Clastic/Detrital Sedimentary Rocks

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Clastic/Detrital Sedimentary Rocks
I.G.Kenyon
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Characteristics of Sedimentary Rocks Formed at or
very close to the earths
surface Deposited in layers or beds - often
horizontal Frequently contain fossils
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Classification of Sedimentary Rocks 3 Groups
recognised according to mode of
origin Clastic/Detrital Organic Chemical
Precipitates
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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
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Organic Sedimentary Rocks Formed from the
remains of once-living organisms
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Chemical Sedimentary Rocks Rocks precipitated
directly from solution
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Particle Sizes of Clastic Rocks gt256mm
Boulder 256mm 64mm Cobble 64mm- 4mm Pebble 4mm-
2mm Granule 2mm 1/16mm Sand 1/16 1/256mm
Silt lt1/256mm - Clay
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Clastic Rock Groups Rudaceous - Coarse
gt2mm Arenaceous - Medium 1/16 2mm Argillaceous
Fine lt1/16mm
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Rudaceous Rocks Over 50 of the clasts
(particles) are over 2mm in diameter Primarily
consist of rock fragments If particles rounded
Conglomerate If particles angular Breccia
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Arenaceous Rocks Over 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
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Argillaceous Rocks Over 50 of the particles
are lt1/16mm in diameter Consist of clay minerals
and small quartz grains Rock types are
Siltstone, Mudstone, Clay and Shale
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Clastic Rock Terminology 1
PhenoclastA large clast/rock fragment Matrixthe
finer material often sand,silt and clay
surrounding the phenoclasts Cementmaterial
precipitated from solution to stick
the sediment together.This is often quartz,
calcite or haematite
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Clastic Rock Terminology 2
Well Sorted all of the clasts are
very similar in size (unimodal) Poorly Sorted
clasts show a wide range of particle
sizes (polymodal)
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A Sediment Sorting Comparitor
Very Well Sorted
Well Sorted
Moderately Sorted
Poorly Sorted
Very Poorly Sorted
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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 persons subangular could be
another persons subrounded.
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Conglomerate
Typical deposit of a high-energy shallow marine
environment-beach
Flint showing conchoidal fracture
Clasts range in size 1mm 3cm, poorly sorted,
polymodal
Grey, cream, yellowish cement, no acid reaction
probably quartz
Clasts are all flint pebblesoligomict
Mineralogically mature
Clasts all well rounded texturally mature
1cm
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Breccia
All fragments are angular texturally immature
Contains fragments of limestone, basalt, slate
and quartz polymict mineralogically immature
Produced by a flash flood in a desert environment
Matrix is a micro-breccia fine grained lt0.25mm
1cm
Red colour is haematite (iron oxide) cement
Clasts range in size 1mm - gt3cm poorly sorted,
polymodal
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Limestone Breccia Fault Breccia
Poorly sorted, clasts 1mm 7cm
Calcite cement reacts with acid
Limestone
All clasts are limestone therefore oligomict
Formed adjacent to a fault plane, main process
is cataclasis
Limestone
Zone of Fault Breccia
All clasts are very angular
Texturally very immature
4cm
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Glacial Breccia Boulder Clay/Till, (Tillite
when Lithified)
Wide range of particle sizes from clay lt1/256mm
to boulders gt256mm
Very poorly sorted, texturally and
mineralogically very immature
Direction of Ice Flow
1m
Large boulder showing glacial striations
All clasts are very angular
Long axes (a) of clasts show sub-parallel
alignment
Polymict and Polymodal
Produced by freeze-thaw, plucking, glacial
abrasion and attrition
Photograph courtesy A.Quarterman, Greenhead
College
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Greywacke/Turbidite or Muddy Sandstone
Polymict/polymodal
Clasts are angular rock and mineral fragments
2-6mm
Fossils Rare
Texturally and mineralogically immature
Angular quartz grain
Possible fining upwards sequence/graded bedding
Comprises up to 40 muddy matrix
1cm
Poorly sorted
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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
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Arkose
Comprises angular feldspar and quartz grains and
is texturally and mineralogically immature
Absence of fine material and mica as blown away
by the wind
5mm
Pinkish/purple colour due to high percentage of
feldspar and iron oxide cement
A sandstone containing over 25 feldspar,
produced by mechanical weathering of
granite/gneiss under arid conditions. Main
processes exfoliation and granular disintegration.
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Desert Sandstone
Aeolian/wind blown deposit Formed in a desert
Grains well rounded and texturally mature
Red/brown haematite cement
Cross bedding common but no fossils
Grains have frosted/pitted surfaces due to
constant attritiom/abrasion
All grains are quartz mineralogically mature
Poorly consolidated grains rub off in the
fingers
Well sorted grains 0.25-0.5mm
8mm
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Siltstone
Contains mainly clay minerals such as kaolinite,
illite, serecite plus fine quartz particles
1cm
Reddish brown colour implies haematite cement
Shows laminations-splitting into layers lt1cm thick
Grain size mainly 1/16-1/256 mm
Feels gritty when rubbed gently on the teeth!
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Mudstone
Deposited in a low energy environment such as a
river estuary or marine harbour
1cm
Homogenous/structureless with little evidence of
laminations
Grain size lt1/256mm
Represents a clay that has been consolidated and
the water content reduced
Feels smooth when rubbed on the teeth and implies
absence of quartz
Comprises entirely clay minerals such as
kaolinite, illite and serecite
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Clay
Particles lt1/256mm
This specimen has dried out and has zero
plasticity so is more appropriately called a
claystone or mudstone
Deep Sea or Lake deposit where energy conditions
are very low
1cm
Comprised of clay minerals, chiefly kaolinite
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Black Shale with Graptolites
Composed of clay minerals and carbonaceous
material which results in dark colour
Well laminated
Deep sea, low energy deposit
Well preserved Didymograptus (Tuning Fork
Graptolite) of Ordovician Age
Main clay minerals are kaolinite and illite
Splits into thin layers Fissile
1cm
Grain size lt1/256 mm
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Depositional Environments Sedimentary Rocks
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4
1
3
2
8
6
7
10
9
13
11
15
12
14
Suggest an appropriate sedimentary rock type that
may be forming in the areas labelled 1 to 15 above
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The End