Large-scale structure of Calamus River deposits (Nebraska, USA) revealed using ground-penetrating...

Large-scale structure of Calamus River deposits(Nebraska, USA) revealed using ground-penetrating radar

JOHN BRIDGE*, RICHARD COLLIER and JAN ALEXANDERà§* Department of Geological Sciences, Binghamton University, Binghamton, NY 13902-6000, USA, Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, England, UK,à Department of Earth Sciences, Cardiff University of Wales, Cardiff CF1 3YE, Wales, UK,§Current address: Department of Environmental Sciences, University of East Anglia, Norwich,NR4 7TJ, UK

ABSTRACT

The large-scale (i.e. bar-scale) structure of channel deposits of the braided, low-

sinuosity Calamus River, Nebraska, is described using ground-penetrating radar (GPR)

pro®les combined with vibracores. Basal erosion surfaces are generally overlain by

medium-scale, trough-cross-strati®ed (sets 3±25 cm thick), very coarse to medium

sands, that are associated with relatively high amplitude, discontinuous GPR

re¯ectors. Overlying deposits are bioturbated, small-scale cross-strati®ed (sets <3 cm

thick) and vegetation-rich, ®ne to very-®ne sands, that are associated with low-

amplitude discontinuous re¯ectors. Near-surface peat and turf have no associated GPR

re¯ectors. In along-stream pro®les through braid and point bars, most GPR re¯ectors

dip downstream at up to 2° relative to the basal erosion surface, but some re¯ectors in

the upstream parts of bars are parallel to the basal erosion surface or dip upstream. In

cross-stream pro®les through bars, GPR re¯ectors are either approximately parallel to

bar surfaces or have low-angle inclinations (up to 6°) towards cut banks of adjacent

curved channels. Basal erosion surfaces become deeper towards cut banks of curved

channels. These structures can be explained by lateral and downstream growth of bars

combined with vertical accretion. Convex upwards forms up to 0á5 m high, several

metres across and tens of metres long represent episodic accretion of unit bars (scroll

bars and bar heads). Stratal patterns in channel ®lls record a complicated history of

erosion and deposition during ®lling, including migration of relatively small bars. A

revised facies model for this type of sandy, braided river has been constructed based on

this new information on large-scale bedding structure.

INTRODUCTION

Ground-penetrating radar (GPR) pro®ling com-bined with intensive coring can offer a veryeffective means of documenting river deposits ingreat detail (e.g. Gawthorpe et al., 1993; Huggen-berger, 1993; Alexander et al., 1994; Bridge et al.,1995). GPR is particularly useful for delineatingthe lithofacies, geometry and orientation of thelarge-scale inclined stratasets that are associatedwith channel-bar migration and channel ®lling.Bridge et al. (1995) attributed characteristic strati-graphic patterns in point-bar deposits to speci®c

modes of channel migration, and particularly todiscrete episodes of erosion and deposition thatwere related to ¯oods of varying recurrenceinterval. The high degree of detail in descriptionand interpretation of these point-bar deposits wasmade possible through the use of information onchannel geometry, water ¯ow, sediment trans-port, erosion and deposition, in addition to thecores and GPR data.

Braided-river deposits are much less wellknown than those of sinuous, unbraided rivers(Bridge, 1993a). Although Huggenberger (1993)has studied Pleistocene braided-river deposits in

Sedimentology (1998) 45, 977±986

Ó 1998 International Association of Sedimentologists 977

a gravel pit using GPR, the type of detailed studymentioned above has not yet been reported. As astart in this direction, we performed a GPRsurvey of the deposits of a braided, low-sinuosityreach of the Calamus River, Nebraska Sand Hills(Fig. 1). This reach contains braid bars, pointbars and channel ®lls. The channel and ¯ood-plain morphology, patterns of channel migration,and nature of the deposits were described byBridge et al. (1986). Water ¯ow, sediment trans-port and bedform dynamics were studied byBridge and Gabel (1992) and Gabel (1993). Morethan 50 vibracores were used to show spatialvariations of grain size and sedimentary struc-tures within channel bars and ®lls, and a simpledepositional model was proposed (Bridge et al.,1986; Fig. 2). The large-scale (i.e. bar-scale)stratal geometry and inclination in this modelwere inferred, because they could not be ob-served directly. Furthermore, the strata were onlyrepresented in cross-stream sections. Thus, themain purpose of this study was to run GPRsurveys along some of the previous coring

pro®les (Bridge et al., 1986: their ®g. 5) in orderto document the nature of large-scale strata indifferently oriented sections, and to assess theprevious inferences regarding their character.

