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 orthochemicalIA= 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:

– 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 • >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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• Grain size, sorting, and roundness – interpretation:

• Textural Maturity– Kinetic energy

during transport and reworking

– Transport history

– Dispersal patterns

– Beware:• Mixed sources• Biogenic

reworking

Siliciclastic Rocks: Texture

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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 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 (>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• Lime mud (<0.004 mm)• Largely fragmental algae remains, also chemical precipitate

– Sparite• Crystalline carbonate material (>0.004 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 (<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 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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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 Activity1. 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 evidence3. 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