Download Rivers and Sediments

Path of desire or path of
least resistance?
“Desire Path: A term in landscape architecture used to describe a path that isn’t designed but rather is worn casually away by people finding the shortest distance between two points.” http://www.yourdailyawesome.com/
Running Water
• Running water is the most important geologic agent in eroding, transporting and depositing sediment
• Nearly every landscape on Earth shows the results of stream erosion or deposition
Hydrologic Cycle
• Hydrologic cycle ‐ the movement and interchange of water between the sea, air, and land
– Evaporation
• Solar radiation provides energy
– Precipitation
• Rain or snow
– Transpiration
• Evaporation from plants
– Runoff
• Water flowing over land surface
– Infiltration
• Water soaking into the ground
Running Water
• Stream ‐ a body of running water, confined to a channel, that runs downhill under the influence of gravity
– Headwaters ‐ upper part of stream near its source in the mountains
– Mouth ‐ place where a stream enters sea, lake or larger stream
– Channel ‐ a long, narrow depression eroded by a stream into rock or sediment
• Stream banks ‐ sides of channel
• Streambed ‐ bottom of the channel
– Floodplain ‐ flat valley floor composed of sediment deposited by the stream
Insert revised Fig. 10.2
Yellowstone River Drainage Basin
Mississippi River Drainage Basin
The Big Sandy: a meandering stream
Sediment Deposition
• Meandering streams flow faster along the outside of bends and more slowly along the inside, depositing point bars on the insides of the meanders
• Meander cutoffs may form when a new, shorter channel is cut through the narrow neck of a meander (as during a flood)
Insert Fig. 10.20
Deposition of sand bars, formation of cross bedding and Ox‐bows (not related)
Stream Erosion
•
Stream erosion (and deposition) controlled by flow velocity and discharge
– Stream velocity controlled by stream gradient
(slope), channel shape and channel roughness
• Maximum velocity near center of channel
– Floods involve increased velocity and discharge (volume of water passing a particular point in a stream over time)
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Higher stream velocities promote erosion and transport of coarser sediments
– Erosion of very small particles difficult due to molecular binding forces
Sediment Deposition
•
Sediments are temporarily deposited along stream course as bars and floodplain deposits, and at/near its end as deltas or alluvial fans
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Bars ‐ ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a stream
– Braided streams contain sediment deposited as numerous bars around which water flows in highly interconnected rivulets
Sediment Deposition
• Floodplains are broad strips of land built up by sedimentation on either side of a stream channel
– Floodplain sediments are left behind as flood waters slow and recede at the end of flood events
– Main channel has slightly raised banks with respect to the floodplain known as natural levees
Sediment Deposition
• Delta ‐ body of sediment deposited at the mouth of a river when flow velocity decreases
– Surface marked by shifting distributary channels
– Shape of a delta depends on whether its wave‐dominated, tide‐
dominated, or stream‐dominated
The Ganges River Delta
Sediment Deposition
• Alluvial fan ‐ large, fan‐ or cone‐shaped pile of sediment that forms where stream velocity decreases as it emerges from a narrow mountain canyon onto a flat plain – Well‐developed in desert regions, such as the southwestern U.S.
