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Fluvial

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Fluvial

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Fluvial

  1. 1. Fluvial & Alluvial Fan System Lecture #5
  2. 2. Introduction Why studying fluvial system? Essential to the existence of clastic sedimentary basins  Determine transportation system of sediment from land to coast
  3. 3. • Organized into distinct drainage basin Basin (lake/pond)  water & sediment • Fluvial current correlative to regional topographic slope • Episodic/seasonal flood can reflected in sedimentary structure Alluvial Flood vs. Influx
  4. 4. Fluvial: River & Floodplain
  5. 5. Morphology of River & Floodplain River/Channels  Narrow and funnel  Max thalweg depth 5-20 m  Width: 100 m to 2-3 km  Frequently flanked by levees Floodplain  Adjacent to the channels  Crevasse splay deposits
  6. 6. Hypothetical Model Miall, 1992
  7. 7. Fluvial environments • Bars are sandy or gravelly macroforms in channels that are emergent, mostly unvegetated features at low flow stage, and undergo submergence and rapid modification during high discharge • Point bars form on inner banks and typically accrete laterally, commonly resulting in lateral-accretion surfaces; mid-channel or braid bars accrete both laterally and downstream
  8. 8. • Bars are always associated with channels; a genetically related bar/bar complex and channel/channel complex is known as a storey • Lateral accretion involves higher-order bounding surfaces dipping perpendicular to paleoflow direction and associated lower-order bounding surfaces; in the case of downstream accretion higher-order bounding surfaces dip parallel to paleoflow direction
  9. 9. Fluvial environments • Braided rivers are characterized by a dominance of braid bars exhibiting both lateral and downstream accretion; meandering rivers primarily contain point bars with lateral accretion; in straight (and most anastomosing) rivers bars are commonly almost absent
  10. 10. Fluvial environments • Facies successions in sandy to gravelly channel deposits typically fine upward, from a coarse channel lag, through large-scale to small-scale cross stratified sets (commonly with decreasing set height), and finally overlain by muddy overbank deposits • The geometry and three-dimensional arrangement of architectural elements therefore provides a much better means of inferring fluvial styles from the sedimentary record
  11. 11. Fluvial environments • Channel belts consist of channel-bar and channel-fill deposits; the proportion of the two generally decreases markedly from braided rivers to anastomosing rivers • The geometry of a channel belt (width/thickness ratio) is a function of the channel width and the degree of lateral migration; values are typically much higher for braided systems (>>100) than for straight or anastomosing systems (<25) Sheets have width/thickness ratios of >50 Ribbons have width/thickness ratios of <15 • Residual-channel deposits are predominantly muddy (occasionally organic) deposits that accumulate in an abandoned channel where flow velocities are extremely small
  12. 12. Fluvial environments • Overbank environments are dominated by fine-grained facies (predominantly muds) • Natural-levee deposits are wedges (‘wings’) of sediment that form adjacent to the channel, dominated by fine sand and silt exhibiting planar stratification or (climbing) ripple cross stratification • Crevasse-splay deposits are usually cones of sandy to silty facies with both coarsening-upward and fining-upward successions, and are formed by small, secondary channels during peak flow • Flood-basin deposits are the most distal facies, consisting entirely of muddy sediments deposited from suspension, and are volumetrically very important (mainly in low-energy fluvial settings)
  13. 13. Fluvial environments • Paleosols (well drained conditions) and occasional peats (poorly drained conditions) occur frequently in overbank environments and are important indicators of variations of clastic aggradation rates and the position relative to active channels (proximal vs. distal) • The pedofacies concept refers to the maturity of a paleosol, irrespective of the specific set of pedogenic processes operating, in the case of floodplains mainly controlled by distance to the active channel • Lacustrine deposits can be important in overbank environments characterized by high water tables, and are also found in distal settings; they are more likely to contain primary sedimentary structures (horizontal lamination) than their frequently bioturbated subaerial counterparts
  14. 14. Fluvial environments • Avulsion is the sudden diversion of a channel to a new location on the floodplain, leading to the abandonment of a channel belt and the initiation of a new one • Avulsions are the inevitable consequence of the increase of cross-valley slope (typically through a crevasse channel) relative to down-valley slope along the channel, associated with the growth of an alluvial ridge • An avulsion belt constitutes an extensive network of rapidly aggrading, narrow, crevasse-like channels with genetically associated overbank deposits, that may surround the new channel belt