METHODS

The previous core sites were relocated on detailedtopographic maps (Fig. 3) using ®xed referencepoints and some remaining core-site markers.Vegetation was ¯attened in order to set up theGPR pro®le lines. The GPR system used was aGSSI SIR-SYSTEM 10 with 500 MHz antennae(transmitter and receiver) mounted together in asled. The sled was pulled along the survey linesat a steady, slow walk, and identi®cation markswere inserted every 2 m. Optimal acquisitionparameters were found to be: 52 kHz transmit-tance rate, 512 samples/scan, 32 scans/second,with an 80 ns range. Vertical low-pass andhigh-pass ®lters were applied (80 and 20 cycles/scan, respectively). Gain functions were not

Fig. 1. Location maps. (A) Regionalsetting of study reach within Ne-braska, USA. The Sand Hills regionis stippled. (B) Map of study reachdrawn from USGS 7á5 minute topo-graphic sheet. Black area is modern¯oodplain, and minor terraces arestippled. Areas corresponding todetailed maps of Fig. 3 are identi-®ed by arrows. Highway 183 is in-dicated.

978 J. Bridge et al.

Ó 1998 International Association of Sedimentologists, Sedimentology, 45, 977±986

applied during acquisition in order to allow forlater processing of undistorted digital records.Processing was carried out using Radan Win3software to remove or reduce a prominent, coher-ent `ringing' (machine noise) that occurred on allpro®les. The most effective ®lter proved to be aboxcar ®lter, a simple running average producedby summing the data over a window of the data,determined by the ®lter length, the result beingoutput as a single point at the centre of thewindow. A horizontal high-pass ®lter set at 255scans (equivalent to �32 wiggle traces or �1.8 mwith data stacked at eight scans per wiggle trace)was applied, removing background noise bysubtraction of a low-pass version of the data fromthe original data. A low-pass ®lter set at 11 scanswas applied to reduce high frequency horizontalnoise. Topographic corrections were performedon the pro®les prior to analysis. Pro®les wereplotted in wiggle-trace format following autogainapplication and trace stacking.

Preliminary analysis of the GPR data was donewith plots of two-way travel time against hori-zontal distance. The interpretation of GPR re¯ec-tion pro®les is carried out in a similar fashion toseismic re¯ection interpretation, using stratal(re¯ector) terminations to delineate genetically

related units, and correlating radar facies (re¯ec-tor amplitude, frequency and lateral continuity)with core data.

THE CHANNELS AND THEIR DEPOSITS

Details of the ¯oodplain and channel geometry,patterns of channel migration, and the nature ofchannel bar and ®ll deposits can be obtained fromBridge et al. (1986) and only a brief summary isgiven here. The active channel in the study reachhas low sinuosity (< 1á3) and, in places, mid-stream bars (Fig. 1). Dependent on sinuosity, thechannel is occupied by alternate bars or pointbars, the emergent parts of which form nuclei forbraid bars. Channel migration occurs by: (1)expansion (increasing sinuosity) and downstreamtranslation of channel bends and their associatedpoint bars; (2) chute cut off, and; (3) vertical,lateral and downstream growth of midstreambars. Channel migration rates are up to 2 m peryear. During abandonment, channels are ®lledprogressively from the upstream end, and anumber of relatively small side bars and mid-stream bars may occupy the ®lling channel,especially at its upstream end (Fig. 2). These bars

Fig. 2. Facies model of Bridge et al. (1986) for Calamus River. Sections showing large-scale bedding geometry arenormal to the river, through the upstream, mid, and downstream parts of an abandoned midstream bar and adjacentchannel ®ll. Arrows indicate relative directions of channel segments and bars.

GPR-imaged structure of Calamus River deposits 979


are relatively small because of the reduction indischarge during channel ®lling.

The channel-bar and channel-®ll deposits are upto 1á5 m thick and their base is always a distincterosion surface (Fig. 2). The deposits generally®ne upwards and can be grouped into three main

types of sediment. In order of vertical sequenceabove the basal erosion surface, these are:

1 Medium-scale, trough cross-strati®ed, verycoarse to medium sand. Cross sets are 0á03±0á25 m thick, and the whole interval is typically

Fig. 3. Topographic maps of study area, surveyed in 1984. Areas of maps are shown in Fig. 1B. Map (A) is imme-diately upstream of map (B). Thick unbroken lines are channel banks, with cutbanks indicated by ticks. Broken thicklines are terrace edges. Contours are in metres relative to an arbitrary datum: bankfull level is between ±1 m and ±1á25 m. Vibracore sites are marked by dots. GPR transects surveyed in 1994 are shown by straight, numbered lines.Stippled areas represent sediment deposited above low-water level between 1984 and 1994.



