Transcript Document

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 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 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, Feoxide, 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; 0.0040.063 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:
– Micrite
•
•
–
Sparite
•
•
–
Crystalline carbonate material (>0.004 mm)
Forms by precipitation (often as cement) or recrystallization
Allochems
•
•
–
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
•
•
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
–
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 - <2 mm in
diameter
•
•
–
–
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
1. 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
2. Grain Support: self supporting framework
•
fluid circulation, diagenesis
3. Grain kind: standard microfacies types
4. Grain size, rounding, and coating: hydrologic
interpretations
5. 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
1. 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
2. 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
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