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 GroupsRudaceous - Coarse >2mm

Arenaceous - Medium 1/16 – 2mm

Argillaceous – Fine <1/16mm

Rudaceous RocksOver 50% of the clasts (particles)

are over 2mm in diameter

Primarily consist of rock fragments

If particles rounded = Conglomerate

If particles angular = Breccia

Arenaceous RocksOver 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

Argillaceous RocksOver 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

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 1

Well Sorted – all of the clasts are very similar in size (unimodal)

Poorly Sorted – clasts show a wide range of particle sizes (polymodal)

Clastic Rock Terminology 2

A Sediment Sorting Comparitor

Very Well Sorted Well Sorted Moderately Sorted

Poorly Sorted Very Poorly Sorted

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.

Conglomerate

Clasts are all flint pebbles=oligomict

Mineralogically matureClasts all well rounded

texturally mature

Grey, cream, yellowish cement, no acid reaction probably quartz

Clasts range in size 1mm – 3cm, poorly sorted, polymodal

Flint showing conchoidal fracture

1cm

Typical deposit of a high-energy shallow marine environment-beach

Breccia

Clasts range in size 1mm - >3cm poorly sorted, polymodal

Red colour is haematite (iron oxide) cement

Contains fragments of limestone, basalt, slate and quartz = polymict

mineralogically immature

All fragments are angular texturally immature

Matrix is a micro-breccia fine grained <0.25mm

Produced by a flash flood in a desert

environment

1cm

Limestone Breccia – Fault BrecciaPoorly sorted,

clasts 1mm – 7cm Calcite cement reacts with acid

All clasts are very angular

Texturally very immature

All clasts are limestone therefore oligomict

Formed adjacent to a fault plane, main

process is cataclasis

Zone of Fault Breccia

Limestone

Limestone

4cm

Glacial Breccia – Boulder Clay/Till, (Tillite when Lithified)

Photograph courtesy A.Quarterman, Greenhead College

Wide range of particle sizes from clay <1/256mm to boulders >256mm

Very poorly sorted, texturally and mineralogically very immature

Produced by freeze-thaw, plucking, glacial abrasion and attrition

Large boulder showing glacial striations

All clasts are very angular

1m

Long axes (a) of clasts show sub-parallel alignment

Direction of Ice Flow

Polymict and Polymodal

Greywacke/Turbidite or Muddy Sandstone

Comprises up to 40% muddy

matrix

Possible fining upwards sequence/graded bedding

Clasts are angular rock and mineral fragments 2-6mm

Angular quartz grain

1cm

Polymict/polymodal

Texturally and mineralogically

immature

Fossils Rare

Poorly sorted

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

Arkose

A sandstone containing over 25% feldspar, produced by mechanical weathering of granite/gneiss under arid conditions.

Main processes exfoliation and granular disintegration.

Comprises angular feldspar and quartz grains and is texturally and mineralogically immature

Pinkish/purple colour due to high percentage of feldspar

and iron oxide cement

5mm

Absence of fine material and mica as blown away by the wind

Desert Sandstone

Aeolian/wind blown deposit Formed in a desert

Well sorted grains 0.25-0.5mm

Grains well rounded and texturally mature

All grains are quartz mineralogically mature

Red/brown haematite cement

8mm

Poorly consolidated grains rub off in the fingers

Cross bedding common but no fossilsGrains have frosted/pitted surfaces

due to constant attritiom/abrasion

Siltstone

Contains mainly clay minerals such as kaolinite, illite, serecite

plus fine quartz particles

Feels gritty when rubbed gently on the teeth!

Grain size mainly 1/16-1/256 mm

1cm

Reddish brown colour implies haematite cement

Shows laminations-splitting into layers <1cm thick

Mudstone

Comprises entirely clay minerals such as kaolinite, illite and serecite

1cm

Homogenous/structureless with little evidence of laminations

Represents a clay that has been consolidated and the

water content reduced

Grain size <1/256mm

Feels smooth when rubbed on the teeth and implies

absence of quartz

Deposited in a low energy environment

such as a river estuary or marine harbour

Clay

Comprised of clay minerals, chiefly kaolinite

This specimen has dried out and has zero plasticity so is more appropriately called

a claystone or mudstone

1cm

Particles <1/256mm

Deep Sea or Lake deposit where energy conditions are very low

Black Shale with Graptolites

Grain size <1/256 mm

Composed of clay minerals and carbonaceous material which results in dark colour

Splits into thin layers =

Fissile

Well laminated

1cm

Deep sea, low energy deposit

Well preserved Didymograptus (Tuning Fork Graptolite) of Ordovician Age

Main clay minerals are kaolinite and illite

Depositional Environments – Sedimentary Rocks

12 3

4

5

6 7 89 10

11

12

13

14

15

Suggest an appropriate sedimentary rock type that may be forming in the areas labelled 1 to 15 above

The End