Transcript Non-Terrigenous Sediments and Rocks

Non-Terrigenous
Sediments and Rocks
Carbonate-Chemical-Volcaniclastic
Sediments and Rocks
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No “Simple” Classification Scheme
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Importance of Volcaniclastics
• Recognition of contemporaneous volcanism
– Pyroclastic rocks and volcaniclastics with
admixtures of proclasts
• Voluminous strata at plate boundaries and
hot spots
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Classification of Volcaniclastic Rocks
• Volcanic particulate
material
– Any fragmentation
mechanism
– Any transport process
– Any environment
• Pyroclastic
– Particles broken by
volcanism
• Epiclastic (epiclasts)
– Any fragment of volcanic
(composition) origin
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Classification of Volcaniclastic Rocks
• Pyroclastic rock or sediment
– >75% material fragmented by volcanic eruptions
– Tephra: unconsolidated pyroclastic deposit
• Hydroclastic rocks or sediment
– Water interaction fragmentation
Pyroclastic Ejecta
Juvenile
Cognate
Accidental
Erupting magma,
Crystals and glass
Co-magmatic volcanic
rock
Country rock inclusions
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Classification of Pyroclastic Rocks
• Basic classification otbo (on the basis of) particle size
– Blocks (solid) and bombs (molten) (>64mm)
• Volcanic breccia deposits
– Lapilli (2-64mm)
• Lapillistone
– Ash (<2mm)
• Tuff
• Additional Classification otbo composition
– Crystals
– Lithic
– Vitric fragments
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Composition of Tuffs
• Crystals (intratelluric)
– Euhedral +/- broken
– Compositional zoning
• Vitric (glassey)
fragments
– Bubble wall shards
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Composition of Tuffs
• Vitric (glassy) fragments
– Bubble wall shards
– Hydroclastic shards
• Lithic fragments
– Volcanic rock fragments
(cognate?)
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Fragmentation Processes
• Explosive (gas expansion)
comminution
(fragmentation): mainly
intermediate to silicic
(high silica) magmas.
– Ash fall; Laterally
extensive air fall; Typically
silicic and vitric rich.
• Mantles topography.
• Consists of glass (bubblewall) shards.
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Volcanic Fragmentation Processes and
Products
• Continental silicic (high silica)
magmas; Calderas and
pyroclastic sheet deposits
– Ash flow {nuee ardante or
ignimbrite, as in “great flaming
ignimbrites”.
• Follow topographic lows (high
density fluid).
• Create gigantic pyroclastic sheet
deposits
• Can be hot enough after
deposition to weld, annealed
vitric fragments welded tuff
Kaguyak volcano, Alaska
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Volcanic Fragmentation Processes and
Products
• Hydroclastics; Water
interaction fragmentation
(typically basaltic lavas)
– Great volumes of
hydroclastics on the sea floor
and in the edifice of
submarine volcanoes
– Highly subject to alteration –>
clay minerals, microcrystalline
silica, and zeolite
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Significance of Volcaniclastic Rocks
• Subject to extensive
diagenetic alteration
during burial
– Typically occur in high
heat flow geological
settings
– Typically poor fluid
reservoir rocks
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Orthochemical Sediment:
Evaporites
• Stratified rock consisting of minerals
precipitated from high concentration
brines, typically hypersaline sea water
–
–
–
–
Anhydrite (CaSO4)
Gypsum (CaSO4 )*H2O
Halite (NaCl)
Others
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Evaporites
• Indicative of unusual climatic or oceanographic
conditions
– Severe circulation restriction
– Climatic aridity
• Highly subject to secondary alteration/solution
– Anhydrite<--->gypsum due to hydration/dehydration
– Physical deformation: enterolithic structure
• Occurrence
– Bedded
– Nodular
– Chicken wire
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Siliceous Sediments/Rocks
• Chert/diatomite (SiO2 );
– Opaline tests
– Chalcedony
– microcrystalline quartz
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Siliceous Sediments/Rocks
• Bedded chert (most)
– Pelagic sediment
consisting of siliceous
zoo- and phytoplanktonic
tests
• Siliceous sediment experience a
predictable transformation from
amorphous opal to chalcedony and
eventually to microcrystalline
quartz due to time/temperature
dependant chemical reaction
C= lam chert, s= sandstone layers, f= fractures
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Siliceous Sediments/Rocks
• Nodular Chert; diagenetic
origin (typical)
– Silica derived from the solution
of siliceous fossil material in
predominantly carbonate rich
successions
• Sponge spicules and other siliceous
bioclasts
N=chert nodules, b=bedded chert
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Organic Rich Sedimentary Rock
• Organic compound-rich rocks
– Coal
• Humic coal
– vascular {land} plant derived organic compounds altered by
elevated temperature and burial pressure
• Sapropelic coal
– Formed from non-vascular (algal) plant material
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Organic Rich Sedimentary Rock
– Oil Shale
• Primary, organic carbon (OC)-rich shale (>2% to > 10% OC)
• Formed in low energy environments through suspension and
deposition in stagnant (anaerobic) conditions
– Most common source of long chain, liquid and gaseous
hydrocarbons that can migrate into porous reservoir rocks
and from economic accumulations of petroleum
Spontaneous combustion of
Kimmeridge oil-shale, Dorset, UK.
