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GE0-3112 Sedimentary processes and products Lecture 6. Rivers Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch.

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Presentation on theme: "GE0-3112 Sedimentary processes and products Lecture 6. Rivers Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch."— Presentation transcript:

1 GE Sedimentary processes and products Lecture 6. Rivers Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder Ch. 17. Rivers.

2 RiversGEO Contents ► 6.1 Introduction – importance of fluvial systems ► 6.2 Fluvial channels ► 6.3 Floodplains ► 6.4 Fluvial architecture

3 RiversGEO Importance of fluvial systems ► 1) Rivers are major erosive and sediment transport agents. ► Fluvial sediments are mostly transient but may form thick deposits in several settings. ► Fluvial deposits are sensitive palaeoenvironment indicators.

4 RiversGEO ► 1) Rivers are :  erosive agents  conduits for sediment transport to lacustrine and marine basins.

5 RiversGEO ► 2) Fluvial sediments are mostly transient but form thick deposits in several settings:  coastal plains  intermontane basins  tectonic forelands Modern and Holocene terraced fluvial deposits at Tana, N. Norway.

6 RiversGEO ► 3) Fluvial deposits are sensitive palaeoenvironment indicators:  tectonic slope changes  sourceland geology  climate  sea-level change Postglacial fluvial terraces at Porsanger, N. Norway

7 RiversGEO Fluvial channels ► Size and gradient ► Shape (form) ► Processes ► Bedforms and internal structures

8 RiversGEO Bankfull width ► Channel size is measured as bankfull width. Channel width Bankfull Normal

9 RiversGEO Channel size ► Size varies by four orders of magnitude:  <2 m (small streams)  >20 km (Brahmaputra, Ganges).

10 RiversGEO Channel size vs. discharge ► Discharge increases with increasing width, depth and velocity. ► Discharge, width, depth and velocity all increase downstream. Q = whu Discharge Channel width Channel depth Mean flow velocity

11 RiversGEO Width vs depth ► Depth (h) increases with increasing width (w). ► W/h ratios are higher in low-sinuosity rivers. High sinuosity (low w/h ratios) Low sinuosity (high w/h ratios) NB: Symbols erroneously reversed

12 RiversGEO Long profile ► Downstream changes (in effluent streams):  Discharge increases.  Gradient decreases (the flow is more efficient; with increased discarge the gradient must decrease to maintain equilibrium). Graded river: concave long- profile.

13 RiversGEO Downstream changes: Amazon R.

14 RiversGEO W E Tectonic disturbance of river profiles across the Himalayan front.

15 RiversGEO Channel shape ► Parameters for describing channel planform shape:  Sinuosity (P)  Braiding  Anastomosing Channel types illustrating characteristics of sinuosity, braiding and anastomosing (Galloway & Hobday 1996).

16 RiversGEO ► Sinuosity

17 RiversGEO ► Braiding

18 RiversGEO ► Anastomosing

19 RiversGEO Controls on channel shape ► Sediment load ► Stream power ► Bank stability

20 RiversGEO Meandering on clayey substrate Braiding on sandy substrate

21 RiversGEO Channel variability Gandak River, Nepal-India Braided Meandering

22 RiversGEO River confluences ► Deep scour at confluences. ► May be several times deeper than contributing tributaries. ► Mobile scour-and-fill units at the base of a succession.

23 RiversGEO Jamuna-Ganges confluence, Bangladesh, Scour pool Recent scours Infilled scour

24 RiversGEO Depositional architecture and stacking patterns Masjok, Tana

25 RiversGEO Channel sediment transport and bedforms ► bars (macrofoms) ► dunes ► ripples Point bar Dunes Side bar Mid-channel bar

26 RiversGEO Point bars and meanders ► Helical flow around a meander bend

27 RiversGEO Point bar deposits Lateral accretion (epsilon cross-stratification) Scroll bars

28 RiversGEO Fluvial point-bar depoits in the Spanish Pyrenees

29 RiversGEO ► Fining-upward point-bar successions

30 RiversGEO Channel bars ► Diffluence and confluence ► Downstream accretion

31 RiversGEO

32 RiversGEO Planar cross-bedded unit with sigmoidal foresets (dune bedding), Tana Linguoid dunes, Tana

33 RiversGEO

34 RiversGEO Braided river succession, Masjok, Tana

35 RiversGEO Anastomosing channels ► Vertical accretion dominates

36 RiversGEO Floodplain ► Important processes:  Overbank flooding  Intermittent avulsion

37 RiversGEO Floodplain deposits ► Vertical accretion of overbank muds and organic sediment. ► Lateral accretion on levees and crevasse splays.

38 RiversGEO Avulsion Cumberland Marshes avulsion, Saskatchewan R. Avulsion site, c. 1870

39 RiversGEO Changes

40 RiversGEO Crevasse splays Galloway & Hobday 1996

41 RiversGEO

42 RiversGEO

43 RiversGEO Avulsion and channel belts ► Sudden shift in channel reach (bend cutoff) or whole channel belt. ► Controlled by internal (autocyclicity) or external factors (base-level, climate, tectonics). ► Diversion more likely during extreme flood events or fault movement.

44 RiversGEO Channel belts Palaeochannels of the Holocene Rhine-Meuse. Stacking patterns – fluvial architecture.

45 RiversGEO Incision – aggradation cycles ► Regional cycles of incision and aggradation may occur on the scale of decades or more. ► Causes may be ’intrinsic’ or extrinsic, e.g:  water and sediment discharge variations controlled by climate and catchment characteristics (e.g. ENSO).  eustatic sea level changes.  tectonics.

46 RiversGEO Fluvial incision and knickpoints ► Fall in relative sea-level causes upstream knickpoint migration.

47 RiversGEO Depositional architecture and stacking patterns

48 RiversGEO Depositional architecture and stacking patterns at Tana Masjok, Tana

49 RiversGEO Fluvial architecture

50 RiversGEO Ancient fluvial deposits

51 RiversGEO Further reading ► Cf. Colloquim literature on fluvial deposits.


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