0á5±1á25 m thick. Bridge et al. (1986) referred tothese strata as large-scale cross strata, but the termmedium scale is used here following Bridge(1993b).

2 Small-scale cross-strati®ed, ®ne to very ®nesand, interbedded with vegetation-rich layers.Cross sets are less than 3 cm thick. Flood-related®ning-upward stratasets are centimetres to adecimetre or so thick, and sequences of this kindof strataset are typically 0á1±0á5 m thick. Plantroots are abundant in these strata, resulting insubstantial disruption of strati®cation in places.

3 Peat grading into densely rooted, vegetation-rich, very ®ne sand: generally about a decimetre

thick, but up to several decimetres thick inchannel ®lls.

The vertical sequence and thickness of chan-nel-bar and channel-®ll deposits varies spatiallywith position in bars and ®lls, and with channelsinuosity (Bridge et al., 1986; Fig. 2). For exam-ple, as a channel becomes more sinuous duringbend expansion, the bar sequence becomes pro-gressively thicker. Also, the downstream parts ofabandoned channels tend to contain a relativelylarger proportion of peat. The inferred large-scalestratal geometry and orientation in Fig. 2 weredrawn to re¯ect the vertical and lateral accretion

Fig. 4. GPR pro®les and interpretations ((A) and (B), respectively) for lines 7, 8 and 9, with key to vibracore logs.These lines are on the active midstream bar shown in Fig. 3(B). Vibracore logs are superimposed on the interpretedsections. On these sections, thick black lines represent the basal erosion surface and the land surface. Thick dashedlines mark the upper boundary of medium-scale cross-strati®ed sand. Thin lines are prominent GPR re¯ectors.



of channel bars, and the presence of smallermigrating bars near the entrance to ®lling chan-nels.

GROUND-PENETRATING RADARPROFILES

General character

Close comparison of GPR pro®les and core datagives an average near-surface wave velocity of0á055±0á06 m ns±1, that is typical of water-satu-rated sands (Davis & Annan, 1989). During thesurvey, the ground was saturated, with standingsurface water in places. It was not possible tocheck this velocity with common-mid-point anal-ysis with our equipment set up. There is a closerelationship between radar facies and sedimenta-ry facies in our data (see also studies of Beres &Haeni, 1991; Gawthorpe et al., 1993; Bridge et al.,1995). The basal erosion surface and medium-scale cross-strati®ed sand tend to be associatedwith relatively high amplitude, discontinuous,dipping re¯ectors (e.g. J in Fig. 4). The verticalseparation of re¯ectors in this radar facies trans-lates into sediment thicknesses of� 7±17 cm. Thecross sets in this facies have a similar range of

thickness: therefore, re¯ections may correspondto the changes in grain size and proportion ofdrifted plant material at the bases of medium-scalecross sets. Trough scores (e.g. K in Fig. 4) can beinterpreted on the basis of truncation of under-lying re¯ectors. The small-scale cross-strati®edsand is associated with relatively low-amplitude,discontinuous, wavy re¯ectors (e.g. L in Fig. 5).The vertical separation of these re¯ectors suggeststhat they correspond to the bases of dm-scale,¯ood-related stratasets. The low amplitude ofthese re¯ectors is possibly due to the lack ofgrain-size/compositional contrasts. Peat and verybioturbated, vegetation-rich sand have no obviousre¯ectors (e.g. M in Fig. 6). Re¯ectors are generallygreater in amplitude where a survey was conduct-ed over areas of standing water.

Figures 4±7 show examples of GPR pro®les andvibracore logs with interpretations. All of the GPRpro®les shown on Fig. 3 were analysed, but onlyselected pro®les could be presented here.

Pro®les through active midstream bars

Along-stream GPR pro®les through active mid-stream bars most commonly show re¯ectors thatare inclined downstream at a low angle (up to 2°)

Fig. 5. GPR pro®le and interpretation (upper and lower ®gures, respectively) for line 15. This line is along the stream-parallel axis of the active midstream bar shown in Fig. 3(A).



relative to basal erosion surfaces (Figs 4 and 5).Some re¯ectors in the uppermost parts of bars aremore or less parallel to the bar surface, but thereare distinct undulations in the strata (e.g. Fig. 4).In the lowermost, upstream parts of the bars thereare one or two laterally overlapping, convex-upward forms that are up to 0á5 m high and on theorder of 10 m in length (Figs 4 and 5). Thedepressions between these forms may showevidence of channeling. Convex-upward stratalsurfaces also occur in the upper, downstreamparts of the bars (e.g. Fig. 5).

