Wsambrook Smith Etal 06

7/29/2019 Wsambrook Smith Etal 06

1/22

The sedimentology and alluvial architecture of the sandybraided South Saskatchewan River, Canada

G . H . SA M BRO O K SM I TH *, P. J . A SH W O RTH, J . L. BEST , J . WOODWARD and

C. J . SI M PSO N*School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston,Birmingham B15 2TT, UK (E-mail: [email protected])Division of Geography, School of Environment, University of Brighton, Lewes Road, Sussex BN2 4GJ, UKEarth and Biosphere Institute, School of Earth and Environment, University of Leeds, Woodhouse Lane,Leeds, West Yorkshire LS2 9JT, UKDivision of Geography, School of Applied Science, Northumbria University, Ellison Building, Newcastleupon Tyne NE1 8ST, UKDepartment of Geography, Simon Fraser University, 8888 University Drive, Burnaby, BC, CanadaV5A 1S6

ABS TR ACT

Ground penetrating radar (GPR) surveys of unit and compound braid bars inthe sandy South Saskatchewan River, Canada, are used to test the influentialfacies model for sandy braided alluvium presented by Cant & Walker (1978).Four main radar facies are identified: (1) high-angle (up to angle-of-repose)inclined reflections, interpreted as having formed at the margins of migratingbars; (2) discontinuous undular and/or trough-shaped reflections, interpretedas cross-strata associated with the migration of sinuous-crested dunes; (3) low-angle (< 6) reflections, interpreted as formed by low-amplitude dunes or unitbars as they migrate onto bar surfaces; and (4) reflections of variable dipbounded by a concave reflection, interpreted as being formed by the filling ofchannel scours, cross-bar channels or depressions on the bar surface. The

predominant vertical arrangement of facies is discontinuous trough-shapedreflections at the channel base overlain by discontinuous undular reflections,overlain by low-angle reflections that dominate the deposits near the barsurface. High-angle inclined reflections are only found near the surface of unitbars, and are of relatively small-scale (< 05 m), but can be found at a greaterrange of depths within compound bars. The GPR data show that a high spatialvariability exists in the distribution of facies between different compound bars,with facies variability within a single bar being as pronounced as that betweenbars. Compound bars evolve as an amalgamation of unit bars and othercompound bars, and comprise a facies distribution that is representative of themain bar types in the South Saskatchewan River. The GPR data are comparedwith the original model of Cant & Walker (1978) and reveal a much greater

variability in the scale, proportion and distribution of facies than thatpresented by Cant & Walker (1978). Most notably, high-angle inclined strataare over-represented in the model of Cant and Walker, with many bars beingdominated by the deposits of low- and high-amplitude dunes. It is suggestedthat further GPR studies from a range of braided river types are required toproperly quantify the full range of deposits. Only by moving away fromtraditional, highly generalized facies models can a greater understanding ofbraided river deposits and their controls be established.

Keywords Alluvial facies, ground penetrating radar, sandy braided river,sedimentary architecture, South Saskatchewan River.

Sedimentology (2006) 53, 413434 doi: 10.1111/j.1365-3091.2005.00769.x

2006 The Authors. Journal compilation 2006 International Association of Sedimentologists 413


2/22

INTRODUCTION

Many authors have proposed facies models forsandy braided rivers from study of both contem-porary and ancient rivers (Collinson, 1970; Smith,1971; Cant & Walker, 1978; Miall, 1978; Blodgett& Stanley, 1980; Ethridge & Flores, 1981; Allen,

1983; Crowley, 1983; Bridge et al., 1986, 1998;Bristow, 1987, 1993; Bridge, 1993; Ashworthet al., 2000; Bridge & Tye, 2000; Best et al.,2003; Skelly et al., 2003), with one of the mostinfluential facies models being that of Cant &Walker (1978). This model (derived from thesame reach as is reported in this paper) was basedon repeat aerial photographs, surface mapping,box coring, description of cutbanks and shallowtrenches, echo sounding and measurements ofbedforms within the channels of the SouthSaskatchewan River, Canada, together with ex-

amination of the Devonian Battery Point Forma-

tion (see also Cant, 1976, 1978; Cant & Walker,1976). Cant & Walker (1978) proposed that theSouth Saskatchewan channel-belt could be char-acterized by three different facies profiles (Fig. 1):sand flat (profile A), channel (profile C) and acombination of the two termed mixed influence(profile B). Sand flats are large expanses of sand,

that may be 50 m to 2 km in length when exposedat low flow stages, occupy up to 80% of thechannel-belt width and typically have minorchannels crossing them (Fig. 1). Cant & Walker(1978) envisaged that such sand flats originatefrom deposition on the tops of cross-channelbars. These were judged to form by accretion atthe downstream margin of slipface-bounded bars,which are usually found in zones of flow expan-sion (Fig. 1), are commonly the width of theprimary channel and may be up to hundreds ofmetres long. The key differences between facies

profiles A and C proposed by Cant & Walker

ABC

Fig. 1. Three-dimensional block diagram and vertical facies profiles redrawn from Cant & Walker (1978), showingtheir interpretation of the key morphological and sedimentological features of the sandy braided South SaskatchewanRiver. Note that no scale was included on the 3-D diagram in the original paper of Cant & Walker (1978).

414 G. H. Sambrook Smith et al.

2006 The Authors. Journal compilation 2006 International Association of Sedimentologists, Sedimentology, 53, 413434


3/22

(1978) are the abundance of deposition attributedto bars (represented as large-scale planar cross-stratification) and dunes (represented as medium-scale, trough cross-stratification; see Fig. 1).

The model of Cant & Walker (1978) for deposi-tion in the South Saskatchewan River has beenhighly influential and adopted by a number of

authors as a suitable analogue for ancient sandybraided river deposits (Miall, 1978; Allen, 1983;Miall & Jones, 2003). However, although themodel of Cant & Walker (1978) provides detaildown to 5 m below the bar surface, most of thedata used in its derivation were collected withinonly the top 05 m, with no detailed quantifica-tion of the deeper sub-surface. Data between thisshallow depth and down to the thalweg werepredominantly based on extrapolation of obser-vations of surface processes, sections taken inshallow trenches, box-cores and examination of

cutbank exposures. Additionally, it is also un-clear whether the facies models proposed by Cant& Walker (1978) can be applied to all types of barmorphology (e.g. symmetrical or asymmetrical,attached or isolated), or in a reach with bars thatmay have very different depositional histories(e.g. unit or compound bars). A critical re-examination of the model of Cant & Walker isthus long overdue.

It is now possible to provide much higherresolution records of sub-surface alluvial strati-graphy in sandbed rivers by using groundpenetrating radar (GPR) (Bridge et al., 1998;

Bristow et al., 1999; Ferguson & Brierley, 1999;Fielding et al., 1999; Lehmann & Green, 1999;Vandenberghe & van Overmeeren, 1999; vanDam & Schlager, 2000; Best et al., 2003; Skellyet al., 2003; Woodward et al., 2003; Neal, 2004).The present paper reports on the use of GPR todescribe and quantify the deposits of severalbars within the South Saskatchewan River. Aer-ial photographs taken before and after the GPRsurvey are used to identify surface morphologi-cal changes that are then related to the sub-surface sedimentary architecture, thus allowing

linkage of channel change with depositionalstructure. Specific objectives of this paper areto: (1) describe the principal morphologicalfeatures of a 10 km long reach of the SouthSaskatchewan River; (2) classify the dominantdepositional sedimentary facies and alluvialarchitecture for a range of bar types with differ-ent depositional histories; (3) investigate the linkbetween the depositional architecture and thechannel and bar dynamics; and (4) criticallyanalyse the Cant & Walker (1978) facies model

for the South Saskatchewan River with thesenew data.

