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, Greywacke
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)
14 A Sediment Sorting Comparitor Very Well SortedWell SortedModerately SortedPoorly SortedVery Poorly Sorted
15 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.
16 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
17 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
18 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
19 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
20 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
21 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
22 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.
23 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
24 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!
25 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
26 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
27 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
28 Depositional Environments – Sedimentary Rocks 541328671091311151214Suggest an appropriate sedimentary rock type that may be forming in the areas labelled 1 to 15 above