Sedimentary Geology 143 (2001) 199±217 www.elsevier.com/locate/sedgeo
Department of Geography, Simon Fraser University, Burnaby, BC, Canada V5A 1S6
Received 19 April 2000; accepted 18 December 2000
Abstract
Ground penetrating radar (GPR) surveys have been carried out in order to characterise re¯ection patterns and to assess the method's potential for imaging alluvial fan sediments in southwestern British Columbia, Canada. The results from Cheekye Fan show that GPR achieved over 40 m penetration with the 50 MHz antenna and provided data that allowed identi®cation of 10 radar facies. Glacial till produces a re¯ection free con®guration and bedrock radar facies is characterised by a hummocky pattern often with stacked macro-scale diffractions. Four subaerially deposited alluvial-fan facies were identi®ed. Radar facies of the nearby meandering Squamish River are characterised by a complex, sigmoid oblique pattern that is clearly different from any con®guration observed on Cheekye Fan.
Based on the analysis of over 27 km radar data, the large-scale internal architecture of Cheekye Fan was reconstructed. Fan sediments are underlain by till and hummocky bedrock in the northern and eastern portion of the fan. Sedimentation was initiated by subaqueously deposited delta foresets following deglaciation of Upper Howe Sound. Subaerially deposited sediments can be divided into: (a) massive, matrix-rich diamicton that appears predominant at the core of the fan and at lower stratigraphic horizons, suggesting bouldery rock-slide or debris ¯ow origin; and (b) horizontally-bedded sheet¯ood gravel dominating the upper 20±25 m of the stratigraphic column.
q 2001 Elsevier Science B.V. All rights reserved.
Keywords : Ground penetrating radar (GPR); Alluvial fan; Radar facies; Land slides; Debris ¯ow; Cheekye Fan
1. Introduction
It is widely recognised that alluvial fans form in naturally unique sedimentary environments which can be readily distinguished from ¯uvial environments, including those of gravel-bed rivers, based upon differences in slope, radial pro®le, relief, geomorphic setting, sedimentologic processes and facies assemblages (Blair and McPherson, 1994a,b).
This general assertion leads to particular hypotheses
* Corresponding author. Tel.: 1 1-604-291-3321; fax: 1 1-604-
291-5841.
E-mail address: ekes@sfu.ca (C. EÂkes).
which remain poorly tested. For example, do alluvial fans have distinctive three-dimensional architecture re¯ecting a unique assemblage of the constituent alluvial facies? Can alluvial fans be distinguished from alluvial sediments deposited in valley ®lls by aggrading gravel-bed rivers?
Despite many alluvial fan studies, there remains a lack of understanding of the processes and controls in¯uencing alluvial fan sedimentation (for discussion see Blair and McPherson, 1992, 1994a,b; Hooke,
1993; Blikra and Nemec, 1998). This is partly due to the fact that (a) scienti®c publications dealing with alluvial fans are predominantly based on fans in arid to semi-arid environments, mostly located in
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200 C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217 the south-western United States, and (b) there is a lack of reliable, detailed, subsurface and quantitative data
(Blair and McPherson, 1994a,b).
The paucity of data on the three-dimensional internal architecture of alluvial fans re¯ects the dif®culty of drilling in these sites and of ®nding appropriate sections for study. Although many alluvial fans are incised by the modern channels, the opportunities for ensuring more than a few linear exposures are normally few.
The recent developments and application of ground penetrating radar (GPR) to image the subsurface may provide a means to reduce this observation/sampling problem on alluvial fans. GPR has been used for pro®ling various ¯uvial deltaic and non-deltaic coastal systems (Bridge et al., 1995, 1998; Beres et al., 1999; Huggenberger, 1993; Jol and Smith, 1991;
Jol and Roberts, 1992; Meyers et al., 1996; van
Heteren et al., 1998), however, this is the ®rst account of its successful application for studying alluvial fans.
