Date post: | 27-Nov-2014 |
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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.
• 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
Simplified Model
Braided fluvial systems typically consist of a single major facies association produced by lateral and intrachannel bars within channel-forms.
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.
Model Variability
There is actually a range of braided fluvial styles, each represented by a somewhat different combination of similar facies.
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.
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.
Photo by W. W. Little
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.
Photo by W. W. Little
Structureless (massive) Gravel
Structureless gravel represents rapid deposition as bars within large channels.
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.
Photo by W. W. Little
Imbricated pebbles
Photo by W. W. Little
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..
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.
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.
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.
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
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.
Photo by W. W. Little
Foreset Bedding(down-stream accreting
macroforms)
Foresets are large-scale planar cross-beds.
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
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.
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
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.
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
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little