Weathering - Crc Leme

Weathering - Crc Leme

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 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) Increasing degree of weathering Stone layer Saprolite Core stones Saprock Fresh CRC LEME 2007

Weathering Profiles Saprolite & Core stones Granitic saprolite Profile over basalt CRC LEME 2007 2 100+ m Pedolith 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) Saprolith Pallid/Arenose zone grey clay/sand (saprolite) (kaolinite, smectite) Saprolite weathered rock that retains rock fabric Kaolin, smectite, illite; If ferruginized Fe oxides)

(Mottled; Ferruginized; Silicified) Saprock partly weathered rock fabric retained Fresh Rock 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 cant be broken in the hand and retains rock fabric Fresh rock - shows no signs of weathering CRC LEME 2007 Laterite/Lateritic profiles

Mottled Saprolite-pallid 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 Ultramafic Granite 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 Residual CRC LEME 2007

Gravel colluvium Weathering Profiles: Sedimentation/Stratigraphy Single to multiple unconformities in deep or lateritic profiles Profiles preserve landscape & geological history Image: R Anand 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!

Biomantle biomechanically active material at the top of regolith Bioturbators Conveyor belt organisms (termites, ants, worms) Mix master organisms (moles, wombats, marsupials) Cratering organisms (wombats, tree-fall)

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 Surface landscape events in 1D 1 Weathering of basement 2 Erosion of surface Weathered sediment 3 Deposition of sediment

4 Weathering of sediment Erosional Unconformity Unconformity still recognizable 5 Deeper weathering of sediment obscures unconformity landscape event unrecognizable Weathered Basement Fresh Rock Need to unravel landscape events

in weathering profiles in 1D and 2D CRC LEME 2007

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