It is the general purpose of this paper to explore the degree to which the sedimentary architecture of alluvial fans yields characteristic radar re¯ection patterns.
Speci®cally, we use GPR to describe the facies assemblage and large-scale internal architecture of Cheekye
Fan in southwestern British Columbia, Canada. This paper is a contribution to better understand the complex internal structure of alluvial fans and the
®rst application of GPR to analyse the problem.
Fig. 1. Location of Cheekye Fan in southwestern British Columbia,
Canada.
2. Study area
Cheekye Fan is located within the Coast Mountains of British Columbia (Fig. 1). This environment is characterised by high relief, steep slopes, heavy rainfall and landslide-prone materials (Evans and
Savigny, 1994). Cheekye Fan was initiated by prehistoric, multiple massive landslide events (Mathews,
1952) and debris ¯ow activity which has continued to the present time.
The modern fan has a radius of 3±4 km, an average slope of 5%, and covers an area of about 8.3 km 2
(Thurber Engineering Ltd and Golder Associates
Ltd, 1993). The western edge of the fan is truncated by the Cheakamus and Squamish Rivers, producing a scarp about 15 m high in places. The southern edge of the fan is graded to the Squamish river ¯oodplain at about 10 m asl. A number of bedrock outcrops exist in the northern portion of the fan, at the mouth of
Cheakamus River, but the western and southern portions of the fan have a broadly undulating surface.
The stratigraphy of parts of the lower fan is exposed in a number of pits and cut banks. A 15 m-high exposure along Squamish River, reveals the surfacefan composition to be a complex of sediments consisting of matrix-supported debris-¯ow diamicton and clast-supported planar, weakly bedded to massive sheet¯ood materials. The dominant lithology of fan materials is volcanic rock consisting of red to grey dacite derived from Mount Garibaldi. The texture and structure of fan sediments varies greatly from strati®ed, well-sorted sand and gravel of sheet¯ood origin to massive, poorly sorted, matrix-supported diamicton of debris ¯ow origin. Sheet¯ood sediments have subrounded to rounded clasts, with a typical sample consisting of 5% boulders, 30% cobbles,
40% pebbles, 20% sand, and 5% ®nes. Debris-¯ow facies contain angular to subangular clasts, with a typical sample consisting of 5% boulders, 10%

C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217 cobbles, 60% pebbles, 20% sand, and 5% ®nes
(Thurber Engineering Ltd and Golder Associates
Ltd, 1993). The distinction between sheet¯ood and debris ¯ow facies is clast versus matrix-supported structure, rounded versus angular clasts, and maximum clast size of 0.8 versus 2 m, respectively.
Debris-¯ow diamicton may contain a greater proportion of cobbles versus pebbles. Debris ¯ow units vary in thickness from about 5 m at the proximal part of the fan to 0.5±2 m in distal locations.
3. Methods
Field work included gathering all available outcrop, test pit, water-well log data which were integrated with air photo and GPR data analyses. Ground penetrating radar (GPR) is a surface-geophysical method that depends on the emission, transmission, re¯ection, and reception of an electromagnetic pulse and can produce rapidly and ef®ciently continuous high-resolution pro®les of the subsurface (Beres and
Haeni, 1991). Pro®ling by GPR is similar to sonar and seismic re¯ection pro®ling, except that it is based on the propagation and re¯ection of electromagnetic
(EM) energy (Jol and Smith, 1991, 1992a). The radar produces a short pulse of high frequency
(10±000 MHz) EM energy which is transmitted into the ground. Some of the energy is re¯ected back to the surface due to a change in bulk electrical properties of different subsurface lithologies (e.g. sand and mud) and the character of the interface (Moorman et al.,
1988; Davis and Annan, 1989). At the surface, a receiver monitors re¯ected energy versus delay time. The pulse delay time from the energy transmitted into the ground and re¯ected back to the receiver is a function of the strength of energy transmitted into the ground, the EM propagation velocity through the sediment, the depth of subsurface re¯ectors and the character of the interface (Moorman et al., 1988; Davis and
Annan, 1989; Jol, 1993).
