ClasticDetrital Sedimentary Rocks I G Kenyon Characteristics of

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 precipitated directly from solution

Particle Sizes of Clastic Rocks >256 mm Boulder 256 mm – 64 mm Cobble 64 mm- 4 mm Pebble 4 mm- 2 mm Granule 2 mm – 1/16 mm Sand 1/16 – 1/256 mm Silt <1/256 mm - Clay

Clastic Rock Groups Rudaceous - Coarse >2 mm Arenaceous - Medium 1/16 – 2 mm Argillaceous – Fine <1/16 mm

Rudaceous Rocks Over 50% of the clasts (particles) are over 2 mm in diameter Primarily consist of rock fragments If particles rounded = Conglomerate If particles angular = Breccia

Arenaceous Rocks Over 50% of the particles are 1/16 mm to 2 mm 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 <1/16 mm 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 similar in size (unimodal) very 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 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 b/a 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.

Conglomerate Typical deposit of a high-energy shallow marine environment-beach Flint showing conchoidal fracture Clasts range in size 1 mm – 3 cm, poorly sorted, polymodal Clasts all well rounded texturally mature 1 cm 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. 25 mm 1 cm Clasts range in size 1 mm >3 cm poorly sorted, polymodal Red colour is haematite (iron oxide) cement

Limestone Breccia – Fault Breccia Poorly sorted, clasts 1 mm – 7 cm Limestone Calcite cement reacts with acid All clasts are limestone therefore oligomict Formed adjacent to a fault plane, main process is cataclasis Zon eo f Fa ult Limestone Bre cci a Texturally very immature 4 cm All clasts are very angular

Glacial Breccia – Boulder Clay/Till, (Tillite when Lithified) Wide range of particle sizes from clay <1/256 mm to boulders >256 mm 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 1 m 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 -6 mm Fossils Rare Texturally and mineralogically immature Angular quartz grain Possible fining upwards sequence/graded bedding Comprises up to 40% muddy matrix Poorly sorted 1 cm

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 5 mm 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 4 mm, but still rudaceous and polymodal Texturally and mineralogically immature Graded bedding Fining upwards sequence Well cemented together by a silica cement 5 mm 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 8 mm Cross bedding common but no fossils Poorly consolidated grains rub off in the fingers Well sorted grains 0. 25 -0. 5 mm

Micaceous Sandstone (Flagstone) Moderately well sorted, most grains 0. 25 -1. 25 mm in diameter 1 cm 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 -5 cm in thickness where mica concentrations occur

Orthoquartzite Oligomict and unimodal Quartz cement results in very low porosity Texturally and mineralogically mature Well sorted most grains 0. 25 to 0. 5 mm 2 mm 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 1 cm Moderately well cemented by the pale green mineral glauconite Oligomict and unimodal Comprises entirely quartz grains 0. 50 to 0. 75 mm in diameter A very stable sediment

Siltstone Contains mainly clay minerals such as kaolinite, illite, serecite plus fine quartz particles 1 cm Reddish brown colour implies haematite cement Shows laminations-splitting into layers <1 cm thick Grain size mainly 1/16 -1/256 mm Feels gritty when rubbed gently on the teeth!

Mudstone 1 cm Deposited in a low energy environment such as a river estuary or marine harbour Grain size <1/256 mm 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 <1/256 mm 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 1 cm

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 Grain size <1/256 mm 1 cm

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

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