STUDY SITE

The South Saskatchewan River originates in the

Rocky Mountains, Alberta, Canada, and flowseastwards into Lake Diefenbaker, the downstreamend of which is 25 km upstream of the studyreach at Outlook (Fig. 2). The river is incised byup to 30 m into Cretaceous shales and sandstonesand Quaternary deposits. At Outlook, the channelbelt is approximately 0 6 km wide, has an averagebed slope of 00003 (Cant, 1976) and a braidedplanform. The braiding index (the number ofactive main channels per cross-stream transect atlow-flow stage) within the 10 km long study reachis 22. Fifty-two grain-size samples taken from bar

surfaces and cutbanks in the study reach showthe D50 grain-size ranges from 022 to 044 mm,with a mean of 030 mm, identical to that reportedby Cant (1976). Clay is rarely found within thesediments (< 1% by weight). Gravel is also rarebut can be found in channel thalwegs, withcobbles and boulders near some cutbanks wherethe river has eroded into Quaternary sediments.Scrub grass vegetation, willow bushes and smalltrees stabilize bars within the channel and on thefloodplain. The main (or first-order) channels are25 m deep and 50150 m wide. Cant & Walker(1978) used echo-sounding to show that the

deepest channels are dominated by dunes thatmay be up to 15 m high during floods, but aremore commonly 0305 m high. Ripples are ubi-quitous in shallow areas and on bartops, whilstaeolian reworking of bartop surfaces commonlycreates both aeolian ripples and barchanoiddunes. Winter conditions result in bar surfacesand smaller channels being covered in ice. How-ever, flow continues beneath the ice cover and themajor channels remain ice-free. Cant (1976) notedthat the main impact of ice is to immobilize thecompound bars for the winter months.

In 1967, the South Saskatchewan River wasimpounded by the Gardiner Dam, creating LakeDiefenbaker (Fig. 2), which subsequently causedincision of 05 m up to 5 km downstream of thedam. A series of permanent benchmarks andcross-sections were established by EnvironmentCanada downstream from the dam and have beensurveyed pre-dam (1964) and up to 16 times sincethen. The town of Outlook is located betweenbenchmarks 174 and 210, which are 280 and338 km downstream of the dam respectively.

Sedimentology of the South Saskatchewan River 415



4/22




5/22

Galay et al. (1985) reported that bed degradationwithin 80 km of the dam had slowed signifi-cantly by 1979, and that by this time the river hadlargely adjusted to the introduction of the dam.This conclusion is corroborated by Helfrick(1993) who reported that degradation was onlysignificant within 50 km of the dam and that

beyond this distance any changes in minimumand mean bed elevation could be accounted for bymigrating sand bars. The most recent resurveys inthe summer of 2002 by Phillips (2003) concludethat the study reach reported herein has notexperienced any statistically significant change inmean bed elevation since building of the dam(Fig. 3). The Gardiner dam has reduced some ofthe very largest flood events, with mean annualpeak discharge pre- and post-dam being 1536 and595 m3 sec)1 respectively. The mean annual dis-charge pre- and post-dam is 280 and 203 m3 sec)1

respectively, and most bars become overtopped at230 m3 sec)1. Thus, although the very highestdischarges do not occur anymore, the flow regimestill causes bedload transport over the entirebraidplain during floods and many of the chan-nels are active for large parts of the year. Thus, themore moderate and frequently occurring flowevents continue to shape the channel, and ensurethe South Saskatchewan River near Outlookremains an active braided system and that theresults reported herein can thus be applied toother braided rivers.

DATA COLLECTION

Ground penetrating radar

Approximately 35 km of GPR profiles were col-lected from recently deposited unit and com-pound bars (Fig. 2; see definitions of bars later)in June 2000, and Table 1 provides a summary ofthe GPR methodology employed (a detailed des-cription is given in Woodward et al., 2003). Datawere collected with common offset (CO) surveys

using a PulseEKKO 100 radar system (Sensors &Software Inc., Mississauga, Canada). Antennae(200 MHz) were fixed 075 m apart on a purpose-

built plastic sledge and moved perpendicular tothe profile in step-mode with a step spacing of010 m and trace stack of 64. Twelve commonmidpoint (CMP) surveys allowed calculation ofthe mean radar wave velocity as 0051 0006 m nsec)1. GPR profiles were processedusing Gradix 1.10 software (Interpex, Golden,

CO, USA) and included time-zero correction,dewow and bandpass filtering, backgroundremoval, application of gains, elevation staticsand depth conversion. Migration algorithms didnot optimize the radar signal and, therefore, werenot applied. The sedimentological interpretationof the radar facies is based on ground-truth controlfrom a series of GPR lines over an exposed bar cut-face (Fig. 4; Woodward et al., 2003) and shallowtrenching of profiles immediately after data col-lection (Fig. 5). The cut-face surveys showed thatreflections in the GPR profile are caused by both

clay drapes/layers, varying in thickness from< 00 2 t o 010 m, as well as changes in sandgrain-size, for example, from 030 to 023 mm.This subtle variability in grain-size is caused bygrain-size sorting on individual cross-strata, lowflow deposition, reworking by channel erosionand the presence of gravel lags. As an example,Fig. 5 shows inclined reflections in the GPRsurvey, up to the angle-of-repose, that are pro-duced by grain-size differences in cosets producedat a bar margin, which can be seen both inplanform on the surface and in a shallow trenchcross-section.

Other survey techniques

The topography of each bar and all GPR lineswas surveyed using a total station, and surveysof the GPR lines were used for topographiccorrection of the GPR profiles. To aid interpret-ation of the GPR surveys, the evolution of keymorphological features along the channel reachwas analysed using rectified 1:10,000 scale aerialphotographs flown along the river course before(April 2000) and after (October 2000) the main

GPR data collection period. Information onlonger-term evolution was obtained by compar-ing the aerial photographs from 2000 with

Fig. 2. Location map of the 10 km long study reach on the South Saskatchewan River near Outlook, Saskatchewan.This reach is the same as that studied by Cant & Walker (1978). Also shown is the location of all 200 MHz groundpenetrating radar profiles undertaken on the bars studied. All survey lines are given a three digit code; the first letterrefers to the specific bar (AE), the second letter indicates whether the line was taken down-bar (X) or across-bar (Y).The number discriminates between the different X and Y lines on each bar. Bar edges shown here were defined by thewater edge at the time of survey. Dashed line at edge of Bar E shows the approximate edges of the unit Bar Ea.




6/22

images from 1971 published in Cant & Walker

(1978).

BAR TYPES AND DYNAMICS

A bar is defined herein as a bedform whose lengthis proportional to channel width and whoseheight is comparable with the mean depth of theformative flow (ASCE, 1966). Figure 6 illustratesthe key morphological features of bars that arecommon in the South Saskatchewan River: two

principal bar types are recognized here as eitherunit or compound bars. Unit bars are defined(after Smith, 1978) as having a shape that remainsrelatively unmodified during migration, andbeing simple forms that are not amalgamated/superimposed upon other bar forms. Smith (1978)identified four main shapes of unit bar, althoughin the South Saskatchewan River the overwhelm-ing majority of unit bars have a lobate planformwith a slipface (up to the angle-of-repose) at theirdownstream margin. This style of unit bar wouldbe termed a transverse bar in the scheme ofSmith (1978). Based on the work of Bridge (2003),

compound bars are defined herein as forms thatcomprise more than one unit bar and evolvethrough several erosion and deposition events.Hence, compound bars possess a more complica-ted history than unit bars that is reflected in theirwider range of planform shapes. Other bedforms,most typically ripples and dunes, are also super-imposed on unit and compound bars. Cant &Walker (1978) also refer to sand waves that arecommon in the South Saskatchewan River andare characterized by a high wavelength:heightratio. However, Allen (1982), Ashley (1990) and

Bridge (2003) argue that such features are dunesand this terminology is adopted herein.

Unit bars

Unit bars (labelled UB in Fig. 6) typically have alobate planform, with their highest point at thedownstream end of the bar that terminates in anavalanche face. Cant & Walker (1978) classifiedthis bar type as a cross-channel bar. Unit barshave little topographic relief above the water

Fig. 3. Mean bed elevation change (in m) on the South Saskatchewan River at ranges 17 4 (28.0 km downstream ofdam) and 210 (338 km downstream of dam) from 1964 to 2002. The vertical dashed line is 1967, the year of damcompletion. Horizontal lines mark the approximate mean bed elevation in 1967. Plots adapted from Phillips (2003).

