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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 Micaceous Sandstone, Orthoquartzite, Grit

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) Oligomict – all clasts are of the same type Polymict – clasts are of a variety of types Clastic Rock Terminology 2

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

15 If a rock is texturally and mineralogically mature it has undergone extensive transport and erosion. It is a STABLE sediment and would not change markedly in character if it were to be transported and eroded further. If a rock is texturally and mineralogically immature it has only been transported a short distance and suffered limited erosion. It is an UNSTABLE sediment and would change in character significantly if it were to be transported further. Textural & Mineralogical Maturity

16 Clastic Rock Terminology 3 Mineralogically Mature – the rock consists of clasts of just one type Mineralogically Immature – the rock consists of a wide range of clast types Texturally Mature – all of the clasts are well rounded Texturally Immature – all of the clasts are very angular

17 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.

18 Clast/Particle Shape Rod Zinng classification Involves measuring a, b and c axes of clasts a axis is longest dimension on the clast b axis is widest dimension at right angles to a axis c is shortest axis on which the clast often sits vertically b/a and c/b axial values are plotted as co-ordinates to identify individual clasts as spheres, discs, rods or blades. b/a c/b

19 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

20 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

21 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

22 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

23 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

24 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

25 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

26 Millstone Grit 5mm Most grains between 1 and 4mm, but still rudaceous and polymodal Comprises sub angular to sub rounded grains of quartz and feldspar, polymict Graded bedding Fining upwards sequence Well cemented together by a silica cement Texturally and mineralogically immature

27 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

28 Micaceous Sandstone (Flagstone) Splits into layers quite readily 2-5cm in thickness where mica concentrations occur Moderately well sorted, most grains mm in diameter Mineralogy is quartz and muscovite Bimodal grain size-mica occurs as thin flakes, quartz as sub rounded to rounded grains Well cemented by quartz Mica deposited from suspension when energy conditions reduced environment was a delta with a marked seasonal fluctuation in river flow 1cm

29 Orthoquartzite Comprises over 95% rounded quartz grains Texturally and mineralogically mature Quartz cement results in very low porosity Absence of fossils due to long transport history and prolonged erosion Very resistant to mechanical and chemical weathering Well sorted most grains 0.25 to 0.5mm Oligomict and unimodal 2mm

30 Orthoquartzite (Greensand) Comprises entirely quartz grains 0.50 to 0.75mm in diameter Texturally and mineralogically very mature Oligomict and unimodal Moderately well cemented by the pale green mineral glauconite A very stable sediment 1cm

31 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

32 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

33 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

34 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

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

36 The End


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