Chapter 8- Chronosequences Time periodinterval OIS or MIS Holocene0-10 ka 1 Pleistocene24-10 ka 2 Pleistocene59-24 ka 3 Pleistocene74-59 ka 4 Pleistocene ka 5a-e divided “a” through “e” Chapter 8- Chronosequences Time periodinterval OIS or MIS Holocene0-10 ka 1 Pleistocene24-10 ka 2 Pleistocene59-24 ka 3 Pleistocene74-59 ka 4 Pleistocene 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 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 till140 ka till weathered unweathered weathered

Weathering rinds Reflect rates of rock weathering and therefore are a proxy for rates of soil development 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 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 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