Upload
stuart-moore
View
221
Download
0
Embed Size (px)
Citation preview
© 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