SEDIMENTARY

ROCKS
 The Importance of Sedimentary Rocks
• Start their lives as any rock type – igneous, metamorphic, or sedimentary
• Are broken up and moved around
• Sediments and sedimentary rocks cover approximately 75% of land and
virtually ALL of the ocean basins
• However, those only comprise about 5 percent (by volume) of Earth’s
outer 10 miles

http://volcano.oregonstate.edu/book/export/html/
196
WEATHERING & EROSION

§ Weathering is the mechanical and chemical alteration (breakdown)

of Earth materials at or near the surface

§ Erosion involves removing weathered materials from their place of

origin (wind, water, ice…)
MECHANICAL (PHYSICAL) WEATHERING
Mechanical weathering (a.k.a., physical
weathering)
§ Mechanical breaking and fragmenting of
solid rock without changing its composition
CHEMICAL WEATHERING: DECOMPOSITION OF EARTH MATERIALS
§ Chemical weathering:
§ Minerals in a rock are chemically altered or dissolved
§ Unlike mechanical weathering, the chemistry and mineralogy changes
during weathering

We will return to weathering in

a couple weeks in the
Weathering and Soils lecture.
SEDIMENTARY ROCKS

§ Detrital / clastic sediments are

made of solid particles that are
products of mechanical
(physical) weathering.

§ Chemical sediments are made of

minerals precipitated from solution
by inorganic processes and by the
activities of organisms through
chemical weathering.
DEPOSITIONAL ENVIRONMENTS & ROCKS
§ The majority of the Earth’s surface is covered in sediment or
sedimentary rocks.

§ A depositional environment:
§ Is a geographic setting where sediment
is accumulating.
§ Determines the nature of the sediments
that accumulate (i.e., grain size, grain
shape, etc.)
§ Studying sedimentary rocks tell us about the past environments at the
surface where these rocks we formed.
DEPOSITIONAL ENVIRONMENTS

https://com
Playa mons.wiki
media.org
/wiki/File:
Main_dep
ositional_
environm
ents.svg
HOW TO MAKE A DETRITAL (CLASTIC) SEDIMENTARY ROCK IN JUST 4
EASY STEPS!
§ The clast, or particle – will
be:

1. Weathered
2. Eroded
3. Deposited
4. Lithified

§ Perhaps several times!

CLAST SIZES
¡ What size are
the clasts? Sand: Coarse, medium, fine
¡ Boulder
¡ Cobble
¡ Pebble
ger
Big

¡ Sand
¡ Silt
¡ Clay
Silt and clay
Boulders, cobbles, pebbles
CLAST SHAPE
§ What shape are
the clasts?
§ Rounded clasts
have interacted
with a large Partially rounded
amount of water,
which rolled the
clasts around,
rounding off
edges and corners

Angular
Rounded
CLAST SORTING
§ Are the clasts all about the same size (well
sorted) or different sizes (poorly sorted)?

Moderately sorted
Well sorted

Poorly sorted
EROSION
Wind § Clasts are moved by what agents of transportation?
Sand
§ What’s the biggest particle size each could carry (under
normal circumstances)?
Water
Pebble

Ice
Boulder
DEPOSITED
§ Low areas such as basins accumulate sediment
BURIED AND LITHIFIED INTO ROCK
§ Uncompacted (loose) sediment slowly becomes rock through biological,
chemical, and physical changes.

§ Diagenesis – changes from pressure,

heat, and/or chemistry
§ Physical – compaction – grains are
squished together by surrounding rock
§ Chemical – cementation – grains are
glued together with some substance, often
microcrystalline quartz or calcite à
§ Low temperature process <300°F (150°C)
§ Occurs in the crust ~10-12 kilometers deep
ROCKS FORMED FROM DETRITAL SEDIMENTS
§ Conglomerate: well-
rounded clasts in a
matrix of sand, silt,
and/or clay

§ What is this rock’s

sorting?
§ What is this rock’s
largest clast size?
ROCKS FORMED FROM DETRITAL SEDIMENTS
§ Breccia: angular with a
sand, silt, or clay matrix

§ What is the difference

between breccia and
conglomerate?
Conglomerate and Breccia
WHERE CONGLOMERATES AND BRECCIAS FORM

River channel Beach Alluvial fan

WHERE CONGLOMERATES AND BRECCIAS FORM

Rounded pebbles = lots of water interaction Angular = little water

Alluvial fans

Playa

River channel

Beach
CONGLOMERATE ON MARS
Mars Earth

§ NASA’s Curiosity rover

on Mars found
conglomerate at Gale
crater, indicating that
a river of flowing
water moving fast
enough to round off
pebbles existed there
at one time
ROCKS FORMED FROM DETRITAL SEDIMENTS
§ Sandstone: sand-
sized grains, may
have layers.
Composition of sand
clasts gives the rock
different names.

