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Chapter 5
The Sedimentary Archives
Factors affecting Sedimentary
Characteristics
1. Tectonic setting
2. Physical, chemical, and biological
processes in the depositional
environment
3. Method of sediment transport
4. Rocks in the source area from which the
sediment is derived
5. Climate (and its effect on weathering)
6. Post-depositional processes of
lithification (cementation, compaction)
7. Time
Tectonics
The forces controlling deformation or
structural behavior of a large area of the
Earth's crust over a long period of time.
Structural Behavior
• Tectonically stable - midwestern 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
Principle
tectonic elements
of a continent
•Craton
- Shield
- Platform
•Orogenic belt
Principle tectonic elements of
a continent – the 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.
Principle tectonic elements of
a continent
• Orogenic belts - Elongated regions
bordering the craton which have been
deformed by compression since
Precambrian. Orogenic belts are mountain
belts.
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.
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.
Depositional Environments
There are three broad categories of
depositional environments:
• Marine environments (ocean)
• Transitional environments (along contact
between ocean and land)
• Continental environments (on land)
Depositional Environments
Marine Depositional Environments
1.
2.
3.
4.
Continental shelf
Continental slope
Continental rise
Abyssal plain
Marine Depositional Environments
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
Continental Shelf – cont'd
e. Covered by sand, silt, and clay
f. Larger sedimentary grains are deposited
closer to shore.
g. Locally cut by submarine canyons
(eroded by rivers during Ice Age low sea
level stand)
h. Coral reefs and carbonate sediments
may accumulate in tropical areas
Continental Slope
The steeper slope at edge of the continent.
a. Located seaward of the continental shelf
b. Boundary between continental and
oceanic crust
c. May be about 20 km wide
Continental Slope – cont'd
d. Deeper water
e. More steeply inclined (3 - 6o)
f. Rapid sediment transport down the slope
by dense, muddy turbidity currents
g. Passes seaward into the continental rise
Continental Rise
At the base of the continental slope.
a.
b.
c.
d.
e.
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)
f. Passes seaward into the abyssal plain
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)
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
Deltas
a. Fan-shaped accumulations of sediment
b. Formed where a river flows into a standing
body of water, such as a lake or the sea
c. Coarser sediment (sand) tends to be deposited
near the mouth of the river; finer sediment is
carried seaward and deposited in deeper
water.
d. The delta builds seaward (or progrades) as
sediment is deposited at the river mouth.
Deltas
Mississippi River delta
Niger River delta
Beaches and Barrier Islands
a. Shoreline deposits
b. Exposed to wave
energy
c. Dominated by sand
Beaches and Barrier Islands
d. Marine fauna
e. A few km or less in width but may be more
than 100 km long
f. Separated from the mainland by a lagoon
(or salt marsh)
g. May be associated with tidal flat deposits
Lagoons
a. Bodies of water on the landward side of
barrier islands
b. Protected from the pounding of the
ocean waves by barrier islands
c. Contain finer sediment than the beaches
(usually silt and clay)
d. Lagoons are also present behind reefs,
or in the center of atolls.
Tidal flats
a. Nearly flat, low relief areas that border
lagoons, shorelines, and estuaries
b. Periodically flooded and exposed by
tides (usually twice each day)
c. May be cut by meandering tidal channels
d. May be marshy, muddy, sandy or mixed
sediment types (terrigenous or
carbonate)
Tidal flats
e. Laminations and ripples are common
f. Sediments are intensely burrowed
g. Stromatolites may be present (if
conditions are appropriate)
Estuaries
a.
b.
c.
d.
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
e. Many estuaries formed due to sea level rise as
glaciers melted at end of last Ice Age
f. Some formed due to tectonic subsidence,
allowing sea water to migrate upstream
Continental Environments
1.
2.
3.
4.
5.
Rivers or fluvial environments
Alluvial fans
Lakes (or lacustrine environments)
Glacial environments
Eolian environments
Fluvial Environments
a. Braided and meandering river and stream
systems
b. River channels, bars, levees, and floodplains
are subenvironments
c. Channel deposits are coarse, rounded gravel,
and sand.
d. Bars are sand or gravel.
e. Levees are fine sand or silt.
f. Floodplains are covered by silt and clay.
Alluvial Fans
a. Fan-shaped deposits
at base of mountains.
b. Most common in arid
and semi-arid regions
with rapid erosion.
c. Sediment is coarse,
poorly- sorted gravel
and sand.
Lacustrine Environments (Lakes)
a. May be large or small
b. May be shallow or deep
c. Filled with terrigenous, carbonate, or evaporitic
sediments
d. Sediments are typically fine grained but may
be coarse near the edges
e. Fine sediment and organic matter settling in
some lakes produced laminated oil shales
f. Playa lakes are shallow, temporary lakes that
form in arid regions They periodically dry up as
a result of evaporation
Glacial Environments
a. Sediment is eroded, transported, and
deposited by ice (glaciers)
b. Glacial deposits called till contain large
volumes of unsorted mixtures of
boulders, gravel, sand and clay
Eolian Environments
a. Wind is the agent
of sediment
transport and
deposition
b. Dominated by sand
and silt
c. Common in many
desert regions
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.
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.
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.
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.
Three "textural components" to
most clastic sedimentary rocks:
1. Clasts - the larger grains in the rock (gravel,
sand, silt)
2. Matrix - the fine-grained material surrounding
clasts (often clay)
3. Cement - the "glue" that holds the rocks
together
a.
b.
c.
d.