Contrasts in the dielectric constants of the different sediment types usually cause strong re¯ections from lithologic boundaries in the subsurface (Jol and Smith,
1991). The strength of the re¯ected signal is approximately proportional to the difference in dielectric constants of the sediment interface (Davis and
Annan, 1989). Electrical inhomogeneities are present
201 in most hydrogelogic settings and are determined primarily by water content, dissolved minerals and expansive clay and heavy-mineral content in the subsurface material (Wright et al., 1984; Haeni et al., 1987). Changes in the dielectric constant also affect the rate of attenuation of energy passing through the ground. These effects enable the subsurface stratigraphy and ground-moisture conditions to be inferred from the character of the radar return signals (Jol and
Smith, 1992a).
The re¯ected signal is ampli®ed, transformed to the audio-frequency range, recorded, processed and displayed. The record shows a pro®le of horizontal survey distance in metres versus two way travel time in nanoseconds. By measuring the propagation velocity of the sediment, the depth of the re¯ectors can be determined.
Interpretation of GPR pro®les involves the deciphering of interference patterns rather than discrete re¯ections and diffractions that are characteristic of re¯ection seismic pro®les (Gawthorpe et al., 1993).
The reliability of the interpretation is dependent on the experience of the interpreter and their ability to use reference material.
Radar data were collected from 11 lines oriented parallel to the dip of Cheekye Fan and from six strike oriented lines along existing roads (Fig. 2). A pulseEKKO IV e system was used with a 400 V transmitter. The 50 MHz antenna was selected for maximum penetration with 1 and 2 m separation and
0.5 and 1 m step size. A topographic survey was conducted during pro®ling.
Data were processed using the `pulseEKKO' software package. Topographic corrections were performed on the pro®les prior to analysis. For presentation, Automatic Gain Control was applied.
The number of stacks was kept at 64 per trace.
Low pass temporal ®ltering was applied to reduce random noise. Data are displayed in the wiggle trace format, which is the traditional format for seismic data processing. The horizontal scale of all pro®les is distance in meters, and the vertical scale is shown as both two-way travel time in nanoseconds and depth in metres based on near-surface velocity.
An average near-surface wave velocity of
0.10 m ns 2 1 was calculated from the results of
CMP analysis. The pro®les presented show a 2:1 vertical exaggeration. For brevity local information

202 C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217
Fig. 2. Sur®cial geology map of Cheekye Fan showing layout of GPR survey lines. Thick black lines show location of pro®les shown in Fig. 6.
GD, Garbage Dump exposure; CG, Cheekye Gravel pit exposure; FER, Fernwood Road; EXP, Explosives Road; EPW, East Powerline Road;
LFR, Land®ll Road, GVT, Government Road; SVR, Squamish Valley Road; POW, Powerline Road; H99, Highway 99: SQD, Squamish Dyke.
was excluded and replaced by three letter codes. The codes are explained on the title of Fig. 6.
3.1. Ground penetrating radar calibration
GPR interpretation is based on the identi®cation of re¯ection patterns and is veri®ed by stratigraphic information from outcrop data, well information, test pitting, air photo and topographic cross-section analysis, and familiarity with the Quaternary history of the area. As part of an ongoing project, over 95 km radar data from various sites in southern British
Columbia were collected and analysed. These sites include various types of mass movement deposits,
¯uvial environments and many alluvial fans. Wherever it was possible GPR was calibrated against exposed outcrops. The experience gained from the calibration greatly enhanced our ability to better understand GPR response and aided interpretation where other subsurface information was limited. In addition GPR calibration was carried out at two locations on Cheekye Fan where sediments of known origin are exposed.
