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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

Juvenile Cognate Accidental

Erupting magma,Crystals and glass

Co-magmatic volcanic rock

Country rock inclusions

Pyroclastic Ejecta

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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 (bubble-

wall) 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 reactionC= 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 ofCarbonate 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 ofCarbonate Sediments

• Why? – Any process that decreases [CO3] forces rxn (2) to the

left, CO2 is less soluble in warm water; CaCO3 has retrograde solubility

(1) 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 (bio-buildups)

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Carbonate Minerals in Sedimentary Rocks

• bivalent metal cations– Ca+2

• Calcite, aragonite– Mg+2

• Magnesite, dolomite– Fe +2

• Siderite, ankerite

• + CO3 -2

• The physical and chemical conditions of the environment in which calcites, dolomites and aragonites are formed are reflected in their composition.

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Carbonate Minerals in Sedimentary Rocks

• bivalent metal cations– Ca+2

• Calcite, aragonite– Mg+2

• Magnesite, dolomite– Fe +2

• Siderite, ankerite

• + CO3 -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)

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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)

(Mg,Ca)CO3 (Mg,Fe,Ca)CO3

(Aragonite)

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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”:

– Primary or Secondary/Authigenic??

• Calcite: – <4% Mg++ (CaCO3)

(Mg,Ca)CO3 (Mg,Fe,Ca)CO3

(Aragonite)

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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 sediment-forming 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 pellets4. 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. Intraclasts2. Ooliths 3. Peloids4. 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

3. Grain kind: standard microfacies types4. Grain size, rounding, and coating: hydrologic

interpretations5. 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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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