Lecture 19: Fluvial Facies

Transcription

1 GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits Lecture 19: Fluvial Facies

2 Aims Examine controls on alluvial channel pattern List the types of channel pattern Examine meandering and braided channels Processes Bedforms Facies Depositional models Controls (and links to sequence stratigraphy)

3 Fluvial Channel Patterns Henk Berendsen

4 Meandering Braided Controlling factors Grain size Type of sediment load Fluid discharge Sediment discharge Discharge regularity Size of drainage area Bank stability (strength, vegetation)... and also Regional slope Allocyclic controls such as tectonic tilt Flow duration? Orton & Reading (1993)

5 Henk Berendsen

6 But how complex are channel patterns?

7 Braided or meandering river? i.e. its not as simple as this!!!

8 Meander Development movie

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12 Channels, bars and smaller-scale bedforms ( for all types of rivers..) Small-scale bedforms: e.g. ripples, dunes, uspb, antidunes, bedload sheets. Bars: bed form with length proportional to local channel width and height proportional to channel depth. Simple (Unit) and compound (e.g. point, braid) bars Channels: different orders first (biggest) second..third

13 Fluvial form and stratasets: types and scales Ripples Dunes Low-relief bedwaves on plane beds Antidunes Seasonal flood deposit Unit bars and compound bars Channels Channel belt composed of channels and bars channel fills Floodplain with levees, crevasse splays, channels, lakes, floodbasins Alluvial valley with channel belts and floodplain (or fan or delta) Alluvial river system Small-scale cross stratasets Medium-scale cross stratasets Planar stratasets Low-angle cross stratasets Large-scale (inclined) stratum Large-scale inclined stratasets Channel fills Group of large-scale inclined stratasets and Groups of large-scale inclined stratasets and channel fills Groups of groups: alluvial architecture Basin fill (or part of basin fill)

14 Bridge and Demicco, 2008

15 Bridge, 2006

16 Basic flow in bends Boggs Ch. 8

17 Jorge Abad UIUC

18 The classic facies model for meandering river distal floodplain mud with roots, desiccation cracks, etc. proximal floodplain mud with occasional overbank sand ripple x-lamination and upper stage plane-bed lamination of upper point bar dune trough x-bedding and upper stage plane-bed lamination of lower point bar coarse channel lag deposit Walker (1979)

19 Lateral Accretion characteristics.

20 Preserved Jurassic bend deposits

21 Ph. 2 - site 1 Scroll bars and exposed Jurassic meander bends S

22 Types of cut-off of meander loops AVULSION thin muddy fill gradual abandonment via swale thick muddy fill rapid abandonment via cut bank sand plug Walker & Cant (1984)

23 Meandering river Summary of nomenclature POINT BAR SUBAQUEOUS DUNES CUT BANK RIPPLES SCROLL BARS Sinuosity = total channel length / meander-belt axial length

24 Williams River, Alaska (photo Norm Smith)

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27 Overbank deposits and crevasse splays Sediment-laden flows forming thin sheets/wedges of fine-grained sandstones with characteristic sequences of structures Bridge and Demicco, 2008

28 Overbank deposits: organics, soils and trace fossils Norm Smith Tor Tornqvist Bridge, 2006

29 Overbank deposits and crevasse splays Bristow et al. (1999) Niobrara River

30 Overbank deposits and crevasse splays

31 Styles of bend migration

32 LATERAL ACCRETION SURFACES Bend Migration and Section Orientation (Willis, 1989) Expansion Translation Rotation Wavelength change

33 A point bar is a complex 3D sediment body so the expression of lateral accretion surfaces in outcrop depends on the orientation of the outcrop Let s examine different computer-simulated cross-sections of point bar deposits formed by translating and expanding meander bends Meanders in other environments?

34 Down-valley translating meander bends Lateral-accretion bedset surfaces correspond to migration steps Stippled lines denote lines of equal mean grain size Arrows denote local flow directions (up = into section, down = out of section)

35 Expanding meander bends Same simulation parameters as in previous example 35

36 For other section orientations and combined meander bend translation and expansion, see: Willis, B.J. (1989) Paleochannel reconstructions from point bar deposits: a three-dimensional perspective. Sedimentology, 36, Where can we use this information for? Reconstruction of the geometry of ancient point bars and meandering channel Reconstruction of the course of palaeochannel Reconstruction of the type(s) of meander bed migration

37 Finally, paleohydraulics: empirical relationships between meandering river parameters and flow discharge (for sinuosities > 1.7) = 106 Q 0.46 = 10.9 w 1.01 Q = mean annual discharge = meander wavelength w = channel width w = 6.8 h 1.54 h = channel depth

38 Summary Stratasets and formative bedforms Bend flow processes The classic meandering rivers facies model Complications and additions to this model (cutoff, floodplain, migration pattern etc) Preservation and orientation of sections Is there a definitive meandering river facies? Reading: B&D: Chapter 13 Nichols: Chapter 9 Boggs: Chapter 8 Bridge: Chapter 3 in Facies Models Revisited Leeder: Chapter 17 Bridge: Rivers and Floodplains textbook