Sedimentary Structures and Preliminary to Sedimentary Environments

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Sedimentary Structures and Preliminary to
Sedimentary Environments

• Understanding what sedimentary structures mean in
  terms of physical, biological, and chemical
  processes that occurred during, shortly after,
  and subsequent to deposition is the starting
  point for the analysis of sedimentary rocks and
  the environments in which they were deposited.
  Knowing something about fluid dynamics, ecology,
  and chemical processes is useful!

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Sedimentary Structures

• Features preserved in sedimentary strata that
  indicate.
• Physical,
• Biological, and.
• Chemical processes and conditions during and
  after deposition.
• May (in conjunction with other facies
  characteristics) be indicative of Sedimentary
  Depositional Environment.
• Primary Current Induced structures.
• Biogenic Structures.
• Deformational Structures.
• Secondary Mineralization.

Mud cracks, clear indication of desiccation and
subaerial exposure prior to lithification
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Sedimentary Depositional Environments

• Siliciclastic
• Short
• Long

• Carbonate
• Rimmed margins
• Ramps
• Platforms

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Sedimentary Depositional Environments

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Common Siliciclastic Stratigraphic Successions

• Vertical successions characterized by lithology,
  associations and vertical arrangement of
  sedimentary structures
• Indicative of particular sedimentary depositional
  environments
• Autocyclicity stratigraphic variability inherent
  to a particular depositional environment

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Common Sedimentary Facies Associations in
Carbonate Dominated Environments

• Vertical successions characterized by lithology,
  associations and vertical arrangement of
  sedimentary structures
• indicative of sedimentary depositional
  environments
• typically indicate shoaling or shallowing upwards
  trends

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Primary Current-Induced Sedimentary Structures

• Accretionary and Erosional
• Accretionary/Aggradational
• Physical, chemical, biological accumulation

• Bedding
• lithologic, textural, structural homogeneity
  indicating discrete time intervals of essentially
  constant (or constantly changing) deposition
• Vertical accretion parallel horizontal layers
• Lateral accretion

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Primary Current-Induced Sedimentary Structures

• Bedding
• lithologic, textural, structural homogeneity
  indicating discrete time intervals of essentially
  constant (or constantly changing) deposition

• Vertical accretion parallel, horizontal layers
• Typically suspension settling/chemical
  aggradation
• Lateral accretion Traction transport
• Beds gt 1cm
• Laminations lt 1cm

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Primary Current-Induced Sedimentary Structures

• Erosional (scour)
• Sediment removal by currents
• Bedding contacts
• Change in depositional regime
• Sharp------gt gradational-------gt amalgamated

• Bedding planes
• Non-deposition, erosion, or change in
  depositional regime
• Note the time significance of non-deposited
  strata
• Sedimentary record is sparse More time, with NO
  deposition, than record.

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Primary Current-Induced Sedimentary Structures

• Types of bedding (mechanical aggradation)
• Suspension settling vertical accretion
• Horizontal stratification
• Graded bedding
• Regular variation in particle size in an
  individual bed (constantly changing hydrodynamic
  conditions)

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Primary Current-Induced Sedimentary Structures

• Types of bedding (mechanical aggradation)
• Traction lateral accretion from bed load
• Cross stratification
• Scale
• Cross laminations lt 3cm amplitude, lt 40 cm ?
• Cross beds gt 10cm amplitude, gt 10cm ?
• Planar tabular cross Strat vs trough cross Strat

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Primary Current-Induced Sedimentary Structures

• Types of bedding (mechanical aggradation)
• Traction lateral accretion from bed load
• Cross stratification
• Scale
• Cross laminations lt 3cm amplitude, lt 40 cm ?
• Cross beds gt 10cm amplitude, gt 10cm ?
• Planar tabular cross Strat vs trough cross Strat

Climbing ripples
Planar-tabular x-beds
trough x-beds
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Deformational Sedimentary Structures

• Desiccation
• Mud cracks (subaerial exposure), synaeresis
  cracks (concentration gradients)
• Water escape
• Mechanical instability (rapid deposition, high
  water content)

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Biogenic Sedimentary Structures

• Syndepositional modification of sediment by
  biological agents.
• Ethological approach (environmental conditions
  determine organisms behavior gt sedimentary
  record).
• Biogenic structures influenced by
• Abundance of life.
• Types of organisms (plants, animals Single/
  multicellular etc).
• Mode of living.

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Biogenic Structures Mode of Living

• Discrete burrows
• Form and shape
• Vertical escape artists Skolithos
• Horizontal grazers Zoophycus/Nereites
• 3D miners Cruziana
• Glossifungites
• Hard-rock miners burrows into firm substrate
• Through-going bioturbation
• Obliteration of primary current induced
  structures indicates relatively slow rates of
  sediment accumulation
• Plant rootsgt subaerial deposition

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Biogenic Structures Mode of Living

• Discrete burrows
• Form and shape
• Vertical escape artists Skolithos
• Horizontal grazers Zoophycus/Nereites
• 3D miners Cruziana

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Biogenic Structures Mode of Living

• Discrete burrows
• Form and shape
• Vertical escape artists Skolithos
• Horizontal grazers Zoophycus/Nereites
• 3D miners Cruziana
• Glossifungites
• Hard-rock miners burrows into firm substrate

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Biogenic Structures Mode of Living

• Through-going bioturbation
• Obliteration of primary current induced
  structures indicates relatively slow rates of
  sediment accumulation
• Plant rootsgt subaerial deposition

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Secondary Mineralization

• Typically indicate post-depositional geochemical
  conditions
• May indicate syndepositional geochemistry
• Concretions
• Localized mineralization due to concentration
  gradients/variable geochemical microenvironments
• Veining
• Primary texture induced concentration gradients
  and mineral precipitation
• Liesegang banding
• Stylolites
• Pressure induced differential mineral dissolution

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Secondary Mineralization

• Concretions
• Localized mineralization due to concentration
  gradients/variable geochemical microenvironments

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Secondary Mineralization

• Veining
• Primary texture induced concentration gradients
  and mineral precipitation
• Liesegang banding

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Secondary Mineralization

• Stylolites
• Pressure induced differential mineral dissolution