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Chapter 6
Sedimentary Rocks—
The Archives of Earth History
Sedimentary Environments
and
Structures
Sedimentary Rocks
• formed at or near the surface at relatively low temperatures.
• from sediments which include boulders, cobbles, gravels, sands, silts, and
clay particles.
• OR particles which are suspended and dissolved in water.
• 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: continent, marine, transitional
– environments where sediment is deposited
such as beach, desert, stream, lake, marine
Beach Environment
• Sand deposition
• Sand-size particles are deposited on a
beach along the Pacific coast of the
United States
• After many years and layers of
deposition, sand is compacted, and
eventually cemented to form sandstone.
• Many ancient sandstones
– possess features that indicate they were
also deposited on beaches
• Present day despositional environments
are used as models to help decode the
rocks of the past. (uniformitarianism)
Sedimentary rocks
• Sedimentary rocks may be
– Detrital
– chemical,
including biochemical
– Each rock preserves evidence of how it was formed
Investigating Sedimentary Rocks
• Observation and data gathering
– carefully examine
•
•
•
•
•
textures
composition
fossils (if present)
Thickness of rock bed
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)
Grain Size
• Detrital grain size
– energy conditions
– transport and deposition
• High-energy processes
– streams and waves
• transport gravel
– Rock type: Conglomerate
– Sand transport also requires vigorous currents
• 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
Rounding and Sorting
• A deposit
– of well rounded
– and well sorted
gravel
• Angular, poorly
sorted gravel
Bedding
• Sedimentary rocks generally have bedding or
stratification
– Individual layers
less than 1 cm
thick are
laminations
• common in
Sedimentary
Structures
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
• 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
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 to any one
environment
• 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
Fossils Are Constituents of
Sedimentary Rocks
• This variety of
limestone,
– known as
coquina,
– is made entirely
of shell
fragments
Depositional Environments
Continental environments
Transitional environments
Marine
environments
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
Meandering Stream
• Meandering
stream
deposits
– are mostly fine-grained floodplain
– sediments with subordinate sand bodies
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
– Glacial deposits and 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
– Indicates that glaciers were involved
Moraines and Till
• Origin of glacial drift
Glacial
Dropstone
• Glacial
lake deposits
show
in Varves
– alternating dark and light laminations
• Each dark-light couplet is a varve,
• Dropstones
– representing one year’s accumulation of sediment
– liberated
–
light
layers
accumulate
in
summer
from
– dark in winter
icebergs
– may also be
present
– Varves with
a dropstone
Moraines are made of poorly sorted till
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
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
Stream/River-Dominated Deltas
• Stream/riverdominated
deltas
– have long
distributary
channels
– extending far
seaward
– Mississippi
River delta
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 Island Complex
• Subenvironments of a barrier island complex
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
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