§ What is its sorting?

 Playa

Desert dunes

Beach
Quartz Sandstone

https://c
ommons
.wikimed
ia.org/wi
ki/File:T
heWave
_1600pi
xels.jpg
Quartz Sandstone

Describe these properties of

this rock:

• Sorting
• Rounding
 Playa

Desert dunes

Beach
 Detrital Sedimentary Rocks

• Shale
• Silt- and clay-sized (fine-grained)
particles
• Form from gradual settling of
sediments in quiet,
non-turbulent environments (e.g.,
lakes, floodplains, deep ocean
basins)
• Sediments form in thin layers that
are called laminae
• Has fissility (meaning the rock can
be split into thin layers)
• Crumbles easily and tends to form
gentle slopes
• Most abundant sedimentary rock
https://flic.kr/p/27rWsZU
ROCKS FORMED FROM DETRITAL SEDIMENTS
§ Shale: very fine-
grained, clay-sized
particles that form
layers, breaks in chips
§ Clay – super fine
grained
§ Silt – slightly coarser,
but still so small you
can’t see grains
§ Forms in still or slow-
moving water
 Playa
Delta

Deep Marine

Swamp (not pictured

CHEMICAL / BIOCHEMICAL SEDIMENTARY ROCKS
§ Chemical and biochemical
sedimentary rocks are substances
derived from solution by inorganic
or biochemical processes.

§ Some have a crystalline texture –

composed of interlocking crystals.

§ Others have a clastic texture –

made of fragments, like shells that
are glued together.
EVAPORITES
§ Evaporites, including rock salt and rock gypsum are chemical
sediments formed by precipitation of minerals during the evaporation
of water
 Playa
Playa

Desert
Salt Flats
Salt Flats
CARBONATE ROCKS
§ Carbonate rocks, such as limestone and dolostone, consist primarily
of minerals containing the carbonate ion, such as calcite and dolomite.
§ Dolostone forms when magnesium replaces some calcium in limestone.
Biochemical limestone
Guadalupe Mountains National Park

https://commons.wikimedia.org/wiki/Guadalupe_Mountains_National_Park
 Limestones

https://commons.wikimedia.org/wiki/File:White_Cliffs_of_D https://journals.plos.org/plosbiology/article?id=10.1371/
over_02.JPG journal.pbio.1001087
 Playa

Deep Marine

Lagoon

Reef
 Carbonates
Carbonate
Rocks form in:
• Calm Seawater
• Where it’s warm, near the
tropics https://www.pnas.org/doi/10.1073/pnas.2216019119
• Deep Ocean
• Lagoons
• Reefs
• Limestones formed in reefs
often contain fossils of sea
creatures (shells, coral, etc.)
COAL
§ Coal is a biochemical sedimentary rock composed largely of altered
land plant remains
COAL
§ Coal forms from the deep burial, heating and
dehydration of peat.
§ Peat is partially decayed plant remains found in
low-oxygen waters in swamps.
§ With deep burial, heating, and dehydration, peat
converts into lignite coal.
§ Further burial, heating, and dehydration converts
lignite into bituminous coal, and finally into
anthracite, a metamorphic coal [teaser for Metamorphic
Rocks].
 Swamp
Playa

Lagoon
CHERT
§ Chert is microcrystalline quartz
§ Dark chert is called flint. Red chert is
called jasper
§ Used by ancient people for tools and
to start fires
§ Form in the deep marine
environment
Chert
Playa

Deep Marine
SEDIMENTARY ROCKS TELL A STORY!
§ Textures describe the shape, size
and sorting of grains in sedimentary
rocks.

§ Sedimentary structures are larger-

scale features.

