Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


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 GE0-3112 Sedimentary processes and products Lecture 6. Rivers Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 17. Rivers.

2 RiversGEO-3112 2006Contents ► 6.1 Introduction – importance of fluvial systems ► 6.2 Fluvial channels ► 6.3 Floodplains ► 6.4 Fluvial architecture

3 RiversGEO-3112 2006 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-3112 2006 ► 1) Rivers are :  erosive agents  conduits for sediment transport to lacustrine and marine basins.

5 RiversGEO-3112 2006 ► 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-3112 2006 ► 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-3112 2006 Fluvial channels ► Size and gradient ► Shape (form) ► Processes ► Bedforms and internal structures

8 RiversGEO-3112 2006 Bankfull width ► Channel size is measured as bankfull width. Channel width Bankfull Normal

9 RiversGEO-3112 2006 Channel size ► Size varies by four orders of magnitude:  <2 m (small streams)  >20 km (Brahmaputra, Ganges).

10 RiversGEO-3112 2006 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-3112 2006 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-3112 2006 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-3112 2006 Downstream changes: Amazon R.

14 RiversGEO-3112 2006 W E Tectonic disturbance of river profiles across the Himalayan front.

15 RiversGEO-3112 2006 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-3112 2006 ► Sinuosity

17 RiversGEO-3112 2006 ► Braiding

18 RiversGEO-3112 2006 ► Anastomosing

19 RiversGEO-3112 2006 Controls on channel shape ► Sediment load ► Stream power ► Bank stability

20 RiversGEO-3112 2006 Meandering on clayey substrate Braiding on sandy substrate

21 RiversGEO-3112 2006 Channel variability Gandak River, Nepal-India Braided Meandering

22 RiversGEO-3112 2006 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-3112 2006 Jamuna-Ganges confluence, Bangladesh, 1993-96. Scour pool Recent scours Infilled scour

24 RiversGEO-3112 2006 Depositional architecture and stacking patterns Masjok, Tana

25 RiversGEO-3112 2006 Channel sediment transport and bedforms ► bars (macrofoms) ► dunes ► ripples Point bar Dunes Side bar Mid-channel bar

26 RiversGEO-3112 2006 Point bars and meanders ► Helical flow around a meander bend

27 RiversGEO-3112 2006 Point bar deposits Lateral accretion (epsilon cross-stratification) Scroll bars

28 RiversGEO-3112 2006 Fluvial point-bar depoits in the Spanish Pyrenees

29 RiversGEO-3112 2006 ► Fining-upward point-bar successions

30 RiversGEO-3112 2006 Channel bars ► Diffluence and confluence ► Downstream accretion

31 RiversGEO-3112 2006

32 RiversGEO-3112 2006 Planar cross-bedded unit with sigmoidal foresets (dune bedding), Tana Linguoid dunes, Tana

33 RiversGEO-3112 2006

34 RiversGEO-3112 2006 Braided river succession, Masjok, Tana

35 RiversGEO-3112 2006 Anastomosing channels ► Vertical accretion dominates

36 RiversGEO-3112 2006Floodplain ► Important processes:  Overbank flooding  Intermittent avulsion

37 RiversGEO-3112 2006 Floodplain deposits ► Vertical accretion of overbank muds and organic sediment. ► Lateral accretion on levees and crevasse splays.

38 RiversGEO-3112 2006Avulsion Cumberland Marshes avulsion, Saskatchewan R. Avulsion site, c. 1870

39 RiversGEO-3112 2006 1945 1977 Changes 1945 - 1977

40 RiversGEO-3112 2006 Crevasse splays Galloway & Hobday 1996

41 RiversGEO-3112 2006

42 RiversGEO-3112 2006

43 RiversGEO-3112 2006 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-3112 2006 Channel belts Palaeochannels of the Holocene Rhine-Meuse. Stacking patterns – fluvial architecture.

45 RiversGEO-3112 2006 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-3112 2006 Fluvial incision and knickpoints ► Fall in relative sea-level causes upstream knickpoint migration.

47 RiversGEO-3112 2006 Depositional architecture and stacking patterns

48 RiversGEO-3112 2006 Depositional architecture and stacking patterns at Tana Masjok, Tana

49 RiversGEO-3112 2006 Fluvial architecture

50 RiversGEO-3112 2006 Ancient fluvial deposits

51 RiversGEO-3112 2006 Further reading ► Cf. Colloquim literature on fluvial deposits.


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

Similar presentations


Ads by Google