Sedimentary Rocks and the Origin of Sedimentary Strata

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Sedimentary Rocks and the Origin of Sedimentary
Strata

• Basins to Bedding

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Sedimentary Rocks

• Sedimentary rocks are those rocks which form at
  or near the earth's surface primarily through
• Deposition of weathered silicate material by
  water, wind, or ice (detrital, clastic,
  terrigenous)
• Direct inorganic chemical precipitation from
  water
• Precipitation by organic processes

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Sedimentary Rocks

• Three end-member types

• TTerrigenous
• Residual and secondary weathering products
  (siliciclastic)
• Allogenic (extra-basinal) origin
• A Allochemical
• Chemical or biochemical particles, shell
  fragments
• Authigenic (form within basin) but locally
  reworked
• O Orthochemical
• Primary chemical precipitation from dissolved
  ions
• Authigenic (form within basin of deposition), no
  reworking

IO Impure orthochemical IA Impure allochemical
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Sedimentary Rocks

• T Terrigenous
• Most mudrocks, sandstones, and conglomerates
• 65 to 75 of sedimentary strata
• IA Impure Allochemical
• Very fossiliferous shale, sandy fossiliferous or
  oolitic limestones
• 10-15 of sedimentary strata
• IO Impure Orthochemical
• Clay-rich microcrystalline limestones
• 2-5 of sedimentary strata

• A Allochemical rocks
• Fossiliferous, oolitic, pellet, or intraclastic
  limestone or dolomite
• 10-15 of sedimentary strata
• O Orthochemical Rocks
• Microcrystalline limestone, chert, anhydrite,
  crystalline dolomite
• 2-8 of sedimentary strata

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Sedimentary Rocks Terrigenous

• Terrigenous (clastic, detrital) sediments and
  rocks
• Also called siliciclastic since most particles
  are silicate mineral grains
• Grains created by weathering
• Transported by surface processes
• Water, wind, ice

• Deposited as horizontal, stratified layers in
  sedimentary basins
• Buried and lithified by
• Compaction
• Cementation

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Sedimentary Rocks Allochemical

• Allochemical (mainly carbonate) sediments and
  rocks
• Dominantly biologic origin (shells or bones)
• Carbonate systems develop where siliciclastic
  sourcelands are low and/or very distant
• The water is shallow marine
• Climates are tropical to subtropical

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Sedimentary Rocks Orthochemical

• Orthochemical (chemical precipitate) sediments
  and rocks
• Dominated by limestones and dolostones of
  precipitate origin
• Also includes evaporites, chert, and iron
  formations
• Precipitate from marine or non-marine waters due
  to chemical changes

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Sedimentary Depositional Environments

• In geology depositional environments are defined
  by processes and products
• Physical processes determine
• Grain size, sorting, rounding
• Bedding style (including sedimentary structures)
  and geometry
• Biological processes determine
• Fossil content
• Biological disruption of original stratification
• Chemical processes determine
• Types of minerals formed at the site of
  deposition and during burial
• Study of modern depositional environments used to
  infer how ancient rocks formed (present is key
  to past)

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Sedimentary Depositional Environments Main Types

• Continental (above sea level)
• Fluvial (stream) stream channel and floodplain
• Glacial direct deposits and outwash
• Lacustrine (lake)
• Transitional (Continental and Marine)
• Delta
• Estuary and lagoon
• Beach
• Marine (below sea level)
• Shallow sea (shelf) and reefs
• Submarine canyons (submarine deltas)
• Pelagic environments abyssal plains

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Sedimentary Basins

• Sedimentary rocks form in basins
• Areas of the earths surface subject to long term
  (millions to tens of millions of years)
  subsidence resulting in space to accommodate
  sediment (not subject to erosion)

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Sedimentary Basins

• Basins occur in a wide range of tectonic settings
• Cratonic settings
• Michigan basin
• Convergent plate setting and active plate
  boundaries
• Puget trough
• Divergent plate boundaries
• Passive Atlantic coast basin
• Rift Basins East African Rift

Terrigenous Clastic Basin
Carbonate Basin
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Simple model and classification
Sedimentary Basins and Rocks

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Siliciclastic Rocks Components

• F-M-C-P
• Framework Grains
• gt0.05 mm allogenic mineral grains, rock fragments
• Residual from weathering
• Detrital Matrix
• lt0.05 mm (clay, quartz, feldspar, carbonates,
  organics, oxides)
• Chemical weathering products
• Cement
• Authigenic, post-depositional orthochemical
  component
• Precipitated from circulating pore fluids
  (silica, carbonate, Fe-oxide, clay, feldspar,
  other oxides, zeolite, salts)
• Pores
• Primary (40) or secondary due to
  leaching/dissolution
• Classification based on (1) texture, (2)
  composition

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Siliciclastic Rocks Texture

• Descriptive Textural Classification
• Grain Size
• Uden-Wentworth grain size scale
• Phi -log2 (grain diameter in mm)
• naturally occurring groups
• Gravel rock fragments
• Sand individual mineral grains (particulate
  residues)
• Mud particulate residues /- chemical
  weathering products
• Clay chemical weathering products (clay
  minerals, etc.)