§ Some sedimentary structures, such as

ripple marks, bedding, cross-
bedding, and mud cracks form
shortly after deposition.
SEDIMENTARY ROCKS TELL A STORY! CROSS BEDDING

§ Cross bedding preserves layers

deposited at an angle.
§ Common in depositional environments
like sand dunes, shallow marine deposits
and stream-channel deposits.
§ Understanding how physical features
like cross-beds form today can reveal
important ancient climate information
such as wind / current directions.
Cross bedding

https://www.discovery.co
m/exploration/explore-
these-majestic-sand-
dunes----in-colorado
SEDIMENTARY ROCKS TELL A STORY! RIPPLE MARKS
§ Current ripple marks – streams have a current and leave behind
asymmetric ripple marks.
§ Wave-formed ripple marks – shallow marine ripple marks exhibit a
symmetrical shape from the rocking motion of the waves.
Fluvial (river) Environments

A river entering a lake or ocean

tends to produce a Delta
https://www.theadvocate.com/bat
on_rouge/news/article_4afff67c-
b9e2-5c81-b97e-
1e559f1c9ff1.html
Fluvial (river) Environments

https://en.wikipedia.org/wiki/Moun
tain_stream
 Glacial sediments

Poorly sorted
Limited rounding
Sometimes re-worked
by water afterwards
https://commons.wikimedia.org/wiki/File:
Alpine_glacial_till_%28Pleistocene;_nea
r_Dana_Fork,_Yosemite_National_Park,
_California,_USA%29_5.jpg
SEDIMENTARY ROCKS TELL A STORY! MUD CRACKS

§ Mud cracks are

deposited in playas
and deserts.
SEDIMENTARY ROCKS TELL A STORY! FOSSILS
§ Fossils are remains and traces of ancient life.
§ Usually found in sediments / sedimentary rocks
§ Used by geologists to:
§ Correlate rock layers (follow different rock units – like figuring out plate tectonics by correlating
fossils on different continents)
§ Date rocks (different organisms lived at different times)
§ Interpret depositional environments (different organisms lived in different places)
Shaping
the
landscape
SEDIMENTARY ROCKS TELL A STORY!

§ How do we know that the Navajo

Sandstone formed as a desert dune
deposit?

u Sand-sized particles
u Cross beds and asymmetrical
ripple marks
u Footprints of land animals
SEDIMENTARY ROCKS ARE RESOURCES
§ Sand and gravel
§ Clay
§ Coal
§ Limestone
§ Phosphates
§ Uranium
§ Evaporites (salts)
§ Metallic ores
§ Petroleum (oil and natural gas)
METAMORPHIC
ROCKS
MORPH = TO CHANGE
METAMORPHISM
§ Metamorphism: transformation (change) of rocks without melting,
usually beneath Earth's surface
§ Protolith: the rock before metamorphism (proto – before, lith = rock)

Metamorphic rock
Protolith
Same chemical
Igneous High temperature and/or
composition
Sedimentary high pressure
New minerals
Metamorphic
New structures
METAMORPHISM
§ Metamorphism is a result of one or more
of these agents:

§ Heat
§ Pressure
§ Fluid activity

§ Changes rocks’ Oof.

§ Mineral composition
§ Texture
AGENTS OF METAMORPHISM: HEAT
§ Heat increases the rate of reactions
It’s getting
§ Sources of heat include: hot in here!
§ Lava
§ Magma

He
t
H ea
§ Deep burial

at
MAGMA
§ Temperature increases with depth:
geothermal gradient averages
25°C/km in the crust
AGENTS OF METAMORPHISM: PRESSURE
§ Increasing pressure squeezes the
atoms together to eliminate
unoccupied volume in the crystal

§ These styrofoam cups were taken

to the bottom of the ocean, where
the pressure is much higher than at
the surface à

§ High pressure minerals are denser

than low pressure minerals
AGENTS OF METAMORPHISM: FLUIDS
§ Fluids in sedimentary rocks or coming from magmas can accelerate
chemical changes, which occur during metamorphism and can cause
new minerals to form.

