physical
Weathering
Fracturing
Frost and mineral
wedging
Roots and
other
biologic
activity
Chemical Weathering
• Minerals are broken down causing
new minerals to form.
• 4 processes
–Dissolution – some minerals are
dissolved in water
–Oxidation – some minerals react
with oxygen to form oxide minerals
Chemical Weathering
• Hydrolysis – Silicate minerals
react with water, the hydrogen
ions in the water react with the
minerals
• Biological reactions – decaying
plants produce acids, some
bacteria consume rocks.
Making Sediment: Chemical
Weathering
Hydrolysis
Oxidation
Biological
reactions
What are the characteristics of
Clastic Sediments?
• Clastic sediments are classified in
3 ways:
• Size of clasts
–Boulders, cobbles and pebbles
–Sand
–Fine particles
Observe the range of clast
size (sorting) in each
photograph
Moderately sorted
Poorly sorted
07.04.a
Well sorted
Characteristics of Clastic Sediment
cont’d
• Shape of Clasts
–Angular, rounded, partially
rounded
• Amount of Sorting
–Size and range of clasts
–Poorly to well sorted (poorly has
wide range)
Observe the shape of
these clasts
Partially rounded
Angular
07.04.a
Rounded
Controls on Size, Shape, and
Sorting
Farther from source means rounder and smaller
Steepness of
slope
Strength of current
Agent of transport
07.04.b
Sedimentary Rock Formation
• Clastic Sedimentary rocks form 2
ways
• Compaction- increased pressure from
being buried beneath sediment forces
clasts together.
–Compaction forces out excess
water, sediments lose up to 40% of
their volume.
Formation of Sedimentary Rocks
• Cementation- after sediment is
compacted open pores between
sediment collects water containing
dissolved minerals.
–As water evaporates minerals left
behind act as a cement holding the
clasts together.
How Clastic Sediment Becomes
Rock
Compaction of
sand grains
Compaction of
clay particles
Cementation
07.05.a
Types of Cement
• Rocks are held together by 3 types of
“cement”
• Calcite- CaCO3
– Moderate hold
• Silica- SiO2
– Strong hold
• Iron-oxide Minerals- ex. hematite
– Come out of water and give mineral a
reddish-color
Common Clastic Sedimentary
Rocks
Conglomerate
Breccia
Sandstone
Shale
07.05.b
Nonclastic Rocks
• Form when chemicals dissolved
in water, precipitate. (when water
evaporates)
• Also, can be biochemical
–Extracted by an organism and
reformed into a sedimentary rock
Rocks Formed by Chemical
Reactions
Setting
Rock Salt
Setting
Travertine (limestone)
07.06.a
Rocks Formed by Organisms
Setting
Setting
Limestone with fossils
Coal
07.06.a
Other Nonclastic Sedimentary
Rocks
Gypsum
Chalk
Dolostone
Chert
07.06.b
Difference between Clastic and
Nonclastic
• Clastic rocks are made up of
rocks and minerals
• Nonclastic rocks are made by
dissolved minerals left behind by
evaporated water.
Why do sedimentary rocks have
layers?
• Most sediment is deposited in
layers.
• Classified by thickness and
definition of layers.
–Thinner, smaller layers referred to
as beds.
How layers form
• Discrete event – a one-time
occurrence. Ex. Flood.
• Change in current – Caused by
change in strength or direction of
current that deposited the sediment.
• Sediment Supply – Change in type or
amount of sediment being supplied.
How layers form
• Sea-level change – global rise or
fall in sea level causes
sedimentary environments to
move.
• Climate Change – Boundaries
between layers reflect regional or
global changes in climate.
Types of Beds
• Graded Beds- form when a current loses
velocity and drops sediment. Larger
sediment falls first followed by finer
sediment.
Types of Beds
• Cross Beds- form when sediment moves
over a dune or underwater ripple. At an
angle to other beds.
Types of Beds
• Parallel Beds- form when one layer piling
on another layer. Most common type.
Resources in Sedimentary Rocks
• Groundwater – moves between
pores in rocks
• Petroleum – formed by organicrich sedimentary rocks
• Coal – consolidation of plant
remains in wetland environments
Resources in Sedimentary Rocks
• Cement from limestone – produced
from limestone free of impurities
• Salt – mined from ancient
sedimentary salt layers
• Uranium – Occur in sandstone and
other sedimentary rocks but it is not
formed there…brought by
groundwater
Resources in Sedimentary
Rocks
Groundwater
Petroleum
Coal
Cement from
limestone
Salt
Uranium
07.14.a
Observe this sequence of sedimentary layers
Sandstone, mudstone, coal
Medium to dark gray
shale with fossil marine
organisms
Sandstone with marine
shells
Sedimentary units with
fossilized wood and
dinosaur tracks
07.16.b