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Sedimentary Rocks
Geology 200
Geology for Environmental Scientists
Major Concepts
• Sedimentary rocks form by the processes of
weathering,
g, erosion,, transportation,
p
, and
deposition, or by chemical precipitation.
• Two main types
– clastic: pieces of older rocks (clasts)
– chemical:
h i l precipitated
i i d or formed
f
d in
i place
l
• Sedimentary structures are critical to
interpreting sedimentary rocks.
The Rock Cycle
Major Concepts
• Major sedimentary environments:
–
–
–
–
–
–
–
–
fluvial and alluvial
eolian
li
glacial
delta
shoreline
organic reefs
shallow marine
deep marine
Major Concepts
Sedimentary rock layers are
hierarchically grouped or subdivided
into both:
– rockk stratigraphic
i
hi units
i
– time stratigraphic
g p units
We’ll cover this in the section on
stratigraphic
t ti
hi correlation.
l ti
Clastic Sedimentary Rocks
•
•
•
•
•
•
Produced by weathering of rocks.
Breccia - large,
large angular grains
Conglomerate - large, rounded grains
Sandstone - sand sized grains
Siltstone - silt sized grains
Shale and Mudstone - silt and clay
sized grains
Grain Size
• Grain size can be an indicator of the
energy of the environment.
environment
• Generally speaking, higher energy
water or wind
i d currents are required
i d to
move larger grain sizes.
Grain Size,
Size cont
cont’dd
S e ranges:
Size
a ges: small
s a too large
a ge
• boulders, >256 mm (>1 ft.)
• cobbles,
bbl >64
>64mm (>3 iin.))
• pebbles, >4mm
• granules, 2-4mm
• sand,
sand 1/8
1/8-22 mm
• silt, >1/256 mm
• clay,
l <1/256
1/2 6 mm
Fig. 5.4. Grain size ranges for classification of
common clastic sedimentary rocks
Grain Sorting
• Grain sorting can also be an indicator
of the energy of the environment.
• Well-sorted
W ll
d sediments
di
are deposited
d
i d in
i
high energy environments. Currents
sort the grains by size.
• Poorly
Poorly-sorted
sorted sediments may indicate
weak currents, or transport by glaciers.
Well sorted sand g
grains and an outcrop
p of
well-sorted sandstone.
Poorly sorted glacial deposit ranging from
cobbles to mud-sized particles.
particles
Conglomerate
Conglomerate deposited as an alluvial
f
fan,
Cretaceous
C t
off Utah
Ut h
Sandstone
Sh l
Shale
Chemical Sedimentary Rocks
• Produced by chemical precipitation.
• Evaporites - formed by evaporation of
seawater
– Salt, NaCl
– Gypsum, CaSO4
• Carbonates
– Limestone, CaCO3
– Dolostone, CaMg(CO3)2
Gypsum
yp
Limestone
Fossiliferous
F
ilif
Limestone
Carbonates
• Typically, carbonates form in warm,
clear water free of clastic sediment.
• Carbonate grainstone - composed of
sand-sized grains from invertebrate
skeletons
k l t
or oolites.
lit
• Carbonate mudstone - clayy and siltsized grains from pellets and
calcareous algae.
Carbonate
Grainstone:
Oolitic
Limestone
Oolite Shoals
Carbonate Mudstone:
Micritic Limestone
Carbonate Mud Banks in Florida Bay
Investigating Carbonate Mud Banks in
Fl id Bay
Florida
B
Investigating Carbonate Mud Banks in
Fl id Bay
Florida
B
Coal
• Classified as a chemical rock by most
textbooks But it is not a chemical
textbooks.
precipitate. Instead it forms from plant
debris not consumed by bacteria and
other organisms. It is an organic rock.
• Takes about 10 ft. of peat to form 1 ft.
of bituminous
o
b u
ous co
coal..
Types of coal:
peat
lignite
bituminous
anthracite
th it
Upper Freeport Coal, Preston County, WV
Examples of Sedimentary
Structures
Sedimentary Structures
• Provide clues to depositional
environments Some examples:
environments.
• Cross bedding - rivers, dunes, tidal
channels
h
l
• Graded beddingg - storms and turbidites
• Ripple marks - lower energy
• Mud cracks - subaerial exposure
Bedding planes are primary
sedimentary structures.
structures
Modern Sedimentary Structures:
Mudcracks on a flood plain
plain.
Ancient Sedimentary Structures:
years old.
mudcracks in rocks 1 billion y
Modern Sedimentary Structures: Current
ripples on a modern beach.
beach
Beach in Ireland at low tide
Sedimentary Structures: Current ripples on
an ancient beach,, 400 MY old.
The p
processes by
y which symmetrical
y
and current ripples are formed.
Geometry of sand dunes, both eolian
and aquatic.
TOP
BOTTOM
Fig. 5.9. Large scale cross-bedding formed by eolian dune
migration. Flow direction and top of bed indicated.
Wind blown dune deposits.
p
Mississippian cross beds near Altoona, PA
Mississippian
pp
cross beds near
Altoona, PA
Jurassic Navaho Sandstone, Zion National Park,
wind-blown cross-bedding
cross-bedding.
