Regolith profiles - CRC LEME

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Feb 21, 2014 (3 years and 1 month ago)

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© CRC LEME 2007

Regolith Profiles

Types, Materials, Genesis and Terrestrial
Processes


Mehrooz F Aspandiar

CRC LEME

WASM, Curtin University of Technology

© CRC LEME 2007

© CRC LEME 2007

Weathering and regolith



Reaches great
depths



Regolith is much
more

than soil



Made up of primary
& secondary minerals,
biota, water & gases



Weathering is
central to regolith
development and
evolution

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Weathering profiles

Weathering starts from surface and progresses
downwards into the rock (assuming bioturbation and
erosion are negligible!)

Weathering results in formation of sub
-
horizontal zones
with different physical/chemical/biological characteristics

A 1 D section through the weathered regolith is a
weathering profile

Several types of weathering profiles based on the degree
of weathering and nature of the zones


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A “simple” weathering/regolith profile

Soil

(A,B,BC)



Saprolite




Saprock

Stone layer

Fresh

Core stones

Increasing degree of weathering

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Weathering Profiles


Saprolite & Core stones

Granitic saprolite

Profile over basalt

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“Classic” lateritic profile

Soil


horizons, bio
-
mantle is the uppermost zone of regolith

in which plant roots & fauna live; likely have horizons


Duricrust


Indurated & with fabrics

Fe
-
Al
-
Si
-
Ca cements; Hematite, goethite, gibbsite, calcite

Mottled zone


generally red patches (Fe oxides) in

grey matrix (kaolinite)

Pallid/Arenose zone


grey clay/sand (saprolite)

(kaolinite, smectite)

Saprolite


weathered rock that retains rock fabric

Kaolin, smectite, illite; If ferruginized


Fe oxides)

Saprock


partly weathered rock fabric retained

(Mottled; Ferruginized; Silicified)

2


100+ m

Fresh Rock

Saprolith

Pedolith

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Weathering profiles


Soil/mobile zone/biomantle

-

is the uppermost zone of regolith &
may have horizons, in which plant roots, organism live (bioturbate)


Duricrust


indurated cemented material with various fabrics and
cements (Fe, Si, Ca and Al)


Mottled

zone


composed of mottled (different coloured patches)
material generally red/brown within grey/white matrix


Saprolite
-

is very highly weathered to moderately weathered rock,
easily broken, retains rock fabric


Saprock
-

is slightly weathered rock which can’t be broken in the
hand and retains rock fabric


Fresh rock

-

shows no signs of weathering



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“Laterite”/Lateritic profiles

Saprolite
-
pallid

Mottled

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Weathering profile terminology

Modified from Taylor & Eggleton (2001)

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“Classic” weathering profiles


a few neglected
but critical points

All zones/materials

shown in ‘classic’ profiles
are NOT
present

and every material of profile can crop out at surface

Thickness of zones

varies

laterally within metres

2D & 3D
variations are a norm Not all zones/materials form in the
sequence generally depicted (top to bottom)

Not all zones/materials
form in the sequence

generally
depicted (top to bottom)

Some zones/material may
repeat

in a profile

Not everybody uses the same terminology!

One term to refer
to different materials and different terms for same material


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Weathering Profiles: Residual/In situ regolith

Residual or
In situ
: regolith produced mainly as a result of
underlying parent material (basement)


Degrees of weathered rock, residual sand/clay

Granite

Ultramafic

“lateritic”

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Weathering Profiles: Sedimentation/Stratigraphy


Transported

regolith


Fresh to weathered surficial sediments


Alluvial, aeolian, colluvial, lacustrine …

Weathered Sand
-

aeolian

Neogene fluvial
sediments over
residual profile

Gravel colluvium

Residual

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Weathering Profiles: Sedimentation/Stratigraphy

Image: R Anand

Profiles preserve
landscape &
geological history

Single to multiple
unconformities in deep
or “lateritic” profiles

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Weathering Profiles: Sedimentation/Stratigraphy

Weathering cuts
across or
transgresses
geological layers

Weathering can be
time
-
transgressive

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Weathering Profiles: Biomantle & Stratigraphy!

Bioturbators



Conveyor belt organisms
(termites, ants, worms)



Mix master organisms
(moles, wombats,
marsupials)



Cratering organisms
(wombats, tree
-
fall)

Biomantle



biomechanically active
material at the top of
regolith

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Weathering Profiles: Stratigraphy!


Biomantle


bioturbation
negates law of superposition


Material at base of biomantle
may be younger!


Buried biomantles

(paleosols)

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Genesis of weathering profiles

Regolith forms and evolves by the
interaction between
weathering, erosion, transportation
and

sedimentation

All the terrestrial processes operate at
different rates and
scales

across the landscape and have an impact on the
evolution of a weathering profile
over time

Need to separate terrestrial sediments from weathering
features or character


tease out

landscape history

Need to consider the interaction between weathering,
erosion and sedimentation within the landscape

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Profiles in the landscape through time

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Weathering, weathering profiles & landscape
events

Fresh Rock

Weathered Basement

Erosional

Unconformity

Weathered sediment

1 Weathering of basement

2 Erosion of surface

3 Deposition of sediment

4 Weathering of sediment
Unconformity still recognizable

5 Deeper weathering of
sediment obscures
unconformity


landscape
event unrecognizable

Surface landscape events in 1D

Need to unravel landscape events
in weathering profiles in 1D and 2D