1. The Squamish municipal garbage disposal site offered a type section for debris-¯ow diamicton

C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217 203
Fig. 3. GPR pro®le and photograph of debris ¯ow diamicton at the Squamish municipal garbage dump exposure. Survey rod on photo is 3 m long.
in a freshly exposed pit (Fig. 3). The over 5.5 m thick unit, partially covered with contemporary household garbage was surveyed with various antenna frequencies and step sizes. The GPR pro®les show shallow penetration and generally poor data quality at places obscured by multiples
(Fig. 3). The lack of penetration is believed to be due to the high ®nes content of the unit: 25% silt,
3.5% of which is clay (Thurber Engineering Ltd and Golder Associates Ltd, 1993). The multiples were likely caused by environmental `noise' such as metal objects within the land®ll and to a lesser degree by the vicinity of large earth-moving equipment.The upper 5 m of the pro®le is characterised by poor or chaotic re¯ection pattern which re¯ects the lack of internal structure in the diamicton unit. This pattern is interpreted as matrix rich debris-¯ow diamicton. Below the uppermost
5±8 m the data are obscured by multiples and signal attenuation.
2. A local gravel pit where over 8 m crudely bedded to massive sheet¯ood gravel is exposed providing a favourable location for radar calibration (Fig. 4).
With the 50 MHz antenna, more than 50 ns penetration was achieved (Fig. 4). The overall re¯ection pattern on the strike oriented section is discontinuous and somewhat chaotic (Fig. 4). The dip section shows better horizontal continuity. Individual radar re¯ections could not be tied to depositional features, however, the overall stratigraphy of the 8 m of massive gravel exposed in this pit generally correlates well with the overall radar re¯ection pattern . Leclerc (1995) reached similar conclusions after comparing GPR data collected with various antenna frequencies with a photomosaic of a section of strati®ed sand and gravel deposits.

204 C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217
Fig. 4. GPR pro®le and photograph of exposure at Cheekye Gravel pit showing crudely bedded sheet¯ood gravel and sand. Outcrop is oriented perpendicular to ¯ow direction.
3.2. GPR facies
The full length of all GPR pro®les (Fig. 2) were analysed and interpreted, but only selected pro®les are presented in this paper. Ten macro-scale radar facies distinguished by re¯ection patterns whose appearance differs from that of adjacent units were identi®ed on Cheekye Fan pro®les (Fig. 5). They are described in terms of re¯ection continuity, shape, amplitude, internal re¯ection con®guration and external form using the approach applied by Beres and
Haeni (1991); van Heteren et al. (1998); Beres et al.
(1999). Macro-scale radar facies are de®ned here as a radar facies exceeding ca. 2 m thickness and ca. 40 m in horizontal distance on a two-dimensional GPR pro®le. This scale is based on the resolution of the
50 MHz antenna, which under ideal conditions is capable of imaging objects as small as 0.5±1 m diameter (Jol, 1993). This facies scheme was developed for Cheekye Fan, but with modi®cations can be used for other alluvial fans.
3.2.1. Radar facies 1: re¯ection-free pattern that is not machine related
A re¯ection-free con®guration may signify: (a) massive homogenous lithologic units; (b) the presence of highly conductive dissolved minerals in groundwater; or (c) the presence of sediments containing high amount of clay that attenuates all of the EM signal, preventing it from penetrating deeper units that would otherwise be characterised by distinct re¯ection patterns (van Heteren et al., 1998).
Attenuation of the EM signal is exhibited in some very prominent ways in units characterised by high

C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217 205
Fig. 5. Summary of GPR facies and their interpretation Cheekye Fan. The vertical dimension of the re¯ection patterns varies between 6 and
12 m.
conductivity losses. The Garbage Dump (GDS, GDD)
(Figs. 2 and 3) and LFR pro®les are prime examples of shallow signal penetration and poor data quality.
The Garbage Dump debris-¯ow diamicton which exceeds 5 m in thickness at this location overlies three other diamictons to a depth of about 10 m.