Marine Cross Beds in Ste. Genevieve Ls., Indiana
Tidal Cross Beds in Ste. Genevieve Ls., Indiana
Smaller cross beds deposited in a
river or stream.
A meandering stream with a sandy point bar
on the inside curve, and a cut bank on the
outside curve. The ppoint bar dips
p toward the
stream channel.
St
Stream
Channel
Ch
l
U d
Underwater
sand
d ddunes, Fl
Florida
id Keys
K
Graded
Bedding –
formed by
storms or
turbidite
deposition
Another example of a graded bed
Sedimentary Environments or
Environments of Deposition
Color of Sedimentaryy Rocks
• Color can be useful in the
interpretation of depositional
environments.
• Black color - indicates deposition in
the absence of oxygen in either the
ocean, lakes, or swamps.
• Red color - indicates deposition in the
ppresence of abundant oxygen
yg in a
warm, humid terrestrial environment.
Black Shale, Anoxic Environment
Sideling Hill Outcrop, I-68. What is the
depositional environment of the black layer?
Red Beds, Oxygenated Environment
Environments of Deposition
•
•
•
•
3 Major Environments
C i
Continental
l or Terrestrial
T
i l
Marine
Transitional
Continental or Terrestrial
Environments
•
•
•
•
Glacial - both Alpine and Continental
Alluvial Fans - at the base of mountains
Lakes
Rivers and Floodplains = Fluvial and
alluvial deposits
• Swamps
• Deserts
M i Environments
Marine
E i
• Continental Shelf - most common
marine environment in the rock record
• Shelf Edge
• Slope
• Ocean Floor
Transitional Environments
•
•
•
•
•
•
Beaches
Barrier Islands
Tidal Flats
Lagoons and Bays
Estuaries
River Deltas
Fig. 5.13. Major sedimentary environments.
Alluvial Fan deposits
Fluvial and floodplain deposits
What is the depositional environment?
What is the depositional environment?
Eolian deposits
A modern swamp in Louisiana
A strip coal mine in the western U.S. -an ancient swamp environment
A strip coal mine in the western U.S. -an ancient swamp environment
Beach deposits
Barrier Islands,
bays,
y , and
continental shelf
on the New
Jersey coast.
Barrier Islands, bays, and continental shelf on
the Gulf Coast: Mobile Bay
Seneca Rocks – Paleozoic barrier island deposits
2004 G200 class on top of Seneca Rocks
2005 G200 class on top of Seneca Rocks
Transitional or Marginal Marine Environments,
Tid l Inlets,
Tidal
I l
Marshes,
M h andd Lagoons
L
Marginal
g
Marine Environments,, Tidal
Inlets between Barrier Islands
Marginal Marine Environments, a
Barrier Island
Marginal Marine Environments, a
prograding Spit on a Barrier Island
Transitional
Environment: The
Mississippi River
delta
Transitional
Environment:
The
Mississippi
River delta
Fig. 12.32. Historic flow of the Mississippi
The Nile Delta.
Shaped
p like the
Greek letter
delta.
delta
Sedimentary Facies
• Facies - general appearance or aspect
of sedimentary rocks. Often
correspondd to
t formations.
f
ti
• A reflection of the depositional
p
environment.
• Lithofacies - defined by lithologic
features
• Biofacies - defined by organic features
Sedimentary Facies
• Facies occur laterally adjacent to one
another just as do their depositional
environments.
• Two different
diff
facies
f i can have
h
similar
i il
lithofacies but different biofacies, or
vice-versa.
Sedimentary Facies
• Facies migrate laterally with changes
in sea level.
level
• Rising sea level = transgression.
Marine
i facies
f i overlie
li nonmarine
i
facies.
• Falling sea level = regression.
Nonmarine facies overlie marine
facies.
Facies changes produced by
Marine Transgression
Transgression, regression, and stratigraphic cycles.
Figure 5.26
5 26
Causes of Changes in Sea Level
• Changing ice volumes at the poles.
• Thermal expansion or contraction of
the oceanic crust.
• Continental tectonics or subsidence.
• Shoreline progradation from river and
deltaic sedimentation.
Pennsylvanian Sedimentary
Rocks in the Appalachians
Pennsylvanian
P
l i point
i t bar
b deposits
d
it on WV
Rt. 19 near Oak Hill.
coal seam
point bar surfaces
A meandering stream with a sandy point bar
on the inside curve, and a cut bank on the
outside curve. The ppoint bar dips
p toward the
stream channel.
Stream Deposits
p
at Exit 7, I-68
by Glenmark Centre
Depositional Model for Coal formation
Upper Freeport Coal, Preston County, WV
Upper Freeport Coal with overlying fluvial
sandstones
d
d
deposited
i d by
b meandering
d i river.
i
Coal outcrop at the Morgantown Mall
An idealized
cyclothem
sequence.
Rarely found
in West
Vi i i
Virginia
Pennsylvanian Cyclothems in Kentucky
6
5
4
3
2
1
A modern marine transgression is
exposing Holocene peat deposits on the
Louisiana coastline.
Change of the Louisiana Coastline
Middle Pennsylvanian, Kanawha Fm.,
Rt 19,
Rt.
19 Nicholas Co
Co., WV