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THE EARTH THROUGH TIME
TENTH EDITION
H A R O L D L. L E V I N
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1
CHAPTER 5
The Sedimentary Archives
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FACTORS AFFECTING SEDIMENTARY
CHARACTERISTICS
1.
2.
3.
Tectonic setting
Physical, chemical, and biological
processes in the depositional
environment
Method of sediment transport
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3
FACTORS AFFECTING SEDIMENTARY
CHARACTERISTICS
4.
5.
6.
7.
Rocks in the source area from which the
sediment is derived
Climate (and its effect on weathering)
Post-depositional processes of lithification
(cementation, compaction)
Time
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TECTONICS
Tectonics: The forces controlling deformation
or structural behavior of a large area of the
Earth's crust over a long period of time.
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STRUCTURAL BEHAVIOR
Tectonically stable—mid-western U.S.
 Subsiding (sinking) —New Orleans or Mexico
City
 Rising gently—New England and parts of
Canada after glacial retreat
 Rising actively to produce mountains and
plateaus—parts of Oregon in the Cascade
mountains

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PRINCIPLE
TECTONIC ELEMENTS
OF A CONTINENT
•Craton
- Shield
- Platform
•Orogenic belt
FIGURE 5-1 The tectonic parts of a continent.
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CRATON

Craton—the stable
interior of a continent.


Shields—Large areas of
exposed crystalline rocks.
Platforms—Ancient
crystalline rocks covered
by flat-lying or gently
warped sedimentary
rocks.
FIGURE 5-1 The tectonic parts of a continent.
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OROGENIC BELTS

Orogenic belts—Elongated
regions bordering the
craton which have been
deformed by compression
since Precambrian.
Orogenic belts are
mountain belts.
FIGURE 5-1 The tectonic parts of a continent.
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DEPOSITIONAL ENVIRONMENTS
All of the physical, chemical, biological and
geographic conditions under which sediments
are deposited.
By comparing modern sedimentary deposits
with ancient sedimentary rocks, the
depositional conditions can be interpreted.
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DEPOSITIONAL ENVIRONMENTS
Sediments and sedimentary rocks may be:
 Extrabasinal in origin—formed from the
weathering of pre-existing rocks outside the
basin, and transported to the environment of
deposition.
 Intrabasinal in origin—formed inside the
basin; includes chemical precipitates, most
carbonate rocks, and coal.
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DEPOSITIONAL ENVIRONMENTS
There are three broad categories of depositional environments:



FIGURE 5-2 Marine, transitional, and continental environments of deposition.
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Continental
environments (on
land)
Transitional
environments
(along contact
between ocean
and land)
Marine
environments
(ocean)
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MARINE DEPOSITIONAL ENVIRONMENTS
1.
2.
Continental shelf
Continental slope
3.
4.
Continental rise
Abyssal plain
FIGURE 5-5 Submarine fan built of land-derived sediment emerges from a submarine canyon.
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CONTINENTAL SHELF
The flooded edge of the continent. Flooding
occurred when the glaciers melted about
10,000 years ago.
a. Relatively flat (slope < 0.1o)
b. Shallow water (less than 200 m deep)
c. May be up to 300 km wide (averages 80
km wide)
d. Exposed to waves, tides, and currents
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CONTINENTAL SHELF—CONT'D
e.
f.
g.
h.
Covered by sand, silt, and clay
Larger sedimentary grains are deposited
closer to shore.
Locally cut by submarine canyons (eroded
by rivers during Ice Age low sea level
stand)
Coral reefs and carbonate sediments may
accumulate in tropical areas
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CONTINENTAL SLOPE
The steeper slope at edge of the continent.
a.
b.
c.
Located seaward of the continental shelf
Boundary between continental & oceanic
crust
May be about 20 km wide
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CONTINENTAL SLOPE—CONT'D
d.
e.
f.
g.
Deeper water
More steeply inclined (3–6o)
Rapid sediment transport down the slope
by dense, muddy turbidity currents
Passes seaward into the continental rise
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CONTINENTAL RISE
At the base of the continental slope:
a.
b.
c.
d.
e.
f.
More gradual slope
May be hundreds of km wide
Water depths of 1400 to 3200 m
Submarine fans form off submarine canyons
Turbidity currents transport sediment downslope
from continental shelf (turbidites)
Passes seaward into the abyssal plain
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DEEP MARINE REALM
The deep ocean floor
a.
Nearly flat
b.
Water depths of 3 to 5 km + (2–3 miles +)
c.
Covered by very fine-grained sediment and shells
of microscopic organisms