Till is the other common deposit on the study area with high clay content. Boreholes WW 11 and 12
(Fig. 2) drilled on the northern section of the fan encountered a 20 m thick till unit below 12 m of fan gravel (Thurber Engineering Ltd and Golder
Associates Ltd, 1993). The poorly-de®ned re¯ection pattern and lack of penetration on the FER pro®le and below 10±20 m on the west side of the CBW pro®le was interpreted as till.
3.2.2. Radar facies 2: hummocky or chaotic re¯ection pattern with micro-scale, sometimes stacked diffractions
The macro-scale hyperbolic con®guration was identi®ed on many radar sections, most of which were collected in areas with nearby bedrock outcrops.
Sections with hyperbolic facies are characterised by prominent irregular re¯ections that mark the top of the

206 bedrock unit. Their internal pattern is often one of stacked hyperbolas or diffractions taking the inverted chevron shape (Fig. 6A, C). Penetration within this facies is good, often greater than 800 ns (40 m).
3.2.2.1.
facies 1
Interpretation:
C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217 bedrock.
Hyperbolic con®gurations arise as a result from the response of the diverging GPR beam to an irregular boundary (van
Heteren et al., 1998). EM energy returns to the receiver not only through re¯ection on nearhorizontal surfaces but also through re¯ection of that part of the peripheral GPR beam that forms a
Fresnel zone on subhorizontal to sharply-inclined boundary surfaces (van Heteren et al., 1998).
Examples of bedrock facies are given on the EPW between 310 and 855 m (Fig. 6A), and on EXP between 0 and 190 m (Fig. 6C) sections.
3.2.3. Radar facies 3: chaotic re¯ection pattern with wide crown meso-scale diffractions: alluvial fan
This facies occurs at depths of 300 ns (15 m) or greater and is characterised by re¯ection discontinuity with hyperbolas of meso-scale. No stacking is apparent, neither is the inverted-chevron pattern present.
This pattern is visible on strike sections such as
EPW (1530±1720 m) (Fig. 6A) and POWa. This re¯ection con®guration is found on medial to distal parts of the fan at low stratigraphy horizons.
deeply by the trunk glacier, so that bedrock is unlikely to occur at this relatively shallow depth.
A similar radar pattern is present on the H99 pro®le between 240 and 555 m at 375 ns (18 m) depth (Fig.
6B). This is also interpreted as bouldery fan facies.
Here, due to the closeness of a steep bedrock outcrop
(Fig. 2), this re¯ection pattern is interpreted as imaging rock fall facies; there is ®eld evidence of recent and past rock falls and slides at this location.
3.2.4. Radar facies 4: chaotic high-frequency re¯ection pattern with micro-scale hyperbolas
This facies is prevalent on the southern section of the
GVT pro®le. This pattern is a common feature of records collected along paved roads. These lines run through the community of Brackendale (Fig. 2), where the survey was conducted partly on the asphalt road surface. Individual re¯ections are mostly discontinuous and often chaotic with prominent micro-scale hyperbolas. This facies is characterised by shallow penetration and by many multiples, indicating an environment generally not favourable for GPR surveys.
3.2.4.1. Interpretation: fan facies with buried utility cables and pipes, nearby trees.
The high-frequency chaotic radar facies exclusively occurs on GPR lines from areas where the upper part of the natural stratigraphy has been disturbed. Diffraction patterns caused by overhead power lines, trees and other large objects on or above the surface are common. Some of the individual hyperbolic re¯ections were caused by
®re hydrants along the survey line. Multiples are caused by antenna ringing likely due to the vicinity of metal objects such as fence posts or vehicles
(Moorman, 1990; Rea et al., 1994).
3.2.3.1. Interpretation: alluvial fan facies with boulders.
This signal pattern is interpreted as having originated from bouldery debris-¯ow sediments.
Diffractions point to the presence of boulders and the chaotic pattern indicates the lack of internal structure. The fact that this facies is found at the base of the fan suggests that the bouldery debris
¯ows took place while the lower Squamish Valley was still occupied by ice and these sediments travelled across on the surface of the ice and were dumped as the ice retreated.