– Larger fans show grading from large sediments nearest the mountains to finer sediments farther away
Flooding
•
When water levels rise and overtop the banks of a river, flooding occurs
– Natural process on all rivers – Described by recurrence intervals
• A 100‐year flood is, on average, the size of the largest flood within a 100‐year period of time
– Can cause great damage in heavily populated areas
– High velocity and large volume of water causes flood erosion
– Slowing of waters as flood ends causes flood deposits (usually of silt or clay‐
sized particles) to be deposited in the floodplain
sultan-Landslide.mov
A landslide
FlashFloodJerol.mov
A flash flood
DebrisFlow_Moscardo.mov
A debris flow
DebrisFlow_simualted.mov
Simulated debris flow:
graded bedding
Types of Weathering
• Mechanical weathering
– Physical disintegration
– Frost action, pressure‐release fracturing, plant growth, burrowing animals, salt wedging, thermal cycling
• Chemical weathering
– Decomposition of rock from exposure to atmospheric gases (oxygen, water vapor and carbon dioxide)
– New chemical compounds (minerals) form
– Rate increased by increased rock surface area
Spheroidal weathering
Chemical Weathering
• Oxidation
– Chemically active oxygen from atmosphere
– Iron oxides are common result
• Soil and sedimentary rocks often stained with iron oxides
• Acid dissolution
– Hydrogen cations replace others in minerals
– Carbonic acid from atmospheric CO2
dissolved in water
– Sulfuric, hydrofluoric acids emitted by volcanic eruptions
– Some minerals, such as calcite, may be totally dissolved
– Human activity, such as mining and burning of fossil fuels, produces acids
Chemical Weathering
• Feldspars
– Most common minerals in crust
– Slightly acidic rain water attacks feldspar
– Clay minerals produced
• K+, Na+, Ca++ ions released into water • Other minerals
– Ferromagnesian minerals
• Clays, iron oxides, Mg++ ions produced
– More complex silicate bonds lead to lower weathering susceptibility
• Olivine most susceptible, quartz least • Warm, wet climatic conditions maximize weathering
Chemical Weathering
•
Chemical weathering is driven by thermodynamic energy minimization, just like chemical reactions at high temperature.
– The system seeks the most stable assemblage of phases.
– The differences are that (1) kinetics are slow and metastability is common; (2) the stable minerals under wet, ambient conditions are different from those at high T and P; (3) solubility in water and its dependence on water chemistry (notably pH) are major determinants in the stability of minerals in weathering.
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A fresh rock made of olivine and pyroxenes will end up as clays and iron oxides, with other elements in solution
A fresh rock made of feldspars and quartz will end up as clays, hydroxides, and quartz in most waters.
Stability of minerals at the Earth’s surface is
predicted by Bowen’s reaction series in Reverse, i.e.,
Quartz is most stable and Olivine is least.
Sedimentary Rocks
• Produced from weathering products of pre‐existing rocks or accumulated biological matter
– Detrital (clastic) rocks produced from rock fragments
– Chemical rocks produced by precipitation of dissolved ions in water usually by animals or ‘plants’, i.e., biogenic
– Organic/biogenic ‘rocks’ produced by accumulation of biological debris, such as in swamps or bogs, e.g., coal, oil etc.
• Sedimentary rock types and sedimentary structures within the rocks give clues to past environments
• Fossils in sedimentary rocks give clues to the history of life
• Important resources (coal, oil) are found in sedimentary rocks
Relationship to Earth’s Systems
• Atmosphere
– Most sediments produced by weathering in air
– Sand and dust transported by wind
• Hydrosphere
– Water is a primary agent in sediment production, transportation, deposition, cementation, and formation of sedimentary rocks
• Biosphere
– Biological activity key to formation of sedimentary rocks
– Petroleum and coal resources have biological origin
Sediment
• Sediment ‐ loose, solid particles originating from:
– Weathering and erosion of pre‐existing rocks
– Chemical precipitation from solution, including secretion by organisms in water
• Classified by particle size –
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Boulder ‐ >256 mm
Cobble ‐ 64 to 256 mm
Pebble ‐ 2 to 64 mm
Sand ‐ 1/16 to 2 mm
Silt ‐ 1/256 to 1/16 mm
Clay ‐ <1/256 mm
Gravel
Distinguishing Characteristics of Clastic Sediments:
Grain Size ‐ mud/clay, silt (<0.06mm), fine sand (0.06‐0.25mm), coarse sand (0.25‐2.0mm), pebbles (2‐64mm), cobbles (64‐
256mm), boulders (>256mm); particle size reflects energy
(velocity) of the transport and depositional system.
