RADAR FACIES OF A MEANDERING RIVER FLOODPLAIN,
NORTH THOMPSON RIVER, BRITISH COLUMBIA
Rene F. Leclerc
B.Sc. (Hons), McGill University, 1992
THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
in the Department
of
Geography
O Rene F. Leclerc 1995
SIMON FRASER UNIVERSITY
July 1995
All rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without permission of the author. APPROVAL
Name: Rene Francois kclerc
Degree: Master of Science
Title of Thesis: Radar Facies Of A Meandering River Floodplain, North Thompson River, British Columbia
Examining Committee: Chair: R.D. Moore. Assistant Professor
kl.Hickin, Professor Senior Supervisor
M.C. Roberts, Professor
- - Dr. Brim Ricketls, Research Scientist, Geological Survey of Canada External Examiner
Date Approved: June 26. 1995 PARTIAL COPYRIGHT LICENSE
I hereby grant to Simon Fraser University the right to lend my thesis, project or extended essay (the title of which is shown below) to users of the Simon Fraser University Library, and to make partial or single copies only for such users or in response to a request from the library of any other university, or other educational institution, on its own behalf or for one of its users. I further agree that permission for multiple copying of ths work for scholarly purposes may be granted by me or the Dean of Graduate Studies. It is understood that copying or publication of this work for financial gain shall not be allowed without my written permission.
Title of Thesis/Project/Extended Essay
Padar Facies Of A Meandering River Floodplain, North Thompson River.
British Columbia
Author: - - (signature)
Rene Francois Leclerc (name) ABSTRACT
A ground-penetrating radar (GPR) study of floodplain deposits was conducted on a confined meander of the North Thompson River near Karnloops, British Columbia. A survey grid consisting of 20 individual GPR profiles from 200 m to 900 m in length was constructed on a 0.9 km2 section of the contemporary floodplain. All GPR profiles were collected using a pulseEKKOTMIV GPR system with 100 or 200 MHz antennae, depending on the depth of floodplain sediments. Floodplain deposits consisted of sand and minor gravel overlying silt and clay sediments approximately 8 to 12 m below the floodplain surface.
Vibracore and auger data taken at nine locations in the GPR survey grid showed that changes from fine sand to medium gravel do not produce distinct radar reflections in GPR profiles but that the water table often does result in a distinct, high-amplitude radar reflection. Well log data showed that rapid signal attenuation between 8 and 12 m depths in GPR profiles was caused by underlying silt and clay sediments, useful for mapping 3-D variability in the depth of floodplain deposits.
Prior to the interpretation of GPR profiles from the river floodplain, a calibration study comparing GPR profiles with corresponding geologic exposures at two gravel pit sites was conducted. Results showed that inferences of underlying stratal trends based on GPR profiles were most accurate if derived from a consistent pattern of radar reflections and not from individual reflections, which may not accurately depict local trends.
GPR profiles from the North Thompson river floodplain were divided into macroscale
(> 75 m horizontal distance) radar facies and interpreted based on their orientation relative to surface scroll topography. Two architectural elements were identified, a channel fill (CH) and a lateral accretion (LA) element. The CH element was characterized by channel fill radar facies in cross-section and downstream-dipping reflections in channel-parallel orientation. The LA element showed mainly inclined stratification (IS) and (or) preserved ridge-and-swale (RAS) deposits along the dip direction of surface scroll bars. GPR profiles exhibiting well-developed RAS architecture tended to show little inclined stratification, appearing as individual reflections dipping from the top to the base of the floodplain in the direction of floodplain accretion. Preserved scroll ridge deposits exhibited asymmetry at depth, dipping more steeply in the direction of floodplain accretion. Strike direction profiles, parallel to surface scroll ridges, exhibited either parallel or slightly (0" - 5") inclined reflections dipping in the downstream direction. ACKNOWLEDGMENTS
The author would like to thank his thesis supervisor, Dr. Ted Hickin, for initially suggesting the idea for this study and for providing intellectual and financial support throughout its course, thanks Ted. Thanks also to Dr. Mike Roberts for his many suggestions and helpful advice during thesis production. The following are also recognized for their contributions:
- Lowell Wade, Hugh Baker, Matt Ferguson, Csaba Ekes, Alan Paige, and Mariette Prent for their assistance in collecting data in the field;
- the Puhallo family, for granting unlimited access to the Puhallo Ranch, site of thesis work;
- the Radar Group at the Geophysics Department of the University of British Columbia, especially Mike Knolls, Rob Luzitano, and Jane Rhea for their time and advice in helping a GPR novitiate learn the ropes;
- Dr. Harry Jol for his expertise in GPR, and for directing me to the many helpful references that are now cited in this thesis;
- Dr. John Bridge, for kindly providing a draft copy of his GPR work on meandering streams;
- Lauren Donnelly for assistance with the digital elevation models;
- Matt, John, and Larry for being the best of friends and making my time here seem like I was back in high school. Cool. And thanks mom and dad, for everything. Funding for this thesis was provided by an NSERC grant to Dr. Ted Hickin and by a Simon Fraser University Fellowship. TABLE OF CONTENTS
. . Approval Page ...... 11 ... Abstract ...... 111
Acknowledgments...... v
Table of Contents...... vi
List of Tables ...... ix
List of Figures...... x
Chapter 1 - Introduction
1.1 The Research Problem ...... 1
1.2 Objectives...... 3
Chapter 2 - Literature Review
2.1 Channel Pattern and Floodplain Classification...... 4
2.1.1 Early Models of Meandering River Floodplains...... 9 2.1.2 Sedimentological Criteria for the Identification of Meandering Channel Pattern ...... 10
2.2 Architectural Elements - Description and Identification...... 14
2.2.1 Epsilon Cross-Stratification in Meandering Streams: Modern and Ancient...... 16 2.2.2 Ridge and Swale (RAS) Topography in Meandering River Deposits...... 18
2.3 Ground-Penetrating Radar: Use in Sedimentology...... 20
2.3.1 3-D Radar Stratigraphy of Meandering River Point Bars...... 25 Chapter 3 .Study Area
3.1 Regional Setting...... 28
3.1.1 North Thompson River Study Area...... 31
Chapter 4 .Methods
GPR Theory and Instrumentation ...... 36
4.1.1 High Frequency Electrical Properties of Materials...... 37 4.1.2 GPR Instrumentation...... 44
Interpretation of Radar Reflection Profiles ...... 47
4.2.1 Environmental and Equipment-Related Noise on Radar Profiles...... 47 4.2.2 Ground-Truthing Radar Reflection Data ...... 49
GPR Survey Design: North Thompson River...... 61
4.3.1 Site Selection...... 61 4.3.2 GPR Survey Design ...... 63 4.3.3 Core Sampling Technique...... 67
Chapter 5 .Results
5.1 Core Log Comparisons with GPR Profiles...... 69
5.2 Interpretation of GPR profiles ...... 75
5.2.1 Velocity estimation...... 78 5.2.2 Interpretation of 100 MHz GPR profiles ...... -78 5.2.3 Interpretation of 200 MHz GPR profiles ...... 98
5.3 Three-dimensional (3-D) Distribution of Selected Radar Facies ...... 103
Chapter 6 .Discussion
6.1 Meandering River Floodplain Architecture and Comparisons with Existing Models...... -112
6.2 GPR Observation of Inclined Stratification...... 122
6.3 The Identification of 3-D Radar Facies Boundaries...... 122 6.3.1 The GPR Survey Grid as an Indicator of Floodplain Accretion Direction...... 124
6.4 Limitations of GPR Methodology...... 124
Chapter 7 .Conclusions ...... 128
7.1 Summary ...... 128
7.2 Recommendations for Future Research ...... 129
References ...... 131
Appendix 1...... 142
Appendix 2...... 146
viii LIST OF TABLES
Table 4.1 Typical dielectric constant, electrical conductivity, velocity and attenuation observed in common geological materials at 100 MHz (from Davis and Annan, 1989)...... 42 LIST OF FIGURES
Figure 2.1 Medium.energy. non-cohesive floodplain...... 7
Figure 2.2 The classic sandy. mixed-load meandering river ...... 8
Figure 2.3 Examples of lateral accretion elements ...... 17
Figure 2.4 Interpreted stratal and channel fill patterns from GPR data ...... 22
Figure 2.5 Lithofacies interpretations from radar reflection patterns ...... 23
Figure 2.6 Channel-normal profile of point bar sediments. Madison River ...... 26
Figure 3.1 Location map of the study area...... 29
Figure 3.2 Air photo of the study area ...... 30
Figure 3.3 Location of independent well logs from around the study site ...... 32
Figure 3.4 Photographs of the study area showing scrolled topography of the floodplain ...... 34
Figure 3.5 Scroll topography and digital elevation model of the study site ...... 35
Figure 4.1 A pulseEKKO I11 radar record of bedrock buried beneath a sand overburden...... 38
Figure 4.2 Non-gradational water table boundaries in coarse-grained materials as seen by GPR ...... 39
Figure 4.3 GPR profiles of foreset beds in a late Pleistocene delta ...... 40
Figure 4.4 A photograph of the pulseEKKOTMIV system used in this study ...... 46
Figure 4.5 Prograding delta deposits at Central Aggregate gravel pit. near Peardonville. B.C ...... 51
Figure 4.6 Radar reflection profiles of a highwall exposure at Central Aggregate gravel pit ...... 52
Figure 4.7a Radar interpretation of 200 MHz GPR profile overlaid on a photo of the highwall exposure from Central Aggregate gravel pit ...... 53
Figure 4.7b Radar interpretation of 100 MHz GPR profile overlaid on a photo of the highwall exposure from Central Aggregate gravel pit ...... 54 Figure 4.7~ Radar interpretation of 50 MHz GPR profile overlaid on a photo of the highwall exposure from Central Aggregate gravel pit ...... 55
Figure 4.8 A photomosaic of the exposure at Kirkpatrick Sand and Gravel pit. Maple Ridge. BC ...... 57
Figure 4.9 Interpreted 200 MHz radar reflection profiles of the exposure at Kirkpatrick Sand and Gravel pit ...... 58
Figure 4.10 Interpreted 100 MHz radar reflection profiles of the exposure at Kirkpatrick Sand and Gravel pit ...... 59
Figure 4.1 1 Radar interpretation of profile (A) from Figure 4.9 is overlaid on photomosaic of the exposure at Kirkpatrick Sand and Gravel pit ...... 62
Figure 4.12 The location of radar survey lines at North Thompson River study area...... 64
Figure 4.13 A photograph of the vibracore used in this thesis ...... 68
Figure 5.1 The location of GPR profiles. core sites. and CMP profiles in the study area ...... 70
Figure 5.2 Grain size variability in core logs from the study area ...... 71
Figure 5.3 Locations of the water table and lithologic boundaries from core data are shown on corresponding radar profiles ...... 72
Figure 5.4 The relationship between GPR profiles and surface scroll topography...... 77
Figure 5.5 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line #1 ...... 147
Figure 5.6 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line #2 ...... 148
Figure 5.7 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line #3 ...... 149
Figure 5.8 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line #4 ...... 150
Figure 5.9 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line #5 ...... 151
Figure 5.10 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line #6 ...... 152 Figure 5.11 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line #7 ...... 153
Figure 5.12 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line A ...... 154
Figure 5.13 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line B ...... 155
Figure 5.14 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line C ...... 156
Figure 5.15 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line D ...... 157
Figure 5.16 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line E ...... 158
Figure 5.17 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line F ...... 159
Figure 5.18 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line G...... 160
Figure 5.19 Original (top) and interpreted (bottom) 100 MHz GPR profile of survey line H...... 161
Figure 5.20 Original (top) and interpreted (bottom) 200 MHz GPR profile of survey line dip1 ...... 162
Figure 5.21 Original (top) and interpreted (bottom) 200 MHz GPR profile of survey line dip2...... 163
Figure 5.22 Original (top) and interpreted (bottom) 200 MHz GPR profile of survey line dip3...... 164
Figure 5.23 Original (top) and interpreted (bottom) 200 MHz GPR profile of survey line dip4...... 165
Figure 5.24 Original (top) and interpreted (bottom) 200 MHz GPR profile of survey line strike...... 166
Figure 5.25 The location of ridge-and-swale (RAS) and inclined stratification (IS) deposits in 100 and 200 MHz GPR profiles...... 105
Figure 5.26 The location of ridge-and-swale (RAS), inclined stratification (IS) and channel fill (CH) radar facies in 100 MHz GPR survey lines 1 - 7 ...... 106
xii Figure 5.27 The location of ridge-and-swale (RAS), inclined stratification (IS) and channel fill (CH) radar facies in 100 MHz GPR survey lines A - H ...... 107
Figure 5.28 The location of ridge-and-swale (RAS), inclined stratification (IS) and channel fill (CH) radar facies in 200 MHz GPR profiles...... 108
Figure 5.29 Macroscale inclined bounding surfaces (dashed lines) in ridge-and-swale (RAS) radar stratigraphy of 200 MHz GPR profiles ...... 111
Figure 6.1 Typical radar stratigraphy of the scrolled floodplain in dip and strike sections...... 113
Figure 6.2 Examples of macroscale inclined reflections that occur along dip direction GPR profiles...... 115
Figure 6.3 Examples of steeply dipping inclined stratification in the upper portion of floodplain deposits...... 117
Figure 6.4 Photographs of a secondary channel and chute channel in the contemporary floodplain...... 119
Figure 6.5 Generalized schematic of radar stratigraphy and major features interpreted in dip and strike sections of GPR profiles ...... 120
Figure 6.6 Examples of asymmetric scroll bar deposits that display steeper, channelward-dipping reflections at depth ...... 121
Figure 6.7 Arrows indicate estimated floodplain accretion directions from IS deposits at intersection points in GPR profiles...... 125
... Xlll CHAPTER ONE
INTRODUCTION
1.1 The Research Problem