This radar pattern somewhat resembles that of bedrock although borehole data suggest that bedrock is out of GPR range at 32 m depth in the vicinity of
Garbage Dump and locations upstream. Also, the central and western part of the fan formed over the centre of Squamish Valley that was likely scoured
3.2.5. Radar facies 5: discontinuous, hummocky, wavy re¯ection pattern: alluvial fan facies 2
This faces is characterised by short, discontinuous, irregular or wavy re¯ection patterns and is prevalent on most pro®les (Fig. 6A±D). Closer to the apex of the fan, this faces is generally present on the surface.
With increasing distance from the apex this pattern is found at greater depth.
3.2.5.1. Interpretation: poorly-bedded sheet¯ood sand and gravel or low clay-content matrix-rich debris-
¯ow diamicton.
This facies is interpreted as having

Fig. 6. Cheekye Fan GPR pro®les and their interpretation: (A) East Powerline Road (EPW); (B) Highway 99 (H99) (0±830 m); (C) Explosives
Road (EXP); (D) Squamish Dyke (SQD) (0±900 m).
C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001)199±217 pp. 207±212

C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217 resulted from massive debris-¯ow deposits and/or from thin or poorly-strati®ed sheet¯ood gravels and sands. The lack of strati®cation and limited lateral continuity of the massive debris-¯ow units explain the discontinuous and chaotic nature of the radar re¯ections. The fact that this facies is more prevalent closer to the apex and at greater depth also supports this interpretation, since at early stages of fan growth there were more numerous and larger magnitude debris-¯ow events.
3.2.8. Radar facies 8: trough-shaped re¯ection pattern
Trough-shaped re¯ectors were found on the majority of the strike-oriented pro®les. Most examples of this pattern are found near the fan surface: EPW 440±
570 m (Fig. 6A), FER, H99a and SQD (Fig. 6D).
Their lateral extent is between 30 and 280 m and depth rarely exceeds 5 m.
213 the surface diamicton unit than those of older sediments.
3.2.6. Radar facies 6: horizontally continuous, layered parallel re¯ection pattern: alluvial fan facies 3
This facies is characterised by strong, surfaceparallel, horizontally continuous re¯ections which can be followed over several hundred metres (Fig.
6A±C). This pattern is prevalent on the majority of the pro®les. Generally, it is found at higher stratigraphic positions on the fan and is more common with increasing distance from the apex (Fig. 6A±D).
3.2.8.1. Interpretation: channel ®ll.
The diameter of these features corresponds well with the size of the existing modern channel on Cheekye Fan (50±
100 m). Interpretation is based on this fact and the classic bowl shape (Fig. 6A, D). The fact that most channels are found near the fan surface, suggests that inferred channels on alluvial fans have little preservation potential, or perhaps channel avulsion has been more frequent in the recent past than earlier, or their radar signal is not different from that of the surrounding sediments.
3.2.6.1. Interpretation: strati®ed sheet¯ood gravel and sand and/or matrix-rich debris-¯ow diamicton.
This faces is interpreted as horizontally-bedded sheet¯ood sand and gravel and/or clast-rich debris-
¯ow diamicton. This interpretation is based on information from natural outcrops, well logs and shallow test pits as well as the GPR re¯ection pattern.
3.2.9. Radar facies 9: oblique clinoforms
Steeply inclined (approximately 15±25 8 ), highamplitude clinoforms were found on three pro®les:
POWa, SVr and H99a (630±920 m) (Fig. 6B). This pattern is characterised by 10±20 m thickness and is only found below alluvial-fan sediments.
3.2.7. Radar facies 7: chaotic or poorly de®ned re¯ection pattern: alluvial fan facies 4
This facies is characterised by a chaotic re¯ection pattern and poor signal penetration. The best examples are found on pro®les collected in the vicinity of
Squamish municipal garbage dump: Garbage Dump strike (GDS) and LFR (Fig. 3). Elsewhere, this pattern is found in a near-surface position; e.g. on EPW (60±
120 and 1580±1700) (Fig. 6A) and on POWa pro®les.
