Geol 370: Sedimentology and Stratigraphy Topic 10c: Braided Fluvial Systems

1. Photo by W. W. Little
Braided Fluvial System
(Multichannel/gravel or sand-
dominated)
Braided fluvial systems have multiple, relatively shallow channels with
a sinuosity of less than 2.3, a high bed/suspended load ratio, and non-
cohesive bank material. Braided river deposits can be dominated by
either sand or by gravel and are characteristic of variable-discharge or
ephemeral rivers flowing over high gradients.

2. • High gradient relative to discharge
• Low discharge relative to load
• High bed load relative to suspended load
• Non-cohesive bank materials (commonly sand or gravel)
• Ephemeral discharge with relatively high fluctuation
Conditions Favoring Braiding

3. Simplified Model
Braided fluvial systems typically consist of a single major facies
association produced by lateral and intrachannel bars within
channel-forms.

4. Transverse (side) vs. Longitudinal
(intra-channel) Bars
Transverse bars are
attached to the channel
margin and show some
similarity to point bars
as they build by lateral
migration. Longitudinal
bars form within the
channel and are
dominated by forset
bedding due to
avalanching off the
donwstream end of the
bar.

5. Model Variability
There is actually a range of braided fluvial styles, each represented
by a somewhat different combination of similar facies.

6. Causes of Braiding
Braiding occurs when stream discharge is insufficient to transport the
available load. Conditions that favor braiding over meandering are
high erosion rates (high gradient, proximity to source, non-cohesive
channel margins), coarse grain size, and low or variable discharge.

7. Nodal Migration
Flow from two channels converges at the downstream end of a
bar (node), leading to double helical flow. This accelerates the
flow, causing channel-floor erosion that migrates downstream.
Deposition occurs on the upstream side.

8. Photo by W. W. Little

9. Common Facies
Channel bars are commonly composed of structureless (massive),
horizontally-laminated, and trough or planar cross-stratified gravel
and/or sand. Lateral and down-stream accretion are both common.

10. Photo by W. W. Little
Structureless (massive) Gravel
Structureless gravel represents rapid deposition as bars within large
channels.

11. Photo by W. W. Little
Horizontally-bedded Gravel
Horizontally-bedded gravel is formed as clasts are transported along
a channel base or across a bar top. Elongate or flat clasts are
commonly imbricated.

12. Photo by W. W. Little
Imbricated pebbles

13. Photo by W. W. Little

14. Horizontally-bedded Sand
Horizontally-bedded sand is formed as sand is transported along a
channel base or across a bar top. It is often capped by ripple-bedded
sand; however the rippled sand tends to be eroded upon the next
flood..

15. Photo by W. W. Little
Trough Cross-laminated Sand/Gravel
Braiding occurs when stream discharge is insufficient to transport the
available load. Conditions that favor braiding over meandering are
high erosion rates (high gradient, proximity to source, non-cohesive
channel margins), coarse grain size, and low or variable discharge.

16. Photo by W. W. Little
Planar Cross-laminated Sand/Gravel
Planar cross-bedded sand and gravel represent down-stream deposition
off the distal end of intrachannel bars.

17. Photo by W. W. Little
Channel Elements
Channel elements (macroforms/facies associations) include side and
intrachannel bars, as well as many scales of channel fills.

18. Photo by W. W. Little
Bars that form within the channel tend to be elongated parallel to flow
direction, tapering in a down-stream direction and migrate by erosion
on the upstream end and deposition off the down stream end. Primary
internal structures are horizontally bedded (bar top) and planar cross-
bedded (bar front) sand or gravel. Bar top channel fills are common.
Intrachannel (longitudinal) Bars

22. Facies Association
For introductory purposes, most braided stream deposits can be simplified to a single
complex association of channel margin and intrachannel bars formed within channels
at multiple scales. Overbank deposits are preserved in minor amounts.

23. Photo by W. W. Little
Foreset Bedding
(down-stream accreting macroforms)
Foresets are large-scale planar cross-beds.

24. Photo by W. W. Little
Bars that form along the margin of a channel tend to migrate toward
the channel center, as the flow is deflected away from the bank.
Primary internal structures are horizontally bedded (bar top) and
trough cross-bedded (bar front) sand or gravel. Bar top channel fills
are common.
Channel Margin (transverse) Bars

26. Photo by W. W. Little
Bar Top Channel Fills
During flooding, bar tops are scoured. Subsequent waning flow
often results in deposition of cross-bedded sand and gravel within
these relative small channel forms.

27. Photo by W. W. Little

28. Photo by W. W. Little

29. Photo by W. W. Little

30. Photo by W. W. Little
Overbank Deposits
Floodplains tend to be small and poorly-developed in braided stream
systems. Rapid and repeated lateral migration results in uncommon
preservation for overbank deposits.

31. Large-scale Architecture
Braided fluvial systems are characterized by:
• High sand/gravel:mud ratio
• High sand/gravel body connectivity
• Sheet geometry
• Relatively low accommodation development

32. Photo by W. W. Little

33. Photo by W. W. Little

34. Photo by W. W. Little

35. Photo by W. W. Little

36. Photo by W. W. Little