EARTH MATERIALS VIEARTH MATERIALS VI

The Rock Cycle: The Rock Cycle: Clastic Sedimentary RocksClastic Sedimentary Rocks

Professor Peter DoyleP.doyle@imperial.ac.uk

Profdoyle@btinternet.com

THE ROCK CYCLE

Relative proportions of rock groupsRelative proportions of rock groups

Sedimentary rocks: larger proportion of land surface

The greater proportion created by weathering, erosion & deposition

These are known as clastic sediments

SEDIMENTARY ROCKS• Clastic sedimentsClastic sediments from weathering & erosion

of pre- existing rocks• Weathering can be physical (freeze-thaw) or

chemical• Erosion & subsequent transportation by

water, wind and ice before deposition• Biogenic and chemical sedimentsBiogenic and chemical sediments form as

biochemical or inorganic chemical precipitates • E.g. shelly fragments in limestones; evaporites

Sediments to sedimentary rocks

Weathering structures seen in ancient terranes

Transport shown by sedimentary structures

Lithification due to:1. Compaction

(squeezing out water)2. Diagenesis

(precipitation of cements)

Mineralogy of sedimentary rocks

Clastic sedimentsClastic sediments • siliciclasticssiliciclastics – quartz, feldspars, clay

minerals, rock fragments • calciclasticscalciclastics – calcite, dolomiteCarbonate (biogenic) sedimentsCarbonate (biogenic) sediments• aragonite, calcite • chalcedony silica (flint) occurs as subsiduary Evaporite (chemical) sedimentsEvaporite (chemical) sediments• halite, gypsum and anhydrite • Dolomite, also as dolomitisation of calcite

 Classification of clastic sediments: The Classification of clastic sediments: The Wentworth ScaleWentworth Scale

Grain SizeGrain Size Sediment GradeSediment Grade Rock Rock

>256 mm Boulder Conglomerate 256 – 64mm Cobble (rounded clasts) 64 – 2mm Pebble Breccia  (angular clasts)

2 – 0.063mm Sand Sandstone0.063 – 0.002mm Silt Siltstone< 0.063mm Mud Mudstone / Shale< 0.002mm Clay Claystone 

Conglomerates & BrecciasConglomerates & Breccias• ConglomeratesConglomerates - mainly rounded clasts

– Ortho conglomeratesOrtho conglomerates - clast supported– Para conglomeratesPara conglomerates - matrix supported

• BrecciasBreccias - mainly angular clasts • Clast typeClast type defines rock type:

– MonomictMonomict - clasts of mainly one composition– PolymictPolymict - clasts of various compositions – DiamictDiamict - clasts poorly sorted of various types

• Matrix (sediment between major clasts)– sand and mud grade sediments

• Cement binds clasts/matrix (Crystalline, diagenetic, typically calcite, silica

Grains floating in matrix Grains touching

Finer grains between major clasts

Crystalline cement

Quartz clast

Calcite

cem

ent

Orthoconglomerate

Monomict breccia

Diamict conglomerate

SANDSTONES

• CLASTSCLASTS have modal grain size in the sand grade ( 2mm – 0.063mm)– Coarse sand 2- 0.5mm, – medium 0.5-0.25m – fine 0.25 - 0.063mm

• SiliciclasticSiliciclastic sandssands have clasts composed usually of quartz, feldspar, fine grained rock fragments or lithics

• Mud gradeMud grade matrixmatrix may be present • Crystalline diagenetic cements, Crystalline diagenetic cements, e.g. quartz,

calcite, clay minerals

MaturityMaturity• Maturity Maturity is a function of sediment transport• Textural maturityTextural maturity refers to:

– The degree of roundness of the grains– The amount of sorting of the grain sizes

• Texturally matureTexturally mature sandstones have well-rounded and well-sorted grains, immatureimmature if not

• Mineralogical maturityMineralogical maturity refers to the percentage of refers to the percentage of quartz grainsquartz grains– Feldspars break down with transport– Quartz grains more resistant

• Mineralogically matureMineralogically mature sandstones have mostly quartz grains

• ArkoseArkose is mineralogically immaturemineralogically immature

RoundnessRoundness

Increasing Roundness=increasing maturityIncreasing Roundness=increasing maturity

Dry aeolian (airborne) grainsWell-rounded & heavily abraded

Well sorted Poorly sorted

SortingSorting

Increasing sorting=increasing maturityIncreasing sorting=increasing maturity

Transport structuresTransport structures

Cross-bedding

Ripples

MineralogyMineralogy

Increasing quartz=increasing maturityIncreasing quartz=increasing maturity

Classifying sandstones: mineralogyClassifying sandstones: mineralogy

>95% Quartz =Quartz areniteQuartz arenite

QuartzQuartz

FeldsparFeldspar

Rock Rock fragments fragments (lithics)(lithics)

<95% Quartz - divided on amount of feldspar or lithics

ArkoseArkose

QuartzQuartz

FeldsparFeldspar LithicsLithics

>25% Feldspar = arkosic composition

>25% Feldspar = mineralogically immature

PETTIJOHN CLASSIFICATIONPETTIJOHN CLASSIFICATION

Based on percentage of Based on percentage of matrixmatrix

• Matrix poor (<15%)Matrix poor (<15%) - arenitesarenites• Matrix richMatrix rich (>15%)(>15%) – wackeswackes

• WackesWackes or (greywackes) or (greywackes)• Tend to be dark in colour• Are poorly sorted

Light coloured Light colouredLight coloured

Dark coloured Dark coloured

Examples in hand specimenExamples in hand specimen

Quartz sandstone (arenite)

Red sandstoneCross-bedded sandstone

Polished sandstone

Lithic sandstone

Fine grained siliciclastic rocks• Modal grain size <0.063mm• Grouped as Mudrocks Mudrocks (>50% siliclastic (>50% siliclastic

grains, <0.063mm) grains, <0.063mm) • Typical mineralogy

– Clay minerals (sheet silicates),Clay minerals (sheet silicates),– quartz & feldsparquartz & feldspar

• MUDROCKS MUDROCKS may split may split (fissile) (fissile) or notor not (massive)(massive)

• Siltstones Siltstones non-fissile, coarser grade, mica non-fissile, coarser grade, mica flakes commonflakes common

• MudstonesMudstones - massive, mud-grade• ShalesShales - Fissile (splitting), mud-grade

Siliciclastic rocksIncreasing mud-grade grains

Shale