3.2.7.1. Interpretation: matrix-rich debris-¯ow diamicton.
The Garbage Dump debris-¯ow unit exceeds 5 m thickness at these locations and its high clay content explains the lack of penetration and the poor data quality. Elsewhere, the upper debris-¯ow deposit is thinner ( , 3 m), therefore, its effect on signal penetration is not that dramatic. The grainsize analysis also shows a higher clay content for
3.2.9.1. Interpretation: delta foresets.
Clinoforms indicate near-unidirectional deposition semi-parallel to the GPR record and were identi®ed as delta foresets. Twenty-®ve degrees is the underwater angle of repose for sand and gravel (Smith, 1991), however it is likely that the pro®les were not along maximum dip. The top of the foresets is found at a depth of 18±32 m and can be followed over several hundred metres (e.g. Fig. 6B). Similar re¯ection patterns from late Pleistocene and Holocene river deltas were identi®ed as fan-foresets by Jol and
Smith (1992a,b). High-amplitude re¯ections and deep penetration signify an alternation of gravel and sand with little or no clay and silt (van Heteren et al.,
1998), a fact that also explains good signal penetration achieved within this facies.

214 C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217
3.2.10. Radar facies 10: complex sigmoid oblique re¯ection pattern
Sigmoidal oblique re¯ection con®gurations are marked by high amplitude, continuous re¯ections and good penetration. This pattern is found exclusively on the margin of Cheekye Fan on GVT and
SQD (520±900 m) pro®les (Fig. 6D).
3.2.10.1. Interpretation: modern ¯oodplain.
A complex sigmoidal re¯ection pattern corresponds with a variety of internal structures such as epsilon cross-beds and longitudinal bar foresets which in turn are evidence of channel and bar migration and point bar deposition. These features together are the basis for interpreting this radar facies as ¯oodplain sediments. This interpretation is supported by the fact that the SQD pro®le was collected on an arti®cial dyke built on the Squamish ¯oodplain in the vicinity of the active channel (Fig. 2).
4. Discussion
Ground penetrating radar proved to be a useful survey tool that allowed the collection of over
27 km subsurface data on the Cheekye Fan (Fig. 2).
GPR data interpretation is the most challenging component of GPR work. It relies largely on the availability of reliable subsurface information such as well logs, borehole information, accurate outcrop, well-log and test-pit descriptions and on a good understanding of local Quaternary geology. Interpretation can be greatly aided by calibration. Re¯ection con®gurations from different sediment types, however, can be very similar in appearance, therefore the interpreter's understanding of regional geology is of paramount importance.
The differentiation between sheet¯ood and debris-
¯ow sediments will be brie¯y addressed below.
Although initial GPR calibration surveys showed some promise to distinguish debris ¯ow and sheet-
¯ood facies (Figs. 3 and 4), the results are not consistent. Debris-¯ow sediments produce distinctive radar signals if their conductivity is suf®ciently different from those of adjacent units and if the thickness and depth of a sediment body in question is within the range of the transmitter antenna frequency. This was the case with alluvial fan facies 4, which is characterised by chaotic or poorly de®ned re¯ection con®guration and found locally in near-surface position. This facies is interpreted as matrix-rich debris-
¯ow diamicton with high clay-content and exceeding
3 m thickness.
