Transcript Lecture14_seds_diagensis

Sediments and Diagenesis
The Rock Cycle is a group
of changes. It is an ongoing
process.
Igneous rock can change into
sedimentary rock or into
metamorphic rock.
Sedimentary rock can change
into metamorphic rock or into
igneous rock.
Metamorphic rock can change
into igneous or sedimentary
rock.
Sediments and Diagenesis
Sediment & Diagenesis
75% of all the rocks exposed at
the Earth’s land surface
(only 5% of Earth’s outer 10
miles - so mostly at the
surface)
I. Sediment
Rock fragments created by
weathering and erosion
Accumulates everywhere—
oceans, beaches, lakes,
rivers, deserts, swamps,
caves, base of glaciers
Sediments and Diagenesis
A. Classified based on
its origin.
1. Detrital: composed of
transported solid
fragments (detritus) of
preexisting rocks
2. Chemical: forms when
previously dissolved
minerals either
precipitate from solution
or are extracted from
water by living
organisms and
converted to
shells/skeletons
Sediments and Diagenesis
Sediments
B. Sediment Transport and
Deposition
1. Detrital
Generally move from high
ground to low ground by the
pull of gravity (assisted by
water, wind, or glacial ice)
Deposited when the carrying
material loses it’s capacity to
carry the sediment
2. Chemical
Ions remain in solution until
there’s a change in the
water’s temperature,
pressure, or chemical
composition and then the ions
precipitate
Sediments
C. Sediment Texture:
Detrital sediment and
chemical sediment are
classified based on
composition.
1. Grain size
Grain composition - some
minerals are stronger than
others.
a. Distance - smaller grains
travel longer distances.
b. Energy of the
transportation medium high energy environment
moves larger grains.
Sediments
Sediments
2. Grain shape
a. Distance transported - over far
distances, grained become
more rounded.
b. Transportation medium
High energy river, wind produce
rounded grains
Middle of glacier, mudflow, debris
flow preserve angular grains –
not enough distance to round
grains
Sediments
C. Sediment Texture:
detrital sediment and
chemical sediment are
classified based on
composition.
3. Sorting
Related to the carrying
capacity of the transport
medium
Well sorted = wind
Medium - well sorting =
waves
Medium - poor sorting =
streams
Poorly sorted = glaciers,
debris flows
Sediments
Sediments and Diagenesis
II. Turning sediments into
rock
Eventually accumulated sediment
turns into rock
A. Diagenesis: All the chemical,
physical, and biological changes
that take place after sediments
are deposited.
Burial
Alteration by groundwater
Recrystallization: regrowth of less
stable minerals into more stable
ones.
Lithification: occurs within the upper
few kilometers of the crust at
temperatures < 200C (400F)
Sediments and Diagenesis
II. Turning sediments into
rock
B. Lithification: the process by
which unconsolidated
sediments are transformed
into solid sedimentary rocks
(part of diagensis)
1. Compaction: pressure
(from overlying sediment)
reduces the volume of
sediment—
Compaction forces out air and
water and packs grains
together.
Sediments and Diagenesis
II. Turning sediments into rock
B. Lithification
2. Cementation
Cements grains together - ions
dissolved in water by chemical
weathering may be deposited by
groundwater circulating through
the sediment.
Calcium carbonate = CaCO3
Calcium ions plus CO2 and H2O
Silica = SiO2 is produced by
chemical weathering of silicate
igneous rocks
Iron oxide = Fe2O3
Sediments and Diagenesis
III. Types of Sedimentary
rocks
A. Detrital Sedimentary rocks:
made of sediment that is
transported as solid particles
Particle size is the primary
basis for distinguishing
various detrital sedimentary
rocks.
Particle size indicates
environment of deposition
Water and air currents sort by
size.
Stronger current = larger grain
sizes
Sediments and Diagenesis
III. Types of Sedimentary rocks
A. Detrital Sedimentary rocks:
made of sediment that is
transported as solid particles
1. Shale (mudstone, siltstone)
>50% of all sedimentary rocks:
Silt and clay
Small grain size = settling from
quiet, non-turbulent water
Silt and class form in lakes,
lagoons, swamps, deep
ocean.
Need quiet water depositional
setting
Sediments and Diagenesis
III. Types of Sedimentary rocks
A. Detrital Sedimentary rocks
2. Sandstone: sand sized particles
(1/16 – 2 millimeters)
~25% of all sedimentary rocks
Windblown dunes, some rivers, and
beaches
Shape and sorting important for
determining depositional
environment.
Sorting: well sorted = wind & waves
poorly sorted = streams
Shape: well rounded = water or wind
transported over long distances
Angular = glacier or debris flow
Transported over short distances
Sediments and Diagenesis
III. Types of Sedimentary rocks
A.Detrital Sedimentary rocks
2. Sandstone: sand sized particles
(1/16 – 2 millimeters) (
~25% of all sedimentary rocks
Windblown dunes, some rivers, and
beaches
Shape and sorting important for
determining depositional
environment.
Sorting: well sorted = wind & waves
poorly sorted = streams
Shape: well rounded = water or wind
transported over long distances
Angular = glacier
Transported over short distances
Sediments and Diagenesis
III. Types of Sedimentary rocks
A. Detrital Sedimentary rocks
2. Sandstone: sand sized particles
(1/16 – 2 millimeters) (
~25% of all sedimentary rocks
Windblown dunes, some rivers, and
beaches
Shape and sorting important for
determining depositional
environment.
Sorting: well sorted = wind & waves;
poorly sorted = streams
Shape: well rounded = transported
over long distances (water)
Angular = transported over short
distances (glacier)
Sediments and Diagenesis
III. Types of Sedimentary rocks
A.Detrital Sedimentary rocks
3. Conglomerate and Breccia—
Composed of gravels (pea to large
boulders, >2 mm)
Conglomerate: composed of
rounded grains of difference sizes.
Formed in energetic mountain
streams or coasts (storm deposits)
Breccia: composed of angular
pieces.
Did not travel far: glaciers, landslides
Sediments and Diagenesis
III. Types of Sedimentary rocks
A. Detrital Sedimentary rocks
3. Conglomerate and Breccia
Composed of gravels (pea to large
boulders, >2 mm)
Conglomerate: composed of
rounded grains of difference
sizes.
Formed in energetic mountain
streams or coasts (storm
deposits)
Breccia: composed of angular
pieces.
Did not travel far: glaciers,
landslides, (close to source)