meandering fluvial systems
DESCRIPTION
Geol 370: Sedimentology and Stratigraphy Topic 10b: Meandering Fluvial SystemsTRANSCRIPT
Photo by W. W. Little
Meandering Fluvial System(single channel/mud- or
sand-dominated)
Meandering fluvial systems have a single, relatively deep major channel with a sinuosity of at least 1.7 (often greater than 2.5), a low bed/suspended load ratio, and cohesive bank material. Meandering river deposits can be dominated by either sand or by mud and are characteristic of high-discharge perennial rivers flowing over low gradients.
Photo by W. W. Little
• Low gradient relative to discharge• High discharge relative to load• Low bed load relative to suspended load• Cohesive bank materials (commonly muddy & vegetated)• Perennial discharge with relatively low fluctuation
Conditions Favoring Meandering
Simplified Model
Meandering fluvial systems are typically subdivided into four major facies associations, active channel, abandoned channel, overbank, and splay.
Common Channel Facies
Meandering fluvial systems typically consist of a relatively thin, structureless gravel and trough to ripple cross-bedded sand within channels and laminated mud with thin beds of trough- and ripple-bedded sand on the flood plain. Climbing ripples and flaser bedding can also be present, particularly on point bar tops and levees.
Active vs. Abandoned Channel Fill
Upon avulsion, a channel becomes abandoned and, rather than continued migration of point bars, the channel becomes filled with fine-grained sediment during flooding events.
Photo by W. W. Little
Active Channel Lag Gravel
The coarsest sediment in a meandering stream is transported along the channel floor. This can be a true gravel or consist of mud intraclasts derived from cut-bank caving.
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Trough Cross-bedded Sand
Trough cross-bedded sand as sand sweeps as waves around the point bar. Decreasing velocity results in structures that thin upward.
Photo by W. W. Little
Photo by W. W. Little
Planar cross- and Horizontally-bedded Sand
Planar cross-bedded sand forms by deposition off a bar front; whereas, horizontally-bedded sand represents bar top sedimentation.
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Bioturbated Mud
Flood plain deposits commonly become bioturbated through burrowing and root growth.
Facies Associations
Meandering fluvial systems are dominated by four major facies associations related to active channel fill, abandoned channel fill, overbank deposition, and crevasse-splay development.
Common Channel Facies
Meandering fluvial systems typically consist of a relatively thin, structureless gravel and trough to ripple cross-bedded sand within channels and laminated mud with thin beds of trough- and ripple-bedded sand on the flood plain. Climbing ripples and flaser bedding can also be present, particularly on point bar tops and levees.
Channel Elements
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Point Bars
Point bars are channel elements found mostly on the inside of meander bends. They are formed primarily by lateral accretion, consist of a fining-upward succession of trough cross-stratified sand capped by ripple-bedded sand and overly channel floor gravel.
Photo by W. W. Little
Lateral Accretion Deposits
Lateral accretion occurs in point bars, mostly during flooding, as a channel migrates in association with cut bank erosion. Minor erosion or settling of fine sediment on the sloping surface of the point bar takes place during low flow stage, creating a series of surfaces that dip toward the channel floor.
Lateral Accretion & Facies Assoc.
Facies prograde laterally as channels migrate but maintain a similar vertical succession related to water depth.
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
Scroll Bars
Scroll bars are the surface expression of lateral accretion deposits.
Side-channel Bars
Mid-channel Bars
Photo by W. W. Little
Photo by W. W. Little
Abandoned Channel Fill (channel plugs)
Avulsion leads to abandonment of a channel. Once abandoned, the channel becomes filled with fine-grained sediment as a clay “plug.”
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Chute Channels
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Photo by W. W. Little
Photo by W. W. Little
Floodplain
Levee
Point Bar
Levee
Point Bar
Bar-Top Facies
Point bars are typically capped by fine-grained levee and floodplain deposits. Levees are often dominated by climbing ripples and flaser bedding in fine sand to silt; whereas, overbank sediment consists of laminated silt and clay.
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Overbank Facies
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Paleosols
Following flooding, sediment on the floodplain is subjected to atmospheric weathering. The longer the period between flooding events, the thicker and better developed the soil; therefore, paleosols tend to become fewer in number but more highly developed away from the channel.
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Crevasse-splay Complexes
Flooding can result in levee breaks (crevasses), leading to delta-like deposition on the flood plain (splays). Splays fan out from the crevasse and can consist of simple lenses of sand to complex multiple-channeled complexes, depending upon the degree and length of flooding.
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
Geomorphology
Meandering fluvial systems consist of single, sinuous, typically sand-dominated channels surrounded by broad, flat, mud-dominated floodplains.
Channel Belts
As meandering rivers migrate laterally across their floodplain, they produce a channel belt that is preserved as a sheet of sandstone encased within mudstone.
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Sand:mud ratiosSand body connectivitySand body geometry (lense vs. sheet)Accommodation space and Preservation potential
Transgressive deposits
Lowstand deposit
Early (slow) rise deposits
Moderate risedeposits
Rapid rise deposits
Highstand deposits
Coastal depositsTidally-influenced fluvial deposits
B
Architecture and Subsidence Rate
Low to high sand:mud ratiosLow to moderate sand body connectivitySand body geometry lens to discontinuous sheetModerate to high accommodation and preservation potential
Large-scale Architecture
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little