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Non-Terrigenous
Sediments and Rocks
Carbonate Sediments and Rocks
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No “Simple” Classification Scheme
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Most Common non-Terrigenous
Sedimentary Rocks
• Carbonates (>50% primary carbonate
minerals)
– Limestone (CaCO3)
• Chemical
• biochemical
– Dolomite (CaMg(CO3)2)
• Chemical
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The Origin of
Carbonate Sediments
• Most form as biogenic particles (essentially the only source) in
–
–
–
–
warm (tropical; 30oN to 30oS latitude),
shallow (shelf; within the photic zone), (mostly <10-20 m)
marine water
Also accumulate in deepwater
•
ooze
– limestone (fine-grained) made up of skeletons of pelagic microorganisms such as
Globigerina
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The Origin of
Carbonate Sediments
•
Why?
–
(1)
Any process that decreases [CO3] forces rxn (2) to the left,
CO2 is less soluble in warm water; CaCO3 has retrograde
solubility
H2O + CO2 <---> 2H2CO3 <–->H+ + HCO3- <-->H+ + CO3-2
(2) CaCO3 <-–> Ca+2 + CO3-2
(pH dependant reaction)
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Carbonates: General Characteristics
• The majority of carbonate sediment forms in
subtidal to supratidal environments and gives rise to
widespread tabular deposits along continental
(trailing edge) margins and epicontinental seas
• Important occurrence in reefs, mounds or banks (biobuildups)
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Carbonate Minerals in Sedimentary Rocks
• bivalent metal cations
– Ca+2
• Calcite, aragonite
– Mg+2
• Magnesite, dolomite
– Fe
+2
• The physical and chemical
conditions of the environment in
which calcites, dolomites and
aragonites are formed are
reflected in their composition.
• Siderite, ankerite
• + CO3 -2
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Carbonate Minerals in Sedimentary Rocks
• bivalent metal cations
– Ca+2
• Calcite, aragonite
– Mg+2
• Magnesite, dolomite
– Fe
+2
• Several factors control the crystal
habits and crystal sizes of these
minerals, most important:
– salinity
– ratio of magnesium to calcium of in the
solution (time dependant)
• Siderite, ankerite
• + CO3 -2
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Most modern Carbonate
sediments (mostly biogenic)
• Magnesian Calcite =
small crystals > 4% Mg++
(4-9%),
– Mg++ = 3 x Ca++ in normal
sea water (main site of
CaCo3 ppt)
– “poisons” (interferes
with calcite
crystallization) and
results in formation of
meta-stable Aragonite
(neomorphosis)
• Aragonite (orthorhombic
polymorph of CaCo3)
(Aragonite)
(Mg,Ca)CO3
(Mg,Fe,Ca)CO3
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Most ancient carbonate rock is
• Dolomite
– (CaMg{Co3}2) 42% 58% CaCo3.
• Not readily formed at
low temperatures;
almost always 2nd ary
mineral or very rare
primary ppt “the
dolomite problem”:
• Calcite:
– <4% Mg++ (CaCO3)
(Aragonite)
(Mg,Ca)CO3
(Mg,Fe,Ca)CO3
– Primary or
Secondary/Authigen
ic??
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Other, (mainly diagenetic)
Sedimentary Carbonate Minerals
• Siderite
– Fe Co3
• Ankerite:
– Ca(Mg,Fe)Co3
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Carbonates: General Characteristics
• An antipathetic relationship exists between
carbonate sediments and siliciclastic sediments due
in large part to the biology of carbonate sedimentforming organisms
– high terrigenous sedimentation rates increase turbidity,
which inhibits photosynthesis by benthic organisms
– gill breathers (such as the coral) get clogged up and die
• The structures and textures of carbonate rocks
mostly reflect intrabasinal, biological AND physical
factors
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Carbonates: General Characteristics
• Carbonate sediments are particulate and subject to
physical processes at the site of deposition just like
TC sediments
• Kinetic energy (currents) in the depositional
environment influence:
– grain size and sorting
• BUT, carbonate sediments are mostly biogenic
(“born” not “made”) and may not experience physical
transport
– This can confound grain size-sorting / depositional energy
relationship
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Source and Type of Sediments Produced in
Modern and Ancient Carbonate Environments
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Carbonate Rock Classification
• Allochems
– Carbonate sand
• Micrite
– Micro Crystalline calcite
• Cement (spar)
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Carbonate Rocks Constituents
•
The sand-sized grains that occur in carbonate rocks
are called allochemical particles or allochems.
