Clastic sedimentary rocks

Clastic Sedimentary Rocks
Derived from the Mechanical Breakup and
Redeposition of Older Rocks


Clastic Rocks
Classified by:
• Grain Size
• Grain Composition
• Texture


The Wentworth Scale
• Phi (φ) = -log2(Diameter in mm)
• Best way to remember:

– 1mm: φ = 0
– φ increases as diameter decreases
– Every factor of 2 change in diameter = one
step in φ


Particle Descriptions
Name
Boulder
Cobble
Pebble
Granule
Sand
Silt
Clay

Phi
< -8
-6 to -8
-2 to -6
-1 to -2
-2 to 4
4-8
>8

Diameter mm
256 +
64 - 256
4 - 64
2-4
1/16 - 2
1/256 - 1/16
< 1/256


Sediment Sizes and Clastic Rock Types
Rock Type

Sediment

Grain Size

Shale

Clay

less than 0.001 mm

Siltstone

Silt

.001-0.1 mm

Sandstone

Sand

.01-1 mm

Conglomerate

Gravel

1mm +

Sedimentary rocks made of silt- and clay-sized
particles are collectively called mudrocks, and
are the most abundant sedimentary rocks.


Clastic Terms
Latin

Greek

Gravel

Rudite

Psephite

Sand

Arenite

Psammite

Silt

Lutite

Pelite


Super-Size Me
• Blair and McPherson, J. Sed. Res., v. 69, no. 1, Jan.
1999, p. 6-19.
• Block: 4-65 m (φ = -12 to -16)
• Slab: 65-1000 m (1 km) (φ = -16 to -20)
• Monolith: 1-33 km (φ = -20 to -25)
• Megalith: 33-1000 km (φ = -25 to -30)


Some Special Clastic Rock Types

• Arkose
• Breccia
• Graywacke

Feldspar-Rich
Angular Fragments
Angular, Immature
Sandstone


Sandstone Terminology


Maturity
•
•
•
•

Stability of Minerals
Rock Fragments
Rounding or Angularity
Sorting

Removal of Unstable Ingredients - Mechanical
Working


Diagenesis

Diagenesis
Compaction
Cementing
• Quartz
• Calcite
• Iron Oxide
• Clay
• Glauconite
• Feldspar

Alteration
• Limestone - Dolomite
• Plagioclase – Albite
Recrystallization
• Limestone


Tectonic Settings and Sediment
•
•
•
•

Cratonic: Mature sandstone
Continental slope: Graywacke, turbidite
Trench: Wildflysch (contorted graywacke)
Post-orogenic basin: Molasse (red sandstone and
conglomerate)

Tectonic Settings and Sediment


Lithic Sand, Washington


Lithic Sandsone, Washington


Molasse,
Switzerland


Molasse and the High Alps


Conglomerates
•
•
•
•

Clast Supported vs. Matrix Supported
Polymictic (Outwash, alluvial fans, flood plains)
Oligomictic (Lag deposits)
Diamictic (Tills, Glacio-marine, Submarine
Landslides)

Diamictite, Ontario