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