Table 1. Summary of the methodology used in thecollection, processing and ground-truth control of theSouth Saskatchewan River GPR data.

Data collectionGPR unit PulseEKKO 100Antennae

frequency (MHz)200

Antennae separation (m) 075Station spacing (m) 010Mode of data collection Stop-and-collectStacks 64

ProcessingDewow (MHz) 871Drift removal Yes

Set time-zero YesBandpass filter (MHz) Trapezoidal

with gates at50100220440

Background removal 200 trace windowDepth conversion

velocity (m nsec)1)005

Elevation statics YesAGC Gain (nsec) 25

Ground-truthingCut-face interpretation Yes




7/22

surface, and can possess heights equivalent to thechannel depth (i.e. up to 153 m). The area of thesmallest unit bars is 100 m 100 m, increasingin size to that of some of the smaller compoundbars described below. For example, the maximumlength of unit bars was 300 m during the studyperiod. Unit bars experience significant changeover time periods of 1 year, with the highest ratesof downstream migration (20130 m between

April and October 2000) being recorded for smallunit bars (e.g. 150 m long) in the largestchannels. Unit bars in the South SaskatchewanRiver represent an early stage in the developmentof compound bars, as discussed below and shownin the braid bar model of Bridge (1993).

Compound bars

Although displaying a wide range in planformshape, a distinctive type of compound bar

(labelled CBl in Fig. 6) is that with downstreamelongated limbs or horns (Cant & Walker, 1978;Ashworth et al., 2000). These compound barsappear to be generated from two to four unitbars, in which one unit bar forms the centralcore that then promotes accretion from addi-tional unit bars on either side, thus forming thecharacteristic limbs. Compound bars can developan asymmetric morphology (labelled CBa in

Fig. 6), with one bartail limb becoming longerthan the other, that is associated with developingflow asymmetry within the distributaries (Ash-worth et al., 2000). These newly formed com-pound bars also have little topographic reliefabove the water surface, and average bar dimen-sions are 180 m downstream and 120 macross-stream. The nature and rate of changefor small compound bars are similar to thatrecorded for unit bars, but as compound barsevolve and become larger, their origin from the

A

B

C

Fig. 4. Ground-truth control of ground penetrating radar (GPR) using: (A) photographic montage of the cut-face, (B)the GPR profile for the cut-face and (C) the interpretation of the GPR profile. SHR1: interface between organic-richsoil horizon and underlying sediment. DR1: 002010 m wide clay drape. DR2: clay drape < 002 m thick. DR3:erosion surface between depositional units, defined by an irregular interface between the units above and below thecontact. DR4: an erosional interface defined by a marked change in grain-size (030023 mm) and depositionalcharacteristics (low-angle to angle-of-repose cross-strata). SHR2: thin (< 0 01 m) gravel lag. SHR3: thick (> 010 m)layer of clay at the base of the cut-face. Figure reproduced from Woodward et al. (2003).




8/22

formative discrete unit bars becomes less clear;they may no longer retain the downstream limbsin their morphology (labelled CB in Fig. 6) andtheir rate of migration slows. Compound barsgrow when either other bars migrate and stall ontheir head/margins (see Fig. 7) or the channelseparating a compound bar from an adjacent barbecomes abandoned and fills, resulting in cre-ation of a single, much larger bar. Thus, thelargest compound bars are the result of numer-ous episodes of erosion and deposition, havelengths and widths up to 800 m 400 m, and

may form in the widest areas of the river wherethe bankfull width is 550 m. Cross-bar chan-nels (Bridge, 1993) are formed on these largecompound bars (see Fig. 7), often during floodrecession, and may extend along the entirelength of the bar. However, the depth of thesecross-bar channels is restricted and usually< 05 m, with channel widths of 10 m. Thesedimensions are thus significantly smaller thanthose of the primary channels that may have adepth and width of5 and 150 m respectively.

Compound bars have a similar, but more com-plex, morphology to the sand flats described byCant & Walker (1978). Aerial photographs takenin 1971 and 2000 show that over the decadal timescale, even the largest compound bars can gothrough a sequence of initiation, growth anddestruction. Over an annual period, change isless pronounced, with the maximum erosion ratesbetween April and October 2000 being 50 m onone barhead and maximum deposition being20 m of lateral accretion on the margin of onecompound bar. Vertical accretion was also recor-

ded over the study period, as bar surface depres-sions (see below) present in April were filled byOctober. However, aerial photographs also showthat some compound bars show no discernablechange over time periods of at least 30 years.These bars are exclusively those that are domin-ated by a heavily vegetated surface (labelled CBvin Fig. 6), possess a relatively high elevation (12 m above mean water level), and possibly have asubstantial thickness (> 05 m) of fines (clay andsilt). Such bars are usually found at the margins of

A

B C

Fig. 5. (A) Section of 200 MHzground penetrating radar (GPR)profile from an asymmetric com-pound bar, Bar C (see Fig. 2 forlocation of the profile). Note thehigh-angle inclined reflections(facies 1) at the top left of the profileand enclosed within the white-linedenvelope. The vertical thickness ofthis set is approximately equal tothe flow depth adjacent to the barmargin where this profile was taken.Note that this bar can be seen inFig. 6 labelled as Bar C on theright-hand side of the channel. (B)Photograph of the bar margin overwhich this GPR survey was taken,showing the surface expression ofthe high-angle inclined strata.Trench is approximately 5 m long.(C) Trench along the GPR profileshowing the tops of the high-angleinclined strata imaged by the GPR.Trowel handle is approximately01 m long, numbers on GPR profilesdenote radar facies.




9/22

the floodplain but can also be located mid-channel.

Study of the April 2000 aerial photographs,which were taken at low flow, reveals that thereare approximately equal numbers of unit (67) and

compound bars (69) in the study reach, with 57%of the compound bars possessing downstreamelongated limbs and 9% being stable and veget-ated.

SUB-SURFACE ALLUVIALARCHITECTURE

Classification and interpretation of radarfacies

Ground penetrating radar surveys were conduc-ted over the full range of bar types outlined above(Fig. 2), and the main radar facies detailed beloware summarized in Table 2.

Radar facies 1: high-angle inclined reflectionsFacies 1 consists of inclined reflections with anangle of dip from > 6 up to the angle-of-repose(Figs 5, 79). These reflections are most com-monly truncated at their top and bottom by sub-horizontal reflections, and can extend in sets for

100 m with a maximum thickness of 15 m.Facies 1 can be found throughout a compoundbar, but is more prevalent in the lower sections ofa vertical profile. An example of this facies(Fig. 8), from the left-hand side of a compound

bar, suggests that the formative flow was thusacross the bar limb and into the adjacent deepchannel. The strong reflections at the top andbottom of this radar facies are interpreted asrepresenting episodes of erosion before and afterthe accretion respectively. Less commonly, thisfacies is found with no truncation at either top orbottom, and in these cases the maximum thick-ness is 2 m, similar to the channel depth, andthe lateral continuity is reduced to 20 m. Anexample of this (Fig. 5) from an active bar margindirectly adjacent to a deep channel, shows

inclined strata up to the angle-of-repose that wereformed by bar migration. As outlined above,dunes in the South Saskatchewan reach a maxi-mum height of 15 m and may also be expected toproduce angle-of-respose inclined strata. How-ever, as preserved cross-set thickness is onlyapproximately one-third of the original duneheight (Leclair et al., 1997), inclined cross-strataup to the angle-of-repose and which are greaterthan 05 m in thickness are interpreted herein asbeing the product of bar migration. Facies 1 thus

Fig. 6. Aerial photograph of a section of the study reach taken in April 2000 illustrating the key morphologicalfeatures of the South Saskatchewan River. CB, compound bars; CBv, vegetated compound bars; CBl, compound barswith limbs; CBa, asymmetric compound bars; UB, unit bars. Note Bar C on the right-hand side (east) of the braid beltis that shown in Fig. 5.




