Sandstones and other Clastic Sedimentary Rocks

What geologists want to learn from Sandstones

• Source area – rock typerock type– direction

weathering environment– weathering environment• Transport

di– medium, energy– distance

• Depositional environment– marine or non-marine– physical environment (beach, river, delta,

etc.)

What clues are present in S d t ?Sandstones?

• Grain size• Grain shapeGrain shape• Grain sorting

G i i l• Grain mineralogy• Sedimentary structures

Grain size

• Detrital or clastic rocks have a huge range in grain size

• We need a log scale to represent this wide g psize range

• The Phi (φ) Scale: φ = -log2 (mm)The Phi (φ) Scale: φ log2 (mm)mm = 2- φ (memorize)

Each φ step represents a doubling (smaller• Each φ step represents a doubling (smaller # or more neg.) or halving (larger #) in size

For exampleFor example

• size in mm = 2- φ• size in mm = 2 φ

• -6 φ = 26 mm = 64 mm• 0 φ = 20 mm = 1 mmφ• 2 φ = 2-2 mm = 1/4 mm = 0.25 mm• 4 φ = 2-4 mm = 1/ mm = 0 0625 mm• 4 φ = 2 4 mm = 1/16 mm = 0.0625 mm

Size ranges are givenare given names

Gravel > -1 φ (>2mm)

Sand: 4 φ to -1 φ(0.0625mm to 2mm)

Mud < 4 φ<0 0625<0.0625mm<62.5 μm

Cla < 8 φClay < 8 φ<0.004mm<4 μm

Loose sediments can be separated by sievingsieving

• -2φ (4mm)• -1φ (2mm)φ ( )• 1φ (0.5mm)• 2φ (0 25mm)• 2φ (0.25mm)• 3φ (0.125mm)• 4φ (0.062mm)• closed

Important Textural FeaturesImportant Textural Features

• Bedding and LayeringBedding and Layering• Grain size

G i h• Grain shape– Roundness– Sphericity

• Sortingg• Others?

Grain size comparator for lithified Sandstone

Sorting by comparison

Grain Shape• Sphericity - relative equidimensionality of three

mutually perpendicular axesmutually perpendicular axes• Roundness - lack of sharp corners; larger

grains round faster because of more impactsgrains round faster because of more impacts

Hi hHigh sphericity Increasing roundness

Low sphericity

Minerals of Clastic Sedimentary Rocks

Sandstone Classification

Q

15% to 50% mud,called Wackes, use

Sandstones with less than 15% mud, areF

middle triangle

than 15% mud, arecalled Arenites, use the front triangle

LF

Detrital Grain Types: QuartzDetrital Grain Types: Quartz

Dust rings may show detrital i b d igrain boundaries

oros

itypo

Note optically continuous and euhedral quartz overgrowths

Polycrystalline Quartz in Ss

Chert is a stable lithic fragment grouped with Quartzfragment, grouped with Quartz

Feldspars distinguished fromdistinguished from

quartz by alteration, twinning and perthitetwinning and perthite

What’s the large grain?

Microcline inMicrocline in Sandstone

Lithic Fragments

Volcanic and Plutonic Lithic Fragments inFragments in a Sandstone

Sandstone Classification

Q

15% to 50% mud,called Wackes, use

Sandstones with less than 15% mud, areF

middle triangle

than 15% mud, arecalled Arenites, use the front triangle

LF

Common Accessory Mineralsi S din Sandstones

• Heavy mineralsHeavy minerals• Zircon

T li• Tourmaline• Rutile

• Some micasSome micas• More muscovite than biotite

Detrital Zircon (ZrSiO4) Separate

Detrital Rutile (TiO2) Separate( 2) p

“Textural Maturity” of a Ssy• A measure of the progress of a clastic

sediment in the direction of chemical, mineralogical and textural stability

• Affected by processes that take a long time

• Maturity increases with total input of kinetic energykinetic energy– time of transport, distance of transport– energy of medium– energy of medium

Increasing “Textural Maturity” is indicated by:

• clay removal• increased sortingg• increased rounding• breakdown (absence) of unstable fragments• breakdown (absence) of unstable fragments• breakdown (absence) of unstable minerals• high ZTR (zircon, tourmaline, rutile – super-

stable heavy minerals)

Immature Sandstones - limited transport, rapid deposition and burial

L t f dd t i• Lots of muddy matrix• poorly sorted• poorly rounded fragments and grains• lots of unstable lithics and unstablelots of unstable lithics and unstable

minerals• mostly wackes• mostly wackes• formed in convergent margin settings,

t harc-trench gap

Super-mature Sandstones

• Clean (no mud matrix)• well-sortedwell sorted• well-rounded grains

tl t i• mostly quartz grains• quartz arenites• Cratonic, typically recycled, formed in

beach or other high energy environmentg gy

Increasing Textural Maturity

• Wackes - immature• LitharenitesLitharenites• Arkoses

S b k d blith it• Subarkose and sublitharenite• Quartz arenites - supermature

DiagenesisDiagenesis

• CompactionCompaction• Cementation – Common Cements

Q t– Quartz– Calcite– Hematite – Clay

Calcite-cemented Sandstone

Calcite (and dolomite) C t t i dCement, stained

Hematite Cement

SEM long dimension=1μmWhere does Fe come from?

Clay CoatingsBlue epoxy fills poresBlue epoxy fills pores in some thin sections

Low-Mag. Med.-Mag. Hi-Mag. SEM 5μm g g g μ

Authigenic clay is perpendicular to grain boundariesto grain boundaries

Fe-montmorillonite