§ Common metamorphic fluids:

§ Water
§ CO2
TYPES OF METAMORPHISM
§ The type of metamorphism that results largely depends on which of
the three agents was dominant. The main types are:

§ Contact Contact metamorphism

Regional
§ Regional metamorphism
CONTACT METAMORPHISM
§ High temperature
§ Low pressure
§ Long (intrusive) or short (extrusive)
timescales
§ Localized

He
t
areas

H ea

at
MAGMA
CONTACT METAMORPHISM
Existing rock
§ Magmas or lavas come into contact
with existing rock, baking it
§ Baked fine-grained rock is called
hornfels Baked
zone

He
t

at
H ea
MAGMA
Lava flow

Baked zone
Ash layer
REGIONAL METAMORPHISM

§ Regional
metamorphism
is the most
Accretionary wedge
common type of (low T, high P)
metamorphism
Rock in active mountain
belts (high T, high P)
§ Found at
convergent plate
boundaries
Subducting oceanic crust
(high T, high P)
REGIONAL METAMORPHISM
§ Broad range

§ Long
timescales Accretionary wedge
(low T, high P)
§ High
temperature Rock in active mountain
and pressure belts (high T, high P)

both act as
driving forces for
metamorphic
reactions Subducting oceanic crust
(high T, high P)
PLATE TECTONICS AND METAMORPHISM
§ Metamorphism can
Existing rock around magma
happen at all plate bodies / lava flows (high T, low P)
boundaries but is
most common and Accretionary wedge
extensive at (low T, high P)

convergent
Rock in active mountain
boundaries belts (high T, high P)
§ Two plates smashing
together produces
tremendous amounts
Subducting oceanic crust
of heat and pressure (high T, high P)
INDEX MINERALS AND METAMORPHIC GRADE
§ Metamorphic grade – the degree of metamorphic change a rock
has undergone, usually listed as low, intermediate, or high
Diagenesis – unmodified
[see Sedimentary Rocks]
Low grade – small amount
of change
Intermediate grade –
medium amount of change
High grade – large
Partial melting – magma creation (moving
amount of change
from metamorphic to igneous)
Metamorphic Zones and Index Minerals
INDEX MINERALS AND METAMORPHIC GRADE
§ Index minerals – certain minerals form under specific ranges of
temperatures and pressures. They can be used to determine the
metamorphic grade of a rock
Low Grade High
Clay minerals
Chlorite
Calcite / dolomite
Muscovite
Biotite
Garnet
Staurolite
Kyanite Sillimanite
Feldspars
Quartz
INDEX MINERALS AND METAMORPHIC GRADE
§ Which minerals would Low Grade High
indicate a low grade?
§ Clay minerals, Clay minerals
chlorite, calcite, Chlorite
dolomite, muscovite
Calcite / dolomite
§ Intermediate grade? Muscovite
§ Muscovite, biotite
Biotite
§ High grade?
Garnet
§ Garnet, staurolite,
kyanite, sillimanite Staurolite
§ Which would not be Kyanite Sillimanite
index minerals? Feldspars
§ Feldspars and quartz
Quartz
CLASSIFICATION: TEXTURE

¡ Foliated textures are produced Pressure Pressure

by the preferred orientation of
platy minerals, such as muscovite,
because of pressure

¡ Non-foliated textures do not

have preferred orientation of
minerals (minerals are randomly
oriented.)
Foliation
FOLIATED METAMORPHIC ROCKS
Low grade Shale Slate Phyllite
(unmodified
Shale sedimentary)

Slate

Phyllite

Schist Schist
Gneiss
Gneiss

High grade
Phyllite is a fine-
grained mica-rich rock
that forms as crystals
grow larger in a slate
Basically…Heat Slate
more = phyllite

Clay minerals in the

slate protolith
recrystallize into tiny
micas. Phyllite has a
silky sheen called
phyllitic luster.
NONFOLIATED METAMORPHIC ROCKS
Sandstone

Quartzite

Limestone Marble
NONFOLIATED METAMORPHIC ROCKS
NONFOLIATED METAMORPHIC ROCKS
Basalt Greenstone

Anthracite

Lignite / Bituminous Coal

NATURAL RESOURCES
§ Metamorphic rocks are resources!
Marble
§ Slate
§ Marble
§ Gneiss Slate
§ Graphite
§ Talc
§ Garnet
§ Metallic ores

Gneiss