05. Fluvial.pdf

Rivers/Fluvial

Sedimentologi & Prinsip Stratigrafi

Overview • Fluvial systems • River forms • Flow in channel • Architectural Elements: Channels and overbanks • Fluvial depositional architectural records

Geomorphological zones ➢ the erosional zone ➢ streams are actively downcutting ➢ removing bedrock from the valley floor and from the valley sides ➢ downslope movement of material into the stream bed. ➢ the transfer zone ➢ gradient is lower, ➢ streams and rivers are not actively eroding, ➢ but nor is this a site of deposition. ➢ the depositional zone ➢ sediment is deposited in the river channels and ➢ on the floodplains of a fluvial system or ➢ on the surface of an alluvial fan.

Nichols, 2009

Geomorphological vs depositional zones

Nichols, 2009

Allen & Heller, 2012

Fluvial System • Catchment area or the drainage basin; the area of ground that supplies water to a river system .

• Rivers and streams are mainly fed by ➢ surface run-off and groundwater from subsurface aquifers in the catchment area , following periods of rain. ➢ Soils act as a sponge soaking up moisture and gradually releasing it out into the streams. • Two factors controlling the supply of water to a river system ➢ size of the catchment area ➢ climate

River forms Rivers can have a variety of forms: ➢how straight or sinuous the channel ➢presence or absence of depositonal bars of sand or gravel within the channel ➢number of separate channels that are present in a stretch of the river.

Nichols, 2009

River forms General river types can be recognised based on their channel geometry (Miall 1978; Cant 1982): • A straight channel without bars is the simplest form but is relatively uncommon • A braided river contains mid-channel bars that are covered at bank-full flow • Anastomosing or anabranching river, consists of multiple, interconnected channels that are separated by areas of floodplain (Makaske 2001) • High sinuous rivers that have depositional bars only on the insides of bends are called meandering.

Flow in Channel ➢ most of the time the flow is concentrated within channels.

➢ When the water level is below the level of the channel banks it is at low flow stage. ➢ A river with water flowing close to or at the level of the bank is at high flow stage or bank-full flow. ➢ When the volume of water exceeds the volume that can be contained within the channel, the river flood ➢ overbank flow occurs on the floodplain adjacent to the channel

Flow in Channel As water flows in a channel it is slowed down by friction with ➢ the floor of the channel ➢ the banks and ➢ the air above These frictional effects decrease ➢ away from the edges of the flow ➢ the deepest part of the channel the highest velocity flow. Nichols, 2009

A sandy river channel and adjacent overbank area: the river is The line of the deepest part of the channel is called the at low-flow stage exposing areas of sand deposited in the thalweg. channel. The existence of the thalweg and its position in a channel ➢ important to the scouring of the banks ➢ the sites of deposition in all channels. Nichols, 2009

Thalweg The position of the rivers fastest flowing current (least friction, greater energy, greater erosion)

Flow separation

River forms In ancient fluvial sediments, grain size is the feature most easily diagnosed. It may be expressed in two ways: 1. the grain size of the bedload sediment forming the bedforms and bars of the channel floor will normally fall within the range pebble gravel to fine sand although both coarser and finer examples may be found; and

2. the ratio between bedload and suspended load. after Orton & Reading, 1993

River forms

River Architectural Elements

River Architectural Elements

Nichols, 2009

Mid-channel gravel bars in braided rivers

Nichols, 2009

Point bars in meandering rivers

Nichols, 2009

Mid-channel sand bars in braided rivers

Nichols, 2009

Point bars in meandering rivers

Nichols, 2009

Mid-channel bars in braided rivers

McGeary, 1996

Coarse grained bedload-Braided River Braided river characteristics:

➢ Channel divides around islands (bars); ➢ Tend to form on higher slopes in coarser material ➢ Dominant load - coarser grain sizes (gravels – sands). ➢ Bank – lack cohesive and leads mobile channel courses ➢ Channels – Continually shift and bifurcate, produce rapid network braid channel

Combination of high bedload, low sinuosity rivers ➢ stratigraphic records dominated lenticular, concave upward ➢ sand bodies with cross stratification, ➢ lateral accretion deposits ➢ lack of channel margin

Formation of Braided river • Usually found in areas of high sediment influx, high water influx, and high gradient • Examples include glacial discharge plains, distal portions of alluvial fans, and mountainous regions. • During times of flooding the river is choked with a high amount of sediment and a new channel is formed • Mid channel bars is a result from sediment that too large to be carried

