Surface Processes

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Surface Processes Sediments

• Associate Professor John Worden DEC
• University of Southern Qld

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Surface Processes Sediments

• Rock-forming Processes
• Igneous, Sedimentary, or Metamorphic.
• Next examine Surface Processes which occur at 1A,
  25o C.
• Colliding, rotating, grinding twisting plates
  generate tectonics which thrust up Mountain
  Ranges.
• As steadily as Mountains form, Erosion wears them
  down.
• Erosive agents - Wind, Water and Ice.
• Transport rock debris to depositional sites
• Accumulates as Sediment and,
• Compacted, Lithified to Sedimentary Rocks.
• Time is recorded by Sedimentary Rocks.

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Surface Processes Sediments

• Principles Terms
• Sediments characterised by layers termed
  Strata.
• Discrete recognisable layers termed Beds, and
  these define Bedding.
• Depositional processes produce Stratification, a
  record of changing environmental conditions
  throughout time.
• Law of Original Horizontality
• States that sediments are deposited in Strata
  that are horizontal.
• Principle of Stratigraphic Superposition
• States that in any sequence of sedimentary
  strata,
• The oldest rock is at the bottom, and the
  youngest rockon the top, of the sequence.

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Surface Processes Sediments

• Conformable sequences consist of strata uniformly
  following each other without interruption.
  However, breaks common.
• Sedimentary record incomplete due to
  Unconformities- periods of non-deposition or
  erosion.
• Three types of Unconformities-
• Non-conformity Change in major classes of rock
  types,
• Angular unconformity Period of tectonism between
  rock types,
• Disconformity Irregular erosion surface
• separating two sedimentary layers.
• Unconformities are time gaps in the geological
  record.

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Surface Processes Sediments

• Weathering
• The disintegration decomposition of rock to
  produce Regolith.
• Two types of Weathering Physical and Chemical.
• Physical weathering reduces large masses to much
  smaller units thereby increasing surface areas
  for chemical attack.
• Chemical Biological weathering convert
  mineral components to new components stable at 1A
  and 25o C.
• TheWeathering zone extends down as far as water
  oxygen penetrate, and is due to
• Unloading of rock formations as exposed
• Penetration of soluble salts later
  crystallisation
• Water freezing expansion as frost wedging
• Plants wedging rocks apart along cracks, and
• Repeated bushfires causing unequal heating,
  expansion, and fracturing.

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Surface Processes Sediments

• Soils
• Equate to accumulated Regolith, which is
  composed of decomposed products of rocks
  minerals organic matter.
• Forms a succession of weathered zones termed
  Horizons.
• Not strata, but comprise a soil profile, which
  usually consists of
• An organic-rich surface layer
• Upper A horizon ( dark grey /black, depleted in
  clay, enriched in organic M)
• Middle B horizon ( brown/ red, blocky, enriched
  in clay, Fe-Al hydroxides)
• Possible K horizon ( impregnated with Ca
  Carbonate)
• Underlying C horizon ( yellow/ brown, sl
  oxidised).
• Development dependent on
• Climate, Vegetation cover, Soil organisms,
  Regolith composition, Topography, Time.

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Surface Processes Sediments

• Erosion is the downslope movement of
  regolith/debris under the pull of gravity.
• Termed Mass Wasting.
• Influenced by composition, texture of regolith,
  amount of water/air mixed with debris,
  steepness of slope. All influence velocity.
• Two general types Slope Failure Sediment
  Flow.
• Slope failure is downslope movement of relatively
  coherent masses of rock or regolith slumping.
• Sediment flow is the downslope movement
  ofRegolith/ debris mixed with water air.
• Landslides are examples of mass wasting.
• Once at bottom of slope, other agents
  transportdebris to lower elevations in the
  Landscape.

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Surface Processes Sediments

• Approx 30 of precipitation collects as runoff
  in streams, which move downslope (stream flow).
• This transports debris particles dissolved
  substances along channels.
• Factors controlling channel flow include
• Average width depth
• Channel gradient
• Average velocity of water flow
• Discharge volume and
• Total sediment load.