During the last 10 years fluvial sedimentology has shown an increasing trend away from two-dimensional and toward the three-dimensional (3-D) description and classification of channel-scale sedimentology in meandering river floodplains (Friend, 1983; Miall, 1985; Brierley; 1989a; Bridge, 1993a; Bridge et al., in press). Most studies of modern meandering river deposits have focused on 2-D sections (usually 1 or 2 channel widths) of the floodplain as revealed in cutbank exposures, closely spaced cores, or shallow trenches typically oriented either parallel or normal to channel flow and located near the river channel (e.g. Sundborg, 1956; Frazier and Osanik, 1961; Harms et al., 1963; McGowen and Garner, 1970; Bluck, 1971; Jackson, 1978; Nanson, 1980). As a result, there exists comparatively little information on: i) the appearance and stratal variability of large-scale floodplain deposits, or architectural elements, in varying 2-D orientations, ii) the 3-D assemblage and preservation of architectural elements in river floodplains, and iii) the preservation potential of floodplain deposits, particularly away from the river channel (Fisk, 1944, 1947; Miall, 1985; Brierley, 1989a; Brierley and Hickin, 1991, 1992). The lack of large-scale 3-D floodplain data in the literature is reflected in current models of floodplain architecture. Typically, such models show floodplain deposits in the form of block diagrams, restricted to stratigraphic sections oriented either parallel and (or) normal to the direction of channel flow and based on exposed sections of less than one channel width in extent. Thus, they do not address the macroscale (2 - 3 channel widths) appearance of deposits in different 2-D orientations, nor do they show their 3-D assemblage (Nanson and Croke, 1992; Bridge, 1993a). Three-dimensional studies of floodplain architecture from different fluvial environments (e.g. braided, meandering, and anastomosed) would address these concerns, as well as provide a representative sample of channel-scale structure and 3-D variability of floodplain deposits of a particular fluvial style. More importantly, since fluvial style and floodplain architecture are correlated with certainty only in modern streams, contemporary studies of floodplain deposits are necessary to provide modern analogues for the accurate classification of fluvial style in ancient outcrops (Hickin, 1993). Therefore, the aim of this thesis is to examine the 3-D, channel-scale structure of a contemporary meandering river floodplain, specifically a square kilometre of the North Thompson River floodplain near Karnloops, British Columbia. The method used to acquire sedimentologic data in this study utilizes ground-penetrating radar (GPR), a geophysical tool only recently applied to geomorphological problems (Davis and Annan, 1989; Smith and Jol, 1992a). Similar in principle to high-resolution shallow seismic, GPR uses high frequency radio waves to identify subsurface stratal trends. GPR has the potential of decimetre-scale resolution with depth penetration in excess of 47 m in Quaternary sediments (Jol, 1995). The GPR method is currently the only practical means for acquiring continuous, kilometre-scale data of underlying sedimentology on this scale. More conventional methods, such as coring or trenching, would be impractical for such a large-scale study. Ground-penetrating radar and its application to geomorphology is fully discussed in the Methods section. 1.2 Objectives
This thesis has two main objectives. The first addresses a common concern regarding the testing of a new technology before its application in the field. In geomorphology, the use of GPR to identify underlying stratal trends has far outpaced calibration studies to document its ability to accurately portray stratal variability in visually documented exposures (Jol, 1993; Stephens, 1994; Huggenberger et. al., 1994). Thus, the first objective of this study is to compare GPR profiles against exposed sections of sand and gravel deposits similar in sedimentologic variability to what is expected at the main study site. The second objective of this thesis is to identify and interpret the large-scale stratal geometry of the meandering river floodplain based mainly on interpretations made from radar stratigraphic data. The following sub-objectives are designed to fulfill these goals:
(1) to compare photographs of exposed sections from gravel pit walls with corresponding GPR profiles and qualitatively assess how well GPR identifies meso- and macroscale stratal geometry of the section;
(2) to examine the effects of varying antennae frequency, step size, and station spacing on the interpretation of gravel pit profiles in order to select optimum settings for use on the North Thompson River;
(3) to construct a 1 krn2 survey grid of GPR profiles with 80 to 100 m spacing between grid lines on an alluvial meander of the North Thompson River, BC;
(4) to conduct limited core sampling within the GPR survey grid so that changes in radar stratigraphy can be correlated with grain size variability and the ground water table in floodplain sediments;
(5) to identify and interpret macroscale (>75 m) radar facies in the floodplain and show their 3-D assemblage within the GPR survey grid;
(6) to identify the three-dimensional appearance and assemblage of architectural elements in the meander floodplain. CHAPTER TWO
LITERATURE REVIEW
This chapter is divided into three parts. The first section provides a broad review of the literature on the classification of channel pattern and river floodplains with a major focus on meandering channel planform. Early models of meandering floodplain architecture as well as sedimentological criteria for the classification of meandering channel planform in ancient deposits are discussed. The second section focuses on the description and classification of macroscale deposits, or architectural elements, in meandering river floodplains with specific attention paid on inclined stratification and ridge-and-swale deposits which are primary indicators of meandering channel form in scrolled floodplain deposits. Finally, recent results in the application of ground- penetrating radar (GPR) toward the classification of 3-D floodplain architecture in modern meandering river deposits are examined.
2.1 Channel Pattern and Floodplain Classification
The problem of classifying river floodplains is synonymous with that of classifying fluvial style. It is widely acknowledged that all fluvial styles derive from a continuum of channel patterns, bounded by the end-members: braided, straight, wandering gravel-bed, meandering, anastomosed, and anabranching (Jackson, 1978; Schumm, 1981; Bridge, 1985; Miall, 1992; Knighton and Nanson, 1993). Many rivers exhibit the character of one or more end-member classifications; therefore, it follows that there may exist some difficulty in identifying rivers as simply braided, straight, meandering, or anastomosed. Some authors use a combination of terms to describe river reaches. For example, Bridge (1993b, fig. 4) identified both 'anastomosed single sinuous channels' and 'anastomosed braided channels' in an examination of channel planform on the Brahmaputra River, India. It is similarly acknowledged that floodplain deposits of different fluvial styles may exhibit comparable lithologic facies (Miall, 1977; Jackson, 1978). For example, lateral accretion deposits, most typically associated with meandering rivers, also occur in braided rivers (Bluck, 1979; Ori, 1982; Allen, 1983; Bridge, 1993b), and in some anastomosing channels (Smith, 1983). Thus, river floodplains may also be viewed as deriving from a continuum, in this case bounded by a finite set of floodplain-building processes. Nanson and Croke (1992) identified 6 such floodplain forming processes. They are: 1) lateral point-bar accretion, 2) overbank vertical-accretion, 3) oblique accretion, 4) counterpoint accretion, 5) abandoned channel accretion and 6) braid-channel accretion. Processes (I), (2), and (5) are well-known and self-explanatory. Process (4), counterpoint accretion, occurs in eddy separation zones along the outer banks of confined meanders and forms fine-grained counterpoint bars (also known as concave bank benches; Hickin, 1979, 1986; Nanson and Page, 1982). Process (5), oblique accretion, is described as a muddy fine-grained deposit lapped onto the steeply-dipping convex banks of some channels. Process (6), braid-channel accretion, refers to the complex deposits left from the lateral shifting, aggradation, and infilling of braided stream channels. Nanson and Croke (1992) identified 7 types of meandering floodplains formed from several of these processes, based along a high- to low-energy landform continuum (Nanson, 1986). They are described by the following criteria: 1) specific stream power, 2) sediment calibre, 3) erosional and depositional processes, 4) landforms, 5) channel planform and 6) climatic and geologic environment. All but one of the 7 meandering floodplains are classified as being in medium-energy environments with noncohesive floodplains, the final one being a low-energy cohesive floodplain. Each floodplain type is described and illustrated using a block diagram with references to one or more modern examples from the literature. The river bend in this study would be classified as a medium-energy, laterally accreting scrolled floodplain and is identified in Figure 2.1. Miall (1985) also identifies several types of meandering river floodplains, but from an analytical position that all river floodplains are constructed from one or more of a finite set of depositional elements. His set consists of 8 elements, termed architectural elements, which are as follows: I) channel (CH), 2) gravel bars and bedforms (GB), 3) sandy bedforms (SB), 4) foreset macroforms (FM), 5) lateral accretion deposits (LA), 6) sediment gravity flows (SG), 7) laminated sand sheets (LS) and 8) overbank fines (OF). Each element is identified by its distinctive properties, defined by internal composition and geometry, external geometry, scale, and bounding surfaces. Miall proposes that all fluvially deposited material can be identified as one or more of these 8 elements and therefore that all river floodplains are but an assemblage of them. His example of a mixed-load meandering river floodplain similar to that in this study is shown in Figure 2.2 where lateral accretion deposits (LA), overbank fines (OF), crevasse splays (SB) and occasional channel-fills (CH) constitute the majority of floodplain elements. The only example of element classification for modern river deposits comes from the work of Brierley (1989a, b, 1991a, b) and Brierley and Hickin (1991, 1992) who correlated observed geomorphic processes with channel-scale deposits on the coarse- grained Squamish River, British Columbia. They identified seven elements (distinct depositional zones) in their study of braided, wandering gravel-bed, and meandering river reaches. These elements are: channel framework gravel, platform deposit, ridge deposit, chute deposit, flood cycle, sand sheet, and distal overbank deposit which were identified by mechanism, scale, morphology, and sedimentological characteristics. Meandering floodplain deposits were characterized by discontinuous ridge-and-swale units with occasional chute deposits and extensive platform (developing point bar) deposits adjacent to the main channel. Lateral Migration, Scrolled Floodplain
Figure 2.1 Medium-energy non-cohesive floodplain. Suborder B3b: lateral migration scrolled floodplain (modified from Nanson and Croke, 1992, fig. 2). Figure 2.2 The classic sandy, mixed-load meandering river. CH, OF, SB, and LA indicate channel, overbank fine, sandy bedform and lateral accretion elements respectively. The floodplain also exhibits a scrolled surface. (modified from Miall, 1985, fig. 12) 2.1.1 Early Models of Meandering River Floodplains
Although there is now a general recognition in the literature that several types of meandering river floodplains can be distinguished, many exhibit similar properties which are identified in early 'generalized' models. This section summarizes those studies which have contributed to the development of a conceptualized model for a 'typical' meandering river floodplain (e.g. Walker and Cant, 1984, fig. 1; Miall, 1992, fig. 1). The earliest meandering river floodplain models (Fenneman, 1906; Melton, 1936; Mackin, 1937) distinguish only between lateral and vertical accretion deposits. Happ et al. (1940) improved on the model by identifying channel-lag deposits, lateral accretion deposits, vertical accretion deposits, channel-fills, flood plain splays (crevasse splays) and colluvial deposits (hill wash) as distinct components of meandering floodplains. Two subsequent models by Allen (1964, 1965) and Moody-Stuart (1966) identify levee deposits as well as sedimentological differences between zones, including the now classic and widely used fining-upward depositional model for meandering streams (Allen, 1965). The fining-upward depositional model identifies both epsilon cross-stratification (ECS) (Allen, 1963) and a vertical fining-upward trend as products of lateral accretion. In true-dip (channel-normal) section, ECS appear as large-scale, lithologically heterogeneous, streamward-dipping sigmoidal cross-strata (Allen, 1963, fig. 3). Viewed parallel to channel flow, ECS appear as horizontal, vertically stacked tabular-planar beds. The only study to actually document the valley-scale alluvial stratigraphy of a meandering river floodplain was done by Fisk (1944, 1952) in what remains today the largest-scale study of floodplain sediments in an alluvial valley fill, that of the lower Mississippi River which encompasses an area of over 50,000 sq. miles. Fisk identified four principle depositional zones in the valley fill, namely: deltaic, meandering, alluvial, and backswamp. Meandering fluvial deposits were further subdivided into bar accretions, channel fillings, and natural levees. A later study by Carey (1969) identified 2 other forms of meander belt deposition on the lower Mississippi, defined by their genetic origin as sheet accretion and eddy accretion (same as counterpoint or concave bank-bench accretion, Nanson and Page, 1982; Nanson and Croke, 1992). For a comprehensive literature review of alluvial deposits in meandering rivers see Allen (1965). A historical perspective on the development of depositional models for meandering river floodplains is provided by Miall (1978).
