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

erosion Weathering (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) H 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 P (concentration) y Berrylium 10 and Al 25 are produced (P) in situ by cosmogenic rays interacting with minerals P0 changes with latitude and altitude Y* (~50%) is about 0.5-0.7m bedrock

P (concentration ) of radionuclides is a measure of absolute time y bedrock 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 yr Nile: 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 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

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 Height above sea-level (h) Future erosion (D) Sea-level hc Depth of compensation mantle

Modeling Landform Evolution Isostasy during denudation Sea-level hc-D Depth of compensation mantle

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 Mountain geometry Planar geometry Height above sea-level = h - Height above sea-level = 2(h - ) Sea-level hc-D mantle

Modeling Landform Evolution with 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 Evolution with thermochronometers

Apatite fission track analysis X 200 X 1600 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

Apatite fission track analysis X 1600 X 200 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)

Apatite fission track analysis X 1600 X 200 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 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 Mark erosional tops and bases Identify sequence packages Weight extrapolation

Walther’s Law