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Chapter 7: Sediment Routing This presentation contains illustrations from Allen and Allen (2005) And from Press, Siever, Grotzinger and Jordan 4 th Edition (2003). Sediment Routing. Weathering (in situ) Chemical, Physical an Biological Regolith Sediment Yield Patterns Controls - PowerPoint PPT Presentation
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Chapter 7:
Sediment Routing
This presentation contains illustrations from Allen and Allen (2005)
And from Press, Siever, Grotzinger and Jordan 4th Edition (2003)
Sediment Routing
• Weathering (in situ)– Chemical, Physical an Biological
• Regolith• Sediment Yield
– Patterns– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
erosionWeathering(in-situ)
transportation
Erosion includes BOTH weathering and transportation
Sediment Routing
• Weathering (in situ)– Chemical, Physical an Biological
• Regolith• Sediment Yield
– Patterns– Computational Models– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
Regolith
• Weathered layer between pristine bedrock and the land surface.
• Main chemical agent is water. Water is moved by gravity (down) and capillarity effect (up)
Regolith
Rate of regolith removal: –dH/dt by denudation (top)
Thickness (H) of regolith depends on rate of bedrock decay:
+dH/dt (bottom)
H
bedrock
Regolith
Weathering rate decreases exponentially with depth
H
bedrock
Rate of weathering using Cosmogenic Radionuclide Dating
bedrock
Berrylium 10 and Al 25 are produced (P) in situ by cosmogenic rays interacting with minerals( / *)
0y yP Pe
y
P (concentration)0P
*y
P0 changes with latitude and altitudeY* (~50%) is about 0.5-0.7m
P (concentration ) of radionuclides is a measure of absolute time
E
bedrock
y
P
See Perg et al., 2001 for details of the method
One General Rule of Weathering
• Granites are composed of quartz (25%), micas and feldspars (other ~75%).
• When weathering is intense and the source rock is average continental crust, a basin fill should contain sand, and clay in the same proportion (depending on climate)
(1) Product composition
World Weathering Patterns
Walther’s Law
Sediment Routing
• Weathering (in situ)– Chemical, Physical an Biological
• Regolith• Sediment Yield
– Computational Models– Patterns– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
Run-off
• Run-off (surface water flow) connects land and ocean water reservoirs and moves sediments
• Precipitation = Evaporation ++ Soil water change ++ Groundwater change ++ Run-off
Sediment Routing
• Weathering (in situ)– Chemical, Physical an Biological
• Regolith• Sediment Yield
– Run-off– Computational Models– Patterns– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
Computational Models
Denudation Rate or loss of elevation per unit of time and unit of area in a given catchment area can be calculated from know sediment exit rates from a catchment area:
dh/dt = (1-porosity)/density * total sediment mass discharge /unit time and area (7.7)
(elevation change) is proportional to sediments removed
Sediment Yield = sediment mass/unit / catchment area time
Sediment Yield from artificial traps
Amazon: 79mm/1000 yr NW Himalaya: 400 mm /1000 yrNile: 45 mm/1000 yr
Sediment Yield from preserved stratigraphy
Bay of Bengal: 200 mm/1000 ky
Sediment Routing
• Weathering (in situ)– Chemical, Physical an Biological
• Regolith• Sediment Yield
– Run-off– Computational Models– Patterns– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
Global Pattern of denudation rates
Sediment accumulation thicknesses
Chemical versus Mechanical Denudation Rates
Sediment Routing
• Weathering (in situ)– Chemical, Physical an Biological
• Regolith• Sediment Yield
– Run-off– Patterns– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
Controls on Sediment Yield
• Drainage Area and Tectonic Activity• Vegetative Cover• Precipitation• High vs. low relief
Controls on Sediment Yield• Drainage Area, Tectonics
Low-relief vs. High relief
Low relief: • erosion rates are limited by erosivity of transport processes,
e.g. in dry environments this means availability of water but in LA this means how much sediment can be eroded from the Mississippi River Valley itself.