Braided river deposits • Contain gravel in lower portions of bars and channels and sand throughout the rest of the deposit • mud is almost nonexistent • Longitudinal bars present with some having horizons of plant roots • Linguoid and transverse bars also present • Lateral bars possible, also with possible plants roots • Lateral extent of a braided river deposit can be extensive because lateral migration of the system and the high sedimentation rate

Braided River depositional profiles ➢ Braided rivers have a variety of bar types point bars, alternate bars (may be attached to one bank or mid-channel bars) ➢ Alternate bars form in straight channels; they migrate downstream ➢ Depending on channel width:depth ratio, alternate bar patterns may evolve into braid bars

Braided River depositional models Gravelly braided fluvial system Flow

Irregular fining-up poorly developed

Broad Continuous belt

Downstream accretion (DA)

Sandy braided fluvial system Courtesy of Dardji, 2017

Depositional architecture of a braided river

Nichols, 2009

Depositional architecture of a braided river Deposition on Braid Bars • Bars migrate downstream, but also grow laterally • Will produce lateral accretion surfaces that can be seen in views perpendicular to flow • Downstream-accreting bars will produce large foresets; may be internally large-scale X-stratified

Depositional architecture of a braided bar Deposition on Braid Bars • Bars migrate downstream, but also grow laterally • Will produce lateral accretion surfaces that can be seen in views perpendicular to flow • Downstream-accreting bars will produce large foresets; may be internally large-scale X-stratified

Braided River Depositional Profiles

Model of Braided River Sediments (Miall’s 1977) Scott type shows noncyclic congl., Donjek type congl. and Ss.rough-cyclic, trough X

Platte type made up of Ss. Stacked plannar X bed Bijou Creek type composed superimposed horizontral bedded sheetflood deposits.

• Gravelly braided river

• Sandy braided river • the discharge shows only a moderate variation through the year called perennial fluvial systems.

Facies & Elements

Facies & Architectural Elements

Ramos & Sopena, 1983

Meandering River Characteristics ➢ High sinuosity drainage areas dominated by Low-gradient slopes.

➢ Commonly dominated by a high suspended load ➢ Channels lie on broad meander belt. Complex distribution (active and abandoned) channels ➢ Typically organized into channel and overbank segments

➢ Active deposition is in channel belt resulting alluvial ridge higher than flood plain ➢ Distal margin alluvial ridge form overbank that interfinger with flood plain ➢ Point bars and associated lateral accretion may be constrained by abandoned channel plugs or by lateral migrated channel. ➢ Sediment load – range from fine grained suspended load system to coarse sand gravel pebble rich system

Meandering River

Formation of Meandering River • 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

Meandering River Architectural Elements

Meandering deposits consist ➢ channel deposits and ➢ floodplain (overbank) deposits

Meandering River Architectural Elements

Nichols, 2009

Flood plain deposits Frequent capping the Point Bar

Courtesy of Dardji, 2017

Meandering River depositional models

Meandering river deposits • 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 succession 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

Sedimentation processes in Meander Channel ➢ Characteristics of these deposits is related to flow pattern that develops in river bends. ➢ Flow is pushed toward outer bank of meander bend, Helical flow develops in meander ➢ Flow near bed is directed toward inner bank

➢ Erosion & Deposition in River Meanders ➢ Faster flow and strong bed shear stresses along outer bank causes intense erosion; only coarsest sediment remains on bed as a lag deposit ➢ Slower flow and decreasing bed shear stress toward inner bank results in deposition of a point bar

Deposition of Point Bar in Meandering Channel • Point bar slopes gently toward thalweg: • coarse lag deposit (all but coarsest grains are eroded away) • Most of point bar is covered with dunes: get large-scale cross-stratified sand. • Top of point bar commonly rippled: get small-scale cross-stratified finer sand • Tops of point bar deposits dry out during low flows, become vegetated, and eventually becoming part of the floodplain as channel migrates laterally. • Tops of point bar deposits commonly show desiccation cracks and bioturbation by roots & burrowing organisms.

Deposition of Point Bar in Meandering Channel • Point bar slopes gently toward thalweg: • coarse lag deposit (all but coarsest grains are eroded away) • Most of point bar is covered with dunes: get large-scale cross-stratified sand. • Top of point bar commonly rippled: get small-scale cross-stratified finer sand • Tops of point bar deposits dry out during low flows, become vegetated, and eventually becoming part of the floodplain as channel migrates laterally.

