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 Phi Diameter mmBoulder < -8 256 +Cobble -6 to -8 64 - 256Pebble -2 to -6 4 - 64Granule -1 to -2 2 - 4Sand -2 to 4 1/16 - 2Silt 4 - 8 1/256 - 1/16Clay > 8 < 1/256

Sediment Sizes and Clastic Rock Types

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

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 +

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 • Slab: 65-1000 m (1 km)• Monolith: 1-33 km• Megalith: 33-1000 km

Some Special Clastic Rock Types

• Arkose Feldspar-Rich • Breccia Angular Fragments • Graywacke Angular, Immature

Sandstone

Sandstone Terminology

Maturity

• Stability of Minerals • Rock Fragments • Rounding or Angularity • Sorting

Removal of Unstable Ingredients - Mechanical Working

Diagenesis

Diagenesis

CompactionCementing• Quartz • Calcite • Iron Oxide • Clay • Glauconite • Feldspar

Alteration• Limestone - Dolomite • Plagioclase – AlbiteRecrystallization• 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