2.1.2 Sedimentological Criteria for the Identification of Meandering Channel Pattern
Numerous sedimentological criteria have been identified in the literature as indicators of meandering paleochannel form in ancient deposits. Many of these are summarised by Jackson (1978, table 2) who found all but one to have either exceptions or to be totally invalid. The one indisputable criteria he found was the presence of exhumed meander belts, which can be observed only in large exposures parallel to lateral accretion direction. All the other sedimentological criteria were based on bedform-scale observations and viewed as unreliable indicators of paleochannel form. The limitations of bedform scale observations, specifically in the construction of fluvial facies models, are critically reviewed in the literature by several authors (Miall, 1980; Bridge, 1985; Hickin, 1993). Hickin (1993, p.215) summarised a primary criticism of fluvial facies models stating, "the bedform scale is simply inappropriate to the problem of identifying sedimentological differences arising from differences in the channel-scale properties of river planform." He regarded this as a major limitation of planform facies analysis and identified a departure in the form of larger, channel-scale studies of the internal geometry and shape of fluvial deposits, a type of study termed architectural element analysis (Miall, 1985, 1992). Prior to an investigation of architectural element analysis, four sedimentological indicators of meandering paleochannel form will be considered. They are the fining- upward model, the sand-mud ratio, paleoflow direction and variability, and bedding thickness. Although the first three criteria were found to be unreliable in bedform-scale studies by Jackson (1978), there are more recent findings which examine their utility in channel-scale applications. The final criteria, bedding thickness, was proposed by Friend (1983) and a recent study also examines its utility in large outcrops.
Fining-upward model Jackson (1978), and later Hickin (1993) summarised several instances in ancient and modern meandering stream deposits where Allen's (1965) fluvial fining-upwards model does not apply (e.g. McGowen and Garner, 1970; Bridge and Jarvis, 1976; Nanson, 1977). Nonetheless, it remains commonly referred to as an indicator of meandering channel form in ancient fluvial deposits (Miall, 1992; Bridge and Mackay, 1993; Gibling and Rust, 1993). While many meandering stream deposits do exhibit a vertical fining- upwards, Hickin (1993) recently re-emphasized the point that many do not and he provided examples which show that meandering river floodplains can exhibit a much broader range of sedimentological features than the Allen model allows (e.g. Jackson, 1978; Nanson, 1980). Based on evidence from the literature, Hickin makes it clear that the Allen model is representative of only a small portion of meandering river deposits and therefore cannot be used as a generalized model for meandering streams. Where fining-upwards does exist, Thomas et al. (1987) proposed several distinctions to Allen's vertical fining-upwards model and identified 7 different categories of both vertical and lateral fining sequences which occur in three-dimensional deposits. Their accretion direction was classified relative to the direction of paleoflow as well as being either across or along one or more bedding planes. Thomas et al. (1987) state that these distinctions are useful because they provide a clearer picture of the 3-D depositional processes associated with point-bar formation. Sand-mud ratio
Schurnm (1960) found that channel sinuosity correlated closely with the amount of fine-grained sediments in the channel banks of several rivers in the American mid-west. He suggested that a low sandmud ratio (high mud content) in fluvial deposits was representative of high sinuosity (meandering) streams and that a high sandlmud ratio (negligible mud content) indicates straight or braided streams. Although Schurnrn's classification was based on only 5 data points, it is clear that there exists a gross correspondence between grain size of alluvium and channel gradient (Friend and Moody- Stuart, 1972; Brierley and Hickin, 1984; Leclerc and Lapointe, 1994), and that channel gradient is a geomorphological determinant of channel planform (Leopold et al., 1964; Hey, 1978; Jackson, 1978). In a review of ancient meandering stream deposits, Jackson (1978) identified several cases which do not conform with Schurnrn's classification, exhibiting few or no fine- grained deposits. Jackson (1978) concluded that the overall proportion of fine-grained sediment is not an accurate index of channel pattern. The same observation was reached in a separate review paper by Friend (1983). Furthermore, Bridge (1993a) argued that bank material is not a major control on channel pattern, and suggested that the tendency to identify coarse-grained (sand or gravel) ancient fluvial deposits as braided rather than meandering is unfounded. A study by Kirk (1983) supports this argument having identified atypically fine-grained deposits (fine sands and silt intraclasts) in ancient braided stream deposits in the Mill Coal (Westphalian 'A') deposits of Scotland.
Paleoflow direction and variability The accurate identification of paleoflow direction from ancient fluvial deposits generally requires extensive outcrop data (Friend, 1983). Small-scale paleochannel current indicators such as current ripples can be highly variable and therefore imprecise estimators of true paleoflow direction. Major bedforms are less variable and considered better indicators of paleoflow direction (Jackson, 1978; Bridge, 1985). Thus paleocurrents from the lower parts of channel-bar or fill sequences, those which exhibit the largest flow structures, are expected to give the best indication of local channel orientation. If a sufficiently large outcrop is available, Bluck (1971) suggested that meandering stream deposits may be identified by their wide variability of paleocurrent orientations in sedimentary structure. An example of such variability was identified by Gibling and Rust (1993) who studied a large outcrop of an exhumed meander belt in the Morien Group of the Sydney Basin (Sydney Mines Formation, Westphalian D to Stephanian), Nova Scotia. They estimated paleoflow directions from large-scale trough cross-bedding in two adjacent outcrops and identified local curvature in channel form concomitant with that of a meander bend. Using an equation by Miall (1976), Gibling and Rust estimated channel sinuosity from the degree of paleoflow divergence between the two segments.
Bedding thickness Friend (1983) stated that the thicknesses and patterns of bedding sequences from vertical logs may "suggest a pattern of accumulation that helps to diagnose the sorts of river systems present (Friend, 1983, p.352)." There are few studies in the literature which focus specifically on bedding thickness but recent work on an outcrop of ancient meander point-bar deposits in the Loranca Basin, Spain (Diaz-Molina, 1993) identified bedding thicknesses in 5 different vertical logs. Bed thickness ranged from 1 m to less than 10 cm and several different vertical trends of bed thicknesses were identified. Three logs showed upward-thinning, one showed upward-thickening, and the last no visible trend. Thus significant variability of bed thickness and pattern was found to exist within the meander bend. Therefore, the evidence from this study does not conform with Friend's statement that a particular fluvial style may exhibit a specific range or pattern of bedding thicknesses.
13 2.2 Architectural Elements - Description and Identification
Many authors agree that channel-scale 2-D or 3-D outcrops are necessary to accurately classify paleochannel architecture and form (for example, Allen, 1983; Friend, 1983; Miall, 1985; Bridge, 1993a; Hickin 1993). The identification and classification of the large-scale structures in these exposures is a form of study termed architectural element analysis (Miall, 1985, 1992). Allen (1983) first proposed the term architectural element to describe the internal geometry of sandstone bodies in a study of the Lower Old Red Sandstones in the Anglo- Welsh basin (Welsh Borderland), UK. Miall (1985) later used the term in his classification of 8 distinct architectural elements (reviewed in section 2.1). Miall identified these 8 elements as the primary building blocks of all river floodplains, and suggested that paleochannel form could be discerned through an examination of the architectural assemblage of these elements in ancient fluvial deposits. Bridge (1993a) critically reviewed Miall's architectural elements and identified two major weaknesses. First, he observed that Miall's 8 elements are identified with an inconsistent set of descriptive and interpretive terms. For example, the 'laminated sand sheet' element is an observed lithology while the 'sediment gravity flow' and 'lateral accretion deposit' elements are interpreted processes. Bridge also argued that the 'sediment gravity flow' element is not an element at all but a lithofacies (Gm), as per Miall's (1992, table 2) lithofacies code. Thomas et al. (1987) also addressed the use of descriptive and interpretive terminologies in the classification of fluvial deposits. They suggested that only descriptive, non-genetic terms should be used to identify fluvial deposits thus removing any allusion to depositional process or environment. As a result, they recommend that the term epsilon cross-stratification (ECS) be changed to inclined heterolithic stratification (IHS) where 'inclined' and 'heterolithic' describe dipping structures and the alternating coarse to fine bedding found in ECS. A second term proposed by Thomas et al. (1987) is
inclined stratification (IS) which refers to less steeply dipping homogeneous and generally coarser units than IHS. Bridge's (1993a) second major concern with Miall's (1985) element classification is that he provides only two-dimensional (2-D) descriptions of what are obviously 3-D deposits (Miall, 1985, fig. 1). Thus the accurate identification of these elements in the field is dependent upon orientation of the exposure. In order to bridge the gap between 2- D and 3-D structural classification, Bridge (1993a) illustrates several simple 2-D and 3-D forms of structures including sheet, wedge, lobe, and channel ribbon geometries. However, he acknowledges that our current knowledge of 3-D structure is very limited and notes that details of stratal thicknesses and orientation, grain sizes, internal structures and paleocurrents in 3-D structures are for the most part unknown in the literature. Nonetheless, Bridge stresses that full-knowledge of the 3-D geometry of deposits is necessary for their accurate classification.
Hierarchical ordering of bounding surfaces in architectural elements There are two principal methods reported in the literature for the hierarchical ordering of bounding surfaces in outcrops of ancient fluvial deposits. One is a numerical ordering system and the other is purely descriptive, using the terms micro-, meso-, and macro- as modifiers to define relative scale (Allen, 1966 Bridge, 1993a). Numerical ordering systems by Brookfield (1977, 1992), Allen (1983), and Miall (1985, 1992), all similar in style, are summarised with illustrations in Bridge (1993a). Bridge (1993a) identified several difficulties in the application of these numerical systems in the field. Firstly, he observed that the numerical order of a bounding surface may change laterally with respect to adjacent strata. Second, even with well exposed deposits, Bridge expressed difficulty in distinguishing between Miall's 2nd through 5th order surfaces. Third, numerical orders are implicit references to the relative size of the strata they bound. Bridge proposed a new set of terminology for superimposed strata (Bridge, 1993a, fig. 4) based on a set of more explicit hierarchical terms, namely micro-, meso-, and macro- which define the relative scale and superposition of the strata themselves. This method is adopted for the description of stratal boundaries in this study.
2.2.1 Epsilon Cross-Stratification in Meandering Streams - Modern and Ancient
It is well recognized in the literature that the vast majority of reported ECS in modern streams derives from studies of point bar deposits on small scale meandering channels draining intertidal mud flats (Jackson, 1978; Smith, 1987; Thomas et al., 1987). The presence of a fine-grained (silt) component in almost all modern and ancient reports of ECS shows that it occurs almost exclusively in low-energy cohesive stream environments (Nanson and Croke, 1992), similar to the estuarine channel fill from which the ECS model derives (Allen, 1963; Oornkens and Tenvindt, 1960). Observations of ECS in modern meandering streams are sumrnarised by Jackson (1978) and Thomas et al. (1987). Jackson (1978, p.565) identified only two reports of ECS (unpublished) both of which exhibited channel-bed material finer than coarse sand with bed and banks containing substantial amounts of mud. Thomas et al. (1987, p.134) identified four additional modern examples, three of which contained a mud component. The sole coarse-grained example of ECS in modern meandering streams was identified by Arche (1983) in meander lobe deposits of the Jarama River, Spain. The lack of smooth sigmoidal ECS in coarse-grained river deposits implies a more complex bedding structure. Jackson (1978, p.566-578) stated that this may be because meandering gravelly streams such as those observed by Harms et al. (1963) and McGowen and Garner (1970) have stepped cross-channel profiles from floodplain to thalweg which makes a smooth sigmoidal ECS improbable (see also Nijman and Puigdefebregas, 1978). Figure 2.3 from Miall (1985) illustrates several examples of Figure 2.3 Examples of lateral accretion elements. No vertical exaggeration. A. conglomeratic point bar (lithofacies Gm), with chute channel (lithofacies Gt). B. Element composed of medium grained sandstone, with abundant internal planar-tabular cross-bedding (lithofacies Sp). C. Fine to very coarse sandstone and pebbly sandstone, with cobble to boulder conglomerate lag. Abundant internal crossbed structures (lithofacies Sp, St, Sh and Sl). D. Small sandy point bar with abundant dune and ripple cross-bedding (lithofacies St, Sr). E. Point bar composed mainly of fine sandstone (lithofacies St) at base. F. Giant point bar with thick bedded, fine-grained, trough cross-bedded sandstone at base (lithofacies St), passing up into epsilon set of fine sandstone and argillaceous siltstone (lithofacies Se). (from Miall, 1985, fig. 5) lateral accretion (LA) deposits from fine to coarse-grained meandering streams. Examination of Figure 2.3 shows a clear trend toward greater structural variability in coarse-grained stream deposits as well as the common feature of inclined stratification in each example. Thus it appears that all meandering stream deposits may on some scale exhibit inclined stratification (IS), similar in some ways to ECS (also IHS, Thomas et al., 1987). Miall separated these observed forms of lateral accretion deposits into four groups based on grain-size: gravelly, gravel-sand, sandy, and silt-sand-mud. Clearly, these illustrations show the diversity of lateral accretion deposits in meandering streams and that use of the Allen model as a basis for all meandering streams is inadequate. Moreover, Miall (1985, p. 284) also notes that "LA deposits do not retain a constant geometry or composition around any given meander bend. As a result, the classic fining-upward profile (Allen, 1970) may not be present". Not only do complex bedding structures in coarse-grained streams make it difficult to identify inclined stratification, but very large rivers with high width-depth ratios can exhibit IS with only a 1 or 2 degree dip, making it much more difficult to distinguish (Leeder, 1973). For example, Frazier and Osanik (1961) identified a 2 degree dip angle to the inclined strata in upper point bar deposits of an abandoned channel of the Mississippi River, which they probably would have missed if the excavation had not been for a massive navigation lock over 650 m long and 150 m wide! Point bar sediments in the exposure were otherwise dominated by festoon cross-bedding (large-scale trough cross- stratification).
