The rock cycle clastic sedimentary rocks

Relative proportions of rock groupsSedimentary rocks: larger proportion of land surface The greater proportion created by weathering, erosion & deposition These are known as clastic se

THE ROCK CYCLE

Relative 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

of pre- existing rocks

• Weathering can be physical (freeze-thaw) or

chemical

• Erosion & subsequent transportation by

water, wind and ice before deposition

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:

Mineralogy of sedimentary rocks

Clastic sediments

• siliciclastics – quartz, feldspars, clay siliciclastics

minerals, rock fragments

• calciclastics – calcite, dolomite calciclastics

Carbonate (biogenic) sediments

• aragonite, calcite

• chalcedony silica (flint) occurs as subsiduary

Evaporite (chemical) sediments

• halite, gypsum and anhydrite

• Dolomite, also as dolomitisation of calcite

Classification of clastic sediments: The

Conglomerates & Breccias

– Ortho conglomerates - clast supported Ortho conglomerates

– Para conglomerates - matrix supported Para conglomerates

– Monomict Monomict - clasts of mainly one composition– Polymict Polymict - clasts of various compositions

– Diamict - 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

Calci

te c

ement

Monomict breccia Diamict conglomerate

• Siliciclastic Siliciclastic sands have clasts composed

usually of quartz, feldspar, fine grained rock

fragments or lithics

calcite, clay minerals

• Maturity is a function of sediment transport

• Textural maturity refers to:

– The degree of roundness of the grains

– The amount of sorting of the grain sizes

• Texturally mature sandstones have well-rounded

and well-sorted grains, immature if not

• Mineralogical maturity refers to the percentage of

quartz grains

– Feldspars break down with transport

– Quartz grains more resistant

• Mineralogically mature sandstones have mostly

quartz grains

• Arkose is mineralogically immature

Dry aeolian (airborne) grains Well-rounded & heavily abraded

Well sorted Poorly sorted

Sorting

Increasing sorting=increasing maturity

Transport structures

Cross-bedding

Increasing quartz=increasing maturity

Classifying sandstones: mineralogy

amount of feldspar or lithics

PETTIJOHN CLASSIFICATION

Based on percentage of matrix

• Matrix poor (<15%) - Matrix poor (<15%) arenites

• Matrix rich Matrix rich (>15%) – (>15%) wackes

• Wackes or (greywackes)

• Tend to be dark in colour

• Are poorly sorted

Light coloured Light coloured

Examples in hand specimen

Red sandstone Cross-bedded

Fine grained siliciclastic rocks

• Modal grain size <0.063mm

• Grouped as Mudrocks (>50% siliclastic

grains, <0.063mm)

• Typical mineralogy

– Clay minerals (sheet silicates),

– quartz & feldspar

(massive)

flakes common

Siliciclastic rocks

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Increasing grade grains

mud-Shale