Clastic Rock Groups Rudaceous - Coarse >2mmArenaceous - Medium 1/16 – 2mm Argillaceous – Fine <1/16mm
Rudaceous RocksOver 50% of the clasts (particles) are over 2mm in diameterPrimarily consist of rock fragmentsIf particles rounded = Conglomerate If particles angular = Breccia
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 occur Desert Sandstone, Arkose, Greywacke Micaceous Sandstone, Orthoquartzite, Grit
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
Clastic Rock Terminology 1Phenoclast–A large clast/rock fragmentMatrix–the finer material often sand,siltand clay surrounding the phenoclasts Cement–material precipitated from solution to stick the sediment together.This is often quartz, calcite or haematite
Clastic Rock Terminology 2Well 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
A Sediment Sorting ComparitorVery Well Sorted Well Sorted Moderately Sorted Poorly Sorted Very Poorly Sorted
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.
Clastic Rock Terminology 3 Mineralogically Mature – the rock consists of clasts of just one type Mineralogically Immature – the rockconsists 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
Clast/Particle ShapeIndividual clasts can be assigned to one of six classesbased 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.
Clast/Particle Shape c/b Zinng classification Involves measuring a, b Rod 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 verticallyb/a b/a and c/b axial values are plotted as co-ordinates to identify individual clasts as spheres, discs, rods or blades.
Conglomerate Typical deposit of a high-energy shallow marine environment-beach Flint showing conchoidal fracture Clasts range in size 1mm – 3cm, poorly Grey, cream, sorted, polymodal yellowish cement, no acid reaction probably quartz Clasts are all flint pebbles=oligomict MineralogicallyClasts all well rounded 1cm mature texturally mature
Breccia All fragments are angular texturally immatureContains fragments of limestone,basalt, slate and quartz = polymict mineralogically immatureProduced by a flash Matrix is a micro-breccia flood in a desert fine grained <0.25mm environment 1cm Red colour is haematite (iron oxide) cement Clasts range in size 1mm ->3cm poorly sorted, polymodal
Limestone Breccia – Fault Breccia Poorly sorted, clasts 1mm – 7cm Calcite cement Limestone reacts with acid All clasts are limestone therefore oligomict Formed adjacent to a fault plane, main process is cataclasis Limestone Zo ne of Fau lt B All clasts are rec cia very angular Texturally very immature 4cm
Glacial Breccia – Boulder Clay/Till, (Tillite when Lithified) Wide range of particle sizes from Very poorly sorted, texturally andclay <1/256mm to boulders >256mm mineralogically very immature Direction of Ice Flow 1m Large boulder showing All clasts are glacial striations very angular Long axes (a) of clasts show Polymict and Polymodal sub-parallel alignment Produced by freeze-thaw, plucking, glacial abrasion and attrition Photograph courtesy A.Quarterman, Greenhead College
Greywacke/Turbidite or Muddy Sandstone Polymict/polymodal Clasts are angular rock and mineral fragments 2-6mm Texturally and Fossils mineralogically Rare immature Angular quartz grain Possible fining upwards sequence/graded beddingComprises up to 40% muddy Poorly sorted 1cm matrix
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 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
Comprises angular feldspar and quartz grains Arkoseand 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 bymechanical weathering of granite/gneiss under arid conditions. Main processes exfoliation and granular disintegration.
Millstone Grit Most grains between 1 and 4mm, but still rudaceous and polymodal Texturally and mineralogically immature Graded bedding Fining upwards sequence Well cemented together by a silica cement 5mm Comprises sub angular to sub rounded grains of quartz and feldspar, polymict
Desert Aeolian/wind blown depositSandstone Formed in a desert Grains well rounded and texturally mature Red/brown haematite cement Cross bedding commonGrains have frosted/pitted surfaces but no fossils due to constant attritiom/abrasion All grains are quartz Poorly consolidated grains mineralogically mature rub off in the fingers 8mm Well sorted grains 0.25-0.5mm
Micaceous Sandstone (Flagstone)Moderately well sorted, most Mineralogy is quartz and muscovitegrains 0.25-1.25mm in diameter Bimodal grain size-mica occurs as thin flakes, quartz as sub rounded to rounded grains 1cm Well cemented by quartz Mica deposited from suspension when energy conditions reduced environment was a delta with a marked seasonal fluctuation in river flow Splits into layers quite readily 2-5cm in thickness where mica concentrations occur
Orthoquartzite Texturally and mineralogically mature Oligomict and unimodalQuartz cement resultsin very low porosity Well sorted most grains 0.25 to 0.5mm2mm Very resistant to mechanical Absence of fossils due to and chemical weatheringlong transport history and prolonged erosion Comprises over 95% rounded quartz grains
Orthoquartzite (Greensand) Texturally and mineralogically very mature 1cm Moderately well cemented by the pale green mineral glauconite Oligomict and unimodalComprises entirely quartz grains 0.50 to 0.75mm in diameter A very stable sediment
Siltstone Contains mainly clay mineralssuch as kaolinite, illite, serecite 1cm plus fine quartz particles Reddish brown colour implies haematite cementShows laminations-splitting into layers <1cm thick Grain size mainly Feels gritty when rubbed 1/16-1/256 mm gently on the teeth!
Mudstone Deposited in a low 1cm energy environment such as a river estuary or marine harbour Homogenous/structureless withGrain size <1/256mm little evidence of laminations Represents a clay that hasFeels smooth when rubbed been consolidated and the on the teeth and implies water content reduced absence of quartz Comprises entirely clay minerals such as kaolinite, illite and serecite
ClayParticles <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 Comprised of clay minerals, chiefly kaolinite 1cm
Black Shale with GraptolitesWell laminated Composed of clay minerals and carbonaceous material Deep sea, low which results in dark colour energy deposit Well preserved Didymograptus (Tuning Fork Graptolite) of Ordovician Age Main clay minerals are kaolinite and illiteSplits into thin 1cm layers = Grain size <1/256 mm Fissile
Depositional Environments – Sedimentary Rocks 5 4 1 3 2 6 7 8 9 10 13 11 12 15 14Suggest an appropriatesedimentary rock type thatmay be forming in the areaslabelled 1 to 15 above