8 Clastic Rock Groups Rudaceous - Coarse >2mm Arenaceous - Medium 1/16 – 2mmArgillaceous – Fine <1/16mm
9 Rudaceous RocksOver 50% of the clasts (particles) are over 2mm in diameterPrimarily consist of rock fragmentsIf particles rounded = ConglomerateIf particles angular = Breccia
10 Arenaceous RocksOver 50% of the particles are 1/16mm to 2mm in diameterComprise a high percentage of quartz grainsThese rocks are generally called sandstonesHowever, a wide variety of sandstones occurDesert Sandstone, Arkose, GreywackeMicaceous Sandstone, Orthoquartzite, Grit
11 Argillaceous RocksOver 50% of the particles are <1/16mm in diameterConsist of clay minerals and small quartz grainsRock types are Siltstone, Mudstone, Clay and Shale
12 Clastic Rock Terminology 1 Phenoclast–A large clast/rock fragment Matrix–the finer material often sand,silt and clay surrounding the phenoclastsCement–material precipitated from solution to stick the sediment together.This is often quartz, calcite or haematite
13 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)Oligomict – all clasts are of the same typePolymict – clasts are of a variety of types
14 A Sediment Sorting Comparitor Very Well SortedWell SortedModerately SortedPoorly SortedVery Poorly Sorted
15 Textural & Mineralogical Maturity 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.
16 Clastic Rock Terminology 3 Mineralogically Mature – the rock consists of clasts of just one typeMineralogically Immature – the rock consists of a wide range of clast typesTexturally Mature – all of the clasts are well roundedTexturally Immature – all of the clasts are very angular
17 Clast/Particle ShapeIndividual 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 c/b Zinng classification Involves measuring a, b and c axes of clastsa axis is longest dimension on the clastb axis is widest dimension at right angles to a axisc is shortest axis on which the clast often sits verticallyb/a and c/b axial values are plotted as co-ordinates to identify individual clasts as spheres, discs, rods or blades.Rodb/a
19 ConglomerateTypical deposit of a high-energy shallow marine environment-beachFlint showing conchoidal fractureClasts range in size 1mm – 3cm, poorly sorted, polymodalGrey, cream, yellowish cement, no acid reaction probably quartzClasts are all flint pebbles=oligomict Mineralogically matureClasts all well rounded texturally mature1cm
20 Breccia All fragments are angular texturally immature Contains fragments of limestone, basalt, slate and quartz = polymict mineralogically immatureProduced by a flash flood in a desert environmentMatrix is a micro-breccia fine grained <0.25mm1cmRed colour is haematite (iron oxide) cementClasts range in size 1mm - >3cm poorly sorted, polymodal
21 Limestone Breccia – Fault Breccia Poorly sorted, clasts 1mm – 7cmCalcite cement reacts with acidLimestoneAll clasts are limestone therefore oligomictFormed adjacent to a fault plane, main process is cataclasisLimestoneZone of Fault BrecciaAll clasts are very angularTexturally very immature4cm
22 Glacial Breccia – Boulder Clay/Till, (Tillite when Lithified) Wide range of particle sizes from clay <1/256mm to boulders >256mmVery poorly sorted, texturally and mineralogically very immatureDirection of Ice Flow1mLarge boulder showing glacial striationsAll clasts are very angularLong axes (a) of clasts show sub-parallel alignmentPolymict and PolymodalProduced by freeze-thaw, plucking, glacial abrasion and attritionPhotograph courtesy A.Quarterman, Greenhead College
23 Greywacke/Turbidite or Muddy Sandstone Polymict/polymodalClasts are angular rock and mineral fragments 2-6mmFossils RareTexturally and mineralogically immatureAngular quartz grainPossible fining upwards sequence/graded beddingComprises up to % muddy matrixPoorly sorted1cm
24 Greywacke/Turbidite/Muddy Sandstone A sandstone with a muddy matrix of up to 40%Comprises a wide range of angular rock and mineral fragmentsGraded bedding common, fossils quite rareForms in subsiding marine basins of depositionTexturally and mineralogically immatureTurbidity 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 Absence of fine material and mica as blown away by the wind ArkoseComprises angular feldspar and quartz grains and is texturally and mineralogically immatureAbsence of fine material and mica as blown away by the wind5mmPinkish/purple colour due to high percentage of feldspar and iron oxide cementA sandstone containing over 25% feldspar, produced by mechanical weathering of granite/gneiss under arid conditions. Main processes exfoliation and granular disintegration.
26 Millstone Grit Graded bedding Fining upwards sequence Most grains between 1 and 4mm, but still rudaceous and polymodalTexturally and mineralogically immatureGraded beddingFining upwards sequenceWell cemented together by a silica cementComprises sub angular to sub rounded grains of quartz and feldspar, polymict5mm
27 Desert Sandstone Aeolian/wind blown deposit Formed in a desert Grains well rounded and texturally matureRed/brown haematite cementCross bedding common but no fossilsGrains have frosted/pitted surfaces due to constant attritiom/abrasionAll grains are quartz mineralogically maturePoorly consolidated grains rub off in the fingers8mmWell sorted grains mm
28 Micaceous Sandstone (Flagstone) Well cemented by quartz Moderately well sorted, most grains mm in diameterMineralogy is quartz and muscoviteBimodal grain size-mica occurs as thin flakes, quartz as sub rounded to rounded grains1cmWell cemented by quartzMica deposited from suspension when energy conditions reduced environment was a delta with a marked seasonal fluctuation in river flowSplits into layers quite readily 2-5cm in thickness where mica concentrations occur
29 Orthoquartzite Texturally and mineralogically mature Oligomict and unimodalQuartz cement results in very low porosityWell sorted most grains 0.25 to 0.5mm2mmVery resistant to mechanical and chemical weatheringAbsence of fossils due to long transport history and prolonged erosionComprises over 95% rounded quartz grains
30 Orthoquartzite (Greensand) Texturally and mineralogically very mature1cmModerately well cemented by the pale green mineral glauconiteOligomict and unimodalComprises entirely quartz grains to 0.75mm in diameterA very stable sediment
31 SiltstoneContains mainly clay minerals such as kaolinite, illite, serecite plus fine quartz particles1cmReddish brown colour implies haematite cementShows laminations-splitting into layers <1cm thickGrain size mainly /16-1/256 mmFeels gritty when rubbed gently on the teeth!
32 MudstoneDeposited in a low energy environment such as a river estuary or marine harbour1cmHomogenous/structureless with little evidence of laminationsGrain size <1/256mmRepresents a clay that has been consolidated and the water content reducedFeels smooth when rubbed on the teeth and implies absence of quartzComprises entirely clay minerals such as kaolinite, illite and serecite
33 Deep Sea or Lake deposit where energy conditions are very low ClayParticles <1/256mmThis specimen has dried out and has zero plasticity so is more appropriately called a claystone or mudstoneDeep Sea or Lake deposit where energy conditions are very low1cmComprised of clay minerals, chiefly kaolinite
34 Black Shale with Graptolites Composed of clay minerals and carbonaceous material which results in dark colourWell laminatedDeep sea, low energy depositWell preserved Didymograptus (Tuning Fork Graptolite) of Ordovician AgeMain clay minerals are kaolinite and illiteSplits into thin layers = Fissile1cmGrain size <1/256 mm
35 Depositional Environments – Sedimentary Rocks 541328671091311151214Suggest an appropriate sedimentary rock type that may be forming in the areas labelled 1 to 15 above
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