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Transcript
Chapter 6
Sedimentary Rocks—
The Archives of Earth History
History from Sedimentary Rocks
• How do we know whether sedimentary rocks
were deposited on
– continents—river floodplains or desert sand dunes?
– at the water's edge?
– in the sea?
• Sedimentary rocks
– preserve evidence of surface depositional processes
– also, many contain fossils
– These things give clues to the depositional
environment
• Depositional environments are specific areas
– or environments where sediment is deposited
Beach Environment
• Sand deposition
– on a beach along the
Pacific coast
– of the United States
• Many ancient sandstones
– possess features
– that indicate they were
– also deposited on beaches
Sedimentary rocks
• Sedimentary rocks may be
–
–
–
–
–
detrital
or chemical, including biochemical
and all preserve evidence
of the physical, chemical and biological processes
that formed them
• Some sedimentary rocks are or contain
resources
– phosphorous
– liquid petroleum
– natural gas
Phosphorous
• For instance, phosphorous
– from phosphorous-rich sedimentary rocks
– is used in
•
•
•
•
•
•
metallurgy
preserved foods
ceramics
matches
chemical fertilizers
animal-feed supplements
Investigating Sedimentary Rocks
• Observation and data gathering
– visit rock exposures (outcrops)
– carefully examine
•
•
•
•
•
textures
composition
fossils (if present)
thickness
relationships to other rocks
• Preliminary interpretations in the field
– For example:
• red rocks may have been deposited on land
• whereas greenish rocks are more typical of marine
deposits
• (caution: exceptions are numerous)
Investigating Sedimentary Rocks
• More careful study of the rocks
–
–
–
–
microscopic examination
chemical analyses
fossil identification
interpretation of vertical and lateral facies
relationships
– compare with present-day sediments
• Make environmental interpretation
Composition of Detrital Rocks
• Very common minerals in detrital rocks:
– quartz, feldspars, and clay minerals
• Only calcite is very common in limestones
• Detrital rock composition tells
– about source rocks,
– not transport and deposition
• Quartz sand may have been deposited
– in a river system
– on a beach or
– in sand dunes
Composition of
Chemical Sedimentary Rocks
• Composition of chemical sedimentary rocks
– is more useful in revealing environmental
information
• Limestone is deposited in warm, shallow seas
– although a small amount also originates in lakes
• Evaporites such as rock salt and rock gypsum
– indicate arid environments
– where evaporation rates were high
• Coal originates in swamps and bogs on land
Grain Size
• Detrital grain size gives some indication
– of the energy conditions
– during transport and deposition
• High-energy processes
– such as swift-flowing streams and waves
– are needed to transport gravel
• Conglomerate must have been deposited
– in areas where these processes prevail
• Sand transport also requires vigorous currents
• Silt and clay are transported
– by weak currents and accumulate
– only under low-energy conditions
– as in lakes and lagoons
Sorting and Rounding
• Sorting and rounding are two textural features
– of detrital sedimentary rocks
– that aid in determining depositional processes
• Sorting refers to the variation
– in size of particles
– making up sediment or sedimentary rocks
• It results from processes
– that selectively transport and deposit
– sediments of particular sizes
Sorting
• If the size range is not very great,
– the sediment or rock is well sorted
• If they have a wide range of sizes,
– they are poorly sorted
• Wind has a limited ability to transport sediment
– so dune sand tends to be well sorted
• Glaciers can carry any sized particles,
– because of their transport power,
– so glacier deposits are poorly sorted
Rounding
• Rounding is the degree to which
– detrital particles have their sharp corners and edges
– warn away by abrasion
• Gravel in transport is rounded very quickly
– as the particles collide with one another
• Sand becomes rounded
– with considerably more transport
Rounding and Sorting
• A deposit
– of well rounded
– and well sorted
gravel
• Angular, poorly
sorted gravel
Sedimentary Structures
• Sedimentary structures are
– features visible at the scale of an outcrop
– that formed at the time of deposition or shortly
thereafter
– and are manifestations of the physical and
biological processes
– that operated in depositional environments
• Structures
–
–
–
–
–
seen in present-day environments
or produced in experiments
help provide information
about depositional environments of rocks
with similar structures
Bedding
• Sedimentary rocks generally have bedding or
stratification
– Individual layers
less than 1 cm
thick are
laminations
• common in
mudrocks
– Beds are thicker
than 1 cm
• common in rocks
with coarser grains
Graded Bedding
• Some beds show an upward gradual decrease
– in grain size, known as graded bedding
• Graded bedding is
common in
turbidity current
deposits
