Chapter 7 Parent Material influences on Pedogenesis

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Chapter 7- Parent Material influences on
Pedogenesis Primary influence is on
mineralogy Some minerals more resistant than
others e.g., feldspar vs quartz e.g., olivine vs
quartz
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Measuring influence

• Measure the weathering of key minerals
• Compare those in profile to those that are
  unweathered (i.e., parent material)
• Look for depletion of elements, etching, or clay
  alteration
• Hornblende etching

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Mineral weathering
Augite
Hypersthene
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More to least resistant minerals to weathering
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Chemical composition of igneous vs sedimentary
lithologies
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Chemical composition influence

• Rocks vary in mineralogy
• Changes the chemical composition of the material
• Changes the resistance to weathering
• Resistant minerals harder to weather thinner
  or less well developed soils when compared to
  those developed in rocks with less resistant
  mineralogy and chemical composition

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Influence of texture on soils Can refer to
consolidated or unconsolidated material consolida
ted porosity or fractures are key fine grained
with highly permeable and porous conditions is
preferable
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Depth of leaching varies with permeability and
porosity of parent material
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Extreme control

• Podzolization or not Podzolization. That is the
  question!
• Common in sandy material formed from crystalline
  rock
• Not common in glacial till formed from ground up
  sedimentary rocks
• Common in soils with ultramafics
• Not common in soils with lesser amounts of
  ultramafics
• Sandy (common) clayey (uncommon)

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Limestone soils- terra rosa

• Often contain soils that are nothing like the
  parent rock
• Four common interpretations
• Residual materials in carbonate host rock
• Fluvial or colluvial from higher positions on
  landscape
• Ash sources
• Eolian dust sources
• Lab analysis and field observation can assist in
  source determination
• Mass balance, chemical signatures, topographic
  expression

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Ash soils

• More control over soil formation than any other
  substance
• So unique that they have their own soil order!
• Andisols- melanic epipedon
• Often have unique subhorizons and weathering
  materials
• Volcanic glass- weathers into clays like
  allophane
• Often mistaken for albic subhorizons
• Simple chemical tests often assist in determining
  origins

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Uniformity

• Parent material is very important for assessing
  development
• e.g. PDI relies heavily on parent material values
• Needed to separate pedogenic processes from
  sedimentary processes
• Bedding vs horizonation

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Numerous ways to mix up the parent materials

• Frost heave
• Shrink swell clays
• Colluvial washdown
• Bioturbation
• Preferential weathering

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Dilution by eolian processes
Coarse fractions fine upward due to input from
eolian sources
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Dilution by disintegration

• Preferential disintegration of smaller sized
  fractions relative to larger clasts
• Results in more fines being produced as large
  stuff gets left behind
• Surface to volume ratio dictates this

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River deposits

• Problematic due to episodic variation in
  deposition
• Alternating energy of deposition creates
  stratified materials
• Must separate strata from pedo processes when
  evaluating