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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 earth’s 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
>256mm Boulder 256mm – 64mm Cobble 64mm- 4mm Pebble 4mm- 2mm Granule 2mm – 1/16mm Sand 1/16 – 1/256mm Silt <1/256mm - Clay
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Clastic Rock Groups Rudaceous - Coarse >2mm
Arenaceous - Medium 1/16 – 2mm Argillaceous – Fine <1/16mm
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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 <1/16mm 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 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
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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 person’s subangular could be another person’s 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 pebbles=oligomict 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 <0.25mm 1cm Red colour is haematite (iron oxide) cement Clasts range in size 1mm - >3cm 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 <1/256mm to boulders >256mm 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 % muddy matrix Poorly sorted 1cm
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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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Absence of fine material and mica as blown away by the wind
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 8mm Well sorted grains mm
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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 <1cm thick Grain size mainly /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 <1/256mm 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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Deep Sea or Lake deposit where energy conditions are very low
Clay Particles <1/256mm 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 <1/256 mm
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Depositional Environments – Sedimentary Rocks
5 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
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