Braided Fluvial Systems

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Photo by W. W. Littl 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-

description

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

Transcript of Braided Fluvial Systems

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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.

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• 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

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Simplified Model

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

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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.

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Model Variability

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

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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.

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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.

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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.

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Structureless (massive) Gravel

Structureless gravel represents rapid deposition as bars within large channels.

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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.

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Imbricated pebbles

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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..

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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.

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Planar Cross-laminated Sand/Gravel

Planar cross-bedded sand and gravel represent down-stream deposition off the distal end of intrachannel bars.

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Channel Elements

Channel elements (macroforms/facies associations) include side and intrachannel bars, as well as many scales of channel fills.

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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

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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.

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Foreset Bedding(down-stream accreting

macroforms)

Foresets are large-scale planar cross-beds.

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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

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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.

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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.

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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

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