Chapter 6 Sedimentary Rocks Archives of Earth History

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Chapter 6Sedimentary Rocks Archives of Earth
History

• Sedimentary rocks preserve evidence of surface
  processes and sometimes fossils. Together,
  sediment characteristics and fossils give clues
  to the depositional environment of the specific
  area (the depositional basin and nearby source
  areas) where the sediment was deposited.
• Sediment types detrital (clastic) or chemical
  (precipitate).
• Clastic sediments source is outside of the
  depositional basin, from pre-existing rocks.
• Chemical sediments source is inside of the
  depositional basin, from dissolved minerals in
  the water.
• Both types can be present in different parts of
  the same basin at the same time remember facies
  diagram.

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• Observation and data gathering - visit rock
  exposures (outcrops) and carefully examine
• Sedimentary textures size, sorting, roundness
• Rock composition what minerals are present
• Fossils (if present)
• Thickness
• Relationships to other rocks - interpretation of
  vertical and lateral facies relationships
• compare with present-day sediments
• Make environmental interpretation
• Detrital rocks - common minerals - quartz,
  feldspars, and clay minerals Composition is
  related to source rock and length of transport
  time. Texture (grain size, sorting, roundness)
  is related to environment of transport and
  deposition.
• Quartz sand may have been deposited in a river
  system, on a beach, or in sand dunes.

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• Textures of detrital rocks
• Grain size gives some indication of the energy
  conditions during transport and deposition
• High-energy processes (swift-flowing streams and
  waves)
• are needed to transport gravel
• Conglomerate is deposited where these processes
  prevail.
• Sand is also deposited by vigorous currents
  (rivers, beaches, windy deserts)
• Silt and clay are transported by weak currents
  and accumulate only under low-energy conditions
  (lakes, lagoons, tidal flats, swamps)
• Sorting refers to the variation in size of
  particles, is related to the processes of
  transport and deposition
• Consistent grain sizes (small range)
  well-sorted
• Inconsistent grain sizes (large range)
  poorly-sorted
• Dune beach sands usually well-sorted.
• Landslide glacial deposits usually
  poorly-sorted.

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• Composition of chemical sedimentary rocks
• is more useful in revealing environmental
  information
• Limestone is deposited in warm, shallow seas
• a small amount also originates in lakes. Most of
  the calcite (CaCO3) is precipitated due to
  activity by certain species of algae. Other
  organisms corals, mollusks, bryozoa, sponges -
  precipitate calcite for their exoskeletons.
• Evaporites such as rock salt and rock gypsum
• indicate arid environments high evaporation
  rates.
• Coal originates in swamps and bogs on land, in
  reducing (anoxic, oxygen-poor) environments that
  allow the preservation of organic material.
  Organic material can be preserved in terrestrial
  aquatic and marine environments in basins with
  little water circulation, i.e., anoxic
  conditions. Gives a dark color to sediments.

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• Rounding is related to length of transport

Well-rounded and well-sorted gravel. Likely a
great distance from the original source. Energy
and size contributes to rounding.
Angular and poorly-sorted gravel. Close to the
original source.
Gulf Coast beach sands Great distance from
original source. Well-rounded and well-sorted.
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• Sedimentary structures are outcrop-scale
  features, formed during deposition or shortly
  thereafter. Are related to physical and
  biological processes of that environment.
• External structures seen on the top of the bed,
  examples Ripple marks, footprints, feeding
  trails, mudcracks, etc.
• Internal structures seen in outcrop
  (cross-sectional side view), examples beds ( 1
  cm), laminations (graded bedding.
• These same structures can be seen in modern day
  environment or produced in experiments. Help
  provide information concerning environments of
  deposition.
• Bedding planes form due to the short-term
  cessation of deposition or a change in sediment
  type.