10/22

forms when flow over the bartop causes barmigration into the channel thalweg, with associ-ated slipface accretion at the bar margin (Fig. 5).The orientation of the inclined strata shown infacies 1 is linked principally with flow across thebartop, which is often laterally or obliquely acrossthe bar, rather than with that of the principaldownstream channel flow direction. Addition-

ally, if the bar margin is curved in planform, thenthe resulting strata will also be curved in plan-form (see surface expression of inclined strata,Fig. 5B). The angle of dip of the reflections can bevariable over only a few metres (Fig. 9), a featurenoted in other studies (see Lunt et al., 2004), andis caused by variability in the orientation of thebar relative to the GPR profile, which is most

A

B

C

Fig. 7. (B) Aerial photograph taken in April 2000 of a large compound bar, Bar E (see Fig. 2 for the location of theprofiles). Note the presence of numerous cross-bar channels (labelled CBC) and accretion of a unit bar and smallercompound bar onto the right-hand side of the bar (Bar Ea) and upstream bar-head (Bar Eb) respectively. Also shown are200 MHz ground penetrating radar (GPR) profiles from Bar Ea and Bar Eb (see Fig. 2 for the location of the profiles).The white-dotted lines show the approximate location of Bars Ea and Eb, and the solid lines give the approximatelocation of the GPR survey lines. (C) Bar Ea is a unit bar similar to Bar B and shows a predominance of cross-strata

formed by dune migration. (A) This contrasts with Bar Eb, a compound bar similar to Bar A, which has pronouncedsets of high-angle inclined reflections (facies 1) within its profile. Numbers on GPR profiles denote radar facies.




11/22

pronounced for bars with a curved planform.Alternatively, changes in the dip of the reflectionsmay be caused by a change in the slope of the barmargin, perhaps related to variability of sedimentsupply, as it migrates during high flow. Such aninterpretation can be made for radar facies 1shown in Fig. 9, where the angle of dip at firstincreases, and then decreases to the right beforebeing replaced by discontinuous reflections (seeradar facies 2), which is interpreted here as being

due to of deposition by superimposed ripples/dunes as flow stage dropped (see below).

Radar facies 2: discontinuous undular ortrough-shaped reflectionsIndividual reflections of facies 2 range frombeing trough-shaped, < 3 m across and 05 mhigh, to more horizontal or sub-horizontal and< 5 m long (Figs 5 and 9). Sets of reflections mayextend laterally for tens of metres and be up to3 m thick. The degree of concavity of the trough-shaped reflectors of facies 2 is variable, and they

may grade vertically and laterally into morehorizontal or sub-horizontal undular reflections,with clearly defined troughs generally occurringonly in the bottom part of the profiles (i.e. 255 m below the bar surface). As discussed above,based on the premise that only approximatelyone-third of the dune form is preserved (Leclairet al., 1997), these trough-shaped reflections areinterpreted as representing trough cross-stratifi-cation associated with the largest sinuous-cres-ted dunes (i.e. 15 m high) that can only form

in the deepest channel thalwegs. Inclined cross-strata associated with smaller dunes that arebelow the radar resolution used herein will alsobe present, and will be significantly smaller thanthose associated with bar migration (see radarfacies 1). The variability in the degree of con-cavity of the troughs may be attributed todifferences in the orientation of the radar profilewith respect to the preserved bedform. Thus,where the profile was taken perpendicular to the

direction of dune migration, then the troughshape is most pronounced, but as the profilebecomes more oblique to the migration direction,then the reflections become more undular. In thetop part of the GPR profiles (i.e. 025 m belowthe bar surface), the absence of clear trough-shaped reflections is interpreted as being due tothe decrease in size of the dunes. Although theradar reflections may be horizontal or sub-hori-zontal, this facies represents composite sets ofplanar and trough cross-strata, and their bound-ing surfaces, which formed as a result of the

migration of sinuous-crested dunes. Bartop tren-ches also show the presence of ripples, but theassociated cross-lamination cannot be resolvedby the GPR and hence only the boundingsurfaces between sets are imaged.

Radar facies 3: low-angle reflectionsFacies 3 consists of reflections with an angle ofdip < 6, and that can be arranged in sets that dipeither upstream, downstream, laterally or arehorizontal. Sets are usually extensive, up to a

Table 2. Characteristics of the four radar facies identified in the South Saskatchewan River.

Radarfacies Reflection pattern

Sedimentologicalinterpretation

Examples from other modern andancient sandy braided rivers

1 High-angle inclinedreflections

Large-scale inclinedstrata formed by migrationof bar margin

Blodgett & Stanley(1980), Crowley (1983),Best et al. (2003), Miall &

Jones (2003), Skelly et al. (2003)2 Discontinuous undular

or trough-shaped reflectionsMedium- and small-scale

cross-stratification formedby sinuous crested dunes

Sarkar & Basumallick (1968),Conaghan & Jones (1975),Crowley (1983), Bridge (1993),Bridge et al. (1998), Bridge &Tye (2000), Best et al. (2003),Miall & Jones (2003)

3 Low-angle reflections Strata formed by migrationof low-amplitude dunes orunit bars

Allen (1983), Bristow (1987),Bridge et al. (1998)

4 Reflections of variable dipenclosed by a concavereflection

Channel fills Miall & Jones (2003), Skelly et al. (2003)

Examples of other studies of modern and ancient sandy braided rivers that have recorded facies with similarcharacteristics are listed.




12/22

maximum of 80 m in a downstream direction and25 m in thickness (Fig. 10), with individualreflections being traceable over 10s of metres.This facies is found at all depths within the bar,but becomes more prevalent towards the surface.

Observation of the exposed bar surfaces suggeststhat this facies forms from (1) deposition bydunes with a large wavelength:height ratio or (2)unit bars that migrate from the channel onto thebar surface, an example of which (Fig. 9D) pos-sesses a crest with a clear low-relief slipface. Thelow-angle radar reflections hence represent thebase of the dune or unit bar as it migrates acrossthe bar surface, thus producing a continuoussurface that can be traced both laterally andupstream/downstream. These deposits may also

contain cross-strata similar to the smaller dunesand ripples as discussed above, but again thesecannot be resolved by the GPR.

Radar facies 4: reflections of variable dip

enclosed by a concave reflectionThe key feature of facies 4 is a concave basalreflection that truncates the underlying reflec-tions (Fig. 11), and is overlain by a series ofsteeply dipping inclined reflections, althoughlow-angle and undular reflections may also bepresent (Fig. 11). This facies may extend down-stream and laterally for 20 m, be up to 15 m inthickness and is found throughout the bar depth.Facies 4 is interpreted as having formed by thefilling of channel scours and cross-bar channels.

A

B CApril 2000

Flow

October 2000

Fig. 8. Ground penetrating radar profiles (200 MHz) from tail end of compound Bar A (see Fig. 2 for location of the

profiles). The principal feature is the presence of high-angle inclined strata (facies 1) present in the top section of allprofiles. This facies is present in both the downstream and cross-stream profiles and indicates oblique flow across the

bar surface, a conclusion corroborated by the two aerial photographs (B, C) that show the bar migrated to the left aswell as downstream. The black outline on the October 2000 image shows the approximate location of the bar in April2000, and the lines give the approximate location of the ground penetrating radar (GPR) surveys. Numbers on GPRprofiles denote radar facies.




