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• Most of solid Earth consists of igneous and metamorphic rocks. Geologists estimate these two categories represent 90 to 95 percent of the outer 16 kilometers (10 miles) of the crust.
• Most of Earth's solid surface consists of either sediment or sedimentary rock!
• Across the ocean floor, which represents about 70 percent of Earth's solid surface, virtually everything is covered by sediment.
• Sediments and the rock layers that they eventually form contain evidence of past conditions and events at the surface.
• Sedimentary rocks contain fossils, which are vital tools in the study of the geologic past.
• Coal, which provides a significant portion of our electrical energy, is classified as a sedimentary rock.
• Other major energy sources-oil, natural gas, and uranium-are derived from sedimentary rocks.
• Sediments and sedimentary rocks are also the primary reservoir of groundwater.
• Weathering


  1. Physical disintegration

  2. Chemical decomposition of preexisting igneous, metamorphic, and sedimentary rocks
• Transportation


  1. Soluble constituents carried away by runoff and groundwater

  2. Solid particles moved downslope by gravity, then removed by running water, groundwater, wind, and glacial ice
• Deposition


  1. Solid particles settle out when wind and water currents slow down

  2. Ions precipitate when chemical or temperature changes cause material to crystallize
• Diagenesis


  Burial and transformation of sediments into sedimentary rock by compaction and cementation
• Detrital sedimentary rocks


  Rocks formed from accumulations of solid particles derived from weathering
• Chemical sedimentary rocks


  Rocks formed from precipitation of dissolved ions
• Organic sedimentary rocks


  Rocks formed from the remains of organisms, such as coal
• Detrital sedimentary rocks


  • Clay minerals and quartz are the chief constituents
  • Other common minerals are feldspars and micas
• Particle size


  The primary basis for distinguishing among various detrital sedimentary rocks
• Detrital sedimentary rocks


  • Shale
  • Sandstone
  • Conglomerate or breccia
• Shale


  • A sedimentary rock consisting of silt- and clay-size particles
  • Deposition occurs from gradual settling in quiet, nonturbulent currents
  • Often contains organic matter and forms impermeable barriers to water and petroleum movement
• Compaction of sediment

  1. Clay and silt particles take on a more parallel alignment and become tightly packed

  2. Rearrangement of grains reduces the size of the pore spaces and forces out much of the water

  3. Once the grains are pressed closely together, the tiny spaces between particles do not readily permit solutions containing cementing material to circulate
• Shale


  • Weakly cemented and not well lithified sedimentary rock
  • Impermeable to water and petroleum due to microscopic pore spaces
• Shale often forms barriers to the subsurface movement of water and petroleum
• Rock layers that contain groundwater are commonly underlain by shale beds that block further downward movement
• Underground reservoirs of petroleum are often capped by shale beds that effectively prevent oil and gas from escaping to the surface
• Shale


  • Fine-grained sedimentary rock that exhibits the ability to split into thin layers along well-developed, closely spaced planes (fissility)
  • If the rock breaks into chunks or blocks, the name mudstone is applied
  • Siltstone is a fine-grained sedimentary rock like mudstone but lacks fissility
• Shale is far more common than other sedimentary rocks but does not form prominent outcrops as sandstone and limestone often do
• Shale crumbles easily and usually forms a cover of soil that hides the unweathered rock below
• Shale beds may have economic value, used to obtain raw material for pottery, brick, tile, and china, and when mixed with limestone, used to make Portland cement
• Oil shale may become a valuable energy resource in the future
• Sandstone


  Sedimentary rock in which sand-sized grains predominate
• Sorting

  • The degree of similarity in particle size in a sedimentary rock
  • Well sorted - all grains about the same size
  • Poorly sorted - mixed large and small particles
• Particle shape

  • Angularity - degree to which the clast's edges and corners are rounded
  • Sphericity - how close the shape of the clast is to a sphere
• Sorting and particle shape provide clues about the depositing current and history of the sediment
• • Well-sorted, quartz-rich sandstone with highly rounded grains indicates a great deal of transport and weathering
  • Sandstone with feldspar and angular grains indicates short-distance transport, minimal weathering, and rapid deposition
• Quartz sandstone


  Sandstone in which quartz is the predominant mineral
• Arkose


  Sandstone containing appreciable quantities (25% or more) of feldspar, also contains quartz and mica
• Graywacke


  Sandstone containing abundant rock fragments and matrix (silt- and clay-size particles)
• Graywacke is frequently associated with submarine deposits made by dense sediment-choked torrents called turbidity currents
• Conglomerate


  Sedimentary rock consisting largely of gravels, ranging in size from boulders to pebbles
• Breccia


  Sedimentary rock in which the large particles are angular rather than rounded