– which form when
sediment-water
mixtures flow
along the seafloor
– As they slow,
– the largest
particles settle out
– then smaller ones
Cross-Bedding
• Cross-bedding forms when layers come to rest
– at an angle to the surface
– upon which they accumulate
– as on the downwind side of a sand dune
• Cross-beds result from transport
– by either water or wind
• The beds are inclined or dip downward
– in the direction of the prevailing current
• They indicate ancient current directions,
– or paleocurrents
• They are useful for relative dating
– of deformed sedimentary rocks
Cross-Bedding
• Tabular crossbedding forms by
deposition on sand
waves
• Tabular crossbedding in the Upper
Cretaceous Two
Medicine Formation
in Montana
Cross-Bedding
• Trough cross-bedding
formed by migrating
dunes
• Trough cross-beds in
the Pliocene Six Mile
Creek Formation,
Montana
Ripple Marks
• Small-scale alternating ridges and troughs
– known as ripple marks are common
– on bedding planes, especially in sandstone
• Current ripple marks
–
–
–
–
form in response to water or wind currents
flowing in one direction
and have asymmetric profiles allowing geologists
to determine paleocurrent directions
• Wave-formed ripple marks
– result from the to-and-fro motion of waves
– tend to be symmetrical
• Useful for relative dating of deformed
sedimentary rocks
Current Ripple Marks
• Ripples with an
asymmetrical shape
• In the close-up of
one ripple,
– the internal structure
– shows small-scale
cross-bedding
• The photo shows
current ripples
– that formed in a
small stream channel
– with flow from right
to left
Wave-Formed Ripples
• As the waves
wash back
and forth,
– symmetrical
ripples form
• The photo
shows waveformed ripple
marks
– in shallow
seawater
Mud Cracks
• When clay-rich sediments dry, they shrink
– and crack into polygonal patterns
– bounded by fractures called mud cracks
• Mud cracks require wetting and drying to form,
– as along a
lakeshore
– or a river flood
plain
– or where mud is
exposed at low
tide along a
seashore
Ancient Mud Cracks
• Mud cracks in
ancient rocks
– in Glacier
National
Park,
Montana
• Mud cracks
typically fill in
– with sediment
– when they are
preserved
– as seen here
Biogenic Sedimentary Structures
• Biogenic sedimentary structures include
– tracks
– burrows
– trails
• called trace fossils
• Extensive burrowing by organisms
– is called bioturbation
• It may alter sediments so thoroughly
– that other structures are disrupted or destroyed
Bioturbation
• U-shaped burrows
• Vertical burrows
Bioturbation
• Vertical, dark-colored areas in this rock are
sediment-filled burrows
– Could you use burrows such as these to relatively
date layers in deformed sedimentary rocks?
No Single Structure Is Unique
• Sedimentary structures are important
– in environmental analyses
– but no single structure is unique to a specific
environment
• Example:
– Current ripples are found
• in stream channels
• in tidal channels
• on the sea floor
• Environmental determinations
– are usually successful with
– associations of a groups of sedimentary structures
– taken along with other sedimentary rock properties
Geometry of Sedimentary Rocks
• The three-dimensional shape or geometry
–
–
–
–
–
–
–
–
of a sedimentary rock body
may be helpful in environmental analyses
but it must be used with caution
because the same geometry may be found
in more than one environment
can be modified by sediment compaction
during lithification
and by erosion and deformation
• Nevertheless, it is useful in conjunction
– with other features
Blanket or Sheet Geometry
• Some of the most extensive sedimentary rocks
– in the geologic record result from
– marine transgressions and regressions
• The rocks commonly cover
– hundreds or thousands of square kilometers
– but are perhaps only
– a few tens to hundreds of meters thick
• Their thickness is small compared
– to their length and width
• Thus, they are said to have
– blanket or sheet geometry
Elongate or Shoestring Geometry
• Some sand deposits have an elongate or
shoestring geometry
– especially those deposited in
• stream channels
• or barrier islands
Other Geometries
• Delta deposits tend to be lens shaped
– when viewed in cross profile or long profile
– but lobate when observed from above
• Buried reefs are irregular
– but many are long and narrow
– or rather circular
Fossils—The Biological Content
of Sedimentary Rocks
• Fossils
– are the remains or traces of prehistoric organisms
– can be used in stratigraphy for relative dating and
correlation
– are constituents of rocks, sometimes making up the
entire rock
– and provide evidence of depositional environments
• Many limestones are composed
– in part or entirely of shells or shell fragments
• Much of the sediment on the deep-seafloor
– consists of microscopic shells of organisms
Fossils Are Constituents of
Sedimentary Rocks
• This variety of
limestone,
– known as
coquina,
– is made entirely
of shell
fragments
Fossils in
Environmental Analyses
• Did the organisms in question live where they
were buried?
