ALLUVIAL/FLUVIAL

I. Intro

A. fluvial seds largely classified into 3 dep. environments

1. alluvial

2. braided rivers

3. meandering rivers

http://facstaff.gpc.edu/~pgore/geology/historical_lab/sedenvirons.htm

II. Alluvial FansA. cone or apron shaped

B. form in high relief areas

C. 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 flood

D debris flow common in arid/semiarid regions

E mudflow-mostly sand and finer seds

F. landslides-rk falls, slumps, etc

III. Depositional processes

A debris flow, mudflow, landslides, streamflow

B streamflow dominates; deposits1.elongate channels, coarse. & p. sorted

2. sheet flood- thin, w. sorted, structureless/ or laminated gravel, sand or silt

3. Sieve deposits-gravel lobes; clay

Streamflow

southern 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.

usgs

http://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 sediments

A cone-shaped, anastomozing channels

B concave upward radial profile, cross-section=lens shaped

C fan consists of:

1. upper fan or proximal fan-steep, coarse seds, entrenched channels, debris flows

2. mid fan-less gradient, seds fine, branching shallow channels

3. distal fan- low gradients, fine seds, poorly defined channels

Fan Terminology

geology.cwru.edu/~huwig/catalog/catalog.html

PROXIMAL MEDIAL DISTAL

Most 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 deposits

Fewer gravity flows depositedChannels start splitting farther down the fan

Fewest gravity flows reach this areaSeveral small, shallow channelsFinest grained, best sorted deposits

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

V. Mid Fan

A. 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)

V. Mid Fan

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

VI. Distal Fan

A.Gentle slope gradient

B.Deposition of finer grained sediments

C Deposits merge with sediments on basin floor

D. More development of sedimentary structures

Where on Fan?

Interior of an alluvial fan

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

D thickening and coarsening upward sequence

E inactive fan-thin and fine upward

F fan deposits up to 1000s m

I) Rivers

A. 4 types:

1. braided

2. anastomosing

3. straight to near straight

4. meandering

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

I) Meandering Rivers

A. higher sinuosity, B. less gradients, finer sedsC elements

1. main channel2. pt. bars3. levees4. floodplain5. oxbow lake6. abandoned meander cut-off Allen, 1964

Meandering vs. Braided

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

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

Río 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, USGS

Side-looking radar (SLAR) image of the flood plain between the Rio Japurá 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 lakes

G. crevasse splays

H sediment deposition in:

1. main channel

2. pt. bar

3. natural levees

4. flood basin

5. oxbow lake & meander chute

I channel deposits1. coarse lag material deposited during flood stage

2. gravels and mud chunks

3. 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, Labrador

shows 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 Sequence

O shifting of river, get stacked lateral environments

P lag deposits overlain by fining upward pt. bar sequence

Lynn S. Fichter

K natural levee deposits

1. thick and coarsest near bank, fine and thin into flood basin

2. ripples and planar laminated seds overlain by laminated mud

L flood plain deposits1. fines settling from suspension, plant debris, may be bioturbated

M crevasse splay1. traction and suspension deposition, may be graded

N oxbow lake deposits1. infilled by silt and mud through overbank deposition,laminated, ostracods and fresh water molluscs

Rio Cauto, Cuba

http://en.wikipedia.org/wiki/Meander

Crevasse splay deposits

Lobate in shapeDeposit sand and silt

Sed from traction and suspension

Get coarse bedload and fines forming graded beds

Lobes spread onto floodplain with fingers of sand extending beyond the main lobe

Deep crevasses may tap into lower levels of main channel

allows coarser sediment to escape on floodplain

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

Bryants Creek, MO

Flood Plain Deposits

Composed of:Predominately fine

grain material Which escape

through flooding

Plant debris and bioturbation

Modern Overbank Sands, Mississippi

Oxbow Lakes

Sections of the channel become cut-off

Silt and mud from channel during overbank flood

Laminated with plant debris

Formation of an Ox-bow Lake

II) Ancient example

A) lower part of Devonian Old Red S.S. of Wales and England

Meander Sequence

http://www.geo.umn.edu/courses/4602/Spring01/Slide_List_2.html

Tertiary Caspe Formation, SpainDevonian Catskill Fm, NY

Lateral Accretion Surfaces

Caspe Fm, Spain

http://www.geo.umn.edu/courses/4602/Spring01/Slide_List_2.html

Overbank Deposits

Jurassic, Scalby Fm, Enlgand

http://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 seds

B meandering-shoe string sand bodies elongate in direction of river and enclosed by overbank fines