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