Silica (quartz, SiO2)
Calcite (CaCO3)
Iron oxide
Other minerals
Grain Size
Sedimentary grains are categorized according to
size using the Wentworth Scale.
Gravel > 2 mm
Sand
1/16 - 2 mm
Silt
1/256 - 1/16 mm
Clay
< 1/256 mm
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.
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."
Sorting
Well-sorted sands tend to have higher
porosity and permeability than poorlysorted sands (if they are not tightly
cemented), and may be good reservoirs
for petroleum and natural gas, or good
aquifers.
Sorting
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.
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
• 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.
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
Sedimentary structures can provide
information about the environmental
conditions under which the sediment was
deposited.
Some structures form in quiet water under
low energy conditions, whereas others
form in moving water or high energy
conditions.
Sedimentary Structures
Stratification (= layering or bedding) is the
most obvious feature of sedimentary
rocks. The layers (or beds or strata) are
visible because of differences in the color,
texture, or composition of adjacent beds.
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.
Cross-bedding or cross-stratification
An arrangement of beds or laminations in
which one set of layers is inclined relative
to the others.
water flow
direction
Ripple marks
Undulations of the sediment surface
produced as wind or water moves across
sand.
Symmetric ripple marks are produced by
waves
Ripple marks
Asymmetric ripples form in unidirectional
currents (such as in streams or rivers).
Mud cracks
A polygonal pattern of cracks produced on
the surface of mud as it dries.
Modern mudcracks
Fossil mudcracks
Determining "up direction"
Rocks can be overturned by tectonic forces.
Examine sedimentary structures to determine
"up direction."
•
•
•
•
Graded beds
Cross beds
Mudcracks
Scour marks
•
•
•
•
Symmetrical ripples
Stromatolites
Burrows
Tracks
Sands and Sandstones
Sandstone classification is based on the
composition of the grains.
•Quartz
•Feldspar
•Rock fragments
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.
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.
Sandstone Environmental
Interpretation
Quartz sandstone
• Long time in the depositional basin
• Tectonically stable setting
• Shallow-water environments
Sandstone Environmental
Interpretation
Arkose
• Short time in the depositional basin
• Rapid erosion
• Arid climate
• Tectonic activity
Sandstone Environmental
Interpretation
Graywacke
• Tectonically active source area & basin
• Rapid erosion
Sandstone Environmental
Interpretation
Lithic sandstone
• Deltaic coastal plains
• Nearshore marine environments
• Swamps or marshes
Carbonate Rocks and Sediments
Carbonate rocks are chemical or
biochemical in origin.
•
Limestone
– Calcite (CaCO3)
– Aragonite (CaCO3)
•
Dolostone (or Dolomite)
– Dolomite (CaMg (CO3)2)
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.
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
Origin of carbonate sediments
Much lime mud forms from the disintegration
of calcareous algae
Origin of carbonate sediments
When calcareous algae die, their skeletons
disintegrate, producing aragonite needle
muds. Lime mud lithifies to form finegrained limestone.
Origin of Oöids
• Oöids are tiny spheres
composed of
concentrically laminated
calcium carbonate.
• Oöids form in warm
shallow water with
constant wave agitation.
Origin of carbonate sediments
• Microscopic shells
of marine organisms
• Abrasion of shells
• Precipitation of
calcium carbonate
from seawater as a
result of biologic
activity
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
in a few areas of the world where intense
evaporation of seawater concentrates the
magnesium.
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.
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.
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.
Shale
• A very fine-grained
rock composed of
clay, mud, and silt.
• Shale is fissile splits readily into
thin, flat layers.
Claystone
• A very fine-grained rock composed of tiny
(less than 1/256 mm) clay minerals, mica,
and quartz grains.
• Individual 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.
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.
Formations
• Lithologically homogeneous
• Distinct and different from rock units above
and below.
• Traceable from exposure to exposure, and
of sufficient thickness to be mappable
• Named for a geographic locality where
well exposed.
Other lithostratigraphic units
• Subdivisions within formations are
called members.
• A set of similar or related formations is
called a group.
Virtually all lithostratigraphic units are "time
transgressive" or diachronous (they, or their
contacts, cut across time lines).
Red lines G and O are time lines.
Facies
The characteristics of a particular rock unit,
which we can use to interpret the
depositional environment.
Every depositional environment puts a
distinctive imprint on the sediment, making
a particular facies.
Facies Change
Each depositional environment grades laterally
into other depositional environments.
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.
Sedimentation during a transgression
produces an onlap sequence.
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.
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.
Sedimentation during a regression
produces an offlap sequence.
Causes of Regressions
• Buildup of ice in the polar ice caps
• Formation of glaciers
• Localized uplift of the land in coastal areas
Walther's Law
Sedimentary environments that started out side-byside 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.
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.
Lithostratigraphic correlation
Demonstration of lithostratigraphic correlation
from one exposure to another.
Contacts between Rock Units
There are two basic types of contacts
between rock units:
– Conformable
– Unconformable
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.
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
Types of unconformities
• Angular unconformity
• Nonconformity
• Disconformity
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
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.
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.
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.
Paleogeographic Maps
Interpretive maps
which depict the
geography of an
area at some time
in the past.
Isopach Maps
Isopach maps show
the thickness of
formations or other
units in an area.
Lithofacies Maps
They show the distribution of lithofacies that
existed at a given time over an area.