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ALLUVIAL/FLUVIAL

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 Fans. A. cone or apron shaped B. form in high relief areas

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ALLUVIAL/FLUVIAL

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  1. ALLUVIAL/FLUVIAL

  2. 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

  3. II. Alluvial Fans • A. cone or apron shaped • B. form in high relief areas • C. Common in arid (Death Valley) and semiarid regions

  4. 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

  5. Note Main Alluival Channel • French Pyrenees, decreased velocities as channel mouth widens lead to deposition

  6. 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

  7. III. Depositional processes • A debris flow, mudflow, landslides, streamflow • B streamflow dominates; deposits • 1.elongate channels, coarse. & p. sorted • 2. sheet flood- thin, w. sorted, structureless/ or laminated gravel, sand or silt • 3. Sieve deposits-gravel lobes; clay

  8. Streamflow • southern border of the Taklamakan Desert in Xinjiang . NASA

  9. pr.water.usgs.gov/public/reports/matt.html 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

  10. 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/

  11. 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

  12. IV) Alluvial fan sediments • A cone-shaped, anastomozing channels • B concave upward radial profile, cross-section=lens shaped

  13. 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

  14. Fan Terminology • geology.cwru.edu/~huwig/catalog/catalog.html http://faculty.gg.uwyo.edu/heller/

  15. A. Has a lower slope gradient B. Sediment is intermediate in size and typically better sorted C. Sediments are well stratified and show some cross bedding D. Contains many small shallow braided channels surrounding one main (axial) channel (see photo on right) V. Mid Fan

  16. V. Mid Fan • E. Sieve lobe deposits form at the intersection point of the fan (usually upper mid fan)

  17. A.Gentle slope gradient B.Deposition of finer grained sediments C Deposits merge with sediments on basin floor D. More development of sedimentary structures VI. Distal Fan

  18. Where on Fan? Interior of an alluvial fan

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

  20. D thickening and coarsening upward sequence • E inactive fan-thin and fine upward • F fan deposits up to 1000s m

  21. I) Rivers • A. 4 types: • 1. braided • 2. anastomosing • 3. straight to near straight • 4. meandering

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

  23. I) Meandering Rivers • A. higher sinuosity, • B. less gradients, finer seds • C elements • 1. main channel • 2. pt. bars • 3. levees • 4. floodplain • 5. oxbow lake • 6. abandoned meander cut-off • Allen, 1964

  24. Meandering vs. Braided www.gly.uga.edu/railsback/1121Lxr28.html

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

  26. D channel flow • 1. lateral shifting of currents = helical flow • E current velocity highest along outer bank, • 1. bank undercutting, deepening of channel • 2. coarse lag deposit • 3. remaining sed accreted to pt. bar

  27. 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

  28. F. overbank deposition during floodstage-f. sand/silts-levees, floodplain, oxbow lakes • G. crevasse splays

  29. H sediment deposition in: • 1. main channel • 2. pt. bar • 3. natural levees • 4. flood basin • 5. oxbow lake & meander chute

  30. I channel deposits • 1. coarse lag material deposited during flood stage • 2. gravels and mud chunks • 3. indistinct bedding, thin and discontinuous deposits

  31. J pt. bar deposits • 1. sand over gravels • 2. w/helical flow & flood stage, water & sed transported up pt. bar • 3. velocity decrease up pt. bar, therefore, coarse seds at lower pt. bars, finer grains in upper part • 4. dune bed forms in lower portion, ripples in upper portion • 5. trough x-beds • 6. x-beds have variable dip but altogether dip downstream

  32. Example of Point Bar Deposits • Sebaskachu River, Labrador • shows well developed point bars as it flows across a wide floodplain

  33. Chutes and Lateral Accretion Surfaces

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

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

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

  37. Point Bar Sequence • O shifting of river, get stacked lateral environments • P lag deposits overlain by fining upward pt. bar sequence Lynn S. Fichter

  38. 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

  39. L flood plain deposits • 1. fines settling from suspension, plant debris, may be bioturbated • M crevasse splay • 1. traction and suspension deposition, may be graded • N oxbow lake deposits • 1. infilled by silt and mud through overbank deposition,laminated, ostracods and fresh water molluscs

  40. http://en.wikipedia.org/wiki/Meander Rio Cauto, Cuba

  41. Crevasse splay deposits • Lobate in shape • Deposit 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

  42. Flood Plain Deposits • Composed of: • Predominately fine grain material • Which escape through flooding • Plant debris and bioturbation

  43. Modern Overbank Sands, Mississippi

  44. Oxbow Lakes • Sections of the channel become cut-off • Silt and mud from channel during overbank flood • Laminated with plant debris

  45. Formation of an Ox-bow Lake

  46. II) Ancient example • A) lower part of Devonian Old Red S.S. of Wales and England

  47. Meander Sequence http://www.geo.umn.edu/courses/4602/Spring01/Slide_List_2.html Devonian Catskill Fm, NY Tertiary Caspe Formation, Spain

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