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GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits
Lecture 15: Fluvial facies
(Part 2)
AimsExamine braided river –
•Initiation of braiding•Bedforms•Facies•Depositional models•Controls (and links to sequence stratigraphy)
Sandy braided rivers
William River
How to evolve a straight into a braided channel1. Single row of alternating bars
Initially straight channel
Curved channel with full ‘point’ bar
Adding cross-bar channels Initially straight channel
Curved channel with proto-’point’ bar
Curved channel with full ‘point’ bar
Curved channel with proto-’point’ bar
How to evolve a straight into a braided channel2. Double row of alternating bars
Initially straight channel
Widened channel with proto-bars
Widened channel with full size bars
Adding cross-bar channels
braid bar
channel
channel
Model for braid bar growth (Bridge, 1993)
Potential rising stage erosion, falling stage depositionPotential rising stage deposition, falling stage erosion
bend thalweg confluenceupstream bar head downstream bar end bar top
Potential cross-bar channel directions at high stage
flow
Facies Models for Braided Rivers; the sandy braided South Saskatchewan River, Canada
The sandy braided South Saskatchewan River:A block model
Cant & Walker (1978)
Facies model forbraided river
coarse channel lag deposit
in-channel large-scale trough x-strat.
braid bar within channel duringhigh & low water stage
repetition of braid bar - to -in-channel succession
fine-grained floodplain mud withthin overbank sand layers
in-channel small-scale trough x-strat.
small channel fill on bar topsmall-scale tabular x-strat. on bar top
Walker & Cant (1984)
Cant & Walker, 1976
Three facies profiles?
transverse bar
mid-channel orlongitudinal bars
bank-attachedor
lateral bar
Sandy braided rivertransversebar tabularx-strat.
channeltroughx-strat.scouredbase
longitudinalbar avalanchestratification
channeltrough x-strat.scouredbase
idealisedverticalsections
in-channeltroughx-strat.
continuedchannelaggradation,occasionalbraid bar
finalchannel fill
verticalaggradation
High water stage Intermediate water stage
Low waterstage
Rising waterstage
above bar top below bar topabove bar top below bar topLow waterstage
Rising waterstage
Development of braid barduring falling then rising water stages
Collinson (1970)
Sedimentology of a braid bar in the Jamuna River, Bangladesh…..a large braided river
Avulsion – some physical experiments
Fans -Steeper slopes
Braided River
Henk Berendsen
..anastomosing rivers….
Sedimentsupply
Streampower,
sedimenttransportcapacity
When does a river flow erode or deposit sediment?
Base level is constant
Sediment supply > Transport capacity
Base level rise
Sediment supply < Transport capacity
Base level fall
Fluvial scour and sequence boundarieseffect of sea level on base level
longitudinal river profile
river profile adjuststo new base level
river incision byheadward erosion
sea level fall
incised valley andsequence boundary
When is a scour surface a sequence boundary?
Scour depth exceeds five times the channel depth
Scour is traceable for distances greater than the floodplain width
Scour is traceable for distances greater than the avulsion step length
Ideally, scour is traceable between basins
There should be evidence for ‘interfluve’ paleosols (although this is not necessarily diagnostic)
Points for you to think over…….
•Sand-bed braided river facies•Braided river barforms?•Grain size differences..the same facies model?•Small –vs- big rivers?•Large-scale controls•Base level and sequence boundaries
Reading:B&D: Chapter 13Boggs: Chapter 8Bridge: Chapter 3 in new Facies Models RevisitedLeeder: Chapter 17Bridge: Rivers and Floodplains textbook