2.2.2 Ridge-and-swale (RAS) Topography in Meandering River Deposits
Ridge-and-swale or scrolled topography most commonly occurs in medium-energy, non-cohesive floodplains ranging in textures from sands and minor gravels to sands with abundant silts and organics (Nanson, and Croke, 1992, Table 1). Ridge-and-swale (RAS) structures have been shown to be remarkably well preserved at depth in modern
(Sundborg, 1956; Nanson, 1980) and ancient (Gibling and Rust, 1993) meandering stream deposits. A recent summary of studies on scroll bar forming processes and the frequency of scroll bar formation is given by Nanson and Croke (1992). A commonly described basal feature of RAS structures is an initial ridge of coarse material upon which sediments are vertically accreted to form a scroll bar (ridge). Sundborg (1956) identified these as embryonic point bars from a study of floodplain sediments along a smoothly meandering section of the gravel-bed Klaralven River, Sweden, where embryonic point bars consisted of coarse lobes of bedload material exhibiting cross-bedded sediments which dip toward the inner bank (away from the channel) and lie at the base of some vertically aggraded ridges. In cross-section, RAS deposits appeared as well-preserved vertically stacked layers of alternating coarse-fine units with an overall upward fining trend (1956, Fig. 47). Temporal variation in river stage was considered to be the principal cause of these alternate layers of deposition. Sundborg also noted that strata in ridge-and-swale structures were usually thickest where they were highest, atop ridge crests. Nanson (1980) also identified preserved ridge-and-swale units in the fine-grained Beatton River, a low-energy cohesive stream in northeastern BC. RAS structures were observed in two cutbank exposures each 4 metres high with a thick (2m) cap of vertically accreted floodplain deposits. RAS deposits were vertically stacked but exhibited no change in bed thickness between ridge-and-swale deposits. At the base of each set of vertically accreted ridges, Nanson identified an initial coarse lobe of material which he termed the initial scroll bar. Nanson observed that following the deposition of this stratum the floodplain aggrades and maintains a ridge amplitude determined by the thickness of this initial scroll bar. Nanson suspected that the preservation of scroll patterns in these deposits must be due to the action of secondary currents since primary flow patterns could not account for their presence. While ridge-and-swale structures have often been observed in meandering river floodplains, only recently have workers begun to classify different types of ridge-and- swale geometries. Gibling and Rust (1993) described two main types of RAS deposits, classified in terms of ridge geometry, in an exhumed meander belt in the Morien Group of the Sydney Basin (Sydney Mines Formation, Westphalian D to Stephanian), Nova Scotia. The most common type of ridge, termed an asymmetric ridge, displays a shallower streamward dip in cross-section. Asymmetric ridges were further differentiated by the presence or absence of an initial scroll bar at the base of the deposit. Asymmetric ridges were constructed of upward-climbing, planar cross-beds with a strike parallel to the ridge crests, similar to those described in ancient fluvial deposits by Puigdefabregas (1973). The second form was termed a symmetric ridge. These exhibit an initial scroll bar and may also occur as ridge-swale-ridge units which are the result of chute channel flow located in the swale. A typical vertical section from a ridge deposit consisted of basal cross-stratified sandstones grading into thinner ripples of IS overtopped by more steeply dipping IHS, consisting of alternating sandstone and siltstone beds.
2.3 Ground-Penetrating Radar: Use in Sedimentology
Work using ground-penetrating radar (GPR) in sedimentological studies was first introduced in 1982 (Ulriksen, 1982) and has since been applied in a wide range of geomorphological environments including braided rivers (Bridge, 1993b; Huggenberger et al., 1994; Stephens, 1994), anastomosing channels (Moorman, 1990; Moorman et al., 1991), meandering river point bars (Gawthorpe et al., 1993; Bridge et al., in press), deltas (Jol and Smith, 1992a, b; Smith and Jol, 1992b; Jol, 1993), coastal and lakeshore spits (Smith and Jol, 1992a; Jol et al., 1994), and barrier beaches (Meyers et al., 1994). GPR has also shown similar growth in other areas including archaeology, mining, civil engineering, environmental contamination, groundwater flow, and permafrost and glacier studies. The more than 140 papers presented at the recent Fifth International Conference on GPR in Kitchener, Ontario (Proc. Fifth Int'l. Conf. on GPR, 1994) attest to its application in a wide range of geomorphological and geotechnical applications. GPR data is gathered using electromagnetic (EM) pulses transmitted through the ground surface. Partial reflections of these pulses are caused by changes in dielectric conductivity that typically occur at sedimentary interfaces and the water table and register as reflections on the radar display. Reflection patterns on radar profiles that occur from sedimentary interfaces identify the radar stratigraphy of subsurface deposits, defined as 'the study of stratigraphy and depositional facies from radar data using seismic stratigraphic principles' (Jol, 1993, p.23). For a general overview of seismic stratigraphic principles see Payton (1985) and Berg and Woolverton (1986). The interpretive methods of radar facies analysis also stem directly from seismic reflection facies analysis, and are reviewed by several authors (Mitchum et al., 1977; Roksandic, 1978; Sangree and
Windmier, 1979; Brown and Fisher, 1980; Mitchum et al., 1985). Radar facies analysis is defined as 'the process of describing and interpreting the radar reflection parameters within a depositional sequence (Jol, 1993, p.23)'. A single radar facies is identified as 'a mappable, three-dimensional sedimentary unit composed of reflections whose parameters differ from adjacent units (Jol, 1993, p.23)'. A grouping of radar facies which form a set of genetically related strata is called a radar sequence (Gawthorpe et al., 1993), and is bounded by a radar sequence boundary (Gawthorpe et al., 1993). Several examples of interpreted radar sequences based on seismic stratigraphic methods are shown in Figure 2.4 and a chart showing how radar reflection patterns can be interpreted to deduce lithofacies is shown in Figure 2.5. While a radar data display superficially resembles that of a seismic reflection profile, a primary difference between the two is scale; radar is limited to the shallow subsurface (40m in soft-sediments) but is capable of imaging a higher resolution (decimetre-scale) UPPER BOUNDARY LOWER BOUNDARY
EROSIONAL TOPLAP ONLAP DOWNLAP ONLAP FILL PROGRADED FILL TRUNCATION
CONCORDANCE SIMPLE FAN SLUMP COMPLEX MOUNDED ONLAP FILL CHAOTIC FILL
MIGRATING WAVE COMPOUND FAN CONTOURITE MOUND COMPLEX
DIVERGENT FILL COMPLEX FI~
OBLIQUE PARALLEL OBLIQUE TANGENTIAL SlGMOlD
COMPLEX HUMMOCKY SIGMOID-OBLIOUE SHINGLED CUNOFORMS
Figure 2.4 (A) and (B) show several classifications of interpreted stratal and channel fill patterns from GPR data. All interpretations derive from methods and terms used in seismic reflection facies analysis (from Mitchum et al., 1977). (C) shows how upper and lower boundary classifications described in (A) are used to define a radar sequence boundary (from Gawthorpe et al., 1993). TYPES OF REFLECTION CONFIGURATIONS INTERPRETATION
~-~
1. A??CNU4rKO INKRC? 1. SILT? LACUSTIINt SIOIYKNTS a. SA.0. YASSIVC OR IMICI- .KOOK0 4. TILL. YASSIVK. IKW IOULDCRS
I 1. SILT, L4YlNATKO 70 TWIN-at0010 2. SA.0. LAYI*ATIO 10 rIICI-aKOO~0
i
1. SILT AM0 SAND. @KOOK0 WAV? 1. SANO. 1KOOKD
Figure 2.5 Flowcharts diagramming the interpretation of lithofacies from radar reflection patterns (from Beres and Haeni, 1991). of detail than shallow seismic profiling (metres scale). Hence, radar sequences resolve significantly smaller-scale structures than seismic sequences; therefore, in terms of seismic stratigraphy, radar sequences are comparable to bedsets or parasequences (Van Wagoner et al., 1990). Because of a growing reliance on GPR data to interpret 2-D and 3-D stratal geometries, it is important to compare how well radar profiles can identify subsurface deposits in actual exposures. There are, however, very few studies which delineate radar facies - observed lithofacies comparisons along exposed outcrops (Smith and Jol, 1992b; Huggenberger et al., 1994). Smith and Jol (1992b) performed one such study in a gravel pit of late Pleistocene delta deposits near Brigharn City, Utah. They tested the ability of GPR to infer thickness of deposit, internal structure, angle of bedding and direction of inclined strata along a 200 m x 70 m exposure of delta topset and foreset beds. Results showed a close association between the foreset beds observed in the exposure and the display of radar data. A radar sequence of sand within the otherwise graveliferous foresets was also distinguished in the radar profile by its smaller dip angle. From the results of this experiment, Smith and Jol concluded that large-scale structures in the exposure could easily be identified with GPR. In a similar experiment but on a smaller-scale, Huggenberger et al. (1994) compared GPR data with two 15 m x 10 m exposures of braided river deposits in a gravel pit of Pleistocene Rhine gravels in northeast Switzerland. The two outcrops were oriented normal and parallel to ancient flow direction and showed large-scale trough cross- bedding structures up to 5 m across in normal view. Huggenberger et al. found that large- scale trough cross-beds could easily be discerned from radar profiles in this direction but that along the channel-parallel exposure scour-infill deposits of trough cross-bedding and inclined cross-beds from migrating gravel bars were difficult to distinguish from one another. These two studies and other recent explorations along sedimentary rock outcrops (Pratt and Miall, 1993; Stephens, 1994) suggest that GPR is well-suited to the identification of large-scale strata and stratal trends in coarse-grained (sand or gravel) sedimentary deposits.
