sandstones and other clastic sedimentary...
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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