• Or where their remains or fossils transported
there?
• Example:
–
–
–
–
–
Fossil dinosaurs usually indicate deposition
in a land environment such as a river floodplain
But if their bones are found in rocks with
clams, corals and sea lilies,
we assume a carcass was washed out to sea
Environmental Analyses
• What kind of habitat did the organisms
originally occupy?
• Studies of a fossil’s structure
– and its living relatives, if any,
– help environmental analysis
• For example: clams with heavy, thick shells
– typically live in shallow turbulent water
– whereas those with thin shells
– are found in low-energy environments
• Most corals live in warm, clear,
– shallow marine environments where
– symbiotic bacteria can carry out photosynthesis
Microfossils
• Microfossils are particularly useful
– because many individuals can be recovered
– from small rock samples
• In oil-drilling operations, small rock chips
– called well cuttings are brought to the surface
• These cuttings rarely
– contain complete fossils of large organisms,
– but they might have thousands of microfossils
– that aid in relative dating and environmental
analyses
Trace Fossils In Place
• Trace fossils, too, may be characteristic of
particular environments
• Trace fossils, of course, are not transported
from their original place of origin
Depositional Environments
• A depositional environment
–
–
–
–
is anywhere sediment accumulates
especially a particular area
where a distinctive kind of deposit originates
from physical, chemical, and biological processes
• Three broad areas of deposition include
–
–
–
–
continental
transitional
marine
each of which has several specific environments
Depositional Environments
Continental environments
Transitional environments
Marine
environments
Continental Environments
• Deposition on continents (on land) might take
place in
– fluvial systems – rivers and streams
– deserts
– areas covered by and adjacent to glaciers
• Deposits in each of these environments
– possess combinations of features
– that allow us to differentiate among them
Fluvial
• Fluvial refers to river and stream activity
– and to their deposits
• Fluvial deposits accumulate in either of two
types of systems
• One is a braided stream system
–
–
–
–
with multiple broad, shallow channels
in which mostly sheets of gravel
and cross-bedded sand are deposited
mud is nearly absent
Braided Stream
• The deposits of braided streams are mostly
– gravel and cross-bedded sand with subordinate mud
Braided Stream Deposits
• Braided stream
deposits consist of
– conglomerate
– cross-bedded
sandstone
– but mudstone is rare
or absent
Fluvial Systems
• The other type of system is a meandering
stream
– with winding channels
– mostly fine-grained sediments on floodplains
– cross-bedded sand bodies with shoestring
geometry
– point-bar deposits consisting of a sand body
– overlying an erosion surface
– that developed on the convex side of a meander
loop
Meandering Stream
• Meandering
stream
deposits
– are mostly fine-grained floodplain
– sediments with subordinate sand bodies
Meandering Stream Deposits
• In meandering stream
deposits,
– mudstone deposited in a
floodplain is common
– sandstones are point bar
deposits
– channel conglomerate is
minor
Desert Environments
• Desert environments contain an association of
features found in
– sand dune deposits,
– alluvial fan deposits,
– and playa lake deposits
• Windblown dunes are typically composed
– of well-sorted, well-rounded sand
– with cross-beds meters to tens of meters high
– land-dwelling plants and animals make up any
fossils
Associations in Desert Basin
• A desert basin showing the
association
– of alluvial fan,
– sand dune,
– and playa lake deposits
• In the photo,
– the light colored area in the
distance
– is a playa lake deposit in
Utah
Dune Cross-Beds
• Large-scale crossbeds
– in a Permian-aged
– wind-blown dune
deposit in Arizona
Alluvial Fans and Playa Lakes
