Clastic/Detrital Sedimentary Rocks I.G.Kenyon
Characteristics of Sedimentary Rocks Formed at or very close to the earth s surface Deposited in layers or beds - often horizontal Frequently contain fossils
Classification of Sedimentary Rocks 3 Groups recognised according to mode of origin Clastic/Detrital Organic Chemical Precipitates
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
Organic Sedimentary Rocks Formed from the remains of once-living organisms
Chemical Sedimentary Rocks Rocks precipitated directly from solution
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
Clastic Rock Groups Rudaceous - Coarse >2mm Arenaceous - Medium 1/16 2mm Argillaceous Fine <1/16mm
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
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
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
Clastic Rock Terminology 1 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 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 type Polymict clasts are of a variety of types
A Sediment Sorting Comparitor Very Well Sorted Well Sorted Poorly Sorted Moderately 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 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
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.
Clast/Particle Shape c/b 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 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 sorted, polymodal Clasts all well rounded texturally mature 1cm Grey, cream, yellowish cement, no acid reaction probably quartz Clasts are all flint pebbles=oligomict Mineralogically mature
Breccia All fragments are angular texturally immature Contains fragments of limestone, basalt, slate and quartz = polymict mineralogically immature Produced by a flash flood in a desert environment Matrix is a micro-breccia fine grained <0.25mm 1cm Clasts range in size 1mm >3cm poorly sorted, polymodal Red colour is haematite (iron oxide) cement
Limestone Breccia Fault Breccia Poorly sorted, clasts 1mm 7cm Limestone Calcite cement reacts with acid All clasts are limestone therefore oligomict Formed adjacent to a fault plane, main process is cataclasis Zon e of F a ult Limestone Bre cci a Texturally very immature 4cm All clasts are very angular
Glacial Breccia Boulder Clay/Till, (Tillite when Lithified) Wide range of particle sizes from clay <1/256mm to boulders >256mm Very poorly sorted, texturally and mineralogically very immature Direction of Ice Flow Large boulder showing glacial striations Long axes (a) of clasts show sub-parallel alignment 1m All clasts are very angular Polymict and Polymodal 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 Fossils Rare Texturally and mineralogically immature Angular quartz grain Possible fining upwards sequence/graded bedding Comprises up to 40% muddy matrix Poorly sorted 1cm
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
Comprises angular feldspar and quartz grains and is texturally and mineralogically immature Arkose 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 by mechanical 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 Sandstone Aeolian/wind blown deposit Formed in a desert Grains well rounded and texturally mature Red/brown haematite cement Grains have frosted/pitted surfaces due to constant attritiom/abrasion All grains are quartz mineralogically mature 8mm Cross bedding common but no fossils Poorly consolidated grains rub off in the fingers Well sorted grains 0.25-0.5mm
Micaceous Sandstone (Flagstone) Moderately well sorted, most grains 0.25-1.25mm in diameter 1cm 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 Splits into layers quite readily 2-5cm in thickness where mica concentrations occur
Orthoquartzite Texturally and mineralogically mature Oligomict and unimodal Quartz cement results in very low porosity Well sorted most grains 0.25 to 0.5mm 2mm Absence of fossils due to long transport history and prolonged erosion Very resistant to mechanical and chemical weathering 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 unimodal Comprises entirely quartz grains 0.50 to 0.75mm in diameter A very stable sediment
Siltstone Contains mainly clay minerals such as kaolinite, illite, serecite plus fine quartz particles 1cm Reddish brown colour implies haematite cement Shows laminations-splitting into layers <1cm thick Grain size mainly 1/16-1/256 mm Feels gritty when rubbed gently on the teeth!
Mudstone 1cm Deposited in a low energy environment such as a river estuary or marine harbour Grain size <1/256mm Homogenous/structureless with little evidence of laminations Feels smooth when rubbed on the teeth and implies absence of quartz Represents a clay that has been consolidated and the water content reduced Comprises entirely clay minerals such as kaolinite, illite and serecite
Clay Particles <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 Graptolites Well laminated Deep sea, low energy deposit Composed of clay minerals and carbonaceous material which results in dark colour Well preserved Didymograptus (Tuning Fork Graptolite) of Ordovician Age Main clay minerals are kaolinite and illite Splits into thin layers = Fissile 1cm Grain size <1/256 mm
Depositional Environments Sedimentary Rocks 5 4 1 2 3 6 7 13 8 9 10 11 15 12 14 Suggest an appropriate sedimentary rock type that may be forming in the areas labelled 1 to 15 above
The End