13/22

The angle-of-repose reflections may form fromunit bars migrating within the channels, whereaslow-angle or undular reflections are interpreted torepresent deposition by dunes. The distinctiveconcave reflection that represents the channelbase is most readily seen in radar profiles takenperpendicular to the channel flow direction, withchannel fills viewed in profiles taken parallel tothe flow direction being more difficult to identify.In some cases, facies 4 appears to be laterallyconstrained, cannot be traced for any distance

between survey lines (e.g. < 20 m), occurs only inthe top 50% of the profile and may lack anerosional lower surface. These characteristics areinterpreted as representing a bartop hollow(Ashworth et al., 1994), which are discrete, cir-cular/ovoid depressions in the bartops of theSouth Saskatchewan River, can be up to 15 mdeep and extend 1030 m both down and acrossflow. These bartop hollows form largely by con-vergence of two inwardly accreting bar tail limbsthat create a depression between them that is

A

C

D

E

B

Fig. 9. (A) Section of 200 MHz ground penetrating radar (GPR) profile from unit Bar B, (see Fig. 2 for the location of

the profile) with (B) interpretive sketch. Reflections below 3 m are beneath the current active channel base from thesection of the river where this bar was located. The complex pattern of reflections from 3 to 5 m in the profile mayrelate to channel or confluence scours or the migration of unit bars. Some of the reflections probably also relate tolarger dunes given the more trough-shaped nature of the reflections. Above 3 m, these trough-shaped reflections areoverlain by undular reflections, probably reflecting the decreasing dune size with flow depth. These trough-shapedreflections are then replaced by low-angle reflections (facies 3), interpreted as accretion by low-amplitude duneshigher up the profile. At the very top of the profile, some high-angle inclined reflections are present (facies 1) that are< 05 m in vertical thickness. These relate to the slipface at the leading edge of the bar and are much thinner thanthose associated with bar margin slipface accretion shown in Figs 5 and 8. Note also the variability in the dip of thesereflections, possibly related to a change in the slope of the bar margin during migration. (C)(E) Photographs showingthe surface morphological expression of facies 1, 3 and 2 respectively. Spade in (C) is approximately 12 m long, barwidth in (D) and (E) is approximately 100 m. Numbers on GPR profiles denote radar facies.




14/22

subsequently filled. Facies 4 represents a similarstyle of deposition to the solitary sets of cross-strata described by Lunt et al. (2004) that wereascribed to the filling of scours around ice-blocks

or vegetated mudclasts.

Quantification of the occurrence of facieswithin different types of braid bar

So as to establish the frequency of occurrence ofthe different facies throughout the 35 km ofGPR survey lines, the following methodologywas used. Individual survey lines for each barwere first interpreted based on the facies typesdescribed above. A vertical section was then

sampled every 10 m along each line, thusallowing the proportion of each facies withineach vertical section to be determined for eachsurvey line. By combining data from all the

vertical sections, the overall facies proportionsfor each bar were produced (Table 3). Addition-ally, so as to quantify the vertical variability offacies within each bar, every vertical sectionwas also divided into 05 m intervals measuredupwards from a clear basal erosion surfacepresent within the GPR surveys. Topographicsurveys of the major channels indicate that thissurface is at a depth similar to that of themodern river thalweg. The erosion surface inthe GPR thus represents the channel base above

April 2000

Flow

October 2000

A

B C

Cut Fill

Fig. 10. Ground penetrating radar (GPR) profiles (200 MHz) from compound Bar D (see Fig. 2 for the location of theprofiles). The black outline on the October 2000 image shows the approximate location of the study bar in April 2000,and the lines denote the approximate location of the GPR survey lines. Note the very low angle and continuousnature of the reflections that represent low-amplitude dunes or unit bars accreting onto the bar surface (facies 3).Some steepening of the reflections is evident on the extreme left of profile DY1 (facies 1), and represents limited barmigration towards the channel at this point in the profile. Numbers on GPR profiles denote radar facies.




15/22

which bars initially develop and so identifiesthe likely maximum bar depth. The proportionof each facies within each 05 m vertical inter-val was then determined and summed, thus

enabling the vertical distribution of facies to bequantified. The total number of vertical sectionssampled and facies proportions present for eachbar are given in Table 3.

Fig. 11. Ground penetrating radar (GPR) profiles (200 MHz) from the same compound bar (Bar D) as that shown inFig. 10 (see Fig. 2 for the location of the profiles), together with an interpretive sketch. The sequential aerial pho-tographs in Fig. 10 show that the main change that has occurred on this bar is the filling of a bartop channel (facies 4)that formed towards the left-hand side of the bar tail area (labelled cut and fill). The erosion surface and filling ofthis channel is seen preserved in both the downstream and cross-stream GPR profiles in Fig. 11. Numbers on GPRprofiles denote radar facies.

Table 3. Summary of the faciesproportions in all bars discussed inthe text.

Bar

No. ofverticalprofiles

Facies1 (%)

Facies2 (%)

Facies3 (%)

Facies4 (%)

A 103 23 35 39 3B 9 6 62 32 0C 59 7 59 31 3D 26 1 47 46 6E 99 10 44 39 7Ea 23 9 61 27 3Eb 35 32 38 30 0Average

(AD, Ea and Eb)

255 13 50 34 3

Bars B and Ea are classified as unit bars, whilst Bars A, C, D, E and Eb areclassified as compound bars. Also shown are the average percentage occur-rence figures for all the combined smaller compound and unit bars, andwhich is compared with Bar E, the largest compound bar studied.




16/22

Unit barsData from Bar B (Fig. 2; see also Fig. 9 andTable 3) show the highest proportion (62%) ofradar facies 2 (discontinuous undular or trough-shaped reflections) of all the GPR surveys. Thelack of trough-shaped reflections higher in theprofile is interpreted as representing the decrease

in dune size with decreasing flow depth (i.e. thetrough cross-beds of small dunes cannot beresolved by the GPR). Accretion towards the topof the profile may contain some small sets of high-angle inclined reflections (facies 1, 6%). Thisvertical succession is also displayed in unit BarEa (Figs 2, 7 and 12A; Table 3), that was accretingonto a compound bar (Bar E) when the GPRsurvey was undertaken. Bars B and Ea showsimilarities in the low proportion of high-angleinclined reflections in the upper parts of the

profiles (facies 1, 9%) and the higher amounts ofdiscontinuous undular or trough-shaped reflec-tions (facies 2, 61%), with trough-shaped reflec-tions only found at depth. In contrast to unit BarB, Bar Ea possesses some reflections of variabledip that are enclosed by a concave reflection(facies 4, 3%).

Compound barsA vertical proportion plot (Fig. 12B), based on allsurvey lines from compound Bar A (Fig. 2),shows a consistent trend in the vertical changein facies. The lower 05 m of the bar, with its basedefined by a distinct erosion surface at 2 53 mdepth, is characterized by discontinuous undularor trough-shaped reflections (facies 2, 63%) andhigh-angle inclined reflections (facies 1, 27%).Towards the surface of the bar (top 05 m), facies 2

A

B

Fig. 12. Vertical proportion of facies derived from all ground penetrating radar profiles on (A) unit Bar Ea, and (B)compound Bar A. The principal difference between the two distributions is that the unit bar has a dominance ofcross-strata formed by dune migration at depth (facies 2) with high-angle inclined strata (facies 1) only present higherup in the profile, whereas facies 1 is prevalent throughout the profile of the compound bar. For both figures, faciesdescriptions are as given in Table 2.




17/22

is then progressively replaced by low-anglereflections (facies 3, 84%) as the dominant facies.

The mean frequency of occurrence of facies forall GPR lines on Bar A (Table 3) is 39% (facies 3),35% (facies 2), 23% (facies 1) and 3% (facies 4).However, these averages mask significant spatialvariability within the bar, which was assessed byanalysing each GPR line separately (AX1AX6 and

AY1AY8). The downstream lines (AX; Fig. 13)display the greatest degree of variability, with theline closest to the channel thalweg, AX1, showingthe greatest proportion of high-angle inclinedreflections (facies 1, 39%). Away from the channelthalweg, the proportion of facies 1 drops tobetween 10% and 19% or is absent entirely (AX4and AX6). Discontinuous undular or trough-shaped reflections and low-angle reflections pre-dominate on the right-hand side of the bar (lookingdownstream): for example, line AX6 is dominatedby discontinuous undular or trough-shaped reflec-

tions (facies 2, 65%). The cross-stream lines (AY;Fig. 13) show much less variability than thedownstream lines, although each line does pos-sess some heterogeneity in facies type. For exam-ple, as noted above, high-angle inclinedreflections are dominant on the left-hand side ofthe bar and thus facies 1 is present in the left-handside of the cross-stream lines. Bar A migrated150 m downstream between April and October2000 (Fig. 8), a distance equivalent to the entirelength of the bar, and most of this migration

occurred on the left-hand side of the bar as itprograded into the main thalweg (shown as themuch darker area in the aerial photograph, seeFig. 8). The sets of high-angle, inclined reflections(facies 1) thus formed as sediment, which hadbeen transported across the bar surface, ava-lanched down the slipface of the bar margin.Overbar flow is thus required to produce this type

of deposition and these sets can be expected to bethickest where the bar margin progrades into adeep thalweg, as also shown in the Jamuna Riverwhere such bar-margin slipfaces were recorded asup to 8 m high (Best et al., 2003).

