Sediments and Sedimentary Rocks

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Sediments and Sedimentary Rocks

• Rock Cycle

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Sediments and Sedimentary Rocks

• Sediments classified according to size
• Gravel - greater than 2 mm (pebbles, 2 mm to 64
  mm cobbles, 64 mm to 256 mm boulders, 256 mm)
• Sand - .0625 (1/16) mm to 2 mm.
• Silt - 1/256 mm to 1/16 mm.
• Clay - type of mineral (hydrous aluminosilicates).

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Sediments and Sedimentary Rocks

• Sedimentary Processes
• Transportation - the movement of sediment from
  its point of origin typically results in
  rounding and sorting of sediments
• Deposition - occurs when sediment settles out or
  ceases being transported, or when a chemical or
  organic sediment accumulates the place where
  this occurs is the environment of deposition
• Lithification (consolidation) - the process of
  converting sediment into sedimentary rock by a
  combination of compaction and cementation

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Sediments and Sedimentary Rocks

• Sedimentary Processes
• Compaction - packing grains closely together and
  reducing the open pore space
• Cementation - the precipitation of a mineral on
  the surface of grains, and/or in pore spaces
  (typically carbonate, silica or iron oxides )

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Sediments and Sedimentary Rocks

• Sedimentary Processes
• Clastic texture - sediment grains bound together
  in a rigid framework
• Crystalline texture - an arrangement of
  interlocking crystals that grow and interfere
  with each other

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Sediments and Sedimentary Rocks

• Sedimentary rocks are classified, in part, on how
  they form
• Clastic sedimentary rock formed by
  lithification of particles derived from
  pre-existing rock
• Chemical sedimentary rock formed by
  precipitation from solution
• Organic sedimentary rock formed by accumulation
  and consolidation of organic material

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Sediments and Sedimentary Rocks

• Clastic sedimentary rocks - classified and named
  based on size and type of material
• Breccia gravel-sized particles that are angular
  in shape
• Conglomerate gravel-sized particles that are
  rounded in shape
• Matrix finer-grained particles that surround
  and cement coarser particles

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Sediments and Sedimentary Rocks

• Clastic sedimentary rocks - classified and named
  based on size and type of material
• Sandstone - medium-grained rock formed by
  consolidation of sand grains named according to
  grain size, texture, and composition (e.g. quartz
  sandstone, arkose, graywacke)

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Sediments and Sedimentary Rocks

• Finer-grained sedimentary rocks include shale,
  siltstone, claystone, and/or mudstone
• Shale - contains both silt- and clay-sized
  particles with a notable presence of thin layers
  or laminations along which rock easily splits
  (fissility)
• Siltstone - primarily silt-sized particles,
  slightly coarser than shale, and lacks the
  laminations and fissility of shale
• Claystone - primarily
• clay-sized particles but
• lacks the fissility of shale
• Mudstone similar to
• shale but more massive
• blocky, and lacks
• shale laminations and
• fissility

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Sediments and Sedimentary Rocks

• Chemical sedimentary rocks - formed by
  precipitation from an aqueous environment (most
  predominant type of chemical sedimentary rocks
  are carbonate rocks, e.g. limestone dolomite)
• Biochemical limestone - precipitated by the
  actions of organisms, and often are comprised of
  the fossil remains of organisms
• Inorganic limestone
• precipitated directly
• from water, without
• the aid of organisms
• Limestone can be
• recrystallized,
• destroying original
• texture of the rock

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Sediments and Sedimentary Rocks

• Chemical sedimentary rocks (contd)
• Dolomite like limestone but Mg substitutes for
  Ca.
• Chert - hard, compact sedimentary rock formed
  from silica (may be biochemical or inorganic)
• Evaporites - rocks or minerals with a crystalline
  texture that are formed from precipitation from
  seawater or a saline lake (examples include
  gypsum, halite, borates, and other salts)
• Coal - an organic rock that forms from the
  compaction of plant material that has not
  completely decayed
• Oil and gas form from partially decomposed
  organic material that is buried and subjected to
  elevated heat and pressure in deep basins

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Sediments and Sedimentary Rocks

• Sedimentary Structures - features found within
  sedimentary rocks that formed before the sediment
  was lithified
• Bedding - generally horizontal layering in the
  rock
• Bedding Planes - nearly flat surfaces of
  deposition, separating two rock layers

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Sediments and Sedimentary Rocks

• Sedimentary Structures (contd)
• Cross Bedding - series of thin, inclined layers
  within a thicker rock unit

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Sediments and Sedimentary Rocks

• Sedimentary Structures (contd)
• Graded bedding - sedimentary layer that changes
  grain size vertically within the layer, usually
  coarser at the bottom, and becomes progressively
  finer grained towards the top

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Sediments and Sedimentary Rocks

• Sedimentary Structures (contd)
• Mud cracks - polygonal patterns of cracks formed
  in very fine-grained sediment,due to contraction
  of the sediment as it dries

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Sediments and Sedimentary Rocks

• Sedimentary Structures (contd)
• Ripple marks - small ridges formed on the surface
  sediment layer by moving wind or water
• Wave generated ripple marks are symmetric
• Current generated ripple marks are asymmetric

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Sediments and Sedimentary Rocks

• Sedimentary Structures (contd)
• Fossils - traces of plants or animals preserved
  in sedimentary rock includes replacement by
  calcite or silica, voids spaces or molds
  reflecting the former shape, filling in the molds
  to form casts, flat carbon imprints. footprints,
  trails, and burrows

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Sediments and Sedimentary Rocks

• Formations - very thick bodies of rock with
  characteristics that distinguish it from adjacent
  rock units
• Contacts - a boundary surface between two
  different rock types or ages of rocks, usually
  bedding planes in clastic sedimentary rocks

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Sediments and Sedimentary Rocks

• Source area - the locality from which the
  sediment was derived factors used to evaluate
  source area include rock type, environment of
  deposition, direction (paleocurrents) and
  distance from source area

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Sediments and Sedimentary Rocks

• Depositional environment where sediment is
  deposited. It can be determined by looking at
  sedimentary structures (including fossils), the
  bed shape and vertical sequences within the
  sedimentary layers, and grain composition

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Sediments and Sedimentary Rocks

• Sedimentary environments plate tectonic
  settings
• Convergent boundaries coarse-grained clastic
  sediments with abundant volcaniclastic and felsic
  material

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Sediments and Sedimentary Rocks

• Sedimentary environments plate tectonic
  settings
• Transform boundaries thick sediment piles with
  abundant organic material

Pacific Plate
North American Plate
Lake San Andreas in No. California Sag pond
associated with San Andreas Fault (Transform
boundary between Pacific No. American tectonic
plates)
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Sediments and Sedimentary Rocks

• Sedimentary environments plate tectonic
  settings
• Divergent boundaries - thick wedges of gravel and
  coarse sand along margins,with lake bed deposits
  and associated evaporite rocks possible in
  bottoms of rift valleys