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Siliciclastic Rocks Texture

• Grain size and sorting
• Statistical/graphic presentation of texture
• Quantitative assessment of the of different
  grain sizes in a clastic rock
• Mean average particle size
• Mode most abundant class size

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Siliciclastic Rocks Texture

• Grain size, sorting, and roundness
  interpretation
• Textural Maturity
• Kinetic energy during transport and reworking
• Transport history
• Dispersal patterns
• Beware
• Mixed sources
• Biogenic reworking

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Siliciclastic Rock Classification

• Descriptive textural classification based on
  proportions of
• S (sand 0.063-2mm) - S (silt 0.004-0.063 mm) -
  C (clay lt0.004 mm)
• Sandstones, siltstones, and shales
• G (gravel gt2 mm) - S (sand) - M (matrix lt0.063
  mm)
• Conglomerates and breccias
• gt30 gravel indicates high transport energy
• Further classification based on composition

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Siliciclastic Rocks Sandstone

• Basic classification based on proportions of
• Mineral grains (dominantly quartz)
• Matrix (clay to silt-sized clastic material
  filling spaces between grains
• Arenite lt5-15 matrix
• Clean sandstone
• Depositional agents that sort sediment well
• Wacke gt15 matrix
• Dirty sandstone

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Siliciclastic Rocks Sandstone

• Many classification schemes, but most based on
  relative proportions of framework grains
• Relative abundance a function of mineral grains
• Availability, Chemical Stability, Mechanical
  Durability
• Anything Possible, most common
• Quartz
• monocrystalline, polycrystalline ig, met, or sed
  source
• mechanically chemically stable, abundant
• Feldspar
• K-spar (sandine, microcline), Plag (Na-Ca)
• Abundant and somewhat stable (often altered)
• Rock (Lithic) Fragments
• All kinds (including limestone/dolomite RFs)
• Abundant, less stable (depending on dep
  conditions)
• Also accessory (minor abundance) heavy minerals

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Siliciclastic Rocks Sandstone

• Classification based on normalized (relative
  proportions) of
• Q q/qfr
• F f/qfr
• R (or L) r/qfr
• 7 types of normal sandstones
• Others mineral arenite, i.e. mica-arenite,
  magnetite-arenite

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Siliciclastic Rocks Sandstone

• Sandstone composition is tied to source area and
  tectonic setting
• Ternary System for Sandstone classification

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Siliciclastic Rocks Mudrocks

• Most abundant of all sedimentary rocks
• Composed of silt clay-sized particles
• Dominated by clay minerals (kaolinite, smectite,
  illite)
• Also quartz, feldspar, carbonate, organic matter,
  others
• Composition modified by diagenetic processes
• Variable color
• Gray-black presence of organic matter
• Red-brown-yellow-green oxidation state of Fe

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Siliciclastic Rocks Mudrocks

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Siliciclastic Rocks Conglomerates

• Coarse-grained siliciclastic rock with muddy or
  sandy matrix
• Gravel gt30 of grains
• Provenance easily determined by composition of
  clasts
• Main types
• Conglomerate rounded clasts in sandy matrix
• Breccia angular clasts in sandy matrix
• Diamictite clasts in muddy matrix

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Terrigenous ClasticDepositional Environments

• Long systems
• Complex association of depositional environments
  through which clastic sediment is transported and
  in which some sediment is deposited
• End product is relatively mature sediment

• Sediments are chemically and mechanically stable
  in composition (high temp, unstable minerals are
  not present)
• Sediments are well sorted into the end member
  sizes of sand and clay.
• Sandstones at the end of the long system are
  mature quartz arenites

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Terrigenous ClasticDepositional Environments

• Short systems
• The siliciclastic source land is proximal to
  (close to) the basin
• Commonly observed in tectonically active regions
• Sediments across the entire system are
  mineralogically and texturally immature
• They are generally poorly sorted and range in
  size from gravel to coarse sand

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Carbonates

• Make up 10-15 of sedimentary rocks
• Excellent indicators of depositional
  environments integral to study of past
  environments and earth history
• Important reservoirs for oil and gas
• Carbonates (gt50 primary carbonate minerals)
• Limestone (CaCO3)
• Chemical
• biochemical
• Dolomite (CaMg(CO3)2)
• Chemical

