© CRC LEME 2007

Regolith ProfilesTypes, 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

© CRC LEME 2007

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

© CRC LEME 2007

A “simple” weathering/regolith profile

Soil (A,B,BC)

Saprolite

Saprock

Stone layer

Fresh

Core stones

Incr

easi

ng

deg

ree o

f w

eath

eri

ng

© CRC LEME 2007

Weathering Profiles – Saprolite & Core stones

Granitic saproliteProfile over basalt

© CRC LEME 2007

“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 fabricsFe-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 fabricKaolin, smectite, illite; If ferruginized – Fe oxides)

Saprock – partly weathered rock fabric retained

(Mottled; Ferruginized; Silicified)

2 –

100

+ m

Fresh Rock

Sap

rolit

hP

edol

ith

© CRC LEME 2007

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

© CRC LEME 2007

“Laterite”/Lateritic profiles

Saprolite-pallid

Mottled

© CRC LEME 2007

Weathering profile terminology

Modified from Taylor & Eggleton (2001)

© CRC LEME 2007

“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

© CRC LEME 2007

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”

© CRC LEME 2007

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

© CRC LEME 2007

Weathering Profiles: Sedimentation/Stratigraphy

Image: R Anand

Profiles preserve landscape & geological history

Single to multiple unconformities in deep or “lateritic” profiles

© CRC LEME 2007

Weathering Profiles: Sedimentation/Stratigraphy

Weathering cuts across or transgresses geological layers

Weathering can be time-transgressive

© CRC LEME 2007

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

© CRC LEME 2007

Weathering Profiles: Stratigraphy!

• Biomantle – bioturbation negates law of superposition

• Material at base of biomantle may be younger!

• Buried biomantles (paleosols)

© CRC LEME 2007

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

© CRC LEME 2007

Profiles in the landscape through time

© CRC LEME 2007

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