As well as facies variability within a compoundbar, facies variability is also apparent betweenbars as demonstrated by GPR data from a furtherthree, relatively small, compound bars (Bars Eb, Cand D).

Bar Eb. Two 100 m survey lines were taken over

Bar Eb (Figs 2 and 7; Table 3) that was migratingwithin a deep channel and up onto the head ofBar E. Based on the results from Bar A, it may beexpected that this migration would result in thepresence of high-angle inclined reflections in theGPR profile, which is confirmed (Fig. 7) by 32%of the survey lines for Bar Eb being classified asfacies 1. Bar Eb also clearly demonstrates thatcompound bars grow by an amalgamation ofdifferent bars, as evidenced by the stacked setsof inclined strata in the GPR profile (Fig. 7). The

Flow

Fig. 13. Facies interpretation of ground penetrating radar lines with corresponding vertical proportion curves forcompound Bar A. In both figures facies descriptions are as given in Table 2.




18/22

sets of inclined strata lower in this profile (Fig. 7)are interpreted as representing other unit barsfrom which compound Bar Eb evolved.

Bar C. Flow was directed over the bartop and intothe thalweg on the left-hand side of the bar (seeFig. 6): one bartail limb thus prograded down-

stream further than the other, with a steepslipface developing on one side of the resultingasymmetrical bar. Flow over this asymmetric bar,and subsequent deposition into the channel thal-weg at the bar margin, again resulted in thepresence of high-angle (angle-of-repose) inclinedreflections (facies 1; Fig. 5) in the GPR survey,although the overall abundance (Table 3) of facies1 within this bar was only 7%.

Bar D. In contrast to Bars A, B and C that migrateddownstream over the study period (April 2000

October 2000), Bar D (Figs 2 and 10; Table 3)displayed limited movement and possessed dif-ferent facies characteristics despite the similarityin its planform (Fig. 10). High-angle inclinedreflections were largely absent (facies 1, 1%) fromthe GPR profiles that were dominated by low-angle reflections (facies 3, 46%) and discontinu-ous undular or trough-shaped reflections (facies2, 42%). These characteristics are in contrast tothe more active bar migration and prevalence ofhigh-angle inclined reflections seen in Fig. 8. Theprincipal change in Bar D between April andOctober 2000 was infill of the channel at the

downstream end of the bar (Fig. 10), as highligh-ted in the profiles shown in Fig. 11.

As well as the relatively small compound barsdetailed above, GPR profiles were also taken overa compound bar with a longer and more compli-cated history of erosion, deposition and rework-ing (Bar E; Figs 2 and 7; Table 3). Analysis ofthese data again reveals the major facies are high-angle inclined reflections (facies 1, 10%), discon-tinuous undular or trough-shaped reflections(facies 2, 44%) and low-angle reflections (facies3, 39%). The distribution of facies within Bar E

also reveals a broad similarity to Bar A. High-angle inclined reflections are slightly less com-mon (10%) when compared with the more simplecompound Bar A (23%), whilst the proportions ofdiscontinuous undular or trough-shaped reflec-tions and low-angle reflections are similar (35%and 39%, respectively, for Bar A). Cross-stratainterpreted as having formed by dune migrationare found throughout the vertical profiles fromBar E and diminish in size and occurrencetowards the top of the profiles. High-angle

inclined reflections are more prevalent in thebottom 65% of the profile, while low-anglereflections dominate in the upper sections of theprofile. The amount of facies 4 (fill of channelscours and cross-bar channels) found within BarE (7%) is slightly greater than that of the Bar A(3%), probably because Bar E has evolved by

multiple erosional and depositional events andan amalgamation of different bar remnants, aprocess also documented in gravel-bed rivers byBluck (1976).

DISCUSSION

Comparison of the GPR data presented hereinwith the original work of Cant & Walker (1978)reveals seven important differences:

1 The channel model of Cant & Walker (1978;

profile C in Fig. 1), dominated by cross-strataassociated with dune migration, can applyequally to the deposits of braid bars. Forexample, unit bars are dominated by dune-scalecross-stratification, related to the fact that theyevolve initially through a process of dunestacking. Likewise, compound bars can alsodisplay a similar vertical sequence of faciesfrom thalweg to bartop as that given by Cant &Walkers (1978) channel model (profile C inFig. 1). Cross-strata produced by dune migrationcan thus be found throughout much of the

sequence and decrease in size towards the barsurface, reflecting that dune height is related toflow depth.

2 The avalanche face at the downstream end ofunit bars is of low height (05 m) and only pre-serves minor sets of high-angle inclined strata atthe very top of the profile (Fig. 9). This is incontrast to Cant & Walker (1978) who suggestedthat the dominant facies of unit bars (their cross-channel bars) would be high-angle inclinedstrata, present throughout the full height of thebar form (Fig. 1, profile A).

3 The GPR data demonstrate that large, high-angle inclined strata are most likely to formbecause of sedimentation at the steep margins ofcompound bars. The largest sets will form wherea compound bar migrates into a deep channel, inwhich case the set may extend from bartop tochannel base. Thus high-angle inclined stratashould not be restricted to just 23 m below thesurface of a compound bar, as suggested in thesand flat model of Cant & Walker (1978; see Fig. 1,profile A).




19/22

4 All three profiles given in the model of Cant &Walker (1978; Fig. 1) show high-angle inclinedstrata associated with unit bars (the cross-channelbars of Cant & Walker). However, given that it hasbeen shown herein that unit bars do not preserve12 m thick sets of high-angle inclined strata, thisfacies (facies 1 herein) is probably over-repre-

sented in the model of Cant & Walker (1978).5 The present study has found no evidence for

the substantial (> 01 m thick) deposition of finesthat are preserved at depth (3 m below the barsurface) as shown by Cant & Walker (1978, seeFig. 1). Within a GPR profile, clay is readilyrecognizable by the high-amplitude reflectionsproduced, usually with an associated loss ofresolution beneath (Best et al., 2003). Althoughdrapes of fine-grained material were apparent inbartail areas at the surface, no evidence of thickerdeposits of fines was found at depth within the

GPR profiles presented herein.6 Cant & Walker (1978) state that the faciessequence depicting channel aggradation (labelledC in Fig. 1) in the South Saskatchewan River is aminimum of 5 m thick. The present study sug-gests that these sequences will be much morevariable than this figure, as the profiles scale withthe size of channel in which they formed. Thussome of the GPR profiles presented herein areonly 3 m in thickness from channel base to barsurface. Accurate assessment of such variability isan important consideration when attempting toreconstruct the dimensions of ancient channel

deposits (Bridge & Tye, 2000).7 The facies distributions derived from the

present GPR data demonstrate that there can begreat variability both within bars and betweenbars, and that the three facies profiles presentedby Cant & Walker (1978; Fig. 1) may not capturethe intrinsic variability of sandy braided allu-vium. Additionally, there still remains muchuncertainty concerning the precise vertical andlateral variability in facies across the entire braidbelt, given the logistical problems of collectingGPR data from channels. This area remains a key

region for future research.The deposits of the South Saskatchewan River

are broadly similar to those reported from othermodern sandy braided rivers of different scaleand braid intensities (Sambrook Smith et al.,2005). The main facies identified herein havebeen reported from numerous other studies ofmodern and ancient sandy braided rivers(Table 2). This broad similarity in the facies ofsandy braided rivers (modern and ancient) is not

surprising, if it is assumed that generally the samephysical processes occur in all sandy braidedrivers, regardless of environment or scale (Skellyet al., 2003; Sambrook Smith et al., 2005). How-ever, the present GPR study has revealed avariability in the frequency and sequence offacies that has previously been unreported. Past

work, which has largely been qualitative in itsapproach, has tended to focus on broad similar-ities between different systems rather than inves-tigating the differences that only become evidentwith a more quantitative approach. Although thepresent study possesses limitations in the arealcoverage achieved, it provides the most detailedquantitative assessment of sandy braidedriver facies yet collected and highlights the clearneed for further quantitative analysis of morebraided rivers as new techniques and databecome available.

