Transcript 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:
• T=Terrigenous
• 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 nonmarine 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 earth’s 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:
Terrigenous Clastic Basin
 Puget trough
 Divergent plate
boundaries:
 Passive; Atlantic coast
basin
 Rift Basins; East African
Rift
Carbonate Basin
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Sedimentary
Basins and
Rocks
• Simple model and
classification
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Siliciclastic Rocks: Components
• F-M-C-P
• Framework Grains
• >0.05 mm allogenic mineral grains, rock fragments
• Residual from weathering
• Detrital Matrix
• <0.05 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; <0.004 mm)
• Sandstones, siltstones, and shales
• G (gravel; >2 mm) - S (sand) - M (matrix; <0.063
mm)
• Conglomerates and breccias
• >30% 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 = <5-15% matrix
• “Clean” sandstone
• Depositional agents that sort
sediment well
• Wacke = >15% 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 grain’s
 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 RF’s)
• 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/q+f+r
• F = f/q+f+r
• R (or L) = r/q+f+r
• 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 >30% 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 Clastic
Depositional 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 Clastic
Depositional 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 (>50% 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 <10-20 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:
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Micrite
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Sparite
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Crystalline carbonate material (>0.004 mm)
Forms by precipitation (often as cement) or recrystallization
Allochems
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Lime mud (<0.004 mm)
Largely fragmental algae remains, also chemical precipitate
Transported chemical or biochemical precipitates (fragmental
material)
Include intraclasts, ooliths, peloids, and bioclasts
Biolithic elements
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Formed by organisms in situ
Bound together by precipitated material
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Carbonate Rock Constituents
• Micrite:
• Microcrystalline calcite
particles of clay (<1-4
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
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Reworked, early lithified carbonate fragments
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irregularly-shaped grains that form by syndepositional
erosion of partially lithified sediment
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Carbonate Rock Constituents
Allochems: Ooliths
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Concentrically laminated
carbonate structures
Oolites - <2 mm in diameter
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Thought to be abiogenic in
origin
Layers precipitated onto a
grain during wave agitation
Pisolites - same as oolites,
but >2 mm
Oncolites - spheroidal
stromatolites (> 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 -> 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
grains:mud)
• 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
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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
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fluid circulation, diagenesis
Grain kind: standard microfacies types
Grain size, rounding, and coating: hydrologic interpretations
Biogenically ppt masses bound at time of deposition:
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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
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2.
You have two sandstones (Table, handout)
A. Plot the normalized proportions of Q, F, and L on the ternary diagram.
B. For each sandstone:
1. Classify it (give it a compositional name and indicate arenite vs
wacke)
2. Determine the most likely tectonic setting from which it
originated, and give your evidence
3. 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)
A. Based on the description, for each carbonate:
1. Give it a compositional classification under both the Folk and
Dunham schemes (and indicate allochemical vs orthochemical)
2. Describe the depositional environment as best you can and give
your evidence