Chapter 8- ChronosequencesTime period interval OIS or MISHolocene 0-10 ka 1Pleistocene 24-10 ka 2Pleistocene 59-24 ka 3Pleistocene 74-59 ka 4Pleistocene 130-74 ka 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

20 ka till 140 ka till

weathered

unweathered

weathered

Weathering rindsReflect 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 processesCoarse 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