Comparison of the data from different barswithin the South Saskatchewan River shows thatbars with the same surface planform morphologymay not necessarily have the same sub-surfacealluvial architecture. For example, one com-pound bar (Bar D, Fig. 2 and Table 3) had almosta complete absence of high-angle inclined stratawhereas this facies comprised approximately30% of the deposits of other compound bars(e.g. Bar Eb, Fig. 2 and Table 3). These differencesin facies proportions may be attributed to theexact nature of bar growth from the initial unitbar, which displays a less variable alluvial archi-

tecture. Compound bar growth is controlled bydischarge regime, anabranch width:depth ratio,the abundance of vegetation and the local bar andchannel topography, and therefore, flow direction(Bridge, 1993; Ferguson, 1993; McLelland et al.,1999). These factors will thus control the occur-rence of sediment-laden flow over bar surfacesthat is important for the development of high-angle inclined strata (Sambrook Smith et al.,2005). As the growth of compound bars may beviewed as an amalgamation of smaller bars, thefacies proportions of these large compound bars

may reflect an average of the other smaller barsdiscussed above. In order to test this assumption,if the facies proportions of the four smallercompound bars and two unit bars studied hereinare averaged they yield: facies 1, 13%; facies 2,50%; facies 3, 34% and facies 4, 3%. The sameaveraged data for the large compound bar (Bar E)yield figures of 10%, 44%, 39% and 7% for facies14 respectively (Table 3). The similarity be-tween these figures is not unexpected given that




20/22

compound bars grow as other bars accrete ontothem. The slightly higher proportion of facies 4 inthe large compound bar is also to be expected,reflecting the greater presence of cross-bar chan-nels and the generally more complex reworking ofthe bar that may have occurred. This comparisonalso suggests that larger compound bars will

provide a better characterization of the overallfacies proportions within the river when com-pared with the smaller bars that have been shownto be more variable. However, for logistical rea-sons, it is generally the smaller compound andunit bars that attract study, again highlighting theneed for full braidplain characterization of faciesto achieve a representative facies model.

CONCLUSION

This paper has used GPR to describe and quantifythe deposits of unit and compound bars withinthe sandy braided South Saskatchewan River,enabling an assessment of the influential faciesmodel proposed by Cant & Walker (1978). On thebasis of this new evidence, six principal conclu-sions can be drawn:

1 Bars in the South Saskatchewan River aredominated by the following facies: (1) large-scaleinclined strata formed by migration of bar mar-gins; (2) medium- and small-scale cross-stratifi-cation formed by sinuous crested dunes and

ripples; (3) low-angle strata formed by themigration of dunes or unit bars; and, (4) cross-barchannel fills.

2 Bars possess a characteristic vertical sequenceof facies, with trough cross-stratification mostprevalent above the basal erosive surface. Higherup in the profile, cross-strata associated withsmaller dunes (and ripples revealed in shallowtrenches) becomes more common with low-anglestrata dominating the deposits near the bar sur-face. High-angle inclined strata are only foundnear the surface of unit bars, and are relativelysmall-scale, but can be found at a wider range ofdepths in compound bars.

3 Unit bars are dominated by cross-stratificationformed by dune migration (60% of facies 2),which may merge laterally to form high-angleinclined strata. Cross-stratification associatedwith dunes decreases in size from thalweg tobartop, reflecting the decreasing formative flowdepths. Unit bars possess only a minor amount ofhigh-angle inclined strata (< 10% of facies 1),generally found at bar margins.

4 Flow across the top of a compound bar gen-erates high-angle cross-stratification (facies 1) atthe bar margin, whereas if a compound bar doesnot experience overbar flow, then low-angle pla-nar stratification (facies 3) will predominate theresultant facies in the middle-upper parts of theprofile.

5 The largest compound bars comprise anamalgamation of other smaller bars (both com-pound and unit), and their facies distribution isthus similar to the average for all other bars.

6 The features described herein for the SouthSaskatchewan River possess similarities to manyother modern and ancient sandy braided riversdescribed in the literature. However, the GPR datademonstrate a high variability in facies bothwithin and between bars, with this variabilitybeing greatest within compound bars.

The present study suggests there is a need for

more GPR studies from a range of other braidedrivers to help elucidate the key relationshipsbetween formative conditions and alluvial archi-tecture, assess if a single generic facies model canbe applied to all braided rivers, or conversely, if arange of models is required. Future work shouldfocus on establishing a direct link between theprocesses operating in braided rivers, the chan-ging morphology through successive flood eventsand the resultant sub-surface alluvial architectureacross the entire braidplain.

ACKNOWLEDGEMENTS

This work was carried out with the support ofNERC grant GR9/04273 to P. J. A., J. L. B. andG. H. S. S. The NERC Geophysical EquipmentPool kindly loaned a PulseEKKO 1000 GPRsystem to the project. We wish to thank DavidAshley, for invaluable assistance in the field, TaviMurray (University of Leeds) for supply of aPulseEkko100 GPR system, Derald Smith (Uni-versity of Calgary) for the loan of a boat, outboardmotors and coring equipment, Dirk De Boer(University of Saskatchewan) for help with locallogistics and supply of water discharge data, andBob and Sandy Stephenson for their vital logis-tical support in Outlook. J. L. B. is grateful foraward of Leverhulme Trust Research Fellowship,which was partly conducted at the Ven Te ChowHydrosystems Laboratory, University of Illinois atUrbana-Champaign, which aided completion ofthis paper. Ian Lunt is thanked for discussionsthat led to improvements in the clarity of the




21/22

manuscript. The Graphics Unit at the Universityof Birmingham are thanked for producing thefigures. Gary Brierley, and in particular JohnBridge, provided a range of thorough, criticaland perceptive review comments that greatlysharpened our interpretation of the GPR data.

REFERENCES

Allen, J.R.L. (1982) Sedimentary Structures: Their Characterand Physical Basis, Volume 1. Developments in Sedimen-tology, vol. 30. Elsevier Science Publishers, Amsterdam, 593pp.

Allen, J.R.L. (1983) Studies in fluviatile sedimentation: bars,bar-complexes and sandstone sheets (low-sinuosity braidedstreams) in the Brownstones (L. Devonian), Welsh Border-lands. Sed. Geol., 33, 237293.

American Society of Civil Engineers (1966) Nomenclature forbed forms in alluvial channels. Task force on bedforms inalluvial channels. J. Hydraul. Eng., 92, 5164.

Ashley, G.M. (1990) Classification of large-scale subaqueousbedforms: a new look at an old problem. J. Sed. Petrol., 60,160172.

Ashworth, P.J., Best, J.L., Leddy, J.O. and Geehan, G.W.(1994) The physical modelling of braided rivers anddeposition of fine-grained sediment. In: Process Models andTheoretical Geomorphology (Ed. M.J. Kirkby), pp. 115139.Wiley, Chichester.

Ashworth, P.J., Best, J.L., Roden, J.E., Bristow, C.S. and Kla-assen, G.J. (2000) Morphological evolution and dynamics ofa large, sand braid-bar, Jamuna River, Bangladesh. Sedi-mentology, 47, 533555.

Best, J.L., Ashworth, P.J., Bristow, C.S. and Roden, J.E. (2003)Three-dimensional sedimentary architecture of a large, mid-channel sand braid bar, Jamuna River, Bangladesh. J. Sed.

Res., 73, 516530.Blodgett, K.H. and Stanley, K.O. (1980) Stratification, bed-

forms and discharge relations of the braided Platte Riversystem, Nebraska. J. Sed. Petrol., 50, 139148.

Bluck, B.J. (1976) Sedimentation in some Scottish rivers oflow sinuosity. Trans. R. Soc. Edinburgh, 69, 425456.

Bridge, J.S. (1993) The interaction between channel geometry,water flow, sediment transport and deposition in braidedrivers. In: Braided Rivers (Eds J.L. Best and C.S. Bristow).Geol. Soc. London Spec. Publ., 75, 1371.

Bridge, J.S. (2003) Rivers and Floodplains. Blackwell Pub-lishing, Oxford, 491 pp.

