Clastic sedimentary rocks

Clastic Sedimentary Rocks Derived from the Mechanical Breakup and Redeposition of Older Rocks... Sediment Sizes and Clastic Rock TypesSedimentary rocks made of silt- and clay-sized part

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

Boulder < -8 256 +

Cobble -6 to -8 64 - 256

Pebble -2 to -6 4 - 64

Granule -1 to -2 2 - 4

Sand -2 to 4 1/16 - 2

Silt 4 - 8 1/256 - 1/16 Clay > 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

Shale Clay less than 0.001 mm

Siltstone Silt 001-0.1 mm

Sandstone Sand 01-1 mm

Conglomerate Gravel 1mm +

Clastic Terms

Sand Arenite Psammite

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 Feldspar-Rich

• Breccia Angular Fragments

• Graywacke Angular, Immature

Sandstone

Sandstone Terminology

• Stability of Minerals

• Rock Fragments

• Rounding or Angularity

• Sorting

Removal of Unstable Ingredients - Mechanical Working

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

• Clast Supported vs Matrix Supported

• Polymictic (Outwash, alluvial fans, flood plains)

• Oligomictic (Lag deposits)

• Diamictic (Tills, Glacio-marine, Submarine

Landslides)

Diamictite, Ontario