For most of the sediments contained on the
Cheekye Fan, the distinction between sheet¯ood and debris-¯ow sediments was not so clear-cut. Clast-rich debris-¯ow sediments and sheet¯ood gravels can have very similar appearance in outcrops or in test pits and similar conductivity, and, therefore, these units may produce very similar radar signals. Moreover, the thickness of individual units is often below the resolution of the 50 MHz antenna. Therefore, fan facies 2 and 3 could not be conclusively interpreted as either debris ¯ow or sheet¯ood origin, only as matrix-rich diamicton or poorly-bedded sheet¯ood sand for facies
2 and as horizontally-bedded sheet¯ood sand and gravel or clast-rich debris-¯ow diamicton for facies
3. The distinction between fan facies 2 and 3 is based on the presence or absence of horizontally continuous and layered elements re¯ecting the likely predominance of sediments within that facies. The interpretation could be further re®ned in the future with the use of higher antenna frequencies (100 or 200 MHz) resulting in better resolution at the expense of penetration depth.
GPR distinguished reliably between alluvial fan and non alluvial fan sediments. There is a clear separation between delta foresets and overlying fan sediments as well as bedrock and overlying fan sediments (Fig. 6A±D). The top of the underlying till can only be tentatively delineated. This is still useful information if it can be veri®ed by test-pits or drilling.
With suf®cient GPR penetration, the underlying topography can be inferred and the sediment volume stored on the fan can be calculated (Friele et al.,
1999).
It also became apparent that none of the ¯uvial
GPR facies was present within or under Cheekye
Fan sediments. This implies that alluvial fans produce distinctly different radar signals from those of ¯uvial sediments. It also has important implications for the local Quaternary history (see Friele et al., 1999). This study has also demonstrated that
GPR can be successfully used to detect landslide sediments, a fact that has far reaching social and economic importance.

C. EÂkes, E.J. Hickin / Sedimentary Geology 143 (2001) 199±217 215
5. Internal architecture of Cheekye Fan
Identi®cation of ¯oodplain, bedrock, fan-delta foresets, till, channels and four alluvial fan facies in conjunction with sea level and test pit data enabled the construction of a architecture (Fig. 7).
Fig. 7. Schematic cross section of Cheekye Fan showing its internal architecture.
generalised model of internal fan
Near the apex in the eastern fan sector and at the mouth of Cheakamus River in the northern fan sector, fan deposits overlie till and hummocky bedrock
(Fig. 7). It is also likely that hummocky, ice-lowered rock-avalanche debris underlies a portion of the fan near the apex. The topography of this complex underlying surface is not fully known. A well log near the apex (WW 15; Fig. 2) indicates bedrock at 30 m depth.
GPR penetration increased downfan to 40 m and did not encounter bedrock (Fig. 6B) suggesting a greater thickness of alluvial fan deposits in that direction. Two well logs (WW 11 and 12; Fig. 2) in conjunction with
GPR data at the mouth of Cheakamus River indicate
10±20 m depth to till at that locality.
Two basic architectural components were de®ned within fan sediments: subaqueously deposited fan deltaic sediments and subaerial alluvial fan sediments.
Foreset beds are prominent on the H99 (Fig. 6B),
POW and SVR pro®les.
Fan sediments can be further divided into four GPR facies (Fig. 5). Fan facies 2, interpreted as predominantly matrix-rich debris-¯ow diamicton or poorlybedded sheet¯ood sand and gravel is predominant at lower stratigraphic horizons and at the core of the fan.
Fan facies 1, is also most common at this horizon.
This suggests that the early development of the fan is characterised by bouldery debris-¯ow, rock slide or rock fall deposits.
Alluvial fan facies 3, interpreted as chie¯y horizontally-bedded sheet¯ood sand and gravel or clastrich debris-¯ow diamicton become the dominant facies in the upper 20±25 m of the fan stratigraphic column in the western, central and southern fan sections indicating that sheet¯ood activity and
¯uvial reworking were more dominant processes in the later development of the fan. With decrease in sediment supply there was a tendency toward incision and channel abandonment, thus bowlshaped re¯ections interpreted as channels are seen at shallow depth (Fig. 6A, D).
Acknowledgements
This project was funded by the Natural Sciences

216 and Engineering Research Council of Canada and by Simon Fraser University. The authors wish to thank the referees Peter Huggenberger and R.A.
Overmeeren for their suggestions which have improved the quality of the manuscript.
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