The rock cycle clastic sedimentary rocks

EARTH MATERIALS VI

The Rock Cycle:
Clastic Sedimentary Rocks
Professor Peter Doyle




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
• Clastic 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 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
Lithification due to:
1. Compaction
(squeezing out water)
2. Diagenesis
(precipitation of
cements)

Transport shown by
sedimentary structures


Mineralogy of sedimentary rocks
Clastic sediments
• siliciclastics – quartz, feldspars, clay
minerals, rock fragments
• calciclastics – calcite, dolomite
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
Wentworth Scale
Grain Size Sediment Grade
>256 mm
256 – 64mm
64 – 2mm
2 – 0.063mm
0.063 – 0.002mm
< 0.063mm
< 0.002mm

Rock

Boulder 
Cobble 
Pebble 

Conglomerate
(rounded clasts)
Breccia
(angular clasts)

Sand
Silt
Mud
Clay

Sandstone
Siltstone
Mudstone / Shale
Claystone


Conglomerates & Breccias
• Conglomerates - mainly rounded clasts
–
–

Ortho conglomerates - clast supported
Para conglomerates - matrix supported

• Breccias - mainly angular clasts
• Clast type defines rock type:
– Monomict - clasts of mainly one composition
– 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


Finer grains between
major clasts

Grains floating in matrix

Crystalline
cement

Grains touching


Ca
lc
i

te

ce
m

Quartz clast
en
t


Diamict conglomerate

Orthoconglomerate

Monomict breccia

SANDSTONES
• CLASTS 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

• Siliciclastic sands have clasts composed
usually of quartz, feldspar, fine grained rock
fragments or lithics
• Mud grade matrix may be present
• Crystalline diagenetic cements, e.g. quartz,
calcite, clay minerals


Maturity
• 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

Roundness

Dry aeolian
(airborne) grains
Well-rounded &
heavily abraded
Increasing Roundness=increasing maturity


Sorting

Well sorted

Poorly sorted

Increasing sorting=increasing maturity


Transport structures
Cross-bedding

Ripples


Mineralogy

Increasing quartz=increasing maturity

Classifying sandstones: mineralogy
Quartz

>95% Quartz =
Quartz arenite

<95% Quartz divided on
amount of
feldspar or lithics

Feldspar

Rock
fragments
(lithics)


Arkose

>25% Feldspar = arkosic composition
>25% Feldspar = mineralogically
immature
Quartz

Feldspar

Lithics


PETTIJOHN CLASSIFICATION

Based on percentage of
matrix
• Matrix poor (<15%) - arenites
• Matrix rich (>15%) – wackes
• Wackes or (greywackes)
• Tend to be dark in colour
• Are poorly sorted


Examples in hand specimen
Light
Light coloured
coloured

Dark coloured

Light coloured

Dark coloured


Cross-bedded
sandstone

Red sandstone

Polished sandstone

Lithic sandstone

Quartz sandstone (arenite)


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

• MUDROCKS may split (fissile) or not
(massive)
• Siltstones non-fissile, coarser grade, mica
flakes common
• Mudstones - massive, mud-grade
• Shales - Fissile (splitting), mud-grade


Siliciclastic rocks

QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.

Increasing mudgrade grains

Shale