Sedimentary Rocks The Archives of Earth History

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Chapter 6
Sedimentary RocksThe Archives of Earth History
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Sedimentary rocks

• Sedimentary rocks
• preserve evidence
• of the physical, chemical and biological
  processes
• that formed them
• Some sedimentary rocks are resources, or contain
  resources
• phosphorous
• liquid petroleum
• natural gas

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Investigating Sedimentary Rocks

• Observation and data gathering
• by visiting rock exposures (outcrops)
• and carefully examining
• textures
• composition
• fossils (if present)
• thickness
• relationships to other rocks
• Preliminary interpretations in the field may be
  made
• For example
• red rocks may have been deposited on land
• whereas greenish rocks are more typical of marine
  deposits
• (Caution Exceptions are numerous!)

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Composition of Detrital Rocks

• Very common minerals in detrital rocks
• quartz, feldspars, and clay minerals
• 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

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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

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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 high-energy
  transport
• Silt and clay are transported
• by weak currents and accumulate
• only under low-energy conditions
• as in lakes and lagoons

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Sorting and Rounding

• Texture refers to the size, shape, and
  arrangement of clasts
• 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

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Rounding and Sorting

• A deposit
• of well rounded
• and well sorted gravel

• Versus a deposit
• of angular
• poorly sorted gravel

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Sedimentary Structures

• Sedimentary structures are
• features 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

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Bedding

• Sedimentary rocks generally have bedding or
  stratification

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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

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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

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Cross-Bedding

• Individual beds are deposited at an angle

• Cross-bedding in sandstone at Natural Bridges
  National Monument, Utah

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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
• and tend to be symmetrical
• Useful features for relative dating of deformed
  sedimentary rocks

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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

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Wave-Formed Ripples

• As the waves wash back and forth,
• symmetrical ripples form
• The photo shows wave-formed ripple marks
• in shallow seawater

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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

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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

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Biogenic Sedimentary Structures

• Biogenic sedimentary structures include
• tracks
• burrows
• trails
• called trace fossils
• Extensive burrowing by organisms
• is called bioturbation
• and may alter sediments so thoroughly
• that other structures are disrupted or destroyed

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Bioturbation

• U-shaped burrows

• Vertical burrows

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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 groups of sedimentary structures
• taken along with other sedimentary rock properties

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FossilsThe 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 important constituents of rocks, sometimes
  making up the entire rock
• and provide evidence for 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

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Fossils Are Constituents of Sedimentary Rocks

• This variety of limestone,
• known as coquina,
• is made entirely of shell fragments

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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

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Environmental Analyses

• What kind of habitat did the organisms originally
  occupy?
• Studies of a fossils 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

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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

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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

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Depositional Environments

• Continental environments

Transitional environments
Marine environments
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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

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Fluvial

• Fluvial refers to river and stream activity
• and to their deposits
• Fluvial deposits accumulate in either of two
  types of systems
• Braided stream system
• with multiple broad, shallow channels
• in which mostly sheets of gravel
• and cross-bedded sand are deposited
• mud is nearly absent

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Braided Stream

• The deposits of braided streams are mostly
• gravel and cross-bedded sand with subordinate mud

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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
  geometries
• point-bar deposits consisting of a sand body
• overlying an erosion surface
• that developed on the convex side of a meander
  loop

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Meandering Stream

• Meandering stream deposits

• are mostly fine-grained floodplain
• sediments with subordinate sand bodies

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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

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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

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Associations in Desert Basin

• Huge alluvial fans formed at the base of the
  Panamint Mountains, Death Valley
• Sand dunes also are present in Death Valley

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Dune Cross-Beds

• Large-scale cross-beds
• in a Permian-aged
• wind-blown dune deposit in Arizona

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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

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Moraines and Till

• Moraines and poorly sorted till

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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
• barrier islands and lagoons
• tidal flats

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Transitional Environments
Transitional environments
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Simple Deltas

• topset beds
• foreset beds
• bottomset beds

• The simplest deltas are those in lakes. They
  consist of

• 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

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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

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Stream/River-Dominated Deltas

• Stream/river-dominated deltas
• have long distributary channels
• extending far seaward
• Mississippi River delta

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Wave-Dominated Deltas

• Wave-dominated deltas
• such as the Nile Delta of Egypt
• also have distributary channels
• but their seaward margin
• is modified by wave action

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Tide-Dominated Deltas

• Tide-Dominated Deltas,
• such as the Ganges-Brahmaputra delta

• have tidal sand bodies
• along the direction of tidal flow

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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

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Barrier Island Complex

• Subenvironments of a barrier island complex

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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 herringbone cross-bedding
• or sets of cross-beds that dip in opposite
  directions

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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

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Marine Environments
Marine environments
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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

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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

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Slope and Rise

• Once sediment passes the outer margin
• of the self, the shelf-slope break,
• turbidity currents transport it
• So sands with graded bedding are common
• as well as mud that settled from seawater

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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
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Deep Sea

• Beyond the continental rise, the seafloor is
• nearly completely covered by fine-grained
  deposits
• pelagic clay and ooze
• with no sediment at all
• near mid-ocean ridges
• sand and gravel are notably absent
• The main sources of sediment are
• windblown dust from continents or oceanic islands
• volcanic ash
• shells of microorganisms that dwelled in surface
  waters of the ocean

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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

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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,
• instead of being transported there

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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 ooids
• which make up the grains in oolitic limestone

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Carbonate Shelf

• The carbonate shelf is attached to a continent
• Examples occur in southern Florida and the
  Persian Gulf

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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

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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

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Navajo Sandstone
Checkerboard Mesa, Zion National Park, Utah

• Vertical fractures
• intersect cross beds of desert dunes
• making the checker-board pattern

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Paleogeography

• Paleogeography deals with
• Earths geography of the past
• Using interpretations
• of depositional environment
• such as the ones just discussed
• we can attempt to reconstruct
• what Earths 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

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Paleogeography

• and from Late Precambrian to Middle Cambrian
• the shoreline migrated inland from east and west
• during a marine transgression