2.3.1 3-D Radar Stratigraphy of Meandering River Point Bars
There are only two studies which use GPR to examine the internal structure of meandering river deposits, both of which investigate point-bar deposits in small coarse- grained streams (Gawthorpe et al., 1993; Bridge et al., in press). Gawthorpe et al. (1993) profiled two channel-normal sections of a modern point-bar on the Madison River, Montana, U.S.A. An example of the radar stratigraphy of these deposits is shown in Figure 2.6. Note that the river channel is north of the profile and flows out of the page. The interpretation of radar reflections in Figure 2.6 shows that upper radar sequence boundaries (X, Y, and Z) exhibit a sigmoidal geometry with a channelward dip that are defined by either erosional truncations or changes in the continuity of reflectors. In an examination of reflection patterns, Gawthorpe et al. identified two main radar facies, namely: high-amplitude oblique clinoforms (below the
'X' boundary from 18 - 42 m) and hummocky facies (above the 'Y' boundary from 4 - 13 m). Based on these data, they concluded that radar facies may be used to identify the major sedimentological units within point-bar deposits and may also be used to reconstruct temporal evolution of the point bar, based on a chronological ordering of radar stratigraphy. Bridge et al. (in press) performed a similar but much more thorough investigation of point-bar sediments on the meandering South Esk River, Scotland, where a GPR survey grid of closely spaced (10 metres or less) radar profiles was laid across a modern point- bar (80 m x 50 m area) in a single meander bend. Radar data was supplemented by trench and closely spaced core data within the radar grid. Point bar deposits were found to be constructed of upper bar sands overlying lower bar sands and gravels and were easily distinguished on radar profiles by continuous (upper bar) and discontinuous (lower bar) reflections. A lobate ridge of sediment termed a unit bar was also identified in the lower bar sands. The 3-D stratal geometries of unit bar, upper, and lower point bar deposits were constructed to determine their 3-D variability and preservation potential in the floodplain. Lower point bar deposits were found to thin in the downstream direction while upper bar sediments thickened, concomitant with a decrease in land surface elevation. Bridge et al. also identified a preferential preservation in the floodplain of bar- tail deposits exhibiting a well-developed fining-upward sequence, likely a result of bar- head deposits being rapidly eroded as the bend migrated downstream. When compared with core data, Bridge et al. found that GPR reflections seemed to correlate to some degree with changes in mean grain size and composition, particularly with vegetation-rich layers. They concluded, however, that there was only partial evidence to suggest that reflection amplitudes (signal strength) could be correlated with lithofacies types and that for the most part, radar reflections could not be correlated with individual lithofacies from core data. CHAPTER THREE
STUDY AREA
3.1 Regional Setting
The study area is located on a single meander bend of the North Thompson River located about 10 krn north of Kamloops, BC, approximately 300 km northeast of Vancouver (Figures 3.1 and 3.2). The North Thompson River Valley lies in the Interior Plateau of British Columbia, characterized by a series of rolling uplands separated by deep valleys (Bostock, 1948). In the vicinity of Kamloops the uplands attain elevations up to 2,000 m a.s.1. while the greatest proportion lie between 800 and 1200 m a.s.1. (Fulton,
1967). Climate in the region is semi-arid with 30 - 50 mm of precipitation per year (Dept. of Mines, Energy and Resources, 1974), although precipitation increases significantly with altitude so that tributary basins are considerably more humid (Ryder, 1971). Upland areas are characterized by shrub and grassland while mature Ponderosa pine, cottonwoods, and aspens characterize valley areas. Land use in the lower North Thompson Valley consists mainly of cattle grazing and hay farming. Drainage area of the North Thompson River is approximately 20,000 km2. Maximum annual discharge at McLure Station, 15 krn north of the study site, is 1860 m3/s based on 32 years of record (Environment Canada, 1992). The time of peak discharge is controlled by snowmelt and usually occurs in early June. Minimum flow discharge is reached in February and averages 63.3 m3/s while mean annual discharge over the thirty-two year period of record is 428 m3/s (Environment Canada, 1992). Bedrock in the study area, and in much of the lower North Thompson Valley, is covered by a variable mantle of post glacial alluvium resulting from the Fraser glaciation (Fulton, 1963, 1967). These sediments, termed Kamloops Lake Drift by Fulton and Smith Figure 3.1 Location map of the study area. The study site is located on a single meander bend of the North Thomspon River floodplain about 10 km north of Karnloops, BC. 0 150 300 450 600 I I I I I Metres Scale 1 :I5,000 f
Figure 3.2 The North Thompson River study area. Channel flow is from north to south. (1978), are comprised mainly of silt and clay deposited by stagnant ice blocking the valley immediately to the south of the study area during deglaciation (Fulton, 1967,
Fulton and Smith, 1978). Kamloops Lake Drift sediments here are characterized by glaciolacustrine silt containing occasional minor clay rhythmites (Fulton and Smith, 1978). The upper 10 m of these deposits at the base of the valley, however, have been heavily reworked by the North Thompson River and, to some extent, by paraglacial alluvial fans stemming from the base of steep erosional gullies that bisect the valley walls (Ryder, 197 1). Consequently, these reworked surficial deposits contain a coarse-grained component, consisting mainly of sand and gravel that form a cap over glaciolacustrine silt and clay. Independent well logs from the area illustrate this facies sequence (Figure 3.3). Eight out of nine well logs exhibit some form of downward fining, typically from sand or silty sand to increasing silt and clay deposits at depth. Cores 1, 3, and 6 which are located on the modern floodplain in Figure 3.3 show that downward fining occurs between 8 and 12 m depth. Note that cores 5 through 9 show somewhat smaller grain sizes than do cores 1, 3, and 6, reflecting the progression of concave bank-bench deposits in this part of the floodplain (see Figure 3.1). Concave bank-benches (Hickin, 1979; Nanson and Page, 1982) in the study area are located on the upstream bank at sharp bends in the river channel.
3.1.1 North Thompson River Study Area
The North Thompson River in the study area forms a series of confined meanders, bordered by valley walls approximately 2.0 km wide at the study site. The river varies between 150 and 300 m in width near the study site and thalweg depth at bankfull stage is between 9 and 10 m, although it reaches 14 m in a narrow section of the channel east of the study site (Ministry of Environment, 1982) (Figure 3.1). Local deepening is attributed to scour caused by channel confinement. Channel cross-sections at inflection points Grain size scale clay silty clay silt silty sand sand gravelly sand gravel gravel with cobbles
Figure 3.3 Location of independent well logs from around the study site. Lithologic data from the top 28 metres of each core show high silt and clay concentrations, especially below 7 m. All cores are set to a datum corresponding to the surface elevation of the study site. upstream and downstream of this location show more moderate thalweg depths, 9.4 and 10.7 m respectively (BC Environment, 1982). The meander floodplain at this site was selected because of its suitability for GPR work. It presents a fairly level ground surface with relatively few obstructions and is readily accessible. The dimensions of the study area are approximately 0.8 krn by 1.1 km yielding a total area of 0.9 km2, roughly 3 by 5 channel widths in size. The study area consists of a series of hay fields that, once harvested, reveal scrolled topography (Figure
3.4). Scrolled topography at the study site is not entirely regular, however. The meander floodplain shows several changes in the orientation of surface scroll bars (Figure 3.5a), interpreted as different stages of floodplain growth, as well as three distinct terraces on the floodplain surface (Figure 3.5b). The northwestern-most terrace in Figure 3.5, appears to be the result of a chute channel erosion as the base of the terrace is the lowest part of the floodplain in this area. The other two terraces form steps in the floodplain surface that are the result of local erosion and possible channel degradation at these sites. Overall topographic variability on the floodplain surface is shown in Figure 3.5b. 1 Figure 3.4 Photographs of the study area showing scrolled topography of the L floodplain. Figure 3.5 The study site showing (a) surface scroll topography and (b) topographic variability on the floodplain surface. Thick dashed lines in (a) indicate changes in the orientation of surface scroll topography. Dark bands in (b) show terraces on the contemporary floodplain. Channel flow is to the South. CHAPTER FOUR METHODS
This chapter is divided into three parts. The first part introduces ground-penetrating radar (GPR) and reviews GPR principles and the instrumentation used to collect the radar reflection data presented in this thesis. The second part addresses the interpretation of GPR reflection profiles and is further divided into two sections. The first section identifies sources of spurious radar reflections such as electronic signal noise, multiple reflections, and above ground reflectors such as trees and telephone poles which may adversely affect the interpretation of radar profiles. The second section examines the more fundamental question, 'do radar reflection profiles provide an accurate portrayal of subsurface stratal trends?' The examination of geologic sections and their appearance in radar reflection profiles is used to answer this question. Because few studies have been made in this regard, the results of two such radar surveys performed during the course of this thesis are presented here. The third and final part of the Methods section outlines the survey design for GPR profiling and vibracoring undertaken at the North Thompson River study area.
4.1 GPR Theory and Instrumentation
GPR systems operate by emitting short duration pulses of high frequency (10 - 1000 MHz) electromagnetic (EM) energy into the ground and recording reflected signals as a function of time. GPR is commonly used to construct 2-D radar cross-sections of the near subsurface (< 50 m) where radar reflections indicate changes in the high frequency dielectric properties of underlying materials (Greenhouse et al., 1987; Davis and Annan, 1989). Because there exists a strong relationship between electrical and physical properties of materials, radar reflections may be used to identify physical boundaries such as i) geologic interfaces, ii) the water table, and iii) erosional contacts (Scott et al., 1978; Davis and Annan, 1989; Moorman et al., 1991; Arcone et al., 1992; Smith and Jol, 1992b; Huggenberger, 1993; Meyers et al., 1994). Typically, geologic boundaries will appear as strong, continuous reflections in the radar cross-section, such as the case of a bedrock beneath a sand overburden (Figure 4.1). Although not a material boundary, the water table also has a strong effect on electrical properties and exhibits a similarly strong reflection in cases where the material above the water table is unsaturated (Davis and Annan, 1989; Madsen et al., 1994; Meyers et al., 1994) (Figure 4.2). The most common sources of radar reflections, however, occur along erosional contacts. These reflections may be used to interpret the overall pattern of subsurface stratal geometries, such as the foreset beds of a Holocene delta (Figure 4.3). These generally weaker reflectors, compared to types i and ii, are the main focus of geomorphic studies where the metre- scale radar stratigraphy of a sediment body is of primary interest. Radar reflections do not always represent physical boundaries, they may represent changes in the electrical properties of subsurface materials resulting from various factors including relative changes in porosity, grain size, organic content, and moisture conditions within the vertical column (Olhoeft, 1978; Greenhouse et al., 1987; Davis and Annan, 1989; Moorman, 1990; Jol, 1993; Bridge et al., in press). Greenhouse et al. (1987) comprehensively examine the relationship between radar reflections and the material and electrical properties of subsurface sediments. Factors which affect signal quality and the strength of signal returns are surnmarised by Davis and Annan (1989) and Jol(1993).
4.1.1 High Frequency Electrical Properties of Materials
The two electrical properties that describe the propagation and attenuation of radar waves in geologic materials are dielectric permittivity (K) and electrical conductivity (0)
Figure 4.2 A non-gradational water table boundary in coarse-grained materials appears as a high-amplitude, continuous reflector in GPR reflection profiles: (a) horizontal water table reflector at 1.5 m depth in fine-grained sand deposits from Willapa barrier spit, Washington State, U.S.A. (modified from Meyers et al., 1994), (b) a strong water table reflector sloping upward toward the right in a GPR profile of glaciofluvial sand and gravel deposits, Oak Ridge morraine, Ontario (modified from Pilon et al., 1994). W \D
(Davis and Annan, 1989). A third property, magnetic permeability, is also important but only in cases where the material exhibits strong magnetic properties, a situation generally not encountered in GPR applications (Moorman, 1990). This section examines how the dielectric permittivity and electrical conductivity of different materials affect GPR performance and how they dictate site selection in GPR survey design. Reviews of GPR theory and the electromagnetic principles of subsurface radar profiling are given by
Morey (1974), Annan and Davis (1976), Arcone and Delaney (1982), Ulriksen (1982), Wright et al. (1984), Daniels et al. (1988), Olhoeft (1988), Davis and Annan (1986, 1989), Moorman (1990), and Sutinen (1992). Electrical conductivity (o)is defined as the ability of a material to conduct a direct current (Davis and Annan, 1989). Although influenced by several factors, the electrical conductivity of a material is controlled mainly by the presence of dissolved ions in solution (Ulriksen, 1982). Materials exhibiting high concentrations of dissolved ions, for example sea water or wetted clays, have highest electrical conductivities resulting in rapid radar signal loss (attenuation a) with depth. Davis and Annan (1989) and Jol (1993) summarize those factors which influence electrical conductivity. Dielectric permittivity or the dielectric constant 'K' is defined as the ability of a material to store an electric charge when an electric field is applied and is strongly associated with moisture content (Topp et al., 1980; Sheriff, 1984). The higher the dielectric constant (K) of a material the greater its ability to absorb EM energy, thus the higher the rate of signal attenuation with depth. Therefore, materials with high dielectric constants (i.e. wetted materials) tend to experience higher rates of signal attenuation and therefore less depth penetration by radar waves than materials with low dielectric constants. The dielectric constants and electrical conductivities of several different materials are shown in Table 4.1 for comparison. In general terms, signal attenuation a increases as Material Dielectric Electric Velocity Attenuation Constant Conductivity K (no units) o (mS/m) V (mlns) a (dB/m)
Air
Distilled water
Fresh water
Sea water
Dry sand
Saturated sand
Limestone
Shales
Silts
Clays
Granite
Dry salt
Ice
Table 4.1 Typical dielectric constant, electrical conductivity, velocity and attenuation observed in common geological materials at 100 MHz (from Davis and Annan, 1989). dielectric permittivity and electrical conductivity increase. The propagation velocity V of radar waves in Table 4.1 can be directly related to dielectric constant K (Moorman, 1990). A good example of the combined effects of both high electrical conductivity (o)and high dielectric permittivity (K) on signal attenuation is shown in wetted clays (see Table 4.1) where the rate of signal attenuation (a)is very high. As a result, depth penetration of radar signals is poor, and can be as low as 1 or 2 metres (Moorman, 1990; Moorman et al., 1991). GPR investigations which require depth penetration greater than 10 m are therefore limited to lithologies which exclude clay and have low water saturations (Jol and Smith, 1991; Gawthorpe et al., 1993; Stephens, 1994; Bridge et al., in press). In contrast, very low attenuation rates and excellent depth penetration is experienced in dry, porous sediments with no clay fraction, such as clean quartzose sands, where the penetration of radar signals can be in excess of 30 m (Smith and Jol, 1992b; Jol, 1995). Thus, the study site in this thesis was chosen in part because of its high sand content and negligible clay component. Signal attenuation also depends on antennae frequency. Transmissions from higher frequency antennae attenuate more rapidly with depth, thus providing poorer depth penetration but better vertical resolution than lower frequency antennae (Davis and Annan, 1989). There is thus a trade-off between vertical resolution and depth penetration when selecting GPR antennae. The relationship between antennae frequency and signal attenuation is discussed by Davis and Annan (1989). Theoretical equations relating dielectric permittivity, electrical conductivity, propagation velocity, depth penetration, signal frequency and attenuation are given by Olhoeft (1978), Davis and Annan (1989), and Moorman (1990). 4.1.2 GPR Instrumentation pulseEKKOTM IV system hardware