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Carbonate Sediment Origin

• Most primary carbonate sediments form as biogenic
  particles in shallow marine environments
  (secreted as shells of invertebrates and algae)
• Warm water (tropical 30oN to 30oS latitude)
• Shallow shelf within the photic zone (mostly
  lt10-20 m)
• Also accumulate in deep water (pelagic oozes)
• Inorganic precipitates from sea water also occur
• Can form in continental settings (lacustrine,
  desert, soil, springs)

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Carbonate Rock Constituents

• Carbonate rocks mainly composed of
• Micrite
• Lime mud (lt0.004 mm)
• Largely fragmental algae remains, also chemical
  precipitate
• Sparite
• Crystalline carbonate material (gt0.004 mm)
• Forms by precipitation (often as cement) or
  recrystallization
• Allochems
• Transported chemical or biochemical precipitates
  (fragmental material)
• Include intraclasts, ooliths, peloids, and
  bioclasts
• Biolithic elements
• Formed by organisms in situ
• Bound together by precipitated material

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Carbonate Rock Constituents

• Micrite
• Microcrystalline calcite particles of clay (lt1-4
  micron) size (subtranslucent matrix) formed by
• Chemical or biochemical ppt
• Abrasion of allochems
• Implies deposition in a low energy environment
  just like in terrigenous mudstone

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Carbonate Rock Constituents

• Sparite (cement)
• Clear granular (sugary) carbonate crystalline
  orthochemical material
• Formed in interstitial pore spaces of carbonate
  sediment
• Cement in pores indicates original void space
• Also commonly forms during diagenesis
• Recrystallized allochems or micrite

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Carbonate Rock Constituents

• Allochems Intraclasts
• Reworked, early lithified carbonate fragments
• irregularly-shaped grains that form by
  syndepositional erosion of partially lithified
  sediment

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Carbonate Rock Constituents

• Allochems Ooliths
• Concentrically laminated carbonate structures
• Oolites - lt2 mm in diameter
• Thought to be abiogenic in origin
• Layers precipitated onto a grain during wave
  agitation
• Pisolites - same as oolites, but gt2 mm
• Oncolites - spheroidal stromatolites (gt 1-2 cm)

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Carbonate Rock Constituents

• Allochems Pelloids
• silt to fine grained, sand-sized carbonate
  particles with no distinctive internal structure
• most thought to be fecal pellets

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Carbonate Rock Constituents

• Allochems Skeletal particles (bioclasts)
• whole microfossils, whole megafossils, broken
  shell fragments
• Marine invertebrates algae, forams, corals,
  bryozoans, brachiopods, gastropods, mollusks,
  ostracods, etc.
• Standard microfacies (fossil fragment type -gt
  environment)

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Carbonate Rock Classification

• Based on depositional texture (mainly proportion
  of allochems)
• Two main classification schemes
• Folk
• and type of allochem
• Micrite vs sparite matrix
• Dunham
• Abundance of allochems (ratio grainsmud)
• Original components bound together
• Both overlook some types of carbonates

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Carbonate Rock Classification Dunham

• Dunham Classification
• Texture and allochem type incorporated into
  classification
• Sediment deposited in calm vs agitated waters

• Mud-bearing vs mud-free sediment
• Grain vs mud support
• Original components bound (biologically)
• Depositional texture recognizable

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Carbonate Rock Classification Dunham

• Presence or absence of lime mud is there any mud
  at all. Calm waters allow for the accumulation of
  lime mud and indicates the absence of current
  induced agitation
• Grain Support self supporting framework
• fluid circulation, diagenesis
• Grain kind standard microfacies types
• Grain size, rounding, and coating hydrologic
  interpretations
• Biogenically ppt masses bound at time of
  deposition
• Boundstone
• organic framework
• laminations not consistent with gravity
  (stromatolite)
• roof over sediment filled cavities

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Carbonate Depositional Systems

• In the warm, clear, shallow water organisms
  create sediment
• Calcareous algae flourish and generate micrite
• Invertebrate animal skeletons accumulate as
  sedimentary particles (bioclasts)

• Also, particles created indirectly by biological
  or chemical activity
• Oolitic, pelletal, and intraclastic allochems are
  also produced locally, depending on conditions

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Carbonate Depositional Environments

• Generic rimmed carbonate shelf platform basin
  margin

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Collaborative Activity

• You have two sandstones (Table, handout)
• A. Plot the normalized proportions of Q, F, and L
  on the ternary diagram.
• B. For each sandstone
• Classify it (give it a compositional name and
  indicate arenite vs wacke)
• Determine the most likely tectonic setting from
  which it originated, and give your evidence
• Determine the depositional environment (general -
  long system, short system be more specific if
  you can) in which it most likely formed, and give
  your evidence
• You have three carbonates (handout)
• Based on the description, for each carbonate
• Give it a compositional classification under both
  the Folk and Dunham schemes (and indicate
  allochemical vs orthochemical)
• Describe the depositional environment as best you
  can and give your evidence