• Alluvial fans form best along the margins of
desert basins
–
–
–
–
where streams and debris flows
discharge from mountains onto a valley floor
They form a triangular (fan-shaped) deposit
of sand and gravel
• The more central part of a desert basin
– might be the site of a temporary lake, a playa lake,
– in which laminated mud and evaporites accumulate
Glacial Environments
• All sediments deposited in
– glacial environments are collectively called drift
• Till is poorly sorted, nonstratified drift
– deposited directly by glacial ice
– mostly in ridge-like deposits called moraines
• Outwash is sand and gravel deposited
– by braided streams issuing from melting glaciers
• The association of these deposits along with
– scratched (striated) and polished bedrock
– is generally sufficient to conclude
– that glaciers were involved
Moraines and Till
• Origin of glacial drift
• Moraines and poorly sorted till
Glacial Varves
• Glacial lake deposits show
– alternating dark and light laminations
• Each dark-light couplet is a varve,
– representing one year’s accumulation of sediment
– light layers accumulate in summer
– dark in winter
• Dropstones
– liberated from
icebergs
– may also be
present
– Varves with a
dropstone
Transitional Environments
• Transitional environments include those
– with both marine and continental processes
• Example:
–
–
–
–
Deposition where a river or stream (fluvial system)
enters the sea
yields a body of sediment called a delta
with deposits modified by marine processes,
especially waves and tides
• Transitional environments include
–
–
–
–
deltas
beaches
barrier islands and lagoons
tidal flats
Transitional Environments
Transitional environments
Simple Deltas
• The simplest deltas are those in lakes and
consist of
– topset beds
– foreset beds
– bottomset
beds
– As the delta
builds
outward it
progrades
–
–
–
–
and forms a vertical sequence of rocks
that becomes coarser-grained from the bottom to top
The bottomset beds may contain marine (or lake) fossils,
whereas the topset beds contain land fossils
Marine Deltas
• Marine deltas rarely conform precisely
– to this simple threefold division because
– they are strongly influenced
– by one or more modifying processes
• When fluvial processes prevail
– a stream/river-dominated delta results
• Strong wave action
– produces a wave dominated delta
• Tidal influences
– result in tide-dominated deltas
Stream/River-Dominated Deltas
• Stream/riverdominated
deltas
– have long
distributary
channels
– extending far
seaward
– Mississippi
River delta
Wave-Dominated Deltas
• Wavedominated
deltas
– such as the Nile
Delta of Egypt
– also have
distributary
channels
– but their
seaward margin
– is modified by
wave action
Tide-Dominated Deltas
• Tide-Dominated Deltas,
– such as the Ganges-Brahmaputra delta
– of Bangladesh
– have
tidal
sand
bodies
– along the
direction
of tidal
flow
Barrier Islands
• On broad continental margins
– with abundant sand, long barrier islands lie offshore
– separated from the mainland by a lagoon
• Barrier islands are common along the Gulf
– and Atlantic Coasts of the United States
• Many ancient deposits formed in this
environment
• Subenvironments of a barrier island complex:
– beach sand grading offshore into finer deposits
– dune sands contain shell fragments
• not found in desert dunes
– fine-grained lagoon deposits
– with marine fossils and bioturbation
Barrier Island Complex
• Subenvironments of a barrier island complex
Tidal Flats
• Tidal flats are present
– where part of the shoreline is periodically covered
– by seawater at high tide and then exposed at low tide
• Many tidal flats build or prograde seaward
– and yield a sequence of rocks grading upward
– from sand to mud
• One of their most distinctive features
– is sets of cross-beds that dip in opposite directions
Tidal Flats
• Tidal-flat deposits showing a prograding
shoreline
– Notice the distinctive cross-beds
– that dip in opposite directions
– How could this happen?