  15. 15. Fluvial environments • Alluvial architecture refers to the three-dimensional arrangement of channel-belt deposits and overbank deposits in a fluvial succession • The nature of alluvial architecture (e.g., the proportion of channel-belt to overbank deposits) is dependent on fluvial style, aggradation rate, and the frequency of avulsion • When alluvial architecture is dominated by channel-belt deposits, the separation of channel belts from storeys can be extremely difficult
  16. 16. Channel Patterns
  17. 17. Braided Rivers • Morphology & Sediment Patterns ▫ High gradients ▫ Bed-load dominant rivers ▫ Tend to migrate laterally • Facies & Properties ▫ Extremely variable  Conglomerates, sand, mudstone  Upward fining over erosion surface ▫ Thick and laterally extensive ▫ No regular internal pattern
  18. 18. Meandering Rivers • Morphology & Sediment Patterns ▫ Low gradients ▫ Finer grained and better sorting ▫ Levee & crevasse splay • Facies & Properties ▫ Ribbon-like deposit  Upward fining over sharp-based boundary ▫ Lack of numerous internal erosion surfaces ▫ Internal pattern:  Laterally downlapping beds (lateral accretion)
  19. 19. Meandering River Facies 1. General Characteristics - High sinuosity - Composed mostly of sand and mud - Confined to a single channel
  20. 20. 2. Formation - The transition between a braided river system and a meandering river system is a difficult one to draw a line through. - Further down stream the river has a much less sediment influx, and therefore, does not form bars as a result of sediment choking, but starts to deposit the smaller sediment in its system and also erodes the surrounding banks - The cut bank will erode the outside bank and cause the river to expand laterally while the point bar will deposit sediment from the system and accrete the river laterally with sandy silt deposits and sometimes mud - A meander will sometimes meet another and then form a faster way down stream so the abandoned channel will become an ox-bow lake - During flooding stages the river will spill over its banks and deposit on the levee and also on the flood plain depositing silts and muds
  21. 21. Structure of a Meandering river deposit - The majority of the deposit will consist of the accretion of the point bar - Some abandoned channels and deep channels will be preserved as dish shaped structures, up to hundreds of meters wide, in the outcrop - The point bar will have a fining upward sequence starting from a channel deposit rising through trough cross bedding and sand stone lenses up to ripples and finally a flood plain deposit - The flood plain forms by the deposition of fine material from the river during flood stages - Deposits are usually laminated and may be oxidized - Paleosols may also be present on floodplain, levee, and point bar (though much less common here) - A crevasse-splay deposit will consist of a sheet flow with some cross bedding towards the upper section with rip up clasts present in the bottom of the section
  22. 22. Anastomosed Rivers Not extensively studied ▫ Interconnected narrow channels ▫ Very table bank ▫ Very low gradient ▫ Very high mud/sand ratio ▫ Sediment mostly from vertical aggradation
  23. 23. Morphology Braided Meandering Anastomosing Channel-belt width/thickness ratio High Intermediate Low Channel-deposit proportion High Intermediate Low Overbank-deposit proportion Low Intermediate High Overbank-deposit geometry Wedge-shape Highly irregular due to numerous crevasse channel Overbank facies Well- drain paleosols common Peats and lacustrine deposits common
  24. 24. Alluvial Fans • Consist of fan-shaped fluvial distributive system ▫ Accumulated adjacent to areas of high relief  Tectonically active; Along fault scrap ▫ Mostly coarse-grained to conglomerate ▫ Distinctive deposits:  Debris flow, braided stream, sheet flood  Generally upward coarsening and thickening ▫ Reservoir quality  Ranging from excellent (distal) to poor/ immature (proximal)
  25. 25. Alluvial Fan Facies Alluvial fans can basically be described as fan shaped deposits that are fed by a channel emanating from a very narrow, steep valley in a region of high topographic relief. This high relief is often due to the fact that most alluvial fans are found in association with regions of active faulting.
  26. 26. Fan Types - Basic sedimentological aspects. Gravity-flow Fans - Found in semi-arid settings and are also known as dry fans •Small to moderate in size •Develop along structurally active basin margins •Dominant Processes: Rockfalls, Rock Avalanches, Gravity Slides, Debris Flows •Deposits are normally random and sporadic. Fluvial Fans - Found in humid settings and are also known as wet fans •Large fans •Permanent or ephemeral channelized stream •Deposits progress from coarse to finer sediments distally, which are dominated meandering channels and overbank deposits. •Terminal Fans - A unique class designated because of downfan water loss •Water discharge decreases downfan
  27. 27. Summary of Fluvial Deposits Allen, 1996

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