A pulseEKKOTMIV ground penetrating radar (GPR) system, manufactured by Sensors and Software Incorporated of Mississauga, Ontario, was used to collect all GPR data presented in this thesis. The pulseEKKOTMIV system is lightweight, portable, and fully digital. It functions in temperatures ranging from -25 "C to +35 "C (Jol, 1993) and has been applied in environments covered by ice, snow, water and dense vegetation although additional preparation time may be required under these conditions (Moorman et al., 1991; Finnish Geotechnical Society, 1992; Jol, 1993). The pulseEKKOTMIV system combines many of the advances of GPR made since its inception in the 1960's, a 30 year period of progress reviewed by several authors (Daniels et al., 1988; Davis and Annan, 1989; Moorman, 1990; Sensors and Software, 1992; Jol, 1993). Maximum radar efficiency, portability, and ease of use are primary advantages of the pulseEKKOTMIV design which consists of 5 main elements, i) a console unit, ii) laptop computer, iii) 12 V battery and, iv) two antennae consisting of a 400 V transmitter and a receiver. Power is supplied to the laptop and console units by the 12 V battery and the transmitter and receiver are each powered by two 6 V batteries housed in the antennae. The pulseEKKOTMIV system used in this study utilizes 50, 100, and 200 MHz antennae, although additional 12.5 and 25 MHz antennae may be obtained with a 1000 V transmitter for added depth penetration at low frequencies. All commands for the pulseEKKOTMIV system are controlled by the system software (version 3.1) run on a portable laptop computer which is connected to the console by an RS232 cable. The transmitter and the receiver are each connected to the console unit by fibreoptic cable. In this study the 12 V battery, console, and laptop were mounted on a backpack for mobility in the field (Figure 4.4). pulseEKKOTM IV system software
The parameter settings and processing techniques available for use with the pulseEKKOTMIV system (v. 3.1) are explained in detail in the systems manual (Sensors and Software, 1992) and are also reviewed by Jol (1993). All radar reflection profiles presented in this thesis were obtained using the common-offset, single-fold reflection profiling mode with the antennae placed in a perpendicular broadside position (Sensors and Software, 1992). Antennae separation and station spacing varied between 0.25 m and 2.0 m for radar profiles, depending on the degree of resolution desired, but values of either 1.0 or 0.5 m for both parameters typically were selected. All radar profiles were collected with either 64 or 128 stacks per trace and at the recommended sampling interval, dependent on the frequency of antennae used (Sensors and Software, 1992). Signal processing of each radar profile included the following 5 steps: i) signal saturation and dewow, ii) timezero adjust, iii) Automatic Gain Control (AGC=1000), iv) signal filtering (running average 3, mix 3), and v) topographic correction. Unless otherwise stated, a vertical depth axis for each profile was based on velocity estimates made from common mid-point profiles performed on location, a process explained in detail by Moorman (1990), Jol (1993), and Huggenberger et al. (1994). Migration of signal returns was not performed in this study, although all reported dip angles of radar reflections are corrected for migration based on the equation x = sin-l(tan X) where X is the measured angle in the radar profile and x is the actual angle corrected for signal migration (Sensors and Software, 1992). Figure 4.4 A photograph of the pulseEKK0 IV system used in this thesis. The battery, console, and laptop are mounted on a backpack (A) for greater P mobility in the field. The transmitter and receiver (B) are connected to the console unit by fibreoptic cable and separated at least 5 m from the backpack during operation to minimize electronic interference. 4.2 Interpretation of Radar Reflection Profiles
This section is divided into two parts. The first part examines the identification of radar reflections on 2-D radar profiles caused by sources other than genuine stratal boundaries. These radar reflections include hyperbolic diffractions, multiple reflections, reflections caused by above ground reflectors such as trees and telephone poles, and external sources of signal noise caused by electronic equipment. The second part of this section examines the more fundamental question: 'Do GPR profiles provide an accurate portrayal of subsurface stratal trends?' To investigate this question, the results of two radar surveys of exposed geologic sections are presented here. Both sections were selected because of their structural and textural similarity to deposits in the North Thompson River floodplain, the study site. Hence, the results of each survey will be used to establish a confidence level with which to interpret stratal trends from GPR profiles of the study site.
4.2.1 Environmental and Equipment-Related Noise on Radar Profiles
Any interpretation of radar reflection profiles must recognize that not all GPR reflections directly represent bedding in geologic deposits. Strictly speaking, individual radar reflections are not caused by material interfaces but by changes in the electrical properties of underlying materials as well as a combination of signal noise, scattering effects, changes in moisture conditions, and multiple reflections. As a result, radar facies analysis must contend with the interpretation of real stratal boundaries as well as with these spurious radar reflectors. Because the pulseEKKOTMIV system is designed with unshielded antennae, it is
sensitive to nearby sources of EM interference, particularly from metallic objects. Common sources of anomalous signal returns are overhead objects such as power cables
47 and trees, and ground-level objects like trees, telephone poles, buildings, fences, heavy machinery, and vehicles (Huggenberger, 1993; Young and Sun, 1994). The exact form of anomalous reflections on GPR records depends on the orientation of the survey line with respect to the object causing interference (Moorman, 1990; Huggenberger et al., 1994). It is important therefore, to recognize possible sources of surface scattering when obtaining GPR profiles because they may be indistinguishable from other GPR reflections (Young and Sun, 1994). Unshielded antennae are also susceptible to interference caused by high-frequency noise from nearby electronic equipment (Davis and Annan, 1989, Sensors and Software, 1992). To minimize these effects, the equipment surrounding the transmitter and receiver of the pulseEKKOTMIV radar system (the survey tape, antennae frame, and fibreoptic
cables connecting the antennae to the console unit) are non-metallic. Also, the antennae are kept at least 5 m from the backpack unit during operation in order to minimize the effects of equipment noise from the backpack (Sensors and Software, 1992). More common sources of interference on GPR profiles are hyperbolic diffractions associated with point sources or short lateral discontinuities beneath the ground surface such as large boulders or buried logs. The pulseEKKOTMIV system software (v. 3.1) does not provide any processing techniques to eliminate these hyperbolic diffractions; however, radar reflection data from the pulseEKKOTMIV may be imported into standard
seismic processing packages for their removal (Fisher et al., 1992). Techniques specifically designed to eliminate anomalous signal returns from radar profile are also discussed by Young and Sun (1994). Hyperbolic diffractions in this study, however, were left unprocessed as they were infrequent and did not pose a significant obstacle to the interpretation of radar reflection profiles. Multiple reflections are another common form of anomalous radar reflections. They appear as successive, evenly spaced bands within the profile and are caused when a transmitted radar signal reverberates between two reflectors before returning to the surface. Moorman (1990) and Telford et al. (1977) illustrate several different types of multiple reflections and explain the material conditions where their occurrence is most likely. Other sources of multiple reflections, such as antennae ringing, are discussed by
Young and Sun (1994).
4.2.2 Ground-Truthing Radar Reflection Data
As previously described in the Literature Review (Chapter 2), radar facies analysis involves the ordering of individual radar reflections into radar sequences. Although there is no consensus in the literature regarding the genetic interpretation of radar stratigraphy in GPR reflection profiles, this thesis adopts the techniques used in the radar facies analysis of fluvial deposits from previous studies (Moorman, 1990; Moorman et al., 1991; Gawthorpe et al., 1993; Bridge et al., in press; Huggenberger, 1993; Huggenberger et al, 1994). None of these studies, however, save that by Huggenberger (1993) and Huggenberger et al. (1994), attempt to test the accuracy of radar reflection profiles by validating them against geologic exposures. Due to the overall paucity of testing in this regard, this section presents the results of two such studies undertaken for this thesis, in this case the GPR survey of geologic sections similar in structure and texture to the North Thompson River floodplain. The aim of these surveys is to establish how well GPR portrays underlying stratal geometries of the floodplain at the main study site. Each survey will provide visual evidence to establish the degree to which radar stratigraphic data may be reliably interpreted as stratal trends. Both geologic sections were located in gravel pits in southwestern British Columbia, the first at Central Aggregate near Peardonville, BC and the second at Kirkpatrick Sand and Gravel Co. Ltd. in Maple Ridge, BC. Both sections consisted of sand and gravel deposits exhibiting either inclined stratification, parallel or subparallel tabular bedding, or trough cross-bedding. The objectives of this test were: i) to present a visual demonstration of the ability of GPR to accurately represent the stratal geometry of a geologic section and, ii) to examine the effects of varying GPR survey parameters on the appearance and interpretation of radar reflection profiles. These involved varying antennae frequency, distance between the antennae (antennae separation), and interval between stations (station spacing).
Section 1: Central Aggregate gravel pit, Peardonville, B.C.
Section (1) (Figure 4.5) is approximately 4 m high by 30 m wide and consists of sand and gravel deposits with thin (0.1-0.4m) gravel lenses interspersed within the section. The section exhibits 2 main elements, a sequence of cross-stratified beds inclined at angles from 18 to 25' which is capped along an abrupt erosional contact by a unit consisting of tabular planar beds. The outcrop is part of the Fort Langley Formation (Armstrong, 1981) and is interpreted as a prograding delta front. Three radar reflection profiles were taken of this outcrop, using 50, 100, and 200 MHz antennae frequencies (Figure 4.6). In order to clearly examine the correspondence between GPR profiles and stratal geometry of the exposure, each radar profile was interpreted then scaled to fit a photograph of the section (Figure 4.3, and finally superimposed on top of the photograph (Figures 4.7a, b, c). Both the interpretation and superposition of radar profiles were performed by importing scanned images into a graphic design program. The overall pattern in the three superimposed sections (Figures 4.7a, b, c) shows that the inclined foreset beds in Figure 4.5 are recognizable on GPR profiles. However, there is only gross correspondence between radar stratigraphy and actual strata observed in the section in these diagrams, some radar reflections accurately reflect local stratal trends while others do not seem to correlate at all. In general, best correlation is found on the north side of the Figures (4.7a, b, c), especially in Figures 4.7a and 4.7~where the
50 MHz
100 MHz
200 MHz
Figure 4.6 Radar reflection profiles of highwall exposure at Central Aggregate gravel pit (Figure 4.5) using 50, 100, and 200 MHz antennae. Vertical exaggeration is 1.7~. Figure 4.7a Radar interpretation of 200 MHz profile is overlaid on photo of the highwall exposure from Central Aggreagate gravel pit (Figure 4.5). Each line represents a radar reflection from the original 200 MHz radar reflection profile (Figure 4.6). Uppermost line (paralleling the ground surface) represents the ground wave from Figure 4.6. Figure 4.7b Radar interpretation of 100 MHz profile is overlaid on photo of the highwall exposure from Central Aggreagate gravel pit (Figure 4.5). Each line represents a radar reflection from the original 100 MHz radar reflection profile (Figure 4.6). Uppermost line (paralleling the ground surface) represents the ground wave from Figure 4.6. Figure 4.7~ Radar interpretation of 50 MHz profile is overlaid on photo of the highwall exposure from Central Aggreagate gravel pit (Figure 4.5). Each line represents a radar reflection from the original 50 MHz radar reflection profile (Figure 4.6). Uppermost line (paralleling the ground surface) represents the ground wave from Figure 4.6. erosional contact between topset and foreset beds coincides fairly well between radar and outcrop data and foreset beds are increasingly well defined at depth. In contrast, Figure 4.7b does not show a clear break between topset and foreset beds and the pattern of radar foreset beds begins well below that shown in the photograph. All three radar profiles (Figures 4.7,a,b,c) show increasingly poor correspondence toward the south side of the photo. Radar reflections appear much less steep and exhibit significant waviness in foreset beds that in actuality are not wavy at all. Overall, radar images of the south side of the section do not clearly distinguish foreset beds nor can the erosional contact with overlying topset beds be identified. These results compare poorly with those achieved on similar deltaic deposits by Smith and Jol (1992b) and Jol (1993), shown in Figure 4.3. Some of this discrepancy, however, may be attributed to the close proximity of a conveyor belt (see Figure 4.5) approximately 3 to 4 m behind the exposed section. Poorer correlation between radar and outcrop data on the south side of the section (Figures 4.7a, b, c) could be attributed to the fact that GPR surveys were positioned closer to the conveyor belt on the south side of the section and increasingly further away toward the north.
Section 2: Kirkpatrick Sand and Gravel, Maple Ridge, B. C.
Section 2 is approximately 80 m wide by 5 m high and consists primarily of sub- horizontal, tabular planar sands capped by a thin (0 - 0.5 m), intermittent layer of weakly stratified sand with minor silt and gravel. A photomosaic of section 2 is shown in Figure 4.8. Four radar profiles were taken of the exposure and their parameters are listed below in the following format: (antennae frequency, antennae separation (m), station spacing (m)).The 4 profiles are: i (100 MHz, 1.0, 1.0); ii (100 MHz, 0.5, 0.5); iii (200 MHz, 0.5, 0.5); and iv (200 MHz, 0.25, 0.25). Each profile and its corresponding interpretation is shown in Figures 4.9 and 4.10. Note that the profiles are divided into two sections by a Figure 4.8 Photomosaic of the exposure at Kirkpatrick Sand and Gravel pit, Maple Ridge, BC. The outcrop is approximately 6 m high by 80 m wide and dominated by thinly bedded (0.1-0.4m) sands. Section (A) of the outcrop exhibits some trough cross-stratification with sand clinoforms containing minor silt and gravel at the extreme left edge of VI .l .l the exposure. Figure 4.9 Radar reflection profiles and corresponding interpretation of exposure at Kirkpatrick Sand and Gravel pit (Figure 4.8): (A) 200 MHz, antennae sep.=0.25m, station spacing=0.25m; (B) 200 MHz, antennae sep.=OSm, station spacing=OSm. No vertical exaggeration. The vertical line at 22 m in each interpreted profile represents a 3 m section that was not GPR surveyed due to a fissure at this location, the right edge of section (A) in Figure 3.8. Note that the interpreted area of each profile represents the exposed section of the outcrop.
