Clastic sedimentary rocks

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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 = – φ increases as diameter decreases – Every factor of 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 4-8 >8 Diameter mm 256 + 64 - 256 - 64 2-4 1/16 - 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
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