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• Cross-beds form on the leeward (depositional)
  side of ripples or dunes. Can form in aquatic or
  arid terrestrial environments. Can be disrupted
  by later cycles of erosion. Can be used to
  determine paleocurrent directions and which side
  is up in deformed sediments.
• Asymmetrial ripples form due to a prevailing
  current direction. Symmetrical ripples form to a
  back and forth oscillation of currents.
• Graded bedding forms when a turbidity current (a
  water current with a high concentration of
  suspended sediments) gradually slows. The large
  clasts drop first, followed by a progression of
  decreasing grain sizes.
• May form in submarine fans, river channel, or
  river delta deposits. Can also be used to
  determine which side is up in deformed sediments.

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(Left) Cross-bedded beach sands (internal
structures) Perry Sand, Dooly County, Ga.
(Upper right) Symmetrical Ripple-marked sandstone
New Mexico. (Bottom right) Interbedded shale
and limestone.
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Ripple marks in gypsum sand, White Sands Natl
Monument, New Mexico
Graded bedding occurs when turbidity current
gradually slows.
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Erosion Deposition

• Current ripples asymmetrical current left to
  right.

Mudcracks form because some clays contract as
they dry (lose water).
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• Biogenic structures form due to biological
  action. External seen on bedding plane.
  Internal seen in cross-section. Referred to as
  trace fossils.
• Bioturbation extensive burrowing that may
  destroy existing bedding.
• Structures are not unique to given environments.
  Current ripple marks may occur in stream
  channels, tidal channels, and on the ocean floor.
  Mudcracks may occur on tidal flats or on
  lake-margin mud flats.
• We use these structures and other features to
  determine environments.

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• Three-dimensional shape or geometry of a
  sedimentary rock body may be helpful in
  environmental analyses
• Same geometry may occur in multiple environments.
• May be modified by compaction lithification.
• May be modified by erosion and deformation.

Colorado Plateau layers Blanket or Sheet
geometry.
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Barrier island or stream channel deposits have an
elongate or shoestring geometry.
Delta deposits lobate (fan shaped) in map view
and Lens-shaped (lenticular) in cross-section.
Buried reefs may be irregular, may be long and
narrow, and may encircle a basin.
Capitan Peak (right) Guadalupe Mts. (TX/MN) is
the exposed portion of a basin-encircling series
of Permian-aged reefs.
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Fossils - remains or traces of past organ-isms,
may be used in stratigraphy for relative dating
and correlation are constituents of rocks
provide evidence of depositional
environments. Environmental considerations of
fossils Are fossils preserved where they
lived or have they been transported after
death? What does fossil morphology tell us about
its habitat? Does the fossil seem to fit with
the paleo-environment?
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• If the fossil has living relatives, is the
  ancient environment similar to the modern
  environment?
• If macrofossils are not available, micro-fossils
  may be useful in determinations of geologic age
  and environmental conditions. Especially true in
  cases of oil well cuttings in which most
  macro-fossils would have been destroyed by
  cutting action of drill bit.

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Depositional Environments locations of
sediment accumulation from physical, chemical,
and/or biological processes. Types 1)
Continental 2) Transitional 3) Marine
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Red lettering Continental Environments Blue
lettering Transitional Environments Black
lettering Marine Environments
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Various Depositional Environments have
distin-guishing characteristics, though some
characteristics may be present in more than one
environment. Major Fluvial (stream) Environments
Braided and Meandering.
Braided stream (left) broad, shallow channels
with gravels and sand.
Deposition
Erosion
Meandering (right) finer sediments, migrating
curves, superposition of curves.
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Desert Environments Sand dunes (eolian
deposits), Alluvial fans, Playa lakes. Sand dunes
- large scale cross-beds, very well-sorted.
Playa lakes Great Salt Lake, Dead Sea, Great
Salt Plains Lake, Oklahoma Great Salt Plains Lake
Navajo Sandstone Colorado Plateau see Fig.
6.19, p. 110
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Miniature alluvial fan Dooly County sand
pit, Ga.
Large alluvial fans form adjacent to large,
western mountain ranges. Alluvial fan deposits
are poorly-sorted wide range of grain sizes.