Clay
Volcanic ash
Foraminifera (calcareous)
Radiolarians (siliceous)
Diatoms (siliceous)
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TRANSITIONAL DEPOSITIONAL
ENVIRONMENTS
Environments at or near the transition
between the land and the sea.
1. Deltas
2. Beaches and barrier Islands
3. Lagoons
4. Tidal flats
5. Estuaries
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DELTAS
a.
b.
c.
d.
Fan-shaped accumulations of sediment
Formed where a river flows into a standing
body of water, such as a lake or the sea
Coarser sediment (sand) tends to be
deposited near the mouth of the river; finer
sediment is carried seaward and deposited
in deeper water.
The delta builds seaward (or progrades) as
sediment is deposited at the river mouth.
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DELTAS
FIGURE 5-9 Tale of two deltas: the Mississippi River (A) and Niger River (B) deltas.
Mississippi River delta
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Niger River delta
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BEACHES AND BARRIER ISLANDS
a.
b.
c.
d.
e.
f.
g.
Shoreline deposits
Exposed to wave energy
Dominated by sand
Marine fauna
A few km or less in width but
may be more than 100 km long
Separated from the mainland by
a lagoon (or salt marsh)
May be associated with tidal flat
deposits
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LAGOONS
a.
b.
c.
d.
Bodies of water on the landward side of
barrier islands
Protected from the pounding of the ocean
waves by barrier islands
Contain finer sediment than the beaches
(usually silt and clay)
Lagoons are also present behind reefs, or in
the center of atolls.
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TIDAL FLATS
a.
b.
c.
d.
e.
f.
g.
Nearly flat, low relief areas that border lagoons,
shorelines, and estuaries
Periodically flooded and exposed by tides (usually twice
each day)
May be cut by meandering tidal channels
May be marshy, muddy, sandy or mixed sediment types
(terrigenous or carbonate)
Laminations and ripples are common
Sediments are intensely burrowed
Stromatolites may be present (if conditions are
appropriate)
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ESTUARIES
a.
b.
c.
d.
e.
f.
Mouth of a river drowned by the sea
Brackish water (mixture of fresh and salt)
May trap large volumes of sediment
Sand, silt, and clay may be deposited depending
on energy level
Many estuaries formed due to sea level rise as
glaciers melted at end of last Ice Age
Some formed due to tectonic subsidence,
allowing sea water to migrate upstream
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CONTINENTAL ENVIRONMENTS
1.
2.
3.
4.
5.
Rivers or fluvial environments
Alluvial fans
Lakes (or lacustrine environments)
Glacial environments
Eolian environments
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FLUVIAL ENVIRONMENTS
a.
b.
c.
d.
e.
f.
Braided and meandering river and stream systems
River channels, bars, levees, and floodplains are
sub-environments
Channel deposits are coarse, rounded gravel, and
sand
Bars are sand or gravel
Levees are fine sand or silt
Floodplains are covered by silt and clay
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ALLUVIAL FANS
a.
b.
c.
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Fan-shaped
deposits at base of
mountains.
Most common in
arid and semi-arid
regions with rapid
erosion.
Sediment is
coarse, poorlysorted gravel and
sand.
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LACUSTRINE ENVIRONMENTS (LAKES)
a.
b.
c.
d.
e.
f.
May be large or small
May be shallow or deep
Filled with terrigenous, carbonate, or evaporitic
sediments
Sediments are typically fine grained but may be
coarse near the edges
Fine sediment and organic matter settling in some
lakes produced laminated oil shales
Playa lakes are shallow, temporary lakes that form
in arid regions They periodically dry up as a result
of evaporation
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GLACIAL ENVIRONMENTS
a.
b.
Sediment is eroded,
transported, and deposited
by ice (glaciers)
Glacial deposits called till
contain large volumes of
unsorted mixtures of
boulders, gravel, sand and
clay
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Ashley Cooper/Alamy
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EOLIAN ENVIRONMENTS
a.
b.
c.
Wind is the agent of
sediment transport and
deposition
Dominated by sand and
silt
Common in many desert
regions
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COLOR OF SEDIMENTARY ROCKS
Black and dark gray coloration in
sedimentary rocks generally indicates the
presence of organic carbon and/or iron.
 Organic carbon in sedimentary requires
anoxic environmental conditions.

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COLOR OF SEDIMENTARY ROCKS

Red coloration in sedimentary
rocks indicates the presence of
iron oxides.