• Tops of point bar deposits commonly show desiccation cracks and bioturbation by roots & burrowing organisms.

Deposition of Point Bar

Deposition of Point Bar in Meandering Channel

Meandering River Characters Classic fining upward sequence deposited by lateral migration of meandering river. ➢ Intraclast conglomerate at the base, Trough cross bed, ➢ parallel laminated sands ➢ rippled sands ➢ overbank muds at the top (Allen 1970)

River’s Channel

Log Patterns (Meandering River Deposits)

Nichols, 2009

Nichols, 2009

Overbank (Flood-plain) deposits • Overbank deposition occurs during floods • Floodplain is vegetated; soils develop; sediments become bioturbated • Overbank deposits are finer-grained than channel deposits; • Levee Deposits • Overbank deposition may occur all along the banks of the channel during a flood. • Close to channel, coarser sediment is deposited; may see ripples or dunes • Flow strength decreases through time; levee deposits will show fining in grainsize vertically • Farther from channel, sediments are fine-grained, laminated

Overbank deposits

http://www.seddepseq.co.uk

Burns et al., 2017

Polanco-Boulware & Rice, 2017

Gulliford et al., 2017

Dark organic rich overbank deposits

Facies & Architectural Elements

Anastomosed River System

Anastomosed River Characteristics ➢ Interconnected network low gradient

➢ Relatively deep and narrow channels, variable sinuosity ➢ Very stable and vegetated bank ➢ Fine Siltstone – Clay.

➢ Very high mud/ sand ratio ➢ Sediment mostly from vertical aggradations ➢ Lateral migration limited,

➢ Changes channels occur through avulsion Facies model block diagram for anastomosed river (Smith and Smith, 1980)

➢ Flooding events create progressing breaching, creavassing can lead to form new channel course

➢ Overbank and flood plain consist of narrow natural levees crevassing, vegetated island wet lands.

Anastomosed River Model

Anastomosed River Model

Formation of Anastomosed river • Anastomosing rivers are usually formed by avulsions, i.e., flow diversions that cause the formation of new channels on the floodplain. • As a product of avulsion, anastomosing rivers essentially form in two ways: (1) by formation of bypasses, while bypassed older channel-belt segments remain active for some period; and

(2) by splitting of the diverted avulsive flow, leading to contemporaneous scour of multiple channels on the floodplain. (Makaske, 2001)

Avulsion

after Stouthamer 2001

Avulsion

Avulsion

Soil Forming Processes (Pedogenesis) Soil - a layer of weathered, unconsolidated material on top of bedrock Common soil constituents: clay minerals, quartz, water, organic matter

Soils can be classified according to (Mack et al.1993): •

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the degree of alteration (weathering) of the parent material; . the precipitation of soluble minerals such as calcite and gypsum; . oxidising/reducing conditions (redox conditions), particularly with respect to iron minerals; . the development of layering (horizonation); . the redistribution of clays, iron and organic material into these different layers (illuviation); . the amount of organic matter that is preserved.

Nichols, 2009

Fossil Soil (Paleosol)

Fossil Soil (Paleosol) • Gelisols indicate a cold climate • aridisols are characteristic of arid conditions • oxisols form most commonly under humid, tropical conditions • vertisols form in subhumid to semi-arid climates with pronounced seasonality. • Particular hydrological conditions are required for some soils, such as the waterlogged setting that histosols (peaty soils) formmin. • entisols are very immature • inceptisols show more development, but are less mature than the other types lower in the list • spodosols, alfisols and ultisols are soils formed in forests, whereas • mollisols are grassland soils. • andisols is restricted to volcanic substrates. Nichols, 2009

Fossil Soil (Paleosol) • The precipitation of calcium carbonate within the soil is a feature of some aridisols (semiarid to arid climates) > ‘calcrete soils’ form by the movement of water through the soil precipitating calcium carbonate as root encrustations (rhizocretions) and as small soil nodules (glaebules) (Wright & Tucker 1991). • The nodules grow and coalesce as precipitation continues to form a fully developed calcrete, with tepee structures, i.e. domes in the layer formed by the expansion of the calcium carbonate as it is precipitated (Allen 1974).

Fossil Soil (Paleosol)

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