Fluvial

Fluvial & Alluvial Fan System
Lecture #5

Introduction
Why studying fluvial system?
Essential to the existence of clastic sedimentary basins
 Determine transportation system of sediment from
land to coast

• 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

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

Hypothetical Model
Miall, 1992

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

• 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

• 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

• 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

• 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

• 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)

• 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

• 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

• 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

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

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)

Meandering River Facies
1. General Characteristics
- High sinuosity
- Composed mostly of sand and mud
- Confined to a single channel

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

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

Anastomosed Rivers
Not extensively studied
▫ Interconnected narrow channels
▫ Very table bank
▫ Very low gradient
▫ Very high mud/sand ratio
▫ Sediment mostly from vertical aggradation

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

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)

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.

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

Summary of Fluvial Deposits
Allen, 1996

Fluvial

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