Red beds typically indicate
deposition in well-oxygenated
continental sedimentary
environments. May also be
transitional or marine.
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COLOR OF SEDIMENTARY ROCKS

Green and gray coloration in sedimentary rocks indicates
the presence of iron, but in a reduced (rather than an
oxidized) state.

Ferrous iron (Fe+2) generally occurs in oxygen-deficient
environments.
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TEXTURAL INTERPRETATION OF CLASTIC
SEDIMENTARY ROCKS
Texture = size, shape, sorting, and arrangement of grains in a
sedimentary rock.
The texture of a sedimentary rock can provide clues to the
depositional environment.

Fine-grained textures typically indicate deposition in quiet
water.

In general, it takes higher energy to transport larger grains.
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THREE "TEXTURAL COMPONENTS" TO
MOST CLASTIC SEDIMENTARY ROCKS:
1.
2.
3.
Clasts—the larger grains in the rock (gravel,
sand, silt)
Matrix—the fine-grained material surrounding
clasts (often clay)
Cement—the "glue" that holds the rocks
together
a.
b.
c.
d.
Silica (quartz, SiO2)
Calcite (CaCO3)
Iron oxide
Other minerals
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GRAIN SIZE
Sedimentary grains are categorized according to size
using the Wentworth Scale.
Gravel > 2 mm
Sand
Silt
1/16 - 2 mm
1/256 - 1/16 mm
Clay
< 1/256 mm
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SORTING
Sorting refers to the distribution of grain sizes
in a rock.


The range of grain sizes in a sedimentary rock can
provide clues to help interpret the depositional
environment.
For example, turbulence from waves will winnow
out finer grain sizes such as silt and clay, leaving
sands on the beach.
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SORTING


If all of the grains are the same size, the rock is "well sorted."
If there is a mixture of grain sizes, such as sand and clay, or
gravel and sand, the rock is "poorly sorted."
FIGURE 5-12 Sorting of grains in sandstones as seen under the microscope may range
from good sorting (A) to poor sorting (B).
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SORTING
Well-sorted sands tend to have higher porosity
and permeability than poorly-sorted sands (if
they are not tightly cemented), and may be
good reservoirs for petroleum and natural
gas, or good aquifers.
Poor sorting is the result of rapid deposition of
sediment without sorting by currents.
Examples of poorly-sorted sediment include
alluvial fan deposits and glacial till.
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GRAIN SHAPE
Grain shape is described in
terms of rounding of grain
edges and sphericity (equal
dimensions, or how close it is
to a sphere).
 Rounding results from
abrasion against other
particles and grain impact
during transport.
 Very well rounded sand
grains suggest that a sand
may have been recycled
from older sandstones.
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FIGURE 5-15 Shape of sediment particles.
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SEDIMENTARY STRUCTURES
Some sedimentary structures are created by the water or wind
which moves the sediment. Other sedimentary structures
form after deposition—such as footprints, worm trails, or
mudcracks.
Sedimentary structures can provide information about the
environmental conditions under which the sediment was
deposited.
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SEDIMENTARY STRUCTURES
Some structures form in quiet water under low energy
conditions, whereas others form in moving water or
high energy conditions.
Stratification (= layering or bedding) The layers are visible
because of differences in the color, texture, or
composition of adjacent beds.
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GRADED BEDDING
The grain size in a
graded bed is coarser
at the bottom and finer
at the top.
Graded bedding results
when a sediment-laden
current (such as a
turbidity current) begins
to slow down.
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FIGURE 5-20 Graded bedding results when flowing water sorts particles
by size.
45
RIPPLE MARKS
Undulations of the sediment surface produced as wind or
water moves across sand.
Symmetric ripple marks
are produced by waves
FIGURE 5-22 Profiles of ripple marks. (A) Symmetric ripples. (B) Asymmetric
ripples.
Asymmetric ripples form
in unidirectional currents
(such as in streams or
rivers).
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46
MUD CRACKS
A polygonal pattern of cracks produced on the
surface of mud as it dries.
L. E. Davis
Modern mudcracks
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DETERMINING "UP DIRECTION"
Rocks can be overturned by tectonic forces.
Examine sedimentary structures to determine "up
direction."