3 m wide fissure located on the south side of section A in Figure 4.8 and at the 22 m mark on interpreted profiles in Figures 4.9 and 4.10. This fissure could not be surveyed by GPR due to its irregular surface. The pattern of radar stratigraphy in GPR profiles to the south of the fissure clearly reveals the subparallel bedding of the actual exposure (Figure 4.8). Many radar reflections are laterally extensive (5 - 20 m), reflecting the continuity of bounding surfaces. Nonetheless, some radar reflections exhibit significant degrees of inclination (3-6")' such as the southward dipping reflection located at 2-3 m depth in the 50-60 m region of both 200 MHz profiles (see Figure 4.9). These reflections do not correspond with bounding surfaces in the outcrop photograph (Figure 4.8) where there is no similar dip. Thus, like the profiles from the Central Aggregate pit, some individual radar reflections do not accurately reflect local stratal geometries. Nonetheless, the overall pattern of radar reflections in each profile does correlate well with the outcrop. To the north of the fissure (section A, Figure 4.8) there is a marked change in stratal geometry that is reflected in radar profile data. This section exhibits trough cross- stratification and sand clinoforms, the latter are inclined from north to south in the first 5 m of 100 and 200 MHz radar profiles (Figures 4.9 and 4.10). This change in stratal geometry is expressed by increasing irregularity and lack of lateral continuity in GPR reflections on the south side of the exposure, a clear change from radar stratigraphy to the north. Although the pattern of trough cross-stratification is not clear in the radar profiles, the inclined bedding on the extreme south side of the exposure does appear in all four GPR profiles. Due to the length of this exposure, a radar stratigraphic section could not be overlaid on the outcrop photomosaic because of changes in scale between photographs. Nonetheless, a set of photographs taken of the south side of the outcrop were of
'~11angles (X) measured from radar profiles underestimate the true dip angle (x) and must be corrected by the equation x=sin-l(tan~)(Ulriksen, 1982). sufficiently similar scale to be used for an overlay (Figure 4.11). In general, the overlay
shows good correspondence between patterns of radar reflections and stratal trends observed in the outcrop, however, it does not show clear correlation between individual radar reflections and lithologic contacts observed in the exposure.
Conclusions
Both sections surveyed in these comparison tests lead to similar conclusions. First, inferences of underlying stratal trends from GPR profiles appear to be most accurate when based on a consistent pattern of radar reflections over a large section of the outcrop (10 m x 10 m). Second, individual radar reflections are generally poorer indicators of local stratal geometries and may not accurately depict local trends. Therefore, inferences of local stratal trends made from 1 or 2 radar reflections, rather than a pattern of reflections, should be viewed with skepticism. Third, varying antennae frequency andlor antennae separation and station spacing between the four profiles did not significantly affect the interpretation of large-scale stratigraphic trends in the exposure. Thus, high resolution GPR profiles do not appear to be necessary for the identification of large-scale (>lo m in the horizontal) stratal patterns in these environments.
4.3 GPR Survey Design: North Thompson River
4.3.1 Site Selection
There were four primary criteria in selecting a suitable site for this study. First, previous work using GPR has shown that sediments with high clay content are poorly suited to GPR studies because signal attenuation is very high and results in only 1 or 2 metres of depth penetration (Moorman, 1990). As a result, only floodplain sediments with
61 little or no clay fraction were considered for study. Second, floodplain deposits of at least 5 m in depth were sought to ensure adequate GPR resolution of floodplain architecture. Third, because the focus of this study is on floodplain architecture, a minimum area of 3 to 5 channel widths was considered necessary to acquire a representative sample of the floodplain. Finally, the logistical requirements of GPR profiling needed to be met, including ease of access, reasonably level terrain, and little or no vegetation or obstructions (e.g. fences, trees, brush, and livestock).
4.3.2 GPR Survey Design
The four criteria of the previous section were met on a large meander bend of the North Thompson River, an area used mainly for hay cultivation. A large section of the contemporary floodplain was selected for GPR survey (Figure 4.12). The location of GPR survey lines was based on a Cartesian grid system using cultural boundaries such as fences, field borders, tree lines, and the main road. Survey lines were spaced approximately 100 m apart but their exact location was governed by existing cultural boundaries. Either a radial survey grid or a more precise Cartesian grid were made impossible by the daily passage of farm equipment during harvest, the only period of time local farmers permitted GPR surveying of their fields.
Antennae Frequency Selection
The GPR survey area was investigated in two stages. The first part, represented by undashed lines in Figure 4.12, was examined using 100 MHz antennae and the second part, represented by dashed lines, used 200 MHz antennae. The selection of 100 MHz antennae for the greater portion of the floodplain was based on a test of 50, 100, and 200 MHz antennae on a section of Dairy Road (see Figure 3.1). Comparison of the 3 profiles Figure 4.12 The location of radar survey lines in the study area. Continuous, undashed lines represent region of GPR surveys taken with 100 MHz antennae; dashed lines represent the use of 200 MHz antennae. The two rectangular black boxes show the location of ginseng fields. River flow is from north to south. showed that the 100 MHz antennae provided the best vertical resolution without compromising depth penetration of radar signals in the floodplain. This procedure for antennae frequency selection is detailed in the systems manual (Sensors and Software,
1992) and by Annan and Cosway ( 1994). The second area of radar profiles, shown by dashed lines in Figure 4.12, was surveyed using the 200 MHz antennae. This change in antennae frequency was made because this section of the floodplain was substantially lower than the section surveyed by 100 MHz antennae. As a result, signal penetration with the 200 MHz antennae was sufficient to resolve the entire vertical column of floodplain sediments and provide increased resolution.
All 100 MHz constant-offset GPR profiles of the floodplain were obtained with an ' antennae separation and station spacing of 1 m. Based on the results of Section 4.2.2, this was considered sufficient to accurately identify large-scale (>75 m in the horizontal) floodplain architecture, the primary focus of this study. All 200 MHz GPR profiles were obtained with an antennae separation and station spacing of 0.5 m. The selection of a 0.5 m interval was made to see if greater vertical and horizontal detail in GPR profiles would reveal any significant detail of floodplain architecture that may have been too small to be recorded by a 100 MHz survey.
Collection of topographic data
All GPR profiles were collected in conjunction with topographic data in increments of 100 m, or the total length of the transect line. Thus, each GPR survey line in Figure 4.12 represents a series of connected radar profiles each about 100 m long. After a GPR profile was completed, topographic data along the transect line were collected using an Abney level and survey staff at a minimum of 10 m intervals. Most topographic data were collected at 5 m intervals or less, depending on the topographic features encountered. Error in Abney level readings was +I- 2.5 cm for 10 m intervals and +I- 1 cm for 5 m intervals. Because these data could lead to significant error over kilometre distances, they were used in conjunction with 1:5000 scale orthophoto maps of the study area with 1.0 m contour intervals and point elevations (BC Environment, 1988). Point elevations were accurate to 0.1 m and over 100 were located in the GPR survey grid. For topographic correction of GPR profiles, orthophoto maps were first used to create a base map of large scale topographic variability along each GPR survey line. Abney level data were then used to fill in small scale topographic variability between contour lines, such as the location of minor ridges and swales. This system provided a rapid means of acquiring reasonably accurate, highly detailed topographic data for more than 10 km of GPR profiles in the survey area. Maximum error in topographically corrected GPR profiles was estimated at 0.3 m in worst case conditions.
Velocity Profiling
The propagation velocity V of radar waves through subsurface materials can be determined using several different methods: i) estimating V based on the dielectric constant of the host material, ii) the Common Mid Point (CMP) method, iii) Wide-Angle Reflection and Refraction (WARR), and iv) Velocity Analysis. Methods (i), (ii), and (iii) are reviewed by Moorman (1990) and method (iv) is reviewed by Cross and Knoll (1991). The CMP method (method iii) was used to calculate the propagation velocity of all the materials surveyed in this thesis as it is more accurate than methods (i) and (iii). Method (iv), velocity analysis, is similar to the CMP method except that velocity information is derived from the slope of hyperbolic reflections off point sources in the radar profile, which are not always available. Parameter settings for velocity sounding at the study site were the same as those for reflection profiling except that station spacing and antennae separation were set at 0.5 m when performing a CMP profile. A total of 6
66 CMP profiles were taken at evenly spaced locations within the study area and averaged to provide a mean velocity of radar signals on all GPR profiles.
4.3.3 Core Sampling Technique
Coring in the river floodplain was unsystematic, used primarily to identify grain size variability within specific radar facies and, if possible, to correlate the location of the water table and major geologic boundaries with individual radar reflections. Coring was done using two methods, a hand-powered auger and a vibracorer (Figure 4.13). The hand auger was able to penetrate to a maximum depth of 5.0 m and was used in cases where only a shallow core was needed. The vibracorer was used to access material below the level of the water table where augering was hampered by continual sediment slumping in the auger hole. All core data were subject to grain size analysis in the field using a hand-held grain size chart (Amstrat Canadian Stratigraphic) and a pocket magnifying glass. Auger holes were sampled at least every 15 cm and vibracorer sections were sampled every 10 cm and photographed in cases where the core was well preserved. Core locations were determined from GPR profiles and selected based on an identifiable pattern of radar stratigraphy (e.g. epsilon cross-bedding) or unusually strong reflections in the region which required identification. All cores were taken either along a radar survey line or accompanied by a GPR profile of the immediate area. Figure 4.13 A photograph of the vibracorer used in this thesis. Core pipe diameter is 3 inches, the same as that of the hand auger (not shown). The hand auger was used to core to the water table where the vibracorer was then inserted. The ladder assembly was used for core pipe removal and to vibrate samples into the circular black pipe (P) for grain size analysis. CHAPTER FIVE RESULTS
This chapter is divided into five sections. The first compares lithologic cores from the floodplain with corresponding radar profiles. The object of this comparison is to identify whether or not major lithologic boundaries and the water table produce distinct radar reflections that can be identified on GPR profiles. A second objective of this section is to characterize the sedimentological variability of radar reflectors observed at each core location. The results of common mid-point (CMP) profiles in the study area are presented in section two. The third and fourth sections of this chapter present 100 and 200 MHz GPR profiles from the floodplain survey grid (see Figure 4.12). Each profile is presented in original and interpreted form. Interpreted profiles identify and interpret macroscale radar facies with respect to their two-dimensional (2-D) appearance. The three- dimensional (3-D) arrangement of selected radar sequences and the identification of architectural elements in the floodplain are presented in section five. The location of radar profiles, lithologic cores, and CMP profiles taken in the study area are shown in Figure 5.1.
5.1 Core Log Comparisons with GPR Profiles
A total of nine cores were extracted in the study area (see Figure 5.1). Grain size variability and the location of the water table (where present) at each core site is shown in Figure 5.2. Corresponding radar profiles of each core are shown in Figure 5.3 with the location of major lithologic changes and the water table indicated beside each GPR profile. Note that the water table is identified in only four radar profiles in Figure 5.3 (Cl, C2, C7, and C8). The remaining GPR profiles were not obtained at the same time as the cores, thus the water table could not be correlated with certainty. No core exceeded 7 m in
69 Figure 5.1 The location of GPR profiles, core sites, and CMP profiles in the study area. Cores are denoted by the letter 'C' followed by the core number. CMP profile locations are shown by the letters 'CMP'. Remaining terms identify individual radar profiles. Dark black boxes denote ginseng fields located within the survey grid (Fig. 4.12).