Marine Environments
• Marine environments include:
–
–
–
–
continental shelf
continental slope
continental rise
deep-seafloor
• Much of the detritus eroded from continents
– is eventually deposited in marine environments
• but sediments derived from chemical
– and organic activity are found here as well, such as
• limestone
• evaporites
• both deposited in shallow marine environments
Marine Environments
Marine
environments
Detrital Marine Environments
• The gently sloping area adjacent to a continent
– is a continental shelf
• It consists of a high-energy inner part that is
– periodically stirred up by waves and tidal currents
• Its sediment is mostly sand,
– shaped into large cross-bedded dunes
• Bedding planes are commonly marked
– by wave-formed ripple marks
• Marine fossils and bioturbation are typical
Slope and Rise
• The low-energy part of the shelf
– has mostly mud with marine fossils,
– and interfingers with inner-shelf sand
• Much sediment derived from the continents
– crosses the continental shelf
– and is funneled into deeper water
– through submarine canyons
• It eventually comes to rest
– on the continental slope and continental rise
– as a series of overlapping submarine fans
Slope and Rise
• Once sediment passes the outer margin
– of the self, the shelf-slope break,
– turbidity currents transport it
• So sand with graded bedding is common
• Also common is mud that settled from
seawater
Detrital Marine Environments
• Shelf, slope and rise environments
• The main avenues of sediment transport
– across the shelf are submarine canyons
Turbidity currents
carry sediment
to the
submarine fans
Sand with
graded bedding
and mud settled
from seawater
Deep Sea
• Beyond the continental rise, the seafloor is
– nearly completely covered by fine-grained deposits
• no sand and gravel
– or no sediment at all
• near mid-ocean ridges
• The main sources of sediment are:
–
–
–
–
windblown dust from continents or oceanic islands
volcanic ash
shells of microorganisms dwelling
in surface waters of the ocean
Deep Sea
• Types of sediment are:
– pelagic clay,
• which covers most of the deeper parts
• of the seafloor
– calcareous (CaCO3) and siliceous (SiO2) oozes
• made up of microscopic shells
Carbonate Environments
• Carbonate rocks are
– limestone, which is composed of calcite
– dolostone, which is composed of dolomite
• most dolostone is altered limestone
• Limestone is similar to detrital rock in some
ways
– Many limestones are made up of
• gravel-sized grains
• sand-sized grains
• microcrystalline carbonate mud called micrite
– but the grains are all calcite
– and are formed in the environment of deposition,
– not transported there
Limestone Environments
• Some limestone form in lakes,
–
–
–
–
but most limestone by is deposited
in warm shallow seas
on carbonate shelves and
on carbonate platforms rising from oceanic depths
• Deposition occurs where
– little detrital sediment, especially mud, is present
• Carbonate barriers form in high-energy areas
and may be
– reefs
– banks of skeletal particles
– accumulations of spherical carbonate grains known
as oolites
• which make up the grains in oolitic limestone
Carbonate Shelf
• The
carbonate
shelf is
attached to
a continent
– Examples
occur in
southern
Florida
and the
Persian
Gulf
Carbonate Platform
• Carbonates may be deposited on a platform
– rising from oceanic depths
• This example shows a cross-section
– of the present-day Great Bahama Bank
– in the Atlantic Ocean southeast of Florida
Carbonate Subenvironments
• Reef rock tends to be
– structureless
– composed of skeletons of corals, mollusks, sponges
and other organisms
• Carbonate banks are made up of
– layers with horizontal beds
– cross-beds
– wave-formed ripple marks
• Lagoons tend to have
– micrite
– with marine fossils
– bioturbation
Evaporite Environments
• Evaporites consist of
– rock salt
– rock gypsum
• They are found in environments such as
– playa lakes
– saline lakes
– but most of the extensive deposits formed in the
ocean
• Evaporites are not nearly as common
– as sandstone, mudrocks and limestone,
– but can be abundant locally
Evaporites
• Large evaporite deposits
– lie beneath the Mediterranean Seafloor
• more than 2 km thick
– in western Canada, Michigan, Ohio, New York,
– and several Gulf Coast states
• How some of these deposits originated
– is controversial, but geologists agree
– that high evaporation rates of seawater
– caused minerals to precipitate from solution
• Coastal environments in arid regions
– such as the present-day Persian Gulf
– meet the requirements
Evaporites
• Evaporites
could form
• in an
environment
similar to this
• if the area
were in an
arid region,
– with restricted inflow of normal seawater
– into the lagoon
– leading to increased salinity and salt depositions
Environmental Interpretations
and Historical Geology
• Present-day gravel
deposits
– by a swiftly-flowing stream
– Most transport and
deposition takes place
when the stream is higher
• Nearby gravel deposit
probably less than a few
thousand years old
Environmental Interpretations
and Historical Geology
• Conglomerate
more than 1 billion
years old
– shows similar
features
• We infer that it too was deposited
–
–
–
–
by a braided stream in a fluvial system
Why not deposition by glaciers or along a seashore?
