chapter 7: sediment routing this presentation contains illustrations from allen and allen (2005) and...

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



– Patterns– Computational Models– Controls– Solute and Suspension


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 P e

y

P (concentration)

0P

*y

P0 changes with latitude and altitude

Y* (~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



– Computational Models– Patterns– Controls– Solute and Suspension


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



– Run-off– Computational Models– Patterns– Controls– Solute and Suspension


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



– Run-off– Computational Models– Patterns– Controls– Solute and Suspension


Global Pattern of denudation rates

Sediment accumulation thicknesses

Chemical versus Mechanical Denudation Rates

Sediment Routing



– Run-off– Patterns– Controls– Solute and Suspension


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 .0005

2000 20000.5

20001000

mm mper year

m mmpe

mr years

Sediment Routing



– Run-off– Patterns– Controls– Solute and Suspension


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




Modeling Landform Evolution

• Isostasy during denudation

Depth of compensationmantle

Future erosion (D)

hc

Sea-level

Height above sea-level (h)


• Isostasy during denudation

h

Depth of compensationmantle

hc-D

D h

Sea-level


• 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)


• Planar geometry

mantle

hc-D

Sea-level

Height above sea-level = h - h

Mountain geometry

Height 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


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


X 200 X 1600

U(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 C

Below 100 C track number acts as a geological clock. Track length is larger if the cooling is quick (lava flows)


X 200 X 1600

If 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




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

chapter 7: sediment routing this presentation contains illustrations from allen and allen (2005) and...

Documents

unit of area

given catchment area

sedimentation runoffrun

surface water flow

elevation change

unit of time

ocean water reservoirs

mm1000 yrsediment yield