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