Bridge, J.S. and Tye, R.S. (2000) Interpreting the dimensionsof ancient fluvial channel bars, channels, and channel beltsfrom wireline logs and cores. AAPG Bull., 84, 12051228.

Bridge, J.S., Smith, N.D., Trent, F., Gabel, S.L. and Bernstein,P. (1986) Sedimentology and morphology of a low-sinuosityriver: Calamus River, Nebraska Sandhills. Sedimentology,33, 851870.

Bridge, J.S., Collier, R. and Alexander, J. (1998) Large-scalestructure of Calamus River deposits (Nebraska, USA) re-vealed using ground-penetrating radar. Sedimentology, 45,977986.

Bristow, C.S. (1987) Brahmaputra River: channel migrationand deposition. In: Recent Developments in Fluvial Sedi-mentology (Eds F.G. Ethridge, R.M. Flores and M.D. Har-vey). Spec. Publ. Soc. Econ. Paleontol. Mineral., 39, 6374.

Bristow, C.S. (1993) Sedimentary structures in bar tops in theBrahmaputra River, Bangladesh. In: Braided Rivers (Eds J.L.Best and C.S. Bristow). Geol. Soc. London Spec. Publ., 75,277289.

Bristow, C.S., Skelly, R.L. and Ethridge, F.G. (1999) Crevassesplays from the rapidly aggrading, sand-bed, braided Nio-brara River, Nebraska: effect of base-level rise. Sedimentol-ogy, 46, 10291049.

Cant, D.J. (1976) Braided stream sedimentation in the SouthSaskatchewan River. Unpublished PhD thesis, McMasterUniversity, ON, Canada, 248 pp.

Cant, D.J. (1978) Bedforms and bar types in the South Sas-katchewan River. J. Sed. Petrol., 48, 13211330.

Cant, D.J. and Walker, R.G. (1976) Development of a braidedfluvial facies model for the Devonian Battery Point Sand-stone, Quebec. Can. J. Earth Sci., 13, 102119.

Cant, D.J. and Walker, R.G. (1978) Fluvial processes andfacies sequences in the sandy braided South SaskatchewanRiver, Canada. Sedimentology, 25, 625648.

Collinson, J. (1970) Bedforms in the Tana River, Norway.Geogr. Ann., 52, 3156.

Conaghan, P.J. and Jones, J.G. (1975) The HawksburySandstone and the Brahmaputra: a depositional model for

continental sheet sandstones. J. Geol. Soc. Aust., 22,275283.Crowley, K.D. (1983) Large-scale bed configurations (macro-

forms), Platte River Basin, Colorado and Nebraska: primarystructures and formative processes. Geol. Soc. Am. Bull., 94,117133.

van Dam, R.L. and Schlager, W. (2000) Identifying causes ofground-penetrating radar reflections using time-domain re-flectometry and sedimentological analyses. Sedimentology,47, 435450.

Ethridge, F.G. and Flores, R.M. (1981) Recent and ancientnonmarine depositional environments: models for explora-tion. SEPM Spec. Publ., 31, 349.

Ferguson, R.I. (1993) Understanding braiding processes ingravel-bed rivers: progress and unsolved problems. In:

Braided Rivers (Eds J.L. Best and C.S. Bristow). Geol. Soc.London Spec. Publ., 75, 7387.

Ferguson, R.J. and Brierley, G.J. (1999) Levee morphology andsedimentology along the lower Tuross River, south-easternAustralia. Sedimentology, 46, 627648.

Fielding, C.R., Alexander, J. and McDonald, R. (1999) Sedi-mentary facies from ground-penetrating radar surveys of themodern, upper Burdekin River of north Queensland, Aus-tralia: consequences of extreme discharge fluctuations. In:Fluvial Sedimentology VI (Eds N.D. Smith and J. Rogers).Int. Assoc. Sedimentol. Spec. Publ., 28, 347362.

Galay, V.J., Pentland, R.S. and Halliday, R.A. (1985) Degra-dation of the South Saskatchewan River below GardinerDam. Can. J Civil Eng., 12, 849862.

Helfrick, D. (1993) Bed degradation of South Saskatchewan

River below Lake Diefenbaker. Environment Canada Report,Regina, Saskatchewan, 29 pp.

Leclair, S.F., Bridge, J.S. and Wang, F. (1997) Preservation ofcross-strata due to migration of subaqueous dunes overaggrading and non-aggrading beds: comparison of experi-mental data with theory. Geosci. Can., 24, 5566.

Lehmann, F. and Green, A.G. (1999) Semi-automated georadardata acquisition in three dimensions. Geophysics, 64, 719731.

Lunt, I.A., Bridge, J.S. and Tye, R.S. (2004) A quantitative,three-dimensional depositional model of gravely braidedrivers. Sedimentology, 51, 377414.




22/22

McLelland, S.J., Ashworth, P.J., Best, J.L., Roden, J.E. andKlaassen, G.J. (1999) Flow structure and transport of sand-grade suspended sediment around an evolving braid bar,Jamuna River, Bangladesh. In: Fluvial Sedimentology VI(Eds N.D. Smith and J. Rogers). Int. Assoc. Sedimentol.Spec. Publ., 28, 4357.

Miall, A.D. (1978) Lithofacies types and vertical profile mod-els in braided river deposits: a summary. In: Fluvial Sedi-

mentology(Ed. A.D. Miall). Can. Soc. Petrol. Geol. Mem., 5,597604.Miall, A.D. and Jones, B.G. (2003) Fluvial architecture of the

Hawkesbury Sandstone (Triassic), near Sydney, Australia.J. Sed. Res., 73, 531545.

Neal, A. (2004) Ground-penetrating radar and its use in sedi-mentology: principles, problems and progress. Earth-Sci.Rev., 66, 261330.

Phillips, R.T.J. (2003) Downstream geomorphic impacts ofreservoir construction on the sand-bed braided South Sas-katchewan River, Saskatchewan. Unpublished B.Sc. (Hons)thesis, Simon Fraser University, BC, Canada, 75 pp.

Sambrook Smith, G.H., Ashworth, P.J., Best, J.L., Woodward,J. and Simpson, C.J. (2005) The morphology and facies ofsandy braided rivers: some considerations of spatial and

temporal scale invariance. In: Fluvial Sedimentology VII(Eds M.D. Blum, S.B. Marriott and S.F. Leclair). Int. Assoc.Sedimentol. Spec. Publ., 35, 145158.

Sarkar, S.K. and Basumallick, S. (1968) Morphology, struc-ture and evolution of a channel island in the Barakar River,Barakar, West Bengal. J. Sed. Petrol., 38, 747754.

Skelly, R.L., Bristow, C.S. and Ethridge, F.G. (2003) Archi-tecture of channel-belt deposits in an aggrading shallowsandbed braided river: the lower Niobrara River, northeastNebraska. Sed. Geol., 158, 249270.

Smith, N.D. (1971) Transverse bars and braiding in the Lower

Platte River, Nebraska. Geol. Soc. Am Bull., 82, 34073420.Smith, N.D. (1978) Some comments on terminology for bars inshallow rivers. In: Fluvial Sedimentology (Ed. A.D. Miall).Can. Soc. Petrol. Geol. Mem., 5, 8588.

Vandenberghe, J. and van Overmeeren, R.A. (1999) Groundpenetrating radar images of selected fluvial deposits in theNetherlands. Sed. Geol., 128, 245270.

Woodward, J., Ashworth, P.J., Best, J.L., Sambrook Smith,G.H. and Simpson, C.J. (2003) The use and application ofGPR in sandy fluvial environments: methodological con-siderations. In: Ground Penetrating Radar in Sediments(Eds C.S. Bristow and H.M. Jol). Geol. Soc. London Spec.Publ., 211, 127142.

Manuscript received 29 April 2004; revision accepted 2

August 2005


2006 The Authors Journal compilation 2006 International Association of Sedimentologists Sedimentology 53 413434

Date post:	03-Apr-2018
Category:	Documents
Upload:	bobby-wsk
View:	218 times
Download:	0 times

Wsambrook Smith Etal 06

Documents