1.
Intraclasts (rock fragments):
•
formed, transported and redeposited within the basin
2. Ooliths: concentrically laminated carbonate structures, including:
•
oolites -concentrically laminated structures,less than 2mm in
diameter, thought to be abiogenic in origin
•
pisolites - same as oolites, but greater than 2mm in diameter
•
oncolites - spheroidal stromatolites (> 1-2 cm)
3. Peloids:
•
silt to fine grained sand sized carbonate particles with no
distinctive internal structure; most thought to be fecal pellets
4. Skeletal particles (bioclasts):
•
whole microfossils, whole megafossils, broken shell fragments
– algae, forams, corals, bryozoans, brachiopods, gastropods,
pelecypods, ostracods, etc.
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•
Carbonate Rocks Constituents
The sand-sized grains that occur in carbonate rocks are
called allochemical particles or allochems.
1.
2.
3.
4.
Intraclasts
Ooliths
Peloids
Skeletal particles
(bioclasts)
•
The interpretation of
the depositional setting
of carbonates is based
on grain types, grain
packing or fabric,
sedimentary structures,
and early diagenetic
changes.
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Carbonate Rocks Constituents
•
The sand-sized grains that occur in carbonate
rocks are called allochemical particles or
allochems.
•
Intraclasts (early lithified carbonate fragments):
•
irregularly-shaped grains that form by syndepositional erosion of partially
lithified sediment.
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Carbonate Rocks Constituents
•
The sand-sized grains that
occur in carbonate rocks are
called allochemical particles
or allochems.
•
Ooliths: concentrically laminated
carbonate structures, including:
•
oolites -concentrically laminated
structures,less than 2mm in
diameter, thought to be
abiogenic in origin
•
pisolites - same as oolites, but
greater than 2mm in diameter
•
oncolites - spheroidal
stromatolites (> 1-2 cm)
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•
Carbonate Rocks Constituents
The sand-sized grains that occur in carbonate rocks
are called allochemical particles or 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 Rocks Constituents
The sand-sized grains that occur in carbonate rocks
are called allochemical particles or allochems.
• Skeletal particles (bioclasts):
• whole microfossils, whole megafossils, broken shell fragments
– algae, forams, corals, bryozoans, brachiopods, gastropods,
pelecypods, ostracods, etc.
– Standard microfacies (fossil fragment type -> environment)
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Carbonate Rocks Constituents
• Micrite:
– microcrystalline
carbonate 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
mudstones
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Carbonate Rocks Constituents
• Cement:
– sparry (twinkling crystalline) orthochemical material
formed in interstitial pore spaces of “grainy”
carbonate sediment
• cement in pores indicates original void space
• also recrystallized allochems or micrite
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Carbonate Rocks Constituents
• Insoluble Residues –
– minor amounts of clay minerals and quartz occur in
limestones, as insoluble residues, (so called because
they do not dissolve in HCl)
– Most insoluble material is chert (siliceous)
– chert mostly originates from the shells of silica
secreting organisms including diatoms, radiolarians,
and some sponges.
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Classification Schemes:
Folk Classification
• Type I limestone,
– Sparry Allochemical rocks:
allochems > 50%, spar cement >
micrite mud [4 rock types]
• more energetic environment,
some sorting
• Type II limestone,
– Micritic Allochemical rocks:
allochems >10%, micrite mud >
spar cement [4 rock types]lower
energy environment, more poorly
sorted than Type I
• Type III limestone: Micrite:
allochems < 10%
– very low energy at the site of
deposition (carbonate mudrock)
• “Biolithite”: Reef rock
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Classification Schemes:
Dunham Classification
• 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
• bound (biologically)
• depositional texture
recognizable
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5 Principles of Dunham Classification
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
–
–
–
–
Boundstone
organic framework
laminations not consistent with gravity (stromatolite)
roof over sediment filled cavities
3. Grain kind: standard microfacies types
4. Grain size, rounding, and coating: hydrologic
interpretations
5. Biogenically ppt masses bound at time of deposition:
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Folk Textural Spectrum Classification
• Concocted to incorporate textural
characteristics comparable to textural
maturity in TC sediments
– Mud component
– Sorting
– Rounding
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Dolomitic Rocks
• Typically devoid of primary textures
and structures; if primary textures are
preserved
– <10% dolomite: “dolomitized” (rock name)
– >10% dolomite: dolomitic (rock name)
– recrystallized carbonate: dolostone
• saddle dolomite: “burial” dolomite of
hydrothermal origin
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