From Sediment to Sedimentary Rock
• Transportation
– Movement of sediment away from its source, typically by water, wind, or ice
– Rounding of particles occurs due to abrasion during transport
– Sorting occurs as sediment is separated according to grain size by transport agents, especially running water
– Sediment size decreases with increased transport distance
Distinguishing Characteristics of Clastic Sediments (cont.):
Sorting ‐ Well‐sorted sediment indicates prolonged reworking by wind or water; poorly sorted sediment may indicate rapid deposition, or deposition by ice or mass movement. Angularity/Roundness and Shape – Well rounded sediment also indicate prolonged reworking by transporting agent; the shape of grains often indicates the transport system, but also may be related to the type of mineral or rock fragment
From Sediment to Sedimentary Rock
• Deposition – Settling and coming to rest of transported material
– Accumulation of chemical or organic sediments, typically in water
– Environment of deposition is the location in which deposition occurs
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Deep sea floor
Beach
Desert dunes
River channel
Lake bottom
From Sediment to Sedimentary Rock
• Preservation
– Sediment must be preserved, as by burial with additional sediments, in order to become a sedimentary rock
• Lithification – General term for processes converting loose sediment into sedimentary rock
– Combination of compaction and cementation
Types of Sedimentary Rocks
• Detrital sedimentary rocks
– Most common sedimentary rock type
– Form from cemented sediment grains that come from pre‐existing rocks
• Chemical sedimentary rocks
– Have crystalline textures
– Form by precipitation of minerals from solution
• Organic sedimentary rocks
– Accumulate from remains of organisms
Clastic Sedimentary Rocks
• Breccia and Conglomerate
– Coarse‐grained clastic sedimentary rocks
– Sedimentary breccia composed of coarse, angular rock fragments cemented together
– Conglomerate composed of rounded gravel cemented together
• Sandstone
– Medium‐grained clastic sedimentary rock
– Types determined by composition
• Quartz sandstone ‐ >90% quartz grains
• Arkose ‐ mostly feldspar and quartz grains
• Graywacke ‐ sand grains surrounded by dark, fine‐grained matrix, often clay‐rich
Clastic Sedimentary Rocks
• Shale
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Fine‐grained clastic sedimentary rock
Splits into thin layers (fissile)
Silt‐ and clay‐sized grains
Sediment deposited in lake bottoms, river deltas, floodplains, and on deep ocean floor
• Siltstone
– Slightly coarser‐grained than shales
– Lacks fissility
• Claystone
– Predominantly clay‐sized grains; non‐fissile
• Mudstone
– Silt‐ and clay‐sized grains; massive/blocky
Types of Clastic Sediments & Environments of Formation
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MUDSTONE/SHALE ‐ Well‐sorted, mature, clay‐
sized particles ; generally implies deposition into quiet water •
SANDSTONE ‐ Well‐sorted, mature, commonly bedded, sand‐sized particles typically transported by wind or moderate water movement (e.g. rivers, beaches)
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GREYWACKE ‐ Moderately sorted, immature, clay‐
to sand‐sized particles commonly showing graded bedding. Commonly deposited in deep waters off mountainous coasts. •
CONGLOMERATE ‐ Poorly sorted, immature, clay to boulder‐sized particles transported only a short distance from their source and typically deposited by fast moving water. •
TILL ‐ Very poorly sorted, clay to boulder‐sized particles; non‐bedded; deposited from glaciers.
Sedimentary Structures
• Sedimentary structures
– Features within sedimentary rocks produced during or just after sediment deposition
– Provide clues to how and where deposition of sediments occurred
• Bedding
– Series of visible layers within a rock
– Most common sedimentary structure
• Cross‐bedding
– Series of thin, inclined layers within a horizontal bed of rock
– Common in sandstones
– Indicative of deposition in ripples, bars, dunes deltas
Cross bedding: Zion National Park
Cross bedding: Kaibab Sandstone, Zion
National Park
Sedimentary Structures
• Ripple marks
– Small ridges formed on surface of sediment layer by moving wind or water
• Graded bedding
– Progressive change in grain size from bottom to top of a bed
• Mud cracks
– Polygonal cracks formed in drying mud
• Fossils
– Traces of plants or animals preserved in rock
– Hard parts (shells, bones) more easily preserved as fossils
Clastic Sedimentary Rocks
Composed of fragments of pre‐existing rock that have been weathered, transported,...
by Water
Mature Rivers –
4
Floodplains and Meanders
Effects of Changing
Base Levels
due to tectonic uplift
or changing sea level
Clastic Sedimentary Rocks
Composed of fragments of pre‐existing rock that have been weathered, transported,...
by Wind
Cross‐bedding
Clastic Sedimentary Rocks
Composed of fragments of pre‐existing rock that have been weathered, eroded, and transported
to a site of deposition
Deltaic Environments
deposition of sediment due to loss of transport energy
Near‐shore Environments of Deposition
Deep‐waterDeposition
DebrisFlow_simualted.mov
Chemical Sedimentary Rocks
• Carbonates
– Contain CO3 as part of their chemical composition
– Limestone is composed mainly of calcite
• Most are biochemical, but can be inorganic
• Often contain easily recognizable fossils
• Chemical alteration of limestone in Mg‐rich water solutions can produce dolomite
• Chert
– Hard, compact, fine‐grained, formed almost entirely of silica
– Can occur as layers or as lumpy nodules within other sedimentary rocks, especially limestones
• Evaporites – Form from evaporating saline waters (lake, ocean)
– Common examples are rock gypsum, rock salt
Chemical Sedimentary Rocks
composed of minerals precipitated from water (usually ocean water) due to evaporation or to the metabolic action of organisms (biogenic)
IRON‐FORMATION ‐ Iron oxide minerals, usually magnetite (taconite ore) or hematite (natural ore), interlayered with chert
(microcrystalline quartz) and clay minerals. Common chemical sedimentary rock biogenically formed in shallow marine environments older than about 1.8 billion years.
• LIMESTONE ‐ Calcium carbonate (calcite) typically composed of abundant marine fossils. Most common type of chemical sediment forming today by biogenic processing of seawater. Dolomite (or dolostone) is created by replacement of calcium by magnesium after shallow burial of limestone. Forms in tropical shallow marine environments. • EVAPORITE DEPOSITS (Gypsum, Halite, Anhydrite) – mineral precipitated from saline water in arid environments with high evaporation rates (e.g., playa lakes) • PEAT/COAL – Carbonaceous material created by the accumulation, compaction and heating of organic matter. Forms in temperate to tropical, low energy, terrestrial environments (lagoons, floodplains).
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Organics in Sedimentary Rocks
• Coal
– Sedimentary rock forming from compaction of partially decayed plant material
– Organic material deposited in water with low oxygen content (i.e., stagnant)
• Oil and natural gas
– Originate from organic matter in marine sediment
– Subsurface “cooking” can change organic solids to oil and natural gas
– Can accumulate in porous overlying rocks
Environments of Chemical Sedimentation
Evaporites
Coal
Limestone
Sedimentary Rock Interpretation
• Sedimentary rocks give important clues to the geologic history of an area
• Source area
– Locality that eroded and provided sediment
– Sediment composition, shape, size and sorting are indicators of source rock type and relative location
• Depositional environment
– Location where sediment came to rest
– Sediment characteristics and sedimentary structures (including fossils) are indicators
– Examples: glacial valleys, alluvial fans, river channels and floodplains, lakes, deltas, beaches, dunes, shallow marine, reefs, deep marine
Plate Tectonics and Sedimentary Rocks
• Tectonic setting plays key role in the distribution of sedimentary rocks
• Occurrence of specific sedimentary rock types can be used to reconstruct past plate‐tectonic settings
• Erosion rates and depositional characteristics give clues to each type of tectonic plate boundary