The overall large-scale structure indicates thatmidstream bars grew partly by vertical and lateralaccretion, and partly by downstream extension.Downstream and lateral accretion of medium-scale cross-strati®ed sand is followed by verticalaccretion of ®ner grained facies. This is why thetopographically highest and oldest parts of thebars are at their upstream ends (Bridge et al.,1986). There is little evidence of substantialremoval of bar-head deposits by erosion at theupstream ends of the bars. The convex-upwardforms in the lower, upstream parts of the barsprobably represent the accretion of distinct unitbars forming the nucleus of the midstream bar.Apparently, small channels cut across the mid-stream bar, between the unit bars, as it accreted.Convex-upward forms in the downstream parts ofthe midstream bar suggest episodic growth ofscroll bars during ¯oods (Bridge et al., 1986).

These increments of deposition on the midstreambars are analogous to those observed in thedeposits of the River South Esk by Bridge et al.(1995).

Cross-stream GPR pro®les through one mid-stream bar (Fig. 4) show discontinuous re¯ectorsthat are more-or-less parallel to the bar surface,but with a preferred low-angle inclination to-wards the left-hand (viewed in downstreamdirection) channel, re¯ecting preferential lateralgrowth of the bar in that direction. Marginal scrollbars and a central, ®ne-grained swale ®ll areevident in the pattern of re¯ectors in Fig. 4.

Pro®les through abandoned barsand channel ®lls

Cross-stream GPR pro®les through abandonedbars and adjacent channel ®lls (Figs 6 and 7)show increasing depth of the basal erosionsurface towards the cut bank. Re¯ectors in thecoarser-grained facies are inclined relative to thebasal erosion surface (up to 6°) and dip towardsthe abandoned-channel cut bank (e.g. N in Fig. 6).These features are explained by episodic lateralmigration of bars as the channel bends increasedin amplitude, with a relatively minor componentof downstream translation (see Bridge et al.,1986). With increasing distance from the aban-doned-channel cut banks, the inclined strata ofthe bars may be either truncated by a chute

Fig. 6. GPR pro®le and interpretation (upper and lower ®gures, respectively) for line 20. This line is across theabandoned bar and channel ®ll shown in Fig. 3(A).



channel (Fig. 6), or may ¯atten out and dip in theopposite direction towards another, relativelyshallow channel (Fig. 7B). The latter case indi-cates growth and migration of a mid-channel barwithin a bend. Convex-upward (mound-like)features within these bars indicate increments ofdeposition associated with unit bars.

The thickest parts of the channel ®lls generallycontain the greatest proportion of ®ne-grainedfacies and peat (Figs 6 and 7B). However, mounds(up to 0á5 m high and metres across) of relativelycoarse sand with inclined, convex-upward re¯ec-tors also occur within the thickest parts ofchannel ®lls. These coarse deposits are interpre-ted as due to the migration of relatively small barswithin ®lling channels. The large-scale stratal

patterns shown in Fig. 7 suggest a complicatedsequence of erosion and deposition during theearly stages of channel ®lling.

Along-stream GPR pro®les through abandonedbars (not shown here) are similar to those foractive bars. The presence of low-angle inclina-tions dipping both upstream and downstreamsupports the observation that bars have a compo-nent of vertical and lateral growth in addition tosome downstream translation.

CONCLUDING DISCUSSION

This is the ®rst investigation of the deposits of amodern braided river using GPR. The main valueof GPR was in allowing documentation of the

Fig. 7. GPR pro®les and interpretations ((A) and (B), respectively) for lines 1 and 6. These lines are across theabandoned bar and channel ®ll shown in Fig. 3(B).



large-scale bedding geometry of channel bars andchannel ®lls. The use of the 500 MHz antennaeresulted in unprecedented resolution of strati®ca-tion. The results generally support the inferencesof Bridge et al. (1986) regarding large-scale stratalgeometry and orientation in across-stream sec-tions. However, the bedding structure in along-stream sections was unknown until completion ofthis study. This new information has allowedconstruction of a revised, more comprehensivefacies model (Fig. 8), that shares some of thecharacteristics of the braided-river facies modelspresented by Bridge (1993a; his ®g. 21). The mostsigni®cant improvements in the revised modelare: (1) provision of along-stream cross sections,and; (2) more accurate and detailed delineation ofthe geometry and inclination of large-scale in-clined strata. The detail in this revised facies

model should prove invaluable when the model isused to interpret ancient river deposits.