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Surface Processes Sediments

• Sediment Load transport moves in three ways
• Coarse particles move along stream bed Bed
  load,
• Fine particles move suspended in water
  Suspended Load,
• Material in solution Dissolved load.
• Bed load
• Moves by Saltation- Sand -sized grains
  propelled into suspension, pulled down by gravity
  returned to stream bed. Progress is by short
  intermittent jumps along arcuate paths.
• Suspended Load
• Silt and Clay particles moving in suspension,
• Derived from fine-grained regolith,
• Particles settle when velocity of turbulent flow
  declines.
• Dissolved Load
• Bicarbonate, Ca, sulfate, Na, K, Mg, and Chloride
  ions.

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Surface Processes Sediments

• As particles move downstream, particle size
  decreases due to abrasion and impacts. Gravel in
  headwaters, sand silt at stream mouth.
• Grains are progressively rounded tending toward
  spherical shape. Degree of rounding indicates
  transport distance.
• Grains are also sorted by size or hydraulic
  equivalency.
• Therefore, detrital sediments demonstrate
• Transport distance by degree of rounding
• Source rock type by sediment mineralogy
• Type of transport agency by grain surface
  texture
• Efficiency of transport by degree of sorting.

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Surface Processes Sediments

• Water also exists as ice where T ltlt 0o C.
• Precipitation falls as snow down to snowline (
  lower limit of perennial snow).
• Snow compacts, recrystallises to ice, and moves
  downslope as Glacier.
• Valley glaciers at high latitudes move from high
  to low altitude forming Fjords, where they
  reach sea level.
• Ice sheet flows internally and by basal sliding
  in response to gravity.
• Glacier scrapes bedrock removing soils
  regolith, plucks rocks, and grooves bedrock.
  Bedrock displays striations, etc.
• Debris is transported by ice to depositional
  site
• Glacier terminus as terminal moraine.
• As lateral moraines, icebergs, etc

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Surface Processes Sediments

• Wind also transports sediment
• Dominant in dry climate areas where no vegetation
  to retain regolith.
• Transported particles are mostly sand and finer
  grain sizes.
• Sand particles moved as high as 1 metre above
  surface by saltation.
• Form Dunes of different types when encounter
  obstructions.
• Sand grains move by saltation up windward slope
  of dune( 12o)
• Grains fall over slip face of dune ( 33o) and
  come to rest.
• Dune migrates downwind.
• Process of grain migration forms Cross
  bedding.
• Silt and clay particles carried in suspension
  until
• Wind velocity decreases and particles settle to
  surface.
• Form deposits of Loess uniformly blanket
  surface
• Lack stratification, but can contain fossils, etc.

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Surface Processes Sediments

• Deposition
• Sediment deposited when stream/ wind velocity
  decreases or ice melts.
• Turbulent flow ceases coarse sediment deposits
  first, progressively followed by fines. Hence
  sediment sorted by relative grain size.
• Depositional sites occur from stream floodplain
  to Oceans.
• Meandering streams ( with large loop-like bends)
  in fine- grained alluvium gentle gradients.
• Erosion of outer bank of meander deposition on
  inner bank.
• Produces Point bars
• As stream erodes fine-grained silts, may cut off
  loop.
• Forms Oxbow lake.
• Braided streams form when debris load very high.
• Stream divides into many channels reunites
  often.

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Surface Processes Sediments

• Braided streams form Braid bars consisting of
  coarse debris.
• Such bars and channels are constantly re-worked
  by the stream.
• Deposition at river mouths may form Deltas, or
  be reworked by longshore currents into spits,
  bars,barrier islands, etc.
• In all situations deposited sediment carries
  imprint of transporting processes in its
  texture, rounding, degree of sorting.
• Ice transport produces ill-sorted sediments.
• Diverse grain size range
• Minimal degree of rounding
• Cosmopolitan range of sediment compositions
• Diverse types of resultant landform features.