Because evidence is lacking for either
glacial activity or transitional environment
Interpretation
• Jurassic-aged Navajo Sandstone
– of the Southwestern United states
– has all the features of wind-blown sand dunes:
•
•
•
•
•
•
•
•
the sandstone is mostly well-sorted, well-rounded quartz
measuring 0.2 to 0.5 mm in diameter
tracks of land-dwelling animals,
including dinosaurs, are present
cross-beds up to 30 m high have current ripple marks
like those produced on large dunes by wind today
cross-beds dip generally southwest
indicating a northeast prevailing wind
Navajo Sandstone
Checkerboard Mesa,
Zion National Park,
Utah
– Vertical
fractures
– intersect
cross beds
of desert
dunes
– making the
checkerboard
pattern
Paleogeography
• Paleogeography deals with
– Earth’s geography of the past
• Using interpretations
– of depositional environment
– such as the ones just discussed
• we can attempt to reconstruct
– what Earth’s geography was like
– at these locations at various times in the past
• For example,
– the Navajo Sandstone shows that a vast desert
– was present in what is now the southwest
– during the Jurassic Period
Paleogeography
– and from Late Precambrian to Middle Cambrian
– the shoreline migrated inland from east and west
– during a marine transgression
Paleogeography
• Detailed studies of various
rocks
–
–
–
–
–
in several western states
allow us to determine
with some accuracy
how the area appeared
during the Late Cretaceous
• A broad coastal plain
– sloped gently eastward
– from a mountainous region
– to the sea
Paleogeography
• Later, vast lakes,
– river floodplains, alluvial fans
– covered much of this area
– and the sea had withdrawn
from the continent
• Interpretations the geologic
record
– we examine later
– will be based on similar
– amounts of supporting
evidence
Summary
• The physical and biological features
– of sedimentary rocks reveal something about
– the depositional processes that form them
• Environmental analysis
–
–
–
–
of sedimentary rocks uses
mainly sedimentary structures and fossils
but also textures, rock body geometry
and even composition
• Geologists recognize
– three primary depositional areas
– continental, transitional, and marine
– each with several specific environments
Summary
• Fluvial systems might be braided or
meandering
– Braided streams deposit mostly sand and
gravel,
– whereas deposits of meandering streams are
mostly mud and subordinate sand bodies with
shoestring geometry
• An association of alluvial fan, sand dune,
– and playa lake deposits
– is typical of desert depositional environments
• Glacial deposits consist mostly of till
– in moraines and outwash
Summary
• The simplest deltas, those in lakes,
– consist of a three-part sequence of rocks
– grading from finest at the base,
– upward to coarser-grained rocks
• Marine deltas dominated by
– fluvial processes, waves, or tides
– are much larger and more complex
• A barrier island system includes beach,
– dune, and lagoon subenvironments,
– each characterized a unique association
– of rocks, sedimentary structures, and fossils
Summary
• Inner shelf deposits are mostly sand,
– whereas those of the outer shelf are mostly mud;
– both have marine fossils and bioturbation
• Much of the sediment from land
– crosses the shelves and is deposited
– on the continental slope and rise as submarine fans
• Either pelagic clay or oozes
– derived from the shells of
– microscopic floating organisms cover
– most of the deep seafloor
Summary
• Most limestone originates in shallow,
– warm seas where little detrital mud is present
• Carbonate rocks (just as detrital rocks)
–
–
–
–
may possess cross-beds, ripple marks,
mud cracks, and fossils
that provide information
about depositional processes
• Evaporites form in several environments,
– but the most extensive ones were deposited
– in marine environments
• In all cases, though, they formed
– in arid regions with high evaporation rates
Summary
• With information from sedimentary rocks,
– as well as other rocks,
– geologists determine the past distribution
– of Earth's surface features