It is dif®cult to compare the present resultswith those from other studies of modern sandy,braided-river deposits because it has never beenpossible to describe the large-scale bedding ge-ometry for the entire extent of channel bars andchannel ®lls until now. In these other studies,descriptions have relied upon limited cores, andtrench and cut-bank exposures of the topmostparts of the channel bars and ®lls. However, thereare examples where large-scale inclined strata dipupstream in the upstream parts of braid bars, butgenerally dip downstream and across stream(Bristow, 1993; Singh et al., 1993).

The Calamus River is rather small, and of lowsinuosity and degree of braiding. There is apressing need for studies of the kind presented

Fig. 8. Revised facies model ofchannel bar and ®ll deposits of theCalamus River. (A) Pattern of for-mation and evolution of mid-stream bar, and abandonment and®lling of adjacent curved channel.The nucleus of the midstream baris at its upstream end. This bargrew by downstream, lateral andvertical accretion. Eventually, thecurved, left-hand channel began tolose discharge and ®ll with sedi-ment. Relatively small bars wereformed within this channel during®lling. The right-hand channel si-multaneously increased in dis-charge over time, and continued tomigrate laterally and downstream.Deposits of this channel blockedthe downstream exit of the left-hand channel. (B) Cross-sectionsthrough the abandoned channelbar and channel ®ll (positions lo-cated on A), showing facies andlarge-scale stratal geometry.



here for larger braided rivers with greater degreeof braiding.

ACKNOWLEDGMENTS

We are grateful for the ®nancial support of ARCO.The manuscript bene®tted from the comments ofChris Fielding and an anonymous referee.

REFERENCES

Alexander, J., Bridge, J.S., Gawthorpe, R.L., Leeder,M.R. and Collier, R.E.L. (1994) Holocene meanderbelt evolution in an extensional basin, SW MontanaUSA. J. Sedim. Res., 64 B, 542±559.

Beres, M. and Haeni, F.P. (1991) Application of ground-penetrating-radar methods in hydrogeologic studies.Groundwater, 29, 375±386.

Bridge, J.S. (1993a) The interaction between channelgeometry, water ¯ow, sediment transport and depo-sition in braided rivers. In: Braided Rivers (Ed. by J.L. Best and C. S. Bristow) Geol Soc., London, SpecialPublication, 75, 13±71.

Bridge, J.S. (1993b) Description and interpretation of¯uvial deposits: a critical perspective. Sedimentolo-gy, 40, 801±810.

Bridge, J.S., Alexander, J., Collier, R.E.L., Gawthorpe,R.L. and Jarvis, J. (1995) Ground- penetrating radarand coring used to study the large-scale structure ofpoint-bar deposits in three dimensions. Sedimentol-ogy, 42, 839±852.

Bridge, J.S. and Gabel, S.L. (1992) Flow and sedimentdynamics in a low sinuosity, braided river: CalamusRiver, Nebraska Sandhills. Sedimentology, 39, 125±142.

Bridge, J.S., Smith, N.D., Trent, F., Gabel, S.L. andBernstein, P. (1986) Sedimentology and morphologyof a low-sinuosity river: Calamus River, NebraskaSand Hills. Sedimentology, 33, 851±870.

Bristow, C.S. (1993) Sedimentary structures pre-served in bar tops in the Brahmaputra River,Bangladesh. In: Braided Rivers (Ed. J. L. Best andC. S. Bristow) Geol. Soc., London, Special Publi-cation, 75, 277±289.

Davis, J.L. and Annan, A. (1989) Ground penetratingradar for high-resolution mapping of soil and rockstratigraphy. Geophys. Prospecting, 37, 531±551.

Gabel, S.L. (1993) Geometry and kinematics ofdunes during steady and unsteady ¯ows in theCalamus River, Nebraska, USA. Sedimentology,40, 237±269.

Gawthorpe, R.L., Collier, R.E. Ll., Alexander, J., Bridge,J.S. and Leeder, M.R. (1993) Ground penetrating ra-dar: application to sandbody geometry and hetero-geneity studies. In: Characterization of Fluvial andAeolian Reservoirs (Ed. by C. J. North and D. J.Prosser), Geol. Soc., London, Special Publication, 73,421±432.

Huggenburger, P. (1993) Radar facies: recognition ofcharacteristic braided river structures of the Pleis-tocene Rhine gravel (NE part of Switzerland). In:Braided Rivers (Ed. by J. L. Best and C. S. Bristow),Geol. Soc., London, Special Publication, 75, 163±176.

Singh, H., Parkash, B. and Gohain, K. (1993) Faciesanalysis of Kosi megafan deposits. In: Current Re-search in Fluvial Sedimentology (Ed. by C.R. Field-ing), Sedim. Geol., 85, 87±113.

Manuscript received 18 November 1996; revision ac-cepted 25 October 1997.



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