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Surface Processes Sediments

• Lithification
• Sediment is lithified as it is buried by later
  sediments.
• Formation water is expelled due to load pressure.
• Compaction occurs as void space is reduced.
• Grains frequently cemented together by later
  mineral deposition in void space. Cementing
  agents include Calcite, Silica, Fe oxides, etc.
• Resultant rock becomes indurated or hardened
  strengthened with loss of porosity.
• Sediment generation is related to tectonics
• Thickest sequences form near Mountain Belts.
• These are typically immature sediments.
• Little tectonics means mature sediments.

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Surface Processes Sediments

• Sedimentary Rock Practical
• Important factors when describing identifying
  rocks are
• Texture- shape of fragments and their mutual
  arrangement
• Rounding- degree of reduction towards spherical
  shape
• Sorting- tendency towards unimodal or same
  grain size
• Porosity- percentage of void space in rock
• Cement- nature and type of cement
• Fissility- ability to split along bedding planes
• Friability- tendency to crumble
• Mineral Composition- of fragments, grains,
  matrix
• Structure- bedding, cross bedding, joints, etc

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Surface Processes Sediments

• Sedimentary Rocks
• Classified into two major groups- Detrital or
  Clastic Non-clastic.
• Clastic rocks subdivided on grain size basis into
  Coarse/ Medium/ Fine
• Coarse-grained further sub-divided on shape of
  fragments/grains into
• Conglomerates- with rounded grains
• Breccias- with angular grains or fragments.
• Medium-grained sub-divided on basis of degree of
  sorting rounding of grains into
• Sandstones / Arenites- well sorted, well
  rounded
• Greywacke - poorly sorted, angular fragments.
• Fine-grained divided into
• Shale- bedding visible frequently fissile
• Mudstone- bedding absent, non-fissile, often
  blocky.

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Surface Processes Sediments

• Non-clastic or Chemical Sedimentary rocks
• Include Carbonates, Siliceous, Carbonaceous,
  Ferruginous Evaporites.
• Carbonates are usually of two types
• Limestones- made up of calcite derived from shell
  fragments of organisms, direct precipitation from
  sea water or fresh water, and possibly oolites.
• Dolomites- generally converted limestones that
  have been re-crystallised by solutions to Ca-Mg
  carbonate.
• Siliceous rocks form from siliceous skeletons of
  marine organisms which are later lithified to
  cherts.
• Carbonaceous rocks form from vegetation
• Various types of Coal, Peat, Organic-rich
  Shale.
• Evaporites are direct precipitates from seawater
• Include Rock Salt (NaCl), Gypsum, and Anhydrite.
• Rarer salts such Potash, etc

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Surface Processes Sediments

• Depositional Environments
• Sediments are products of Provenance (Source
  region), Transport history, Depositional
  Environment.
• A hole in the ground is a necessary
  pre-requisite (Need Subsidence!).
• Depositional Environments have gradational
  boundaries.
• Sediments occur in distinct Packages in a
  vertical section sense.
• In conformable sequences, only adjacent
  environment packages can occur immediately
  overlying or underlying particular sediments.
• Results in superimposed sedimentary cycles.
• Examples
• Fluvial/Alluvial Deltaic Coastal Turbidite
  etc.
• Consider only Fluvial Deltaic Environments.
• Point Bar deposits.

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Surface Processes Sediments

• Point Bar Sediments
• Meandering stream deposits.
• Record fining upwards cycles of grain size in
  sediments, ie
• With each flood, deposition commences with
  gravel, grits , coarse sands.
• As current slackens, sands become progressively
  finer-grained.
• Sluggish current enables deposition of silt and
  clay layers.
• Next flood repeats cycle.
• As river meanders over floodplain, deposits
  packages of gravel?sand?clayfining upwards
  cycles.
• Deltaic sediments in contrast, coarsen upwards.
• Deeper water in front of delta is more distal
  from river.
• Results in deposition of fine-grained silts and
  clays.
• As river advances, introduces fine sands, later
  coarsersands and grits. Coarsening Upwards
  cycles.