CORE CORE I CORE I CORE
' 100 MHz - - C1
CORE
sancUgravel contact
sandlclayey sand contact f:
100 MHz
Figure 5.3 Locations of the water table and lithologic boundaries from core data (Figure 5.2) are shown on corresponding radar profiles. Antennae frequency is shown in the lower right corner of each radar profile. The water table (WT) appears as a strong, laterally continuous horizontal or subhorizontal reflector in 4 GPR profiles, but lithologic changes do not appear as well-defined. Rapid signal attenuation at depth is interpreted as fine-grained (silt or clay) sediments underlying the study area (see Figure 3.2). depth, due to the inability of the vibracore to penetrate through coarse-grained sand and gravel in the floodplain. All cores in Figure 5.2 exhibit a vertical upward fining from coarse sand with occasional clay and gravel to very fine-grained sand and silt. This trend corresponds well with lithologic variability observed in core samples from other locations near the study site (see Figure 3.2). The ability of these lithologic changes and the water table to produce distinct radar reflections in GPR profiles varies considerably from one GPR profile to another. Open framework gravel in cores C2, C4, C7, and C9 and a cobble lens in core C1 show varying definition of radar reflections (Figure 5.3). Gravel lenses in profiles C2, C7 and C9 do not produce distinguishable radar returns, although individual reflections that coincide with the depth of gravel lenses suggest that they act as radar reflectors (Figure 5.3). The gravel lens in core C4 does correspond with a strong radar reflection (Figure 5.3); however, this gravel bed is also a major source of well water on the property (Tessier, pers. comrn.). Hence, signal amplitude is most likely influenced by the increase in saturation conditions within the gravel lens than by the gravel lens itself (Davis and Annan, 1989). The final coarse-grained lens, a thin (10 cm) cobble layer in core C1, revealed a 5 cm, well-rounded stone within the vibracorer sample. This stone lay immediately above a clay layer that inhibited further penetration by the vibracore. The radar profile of core C1 (Figure 5.3) shows different characteristics in radar reflections above and below this clay boundary. Apart from changes in radar stratigraphy, radar returns above the clay exhibit stronger individual reflections, with single peaks and clean radar signatures between reflections. In contrast, radar returns below the clay boundary exhibit multiple peaks as well as significant signal noise between each reflection. This could be attributed to the fine laminations observed in the clay sample taken from the base of the vibracore since each laminated bed could produce multiple weak signal returns. A second characteristic of
radar reflections below the clay boundary is their rapid signal attenuation below 10 - 14 m depth, an occurrence that is found at all core sites in Figure 5.3. This is attributed to an increase in silt or clay concentration at these depths, known to exist from lithologic cores of the region (see Figure 3.2). It seems reasonable to assume, therefore, that the depth of rapid signal attenuation in radar profiles may be used as an indicator of the basal contact between coarser floodplain sediments and underlying Kamloops Lake Drift sediments (see Section 3.1). The presence of minor amounts of clay, however, did not result in any significant signal attenuation. For example, clayey sand lenses identified in cores C2 and C8 do not show any noticeable signal loss. Thus, it appears that only abrupt changes in lithology, such as gravel to clay, may be identified using GPR. Grain size variability within two sequences of oblique clinoforms was examined in cores C5, and C6. Oblique clinoform radar facies are prevalent in floodplain sediments, thus two lithologic cores were taken at representative sites to identify their sedimentologic makeup. Both cores show upward-fining from coarse to fine-grained sands within each radar facies. Core C3 was taken to confirm reports from a local resident that clay lay within the top 1 m of the floodplain at this location (see 'C3' Figure 5.1); however, an auger hole to 2.5 m depth revealed no clay at this site. Hyperbolic and inclined reflections below 6 m in this profile (C3 in Figure 5.3) are interpreted as surface reflections caused by a tree and storage shed 1 to 2 m from the GPR survey line. The water table in these four cores taken at the same time as radar profiles (C 1, C2, C7 and C8) all correlate with high-amplitude, laterally continuous reflections in GPR profiles, however, only the water table reflector in core Cl produced a reflection distinguishable from surrounding radar returns. A high-amplitude reflection at 2.5 m depth in GPR profile C3 (Figure 5.3) does correspond to the water table in core C3 (Figure 5.2) but the two cannot be correlated with certainty since core and GPR profile were taken exactly one year apart. Nonetheless, it seems likely that core and GPR data taken one year apart would show similar water table depths. In general, the water table reflector in 100 MHz profiles (Cl and C2) appeared somewhat easier to discern than in 200 MHz profiles (C7 and C8). Water table reflectors are not necessarily horizontal, in cores C1 and C8 in Figure 5.3 they show significant (0.5 - 1.0 m) variability in the vertical column over 20 m horizontal distance. Core C1 exhibits a streamward dip in the water table that is attributed to throughflow from the terrace along the western border of the study site (see Figure 3.5). The water table in core C8 is remarkably well-adjusted to subsurface stratal trends, meaning that this reflection may not be the water table at all but a stratal boundary, which happens to conform with the water table depth at the core site. In addition, cases of significant lithologic variability may change local material properties sufficiently to affect the velocity of radar signals, producing a subhorizontal water table radar reflection from what is actually a horizontal water table surface (Greenhouse et al., 1987). In summary, GPR profile C1 exhibits the most distinctive radar reflection, at the level of the water table, a reflector which could be interpreted in the surrounding area without the need of additional coring. Profile C4 showed clear correlation between a strong radar reflection and a gravel layer but local residents have identified this gravel bed as a major source of well water, thus this reflection could also be attributed to the water table. Gravel and clayey sand lenses did not produce distinctive radar reflections although varved clays at the base of core C 1 did produce a change in radar returns as well as cause significant signal attenuation. Finally, the approximate location of basal silt and (or) clays underlying the floodplain may be interpreted based on rapid signal attenuation with depth, exhibited in radar profiles of each core site (Figure 5.3).
5.2 Interpretation of GPR Profiles
This section presents the results of 100 and 200 MHz GPR surveys of the study area, preceded by a brief description of CMP (common mid-point) calculations in the GPR grid. All GPR profiles in the survey grid (Figure 4.12) are identified in Figure 5.1 where GPR transects are labeled alphabetically along one axis and numerically along the other. Radar profile 1, for example, refers to the entire GPR survey line beside the number '1' and an intersection point such as '1A' refers to the intersection of GPR profiles '1' and 'A'. Note that GPR profiles in Figure 5.1 are not oriented along true dip or strike directions of surface scroll bars (Figure 5.4). Due to the length of GPR transects (200 - 900 m), the orientation of each radar survey line with respect to surface scroll topography changes from one end of the profile to the other. All GPR profiles in this section are presented in original and interpreted form. Interpreted profiles identify and interpret macroscale radar facies2, distinguished by patterns of reflections whose appearance differs from that of adjacent units. The radar reflections which make up a radar facies are described using terminology from the Literature Review (Section 2.7). The genetic interpretation of radar facies is based on the expected internal architecture of a scrolled, meandering river floodplain (Section 2.1). GPR survey lines are examined in two parts. The first part identifies radar facies along the GPR profile and includes some facies level interpretation. The second part, entitled 'interpretation', is reserved for the genetic interpretation of radar facies as they relate to the orientation of surface ridge-and-swale topography. The inclusion of some interpretation in the first part, such as the identification of potential water table reflections, hyperbolic diffractions, multiples, lithologic boundaries, and preserved scroll ridge deposits, is undertaken in order to minimize repetition between 'description' and 'interpretation' sections. Thus, 'interpretation' sections in this chapter refer solely to the genetic interpretation of radar facies as they relate to surface scroll topography. No attempt is made to formulate a set of radar facies for use in the description of GPR profiles in the survey grid. Rather, GPR profiles are examined and interpreted
2a macroscale radar facies is defined as a radar facies exceeding 75 rn in horizontal distance on a two- dimensional GPR profile Figure 5.4 The relationship between GPR profiles and surface scroll topography. Dashed lines indicate the orientation of selected scroll ridges on the floodplain surface. Darker scroll ridges mark changes in the direction of floodplain accretion. Shadowed grid lines and labeling identify GPR profiles from Figure 5.1. independently. Thus, the number of each radar facies in a GPR profile refers to that profile only. Therefore, radar facies 1 from GPR survey line #1 for example, has no relation to radar facies 1 identified in GPR survey line #2; the numbering system exists only to differentiate radar facies in each individual GPR profile. Radar facies are grouped into related patterns of radar reflections and classified genetically in Section 5.3 and in the Discussion (Chapter 6).
5.2.1 Velocity Estimate for GPR Profiles in the Study Area
An average velocity of 0.16 dns (1.6 x 10-8 ds) was calculated from six common mid-point (CMP) profiles located in the GPR survey grid (Figure 5.1). All CMP profiles and calculations are shown in Appendix 1. Individual CMP profile results varied from 0.135 dns to 0.175 dns and both mean and median values are 0.16 dns. This result is greater than typical velocity values for dry and saturated sands (Davis and Annan, 1989) which are 0.15 and 0.06 dns respectively. Nonetheless, a velocity estimate of 0.16 dns results in GPR profiles that exhibit floodplain depths in the study area between 10 to 12 m, in agreement with the depth to the thalweg in the modern river channel (Section 3.1).
5.2.2 Interpretation of 100 MHz GPR Profiles
Radar Profile #I Figure 5.5 shows the original and interpreted profiles of GPR survey line #1, located at the northwestern edge of the study site (Figure 5.1). Macroscale patterns of radar reflections in the profile can be divided into four radar facies. Facies 1 exhibits southward dipping reflections inclined between 2" and 10" that appear as continuous and semi-
continuous reflections from 0 to 330 m in the radar profile (Figure 5.5). Radar facies 2 is
located from 260 - 451 m at 0 - 8 m depth and characterized by laterally continuous, parallel and subparallel reflectors with some oblique parallel, southward dipping reflectors between 3 and 5 m depth from 370 - 400 m. A complex pattern of semi- continuous, subparallel reflections at 10 - 14 m depth between 0 and 90 m is identified as radar facies 3. Radar facies 4 is characterized by an upper boundary of rapid signal attenuation between 5 and 6 m depth except below facies 3 where attenuation occurs at 11 m depth. The reflections in facies 4 are laterally continuous and parallel, although parallel reflections below the uppermost reflection from 102 to 294 m could be interpreted as multiples, potentially caused by a lithologic change from sand and gravel to silt andlor clay deposits at the level of the first parallel reflector (Telford et al., 1977). Although the lack of radar reflections in facies 4 is attributed to higher silt or clay content, this does not imply that the entire vertical extent of this facies consists of these materials. It may be that a thick lens of clay at 7 m depth is responsible for attenuation of the radar signal below this point, preventing any deeper radar stratigraphy (and changes in lithology) from being resolved.
Interpretation: GPR survey line #1 is oriented 65" to 70" from the true dip direction of surface scroll topography (Figure 5.4) thus, parallel or slightly inclined reflectors in this profile are expected since the GPR transect is angled only 20" from the strike face of surface scroll bars, an orientation which should expose parallel tabular beds (Allen, 1963; Bridge et al., in press). The southerly dip of radar reflections in facies 1 (Figure 5.5) indicates floodplain accretion in a southerly direction, in agreement with the orientation of surface scroll bars and the direction of flow in the modern channel. Radar facies 3 exhibits signal penetration better than radar facies 4 but worse than facies 1. Since the rate of signal attenuation may be correlated with clay or silt content (Section 4.1), sediments in facies 3 are interpreted as silty or clayey gravels possibly associated with channel scouring of underlying fine-grained sediments. Radar Profile #2 Figure 5.6 shows the original and interpreted profiles of GPR survey line #2 (location in Figure 5.1). Macroscale patterns of radar reflections in the profile can be divided into four radar facies. Facies 1 delineates a region of southward dipping reflections that are characterized by shingled clinoforms from 220 - 445 m and by laterally continuous, inclined reflections from 30 - 200 m that dip from 2" to 18". Radar facies 2 is located at the southern edge of the profile from 0 - 70 m at 0 - 10 m depth and differentiated from facies 1 by parallel, laterally continuous reflections exhibiting no overall dip. Radar facies 3 is similar in this regard, exhibiting laterally continuous, parallel and subparallel reflectors up to 10 m in depth for more than 300 m. The 'CH' mark at 380 m in Figure 5.6 identifies a bowl-shaped sequence of reflections interpreted as a prograded channel fill. Radar facies 4 extends along the length of the profile and is bounded by an abrupt signal loss at 10 m depth. Repeated parallel reflections throughout this facies, 2-3 metres below the first parallel reflection, are interpreted as multiples since they are equally spaced at depth in a material which is otherwise devoid of radar signatures. Hyperbolic reflections at 40 m and 120 m are likely attributed to surface reflectors, two trees located approximately 15 - 20 m from these locations along the radar survey line.
Interpretation: GPR survey line #2 is oriented between 10" and 20" from the strike face of surface scroll bars in this area (Figure 5.4). Thus, the profile is expected to show macroscale structures in the form of parallel or slightly inclined stratification associated with the strike face of ECS deposits (Allen, 1965). Generally, this is the case in radar facies 2 and 3 although facies 1 exhibits a significant southward dip of reflectors, increasing in depth at 200 m along the survey line (Figure 5.6) at a location corresponding to a change in orientation of the survey line from 10" to 20" off the true strike direction of surface scroll bars (Figure 5.4). Inclined reflections from 200 to 480 m show preserved ridge-and-swale (RAS) units (marked IS' in Figure 5.6.), not expected along a section oriented 10" from the strike face of surface scroll bars.
Radar Profile #3 Figure 5.7 shows the original and interpreted profiles of GPR survey line #3. Macroscale patterns of radar reflections in the profile can be divided into five radar facies. Facies 1 is identified by inclined reflections that dip between 3" and 12" toward the south from 50 to 350 m. Facies 1 also exhibits a radar sequence from