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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.

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Presentation on theme: "Clastic/Detrital Sedimentary Rocks I.G.Kenyon. Characteristics of Sedimentary Rocks Formed at or very close to the earth’s surface Deposited in layers."— Presentation transcript:

1 Clastic/Detrital Sedimentary Rocks I.G.Kenyon

2 Characteristics of Sedimentary Rocks Formed at or very close to the earth’s surface Deposited in layers or beds - often horizontal Frequently contain fossils

3 Classification of Sedimentary Rocks 3 Groups recognised according to mode of origin Clastic/Detrital Organic Chemical Precipitates

4 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

5 Organic Sedimentary Rocks Formed from the remains of once-living organisms

6 Chemical Sedimentary Rocks Rocks precipitated directly from solution

7 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

8 Clastic Rock Groups Rudaceous - Coarse >2mm Arenaceous - Medium 1/16 – 2mm Argillaceous – Fine <1/16mm

9 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

10 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

11 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

12 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

13 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

14 A Sediment Sorting Comparitor Very Well SortedWell SortedModerately Sorted Poorly SortedVery Poorly Sorted

15 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.

16 Conglomerate Clasts are all flint pebbles=oligomict Mineralogically mature Clasts 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

17 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

18 Limestone Breccia – Fault Breccia Poorly 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 4cm

19 Glacial Breccia – Boulder Clay/Till, (Tillite when Lithified) Photograph courtesy A.Quarterman, Greenhead College Wide range of particle sizes from clay 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

20 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

21 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

22 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

23 Desert Sandstone Aeolian/wind blown deposit Formed in a desert Well sorted grains mm 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 fossils Grains have frosted/pitted surfaces due to constant attritiom/abrasion

24 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

25 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

26 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

27 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

28 Depositional Environments – Sedimentary Rocks Suggest an appropriate sedimentary rock type that may be forming in the areas labelled 1 to 15 above

29 The End


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