Braided Channel System edit
Braided rivers, or braided channels, are named for the "braided" appearance created by the splitting and joining of relatively thin channels. These multiple channels are formed by the inability of the water to carry high density of sediments at low flow, the river dumps this material creating islands and bars. The form and position of these islands and bars changes continuously while the channels that surround them migrate.[1] Sediments tend to be moderately coarse-grained and poorly sorted, and channels are typically located on fairly steep gradients. [2]
Lithology edit
The general lithology of a braided river system often includes moderately sorted fine-grained to coarse-grained sandstone with subangular to rounded grains, massive or laminated mudstones, and massive or stratified conglomerates. Conglomerates tend to be sandy and can be predominantly quartz but often contain feldspars or granite fragments. These lithologies can be used in identifying separate sections of the braided system. Mudstones occur during periods of flooding or lower velocity flows, while larger grains require quicker flows.
Mudstones can vary from red to black and organic rich. Red mudstones are often associated with floodplains and can have paleosol development, while black mudstones are often the result of anoxic, lacustrine deposits able to preserve organics.
Sandstones vary from very fine-grained to coarse-grained and tend to be concentrated in channels. [3]
Location of Feature edit
Braided systems tend to form near mountainous regions, especially if glaciation is prevalent. Areas of steep gradient with little vegetation and highly variable discharge allow for easy amalgamation of channels, forming the braided system. The steep gradient and subsequent high flow velocity allows for coarse-grained sediment to be transported. Many bars and bedforms are formed, and channels have low sinuosity.[4]
Changes in the accommodation space or sediment supply can cause a change in lithology, or grain size. This can be viewed as a ratio of Accommodation to Sediment Supply (A/S). The larger the number, 1 or greater, the more likely a system will produce overbank deposits. As the ratio gets closer to 0 channel features, such as Multistory channel sands, will be more prevalent. [3]
Facies Associations edit
A set of facies associations can be used to identify a particular sedimentary environment. The combination of specific associations such as multistory and single-story fluvial channel sands, crevasse splays, floodplains, and lacustine environments can be used to identify a braided river system.[3]
Multistory channel sands edit
These are typically composed of fine to coarse grained sands and can have sedimentary structures such as planar and trough cross-bedding, horizontal lamination, sigmoidal cross-bedding, low-angle cross-bedding, and ripple cross-lamination. Stacked sets of unidirectional cross strata are indicative of fluvial channels. Coarser grained deposits indicate a quicker flow velocity associated with steep gradients. Lateral and vertical accumulation of these elements suggests several bars and channels. [3]
Single-story channel sands edit
Deposition is composed of single, lenticular sand bodies. Floodplain and crevasse splay deposits encase these sands in fine-grained sediments. These are typically fine to coarse grained and form trough cross-bedding and ripple cross-lamination. Unlike multistory channels, these lack lateral accretions. The lack of accretions indicates low sinuosity and a laterally stable channel and are associated with floodplains. [3]
Crevasse splays edit
Tend to have very fine to fine sandstones interbedded with reddish to gray mudstones due to shallow flows and loss of competence and flow leaves the channel. Mud cracks indicate periods of non-deposition between flooding events. [3]
Floodplains edit
The deposition of massive red to gray mudstones containing root traces and formation of paleosols suggest they are well drained and went through periods on non-deposition and oxidation. Periodic waterlogging and reduction of organic material can lead to mottling. Laminated gray mudstones indicate overbank deposition in these conditions.[3]
Lacustrine edit
Deposition occurs in a quiet water environment and results in massive or laminated gray mudstones and organic black shales. Lack of evaporites and roots indicates a constant supply of fresh water and little to no subaerial exposure. Most deposition comes from the settling of mud from suspension, but occasional hyperpycnal flows can result in minor sand beds.[3]
Examples[4] edit
· Son-Kul River, Tien Shan Mtns., Kyrgyzstan
· Kicking Horse River, Yoho National Park, British Columbia
· Resurrection River, Kenai Peninsula, Alaska
· At-Bashi River, Tien Shan Mtns., Kyrgyzstan
· Brahamaputra River
References edit
- ^ Miall, A.D. (1977). "A review of the braided-river depositional environment". Earth-Sci. Rev. 13: 1–62.
- ^ "Braided Rivers". pages.uoregon.edu. Retrieved 2019-02-19.
{{cite web}}: Cite has empty unknown parameter:|dead-url=(help) - ^ a b c d e f g h Scherer, Claiton M.S.; Goldberg; Bardola (2015). "Facies architecture and sequence stratigraphy of an early post-rift fluvial
succession, Aptian Barbalha Formation, Araripe Basin,
northeastern Brazil". Sedimentary Geology. 322: 43–62.
{{cite journal}}: line feed character in|title=at position 76 (help) - ^ a b "Braided Rivers".
{{cite web}}: Cite has empty unknown parameter:|dead-url=(help)