Graded beds
Cross beds
Mudcracks
Scour marks
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



Symmetrical ripples
Stromatolites
Burrows
Tracks
48
SANDS AND SANDSTONES
Sandstone classification is based on the
composition of the grains.
•Quartz
•Feldspar
•Rock fragments
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MAJOR TYPES OF SANDSTONE




Quartz sandstone—dominated by quartz
Arkose—25% or more feldspar
Graywacke—about 30% dark fine-grained
matrix
Lithic sandstone—quartz, muscovite, chert,
and rock fragments. Less than 15% matrix.
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SANDSTONE INTERPRETATION
Minerals provide information on the amount of
weathering and transport of sand grains.


Intense weathering and long transport produce
sandstone dominated by quartz.
Sandstones with abundant feldspars, and
ferromagnesian minerals indicate relatively little
weathering and transport.
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SANDSTONE ENVIRONMENTAL
INTERPRETATION
Quartz sandstone
 Long time in the depositional basin
 Tectonically stable setting
 Shallow-water environments
FIGURE 5-25 Idealized geologic conditions under which the four major categories of sandstones are deposited. (A) Quartz
sandstone.
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52
SANDSTONE ENVIRONMENTAL
INTERPRETATION
Arkose
 Short time in the
depositional basin
 Rapid erosion


Arid climate
Tectonic activity
FIGURE 5-25 Idealized geologic conditions under which the four major categories of sandstones are deposited. (B) Arkose.
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53
SANDSTONE ENVIRONMENTAL
INTERPRETATION
Graywacke
 Tectonically active source area & basin
 Rapid erosion
FIGURE 5-25 Idealized geologic conditions under which the four major categories of sandstones are deposited.
(C) Graywacke.
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SANDSTONE ENVIRONMENTAL
INTERPRETATION
Lithic sandstone
 Deltaic coastal plains
 Nearshore marine environments
 Swamps or marshes
FIGURE 5-25 Idealized geologic conditions under which the four major categories of sandstones are deposited. (D) Lithic sandstone.
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CARBONATE ROCKS AND SEDIMENTS
Carbonate rocks are chemical or biochemical
in origin.

Limestone



Calcite (CaCO3)
Aragonite (CaCO3)
Dolostone (or Dolomite)

Dolomite (CaMg (CO3)2)
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CARBONATE ROCKS AND SEDIMENTS

Most carbonate rocks form in the shallow
marine environment.

Some form in lakes, caves and hot springs.

Most limestones are the direct or indirect
result of biologic activity.
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CHARACTERISTICS OF MOST MARINE
CARBONATE ENVIRONMENTS
Warm water
 Shallow water (less than 200 m deep)
 Tropical climate (30°N–30°S of equator)
 Clear water (low to no terrigenous input)
 Sunlight required for photosynthesis by algae

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ORIGIN OF CARBONATE SEDIMENTS
Much lime mud forms from the disintegration of
calcareous algae
When calcareous algae
die, their skeletons
disintegrate, producing
aragonite needle
muds. Lime mud
lithifies to form finegrained limestone.
Lynn Walters
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ORIGIN OF OÖIDS

Oöids are tiny spheres
composed of
concentrically laminated
calcium carbonate.

Oöids form in warm
shallow water with
constant wave agitation.
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G. R. Thompson & J. Turk
60
ORIGIN OF CARBONATE SEDIMENTS




Microscopic shells of marine organisms
Abrasion of invertebrate organism shells
Fecal pellets
Precipitation of calcium carbonate from
seawater as a result of biologic activity
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61
DOLOMITE




A calcium-magnesium carbonate mineral
(CaMg(CO3)2).
Makes up sedimentary rock dolostone. (Sometimes
the rock is also called dolomite.)
Forms when magnesium in sea water replaces calcium
in calcium carbonate in a limestone.
Dolomite (or high magnesium calcite) only forms
directly in a few areas of the modern world where
intense evaporation of seawater concentrates the
magnesium.
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CLAY
The word "clay" has two definitions:


A grain size term
A layered silicate mineral which behaves plastically
when wet and hardens upon drying or firing.
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CLAY MINERALS
Clay minerals are complex hydrous
aluminosilicates with atoms arranged in
layered or sheet structures.
•
•
•
Kaolinites —Weathering product of feldspars.
Smectites —May contain magnesium, calcium,
and/or sodium ions. Smectites swell when wet.
Illites—The major clay mineral in ancient shales.
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DEPOSITION OF CLAYS

Because of its fine grain size, clay tends to remain
suspended in the water column. It will settle out of
still, quiet water, given enough time.

Clays and shales typically indicate low energy
environments, sheltered from waves and currents.
They are commonly found in lacustrine, lagoon,
and deeper water marine deposits.
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CLAYSTONE & SHALE

Claystone—A very fine-grained rock composed of clay
minerals, mica, and quartz grains (<1/256 mm).




Grains are too small to see with the naked eye or a hand
lens.
Feels smooth to the touch (not gritty).
Not fissile; it breaks irregularly.
Shale—A very fine-grained rock composed of clay,
mud, and silt.

Shale is fissile: splits readily into thin, flat layers.
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LITHOSTRATIGRAPHIC UNIT
A body of sedimentary, extrusive igneous,
metasedimentary, or metavolcanic rock
distinguished on the basis of lithologic
characteristics (texture, color, composition,
etc.) and stratigraphic position.
The smallest lithostratigraphic rock unit is the
bed.
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67
FORMATIONS
Distinct and different from rock units above
and below.
 Composed of a single rock type or
characteristic set of rock types
 Traceable from exposure to exposure, and of
sufficient thickness to be mappable
 Named for a geographic locality where well
exposed.

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OTHER LITHOSTRATIGRAPHIC UNITS
Organization of lithstratigraphic units from
largest to smallest
Super groups  groups formations  members



A set of similar or related formations is called a group.
Subdivisions within formations are called members.
Virtually all lithostratigraphic units are "time transgressive"
or diachronous (they, or their contacts, cut across time
lines).
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69
FACIES
Facies: The characteristics of a particular rock unit,
which we can use to interpret the depositional
environment.
Each depositional environment grades laterally into
other depositional environments.
FIGURE 5-33 Sedimentary facies (lithofacies) developed in the sea adjacent to a land area.
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70
FACIES AND SEA LEVEL CHANGES
A sea level rise is called a transgression.
 A transgression produces a fining-upward
(deepening-upward) sequence of facies.
 Finer-grained (deeper water) facies overlie
coarser-grained (shallower water) facies.
 Sometimes called an onlap sequence.

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TRANSGRESSION PRODUCES AN ONLAP
SEQUENCE
FIGURE 5-34 Sedimentation during a transgression produces an onlap sequence.
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CAUSES OF TRANSGRESSIONS
Melting of polar ice caps
 Displacement of ocean water by undersea
volcanism
 Localized sinking or subsidence of the land
in coastal areas.

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73
REGRESSIONS
A sea level drop is called a regression.
 A regression produces a coarsening upward
(shallowing-upward) sequence of facies.
 Coarser-grained (shallower water) facies
overlie finer-grained (deeper water) facies.
 This is sometimes called an offlap sequence.

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74
REGRESSION PRODUCES AN OFFLAP
SEQUENCE
FIGURE 5-35 Sedimentation during a regression produces an offlap sequence in which coarser nearshore lithofacies overlie finer offshore lithofacies,
as shown in A.
Causes of Regressions:

Buildup of ice in the polar ice caps

Formation of glaciers

Localized uplift of the land in coastal areas
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75
WALTHER'S LAW
Sedimentary environments that started out side-by-side
will end up overlapping one another over time due to
sea level change.
The vertical sequence of facies mirrors the original
lateral distribution of sedimentary environments.
FIGURE 5-36 An illustration of Walther’s Principle, which states that vertical facies
changes correspond to lateral facies changes.
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76
CORRELATION



Lithostratigraphic correlation—Matching up rock
units on the basis of lithology and stratigraphic
position.
Biostratigraphic correlation—Matching up rock
units on the basis of fossils they contain.
Chronostratigraphic correlation—Matching up rock
units on the basis of age equivalence, as
determined by radioactive dating methods or
fossils.
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77
CONTACTS BETWEEN ROCK UNITS
There are two basic types of contacts
between rock units:
 Conformable
 Unconformable
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78
CONFORMABLE CONTACTS
Conformable contacts between beds of
sedimentary rocks may be either:
Abrupt
or
Gradational
Most abrupt contacts are bedding planes
resulting from sudden minor changes in
depositional conditions.
Gradational contacts represent more gradual
changes in depositional conditions.
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79
UNCONFORMITIES
Unconformable contacts (or unconformities)
are surfaces which represent a gap in the
geologic record, because of either:
 Erosion
or
 Nondeposition
The time represented by this gap can vary
widely, ranging from millions of years to
hundreds of millions of years
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80
TYPES OF
UNCONFORMITIES

Angular
unconformity

Nonconformity

Disconformity
FIGURE 5-40 Three types of erosional unconformities.
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81
DEPICTING THE PAST
Various ways in which the distribution of
rocks can be depicted:







Geologic columns
Stratigraphic cross-sections
Structural cross-sections
Geologic maps
Paleogeographic maps
Isopach maps
Lithofacies maps
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82
STRATIGRAPHIC CROSS-SECTIONS
They correlate geologic columns
from different locations to
show how rock units change in
thickness, lithology, and fossil
content in a given area.
A cross-section is a vertical view
of the interior of the Earth. An
example would be to cut a
piece out of a wedding cake
and view the interior of the
cake.
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FIGURE 5-44 How we correlate half-billion-year-old rock units (lower
Cambrian) in western Montana.
83
STRUCTURAL CROSS-SECTIONS
They show the timing of tilting, folding, and faulting of
rock units. Tops and bottoms of rock units are
plotted by elevation. Folds and faults are depicted
clearly.
FIGURE 5-45 Geologic structural cross-section.
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84
GEOLOGIC MAPS
Geologic maps show
the distribution of
various layers and
types of rocks in an
area.
Map symbols
indicate structural
features (folds,
faults, etc.) and
formation names.
FIGURE 5-46 How to create a geologic map.
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85
PALEOGEOGRAPHIC MAPS
Interpretive maps
which depict the
geography of an
area at some time
in the past.
FIGURE 5-47 Paleogeographic map of Ohio and
adjoining states over 300 million years ago (early
Mississippian Period).
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86
ISOPACH MAPS
Isopach maps show
the thickness of
formations or other
units in an area.
FIGURE 5-49 Isopach map of 450 millionyear-old Upper Ordovician
formations in Pennsylvania and adjoining
states.
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87
LITHOFACIES MAPS
They show the
distribution of
lithofacies that
existed at a
given time over
an area.
FIGURE 5-52 Lithofacies map of
430-million-year-old Lower Silurian
rocks in the eastern United States.
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88
IMAGE CREDITS
• FIGURE 5-1 The tectonic parts of a continent. Source: Harold Levin.
• FIGURE 5-2 Marine, transitional, and continental environments of deposition. Source: Harold Levin.
• FIGURE 5-5 Submarine fan built of land-derived sediment emerges from a submarine canyon. Source: Harold Levin.
• FIGURE 5-9 Tale of two deltas: the Mississippi River (A) and Niger River (B) deltas. Source: Harold Levin.
• FIGURE 5-12 Sorting of grains in sandstones as seen under the microscope may range from good sorting (A) to poor sorting
(B). Source: Harold Levin.
• FIGURE 5-15 Shape of sediment particles. Source: Harold Levin.
• FIGURE 5-20 Graded bedding results when flowing water sorts particles
by size. Source: Harold Levin.
• FIGURE 5-22 Profiles of ripple marks. (A) Symmetric ripples. (B) Asymmetric ripples. Source: Harold Levin.
• FIGURE 5-25 Idealized geologic conditions under which the four major categories of sandstones are deposited. Source:
Harold Levin.
• FIGURE 5-33 Sedimentary facies (lithofacies) developed in the sea adjacent to a land area. Source: Harold Levin.
• FIGURE 5-34 Sedimentation during a transgression produces an onlap sequence. Source: Harold Levin.
• FIGURE 5-35 Sedimentation during a regression produces an offlap sequence in which coarser nearshore lithofacies overlie
finer offshore lithofacies, as shown in A. Source: Harold Levin.
FIGURE 5-36 An illustration of Walther’s Principle, which states that vertical facies
changes correspond to lateral facies changes. Source: Harold Levin.
• FIGURE 5-40 Three types of erosional unconformities. Source: Harold Levin.
FIGURE 5-44 How we correlate half-billion-year-old rock units (lower Cambrian) in western Montana. Source: Modified from
Schmidt et al. 1994. Courtesy of U.S. Geological Survey Bulletin 2025., p. 11.
• FIGURE 5-45 Geologic structural cross-section. Source: Harold Levin.
• FIGURE 5-46 How to create a geologic map. Source: Harold Levin.
• FIGURE 5-47 Paleogeographic map of Ohio and adjoining states over 300 million years ago (early Mississippian Period).
Source: Figure courtesy of the U.S. Geological Survey.
• FIGURE 5-49 Isopach map of 450 million-year-old Upper Ordovician formations in Pennsylvania and adjoining states.
Source: After M. Kay, 1951, Geol. Soc. Amer. Memoir 48.
• FIGURE 5-52 Lithofacies map of 430-million-year-old Lower Silurian rocks in the eastern United States. Source: After T. W.
Amsden, 1995, Bull. Amer. Assoc. Petroleum Geologists, 39:60–74
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