Chapter 8- Chronosequences
Time period interval
Holocene
0-10 ka
Pleistocene
24-10 ka
Pleistocene
59-24 ka
Pleistocene
74-59 ka
Pleistocene
130-74 ka
OIS or MIS
1
2
3
4
5a-e
divided “a” through “e”
Rates of weathering
• Rates are dependent upon
– Material chemistry
• i.e. rock type & mineralogy
– Climates
• Warm vs cold
• Wet vs dry
– Chemicals available for weathering
• Acids, bases, water
– Size of materials
• Coarse vs fine
– Surface area of materials
• Exposed or buried
• Large vs small
Boulder weathering as a function of time
weathered
weathered
unweathered
20 ka till
140 ka till
Weathering rinds
Reflect rates of rock weathering
and therefore are a proxy for rates of soil
development
Chronosequences
• 4 kinds of chronosequences
– Post-incisive
• Sequence of deposits where each younger deposit (and therefore
the soil) forms from the beginning of the end of deposition to the
present
– Pre-incisive
• Soils formed in a deposit that has undergone differential burial on
the landscape synchronous with deposition of each sediment package
across the terrain- may be buried, may not be buried, may be
eoroded, may not be eroded
– Time transgressive without historic overlap
• Soils formed on the landscape as parts are buried, such that
sediments and soils are stacked vertically, with the soils recording
times of non deposition and soil development
•
– Time transgressive with historic overlap
• Soils formed on the landscape as parts are buried, such that
sediments and soils are stacked vertically, with the soils recording
times of non deposition and soil development
Influence of texture on soils
Can refer to consolidated or unconsolidated material
consolidated
porosity or fractures are key
fine grained with highly permeable and porous
conditions are preferable to facilitate weathering
and soil development
unconsolidated
Grain size is key
fine grained although highly porous is not
permeable, and therefore inhibits rapid
weathering and soil formation
Depth of leaching & translocation of
clays/silts varies with permeability and
porosity of parent material
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 pulverized
sedimentary rocks
– Common in soils with ultramafics
– Not common in soils with lesser amounts of
ultramafics
– Sandy (common) clayey (uncommon)
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
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
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
Numerous ways to mix up
the parent materials
•
•
•
•
•
Frost heave
Shrink swell clays
Colluvial washdown
Bioturbation
Preferential weathering
Dilution by eolian processes
Coarse fractions fine upward due to input from eolian
sources
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
River deposits
• Problematic due to episodic variation in
deposition
– Alternating energy of deposition creates
stratified materials
– Must separate strata from pedo processes when
evaluating