clastic/detrital sedimentary rocks i.g.kenyon. characteristics of sedimentary rocks formed at or...
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Clastic/Detrital Sedimentary Rocks
I.G.Kenyon
Characteristics of Sedimentary Rocks
Formed at or very close to the earth’s surface
Deposited in layers or beds - often horizontal
Frequently contain fossils
Classification of Sedimentary Rocks
3 Groups recognised according to mode of origin
Clastic/Detrital
Organic
Chemical Precipitates
Clastic Sedimentary Rocks
Clastic is derived from the Greek for ‘broken’
Clastic rocks represent the accumulation of weathered and eroded fragments of older, pre-existing rocks of all types
Organic Sedimentary Rocks
Formed from the remains of once-living organisms
Chemical Sedimentary Rocks
Rocks precipitated directly from solution
Particle Sizes of Clastic Rocks>256mm Boulder
256mm – 64mm Cobble
64mm- 4mm Pebble
4mm- 2mm Granule
2mm – 1/16mm Sand
1/16 – 1/256mm Silt
<1/256mm - Clay
Clastic Rock GroupsRudaceous - Coarse >2mm
Arenaceous - Medium 1/16 – 2mm
Argillaceous – Fine <1/16mm
Rudaceous RocksOver 50% of the clasts (particles)
are over 2mm in diameter
Primarily consist of rock fragments
If particles rounded = Conglomerate
If particles angular = Breccia
Arenaceous RocksOver 50% of the particles
are 1/16mm to 2mm in diameter
Comprise a high percentage of quartz grains
These rocks are generally called sandstones
However, a wide variety of sandstones occur
Desert Sandstone, Arkose, Greywacke
Argillaceous RocksOver 50% of the particles
are <1/16mm in diameter
Consist of clay minerals and small quartz grains
Rock types are Siltstone, Mudstone, Clay and Shale
Phenoclast–A large clast/rock fragment
Matrix–the finer material often sand,silt and clay surrounding the phenoclasts
Cement–material precipitated from solution to stick the sediment together.This is often quartz,
calcite or haematite
Clastic Rock Terminology 1
Well Sorted – all of the clasts are very similar in size (unimodal)
Poorly Sorted – clasts show a wide range of particle sizes (polymodal)
Clastic Rock Terminology 2
A Sediment Sorting Comparitor
Very Well Sorted Well Sorted Moderately Sorted
Poorly Sorted Very Poorly Sorted
Clast/Particle Shape
Individual clasts can be assigned to one of six classes based on visual observation of the clasts in the rock. (After Tucker 1982) Can be subjective as one person’s
subangular could be another person’s subrounded.
Conglomerate
Clasts are all flint pebbles=oligomict
Mineralogically matureClasts all well rounded
texturally mature
Grey, cream, yellowish cement, no acid reaction probably quartz
Clasts range in size 1mm – 3cm, poorly sorted, polymodal
Flint showing conchoidal fracture
1cm
Typical deposit of a high-energy shallow marine environment-beach
Breccia
Clasts range in size 1mm - >3cm poorly sorted, polymodal
Red colour is haematite (iron oxide) cement
Contains fragments of limestone, basalt, slate and quartz = polymict
mineralogically immature
All fragments are angular texturally immature
Matrix is a micro-breccia fine grained <0.25mm
Produced by a flash flood in a desert
environment
1cm
Limestone Breccia – Fault BrecciaPoorly sorted,
clasts 1mm – 7cm Calcite cement reacts with acid
All clasts are very angular
Texturally very immature
All clasts are limestone therefore oligomict
Formed adjacent to a fault plane, main
process is cataclasis
Zone of Fault Breccia
Limestone
Limestone
4cm
Glacial Breccia – Boulder Clay/Till, (Tillite when Lithified)
Photograph courtesy A.Quarterman, Greenhead College
Wide range of particle sizes from clay <1/256mm to boulders >256mm
Very poorly sorted, texturally and mineralogically very immature
Produced by freeze-thaw, plucking, glacial abrasion and attrition
Large boulder showing glacial striations
All clasts are very angular
1m
Long axes (a) of clasts show sub-parallel alignment
Direction of Ice Flow
Polymict and Polymodal
Greywacke/Turbidite or Muddy Sandstone
Comprises up to 40% muddy
matrix
Possible fining upwards sequence/graded bedding
Clasts are angular rock and mineral fragments 2-6mm
Angular quartz grain
1cm
Polymict/polymodal
Texturally and mineralogically
immature
Fossils Rare
Poorly sorted
Greywacke/Turbidite/Muddy Sandstone
A sandstone with a muddy matrix of up to 40%
Comprises a wide range of angular rock and mineral fragments
Graded bedding common, fossils quite rare
Forms in subsiding marine basins of deposition
Texturally and mineralogically immature
Turbidity currents (water-laden sediment flows) on the continental slope cause large volumes of sediment to be deposited rapidly at the base of the continental slope in broad fan-shaped structures
Arkose
A sandstone containing over 25% feldspar, produced by mechanical weathering of granite/gneiss under arid conditions.
Main processes exfoliation and granular disintegration.
Comprises angular feldspar and quartz grains and is texturally and mineralogically immature
Pinkish/purple colour due to high percentage of feldspar
and iron oxide cement
5mm
Absence of fine material and mica as blown away by the wind
Desert Sandstone
Aeolian/wind blown deposit Formed in a desert
Well sorted grains 0.25-0.5mm
Grains well rounded and texturally mature
All grains are quartz mineralogically mature
Red/brown haematite cement
8mm
Poorly consolidated grains rub off in the fingers
Cross bedding common but no fossilsGrains have frosted/pitted surfaces
due to constant attritiom/abrasion
Siltstone
Contains mainly clay minerals such as kaolinite, illite, serecite
plus fine quartz particles
Feels gritty when rubbed gently on the teeth!
Grain size mainly 1/16-1/256 mm
1cm
Reddish brown colour implies haematite cement
Shows laminations-splitting into layers <1cm thick
Mudstone
Comprises entirely clay minerals such as kaolinite, illite and serecite
1cm
Homogenous/structureless with little evidence of laminations
Represents a clay that has been consolidated and the
water content reduced
Grain size <1/256mm
Feels smooth when rubbed on the teeth and implies
absence of quartz
Deposited in a low energy environment
such as a river estuary or marine harbour
Clay
Comprised of clay minerals, chiefly kaolinite
This specimen has dried out and has zero plasticity so is more appropriately called
a claystone or mudstone
1cm
Particles <1/256mm
Deep Sea or Lake deposit where energy conditions are very low
Black Shale with Graptolites
Grain size <1/256 mm
Composed of clay minerals and carbonaceous material which results in dark colour
Splits into thin layers =
Fissile
Well laminated
1cm
Deep sea, low energy deposit
Well preserved Didymograptus (Tuning Fork Graptolite) of Ordovician Age
Main clay minerals are kaolinite and illite
Depositional Environments – Sedimentary Rocks
12 3
4
5
6 7 89 10
11
12
13
14
15
Suggest an appropriate sedimentary rock type that may be forming in the areas labelled 1 to 15 above
The End
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