High relief:• Erosion rates are held back by rock and soil strength.High relief assures availability of materials by rock falls,
landslides. A high relief must be renewed bye.g., tectonic activity
Low-relief vs. High relief
.5 .00052000 20000.52000
1000
mm mper year
m mmpe
mr years
Sediment Routing
• Weathering (in situ)– Chemical, Physical an Biological
• Regolith• Sediment Yield
– Run-off– Patterns– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
Dissolved Solids
• Run-off waters contain dissolved solid concentrations which depend on (1) precipitation, (2) weathering reactions in rocks and soil, and (3) the degree of evaporation.
• Precipitation helps chemical weathering but steep slopes reduce the amount of time water is able to spend in the regolith. So, low-slope areas should produce more chemical weathering (General Rule) BUT that is not so in the Amazon, where most (85%) of solute comes from the Andes.
Concentrations
• Arid areas have saltier waters and hence more dissolved solids e.g. Kazakhstan (1000-6000 mg/l)
…… the Amazon has only 10 mg/l… but the Amazon takes 10% all river water so it may produce more dissolved compounds overall.
Precipitation, weathering rate and evaporation define water type
High concentration of Na(+) through precipitation of CaCO3
Less weathering low concentration of Ca(2+)
Colder climates- less dissolution
Observations
• Principal cations in water are Ca(+2)• Principal anions are HC03 (-), S04 (2-) • Na (+) increases relative to Ca(+2) indicate that the Ca(2+)
is precipitating out of solution • (80% dissolved load in rivers is made of Ca(2+), HC03 (-), S04
(2-), and SiO2)
• Increase of Ca(2+) relative to Na(+) indicates greater chemical weathering, (because Ca is harder to dissolve and require more intense weathering to get into solution).
• But composition reflects availability of ions in the source terrain.
Primary rock origins of solutes and their types
Observations
• Most Na(+) and Ca(2+) ions come from weathering of
secondary sources (salt and carbonates)
• Dissolved SiO2 and K(+) come from silicates
Sediment Routing
• Weathering– Chemical
• Regolith• Sediment Yield
– Patterns– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
Modeling Landform Evolution
• Isostasy during denudation
Depth of compensationmantle
Future erosion (D)
hc
Sea-level
Height above sea-level (h)
Modeling Landform Evolution
• Isostasy during denudation
h
Depth of compensationmantle
hc-D
D h
Sea-level
Modeling Landform Evolution
• Denudation removes material from the surface (drop in the head)
• Rock is uplifted (from below because of isostasy)
Change in elevation = rise of base – drop in head
Surface rises to about 85% of its original height (P. 242)
Modeling Landform Evolution
• Planar geometry
mantle
hc-D
Sea-level
Height above sea-level = h - h
Mountain geometryHeight above sea-level = 2(h - )h
Modeling Landform Evolutionwith thermochronometers
Apatite fission track analysis
Below a certain temperature, natural damage tracks within apatite minerals do not heal. The number of tracks acts as a clock.
Modeling Landform Evolutionwith thermochronometers
Apatite fission track analysis
http://images.google.com/imgres?imgurl=http://faculty.plattsburgh.edu/mary.rodentice/images/research/apatite1.jpg&imgrefurl=http://faculty.plattsburgh.edu/mary.rodentice/research/Fission_Track.html&h=218&w=300&sz=19&hl=en&start=5&tbnid=urHg8C9toErWZM:&tbnh=84&tbnw=116&prev=/images%3Fq%3Dfission%2Btrack%26svnum%3D10%26hl%3Den%26lr%3D%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-US:official%26sa%3DG
X 200 X 1600
Apatite fission track analysis
X 200 X 1600U(238) decay damages mineral structure. Above 100 C and over about 1 My the damage will anneal itself completely. PAZ is the partial annealing zone 60-100 CBelow 100 C track number acts as a geological clock. Track length is larger if the cooling is quick (lava flows)
Apatite fission track analysis
X 200 X 1600If a geothermal gradient for an area is know, AFT analysis can provide information how long ago a sample passed through the PAZ. Several samples can be used to calculate rates of uplift (i.e., not denudation) from which denudation rates can be calculated.
Sediment Routing
• Weathering– Chemical
• Regolith• Sediment Yield
– Patterns– Controls– Solute and Suspension
• Modeling Landform Evolution• Relation between tectonics and sedimentation
Tectonic-Sedimentary model
Walther’s Law
Walther’s Law
Interpretation criteria(1)Mark erosional tops and bases(2) Identify sequence packages(3) Weight extrapolation
Walther’s Law