ALLUVIAL/FLUVIAL

I. Intro

A. fluvial seds largely classified into 3 dep. environments1. alluvial2. braided rivers3. meandering rivershttp://facstaff.gpc.edu/~pgore/geology/historical_lab/sedenvirons.htm

II. Alluvial FansA. cone or apron shaped B. form in high relief areasC. Common in arid (Death Valley) and semiarid regions

D. alluvial from Latin "to wash against" vs. fluvial (Latin "river")E. Bajada = a group of coalesced fans.F. Role of flow expansion - flow unrestricted coming out of canyon, it widens leading to shallowing of flow which reduces velocity. Sedimentation occurs

http://www.agc.army.mil/research/products/desert_guide/lsmsheet/lsbajad.htm

Note Main Alluival ChannelFrench Pyrenees, decreased velocities as channel mouth widens lead to deposition

C streamflow episodic, occurs during floodD debris flow common in arid/semiarid regionsE mudflow-mostly sand and finer sedsF. landslides-rk falls, slumps, etc

III. Depositional processesA debris flow, mudflow, landslides, streamflowB streamflow dominates; deposits1.elongate channels, coarse. & p. sorted2. sheet flood- thin, w. sorted, structureless/ or laminated gravel, sand or silt3. Sieve deposits-gravel lobes; clay

Streamflowsouthern border of the Taklamakan Desert in Xinjiang . NASA

Debris flow deposits on alluvial fan, Los Corales sector of Caraballeda, Vargas, Venezuela. Structure in foreground is tiled rooftop of one-story house, July 2000 pr.water.usgs.gov/public/reports/matt.html

Debris flow deposit

A little scarp cut into the surface of the debris flow shows the finer grained material that makes up the bulk of the debris flow. The coarsening of the material at the surface of the fan is probably in part due to post-depositional winnowing of fine grains as well as some original coarse-tail coarsening in the original debris flow.usgshttp://faculty.gg.uwyo.edu/heller/

Sheet flood

Surface of ancient flood sheet (1250 BP by radiocarbon on detrital wood).Quebrada Rio Seco de Casma, Peru, 1985. geoimages.berkeley.edu/.../geomorph/flood1.html

IV) Alluvial fan sedimentsA cone-shaped, anastomozing channelsB concave upward radial profile, cross-section=lens shaped

C fan consists of:1. upper fan or proximal fan-steep, coarse seds, entrenched channels, debris flows2. mid fan-less gradient, seds fine, branching shallow channels3. distal fan- low gradients, fine seds, poorly defined channels

Fan Terminologygeology.cwru.edu/~huwig/catalog/catalog.html

http://faculty.gg.uwyo.edu/heller/

PROXIMALMEDIALDISTALMost gravity flows deposited (matrix-supported etc)River channels cut down into head of the fan, develops a single large channel cut.Coarsest grained, most poorly sorted depositsFewer gravity flows depositedChannels start splitting farther down the fanFewest gravity flows reach this areaSeveral small, shallow channelsFinest grained, best sorted deposits

V. Mid FanA. Has a lower slope gradientB. Sediment is intermediate in size and typically better sortedC. Sediments are well stratified and show some cross beddingD. Contains many small shallow braided channels surrounding one main (axial) channel (see photo on right)

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V. Mid FanE. Sieve lobe deposits form at the intersection point of the fan (usually upper mid fan)

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VI. Distal FanA.Gentle slope gradient B.Deposition of finer grained sedimentsC Deposits merge with sediments on basin floorD. More development of sedimentary structures

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Where on Fan?Interior of an alluvial fan

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Distal Fan

These deposits exhibit better sorting and may show low angle cross stratification and/or trough stratificationFining upward sequences frequently indicate inactivity of depositional processes Fining upward sequence in an alluvial fan

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D thickening and coarsening upward sequenceE inactive fan-thin and fine upwardF fan deposits up to 1000s m

I) RiversA. 4 types:1. braided2. anastomosing3. straight to near straight4. meandering

www.gly.uga.edu/railsback/1121Lxr28.html

I) Meandering RiversA. higher sinuosity, B. less gradients, finer sedsC elements1. main channel2. pt. bars3. levees4. floodplain5. oxbow lake6. abandoned meander cut-offAllen, 1964

Meandering vs. Braided

www.gly.uga.edu/railsback/1121Lxr28.html

http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/Sedstrat4/sedlect_4.htmRo Socopo flowing off eastern slope of Venezuelan Andes. View is up river (towards WNW Photo ). Octover 24, 1984. Note point bars along inner bends. The channel belt is defined as the region between the red lines wihtin which the active channel tends to meander. from R.H. Meade, USGSSide-looking radar (SLAR) image of the flood plain between the Rio Japur and Rio Solimoes (Amazon River basin), taken in 1971/1972. Flow is towards the lower right. Note the many scroll bars marking the former positions of channels (point bars) across the flood plain. Note these are large rivers with very large meander wavelengths, so that scorsll bars are also very large. SLAR does not see vegetation, overwise this view would only show rain forest.

D channel flow 1. lateral shifting of currents = helical flow E current velocity highest along outer bank, 1. bank undercutting, deepening of channel2. coarse lag deposit3. remaining sed accreted to pt. bar

Fig. 4.23: Highway 44, one of the main routes into Albuquerque (New Mexico, USA) was built in 1969. Part of the project involved the necessity to straighten a section of the upper Rio Puerco River which then flowed parallel to the highway.Fig. 4.24: This photograph taken in 1986 some 17 years after the channel straightening work. It shows how the Rio Puerco, which was perfectly straight in 1969, has now started to meander and threaten to undermine Highway 44 alongside it. In retrospect, it may not have seemed such a good idea to straighten it in the first place! Dr Tim Stott

F. overbank deposition during floodstage-f. sand/silts-levees, floodplain, oxbow lakesG. crevasse splays

H sediment deposition in:1. main channel2. pt. bar3. natural levees4. flood basin5. oxbow lake & meander chute

I channel deposits1. coarse lag material deposited during flood stage2. gravels and mud chunks3. indistinct bedding, thin and discontinuous deposits

J pt. bar deposits1. sand over gravels2. w/helical flow & flood stage, water & sed transported up pt. bar3. velocity decrease up pt. bar, therefore, coarse seds at lower pt. bars, finer grains in upper part4. dune bed forms in lower portion, ripples in upper portion5. trough x-beds6. x-beds have variable dip but altogether dip downstream

Example of Point Bar Deposits

Sebaskachu River, Labradorshows well developed point bars as it flows across a wide floodplain

Chutes and Lateral Accretion Surfaces

www.searchanddiscovery.net/.../images/chptr3.htm

www.searchanddiscovery.net/.../images/chptr3.htm

http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/Sedstrat4/sedlect_4.htm

Point Bar SequenceO shifting of river, get stacked lateral environmentsP lag deposits overlain by fining upward pt. bar sequenceLynn S. Fichter

K natural levee deposits1. thick and coarsest near bank, fine and thin into flood basin2. ripples and planar laminated seds overlain by laminated mud

L flood plain deposits1. fines settling from suspension, plant debris, may be bioturbatedM crevasse splay1. traction and suspension deposition, may be gradedN oxbow lake deposits1. infilled by silt and mud through overbank deposition,laminated, ostracods and fresh water molluscs

Rio Cauto, Cubahttp://en.wikipedia.org/wiki/Meander

Crevasse splay depositsLobate in shapeDeposit sand and siltSed from traction and suspensionGet coarse bedload and fines forming graded bedsLobes spread onto floodplain with fingers of sand extending beyond the main lobeDeep crevasses may tap into lower levels of main channel allows coarser sediment to escape on floodplainhttp://faculty.gg.uwyo.edu/hellerBryants Creek, MO

Flood Plain DepositsComposed of:Predominately fine grain material Which escape through floodingPlant debris and bioturbation

Modern Overbank Sands, Mississippi

Oxbow LakesSections of the channel become cut-offSilt and mud from channel during overbank floodLaminated with plant debris

Formation of an Ox-bow Lake

II) Ancient exampleA) lower part of Devonian Old Red S.S. of Wales and England

Meander Sequencehttp://www.geo.umn.edu/courses/4602/Spring01/Slide_List_2.htmlTertiary Caspe Formation, SpainDevonian Catskill Fm, NY

Lateral Accretion SurfacesCaspe Fm, Spainhttp://www.geo.umn.edu/courses/4602/Spring01/Slide_List_2.html

Overbank DepositsJurassic, Scalby Fm, Enlgandhttp://www.geo.umn.edu/courses/4602/Spring01/Slide_List_2.html

IX) Geometry

A Braided-sheet s.s. of congl w/thin beds or lenses of shales, enclosed in thicker sedsB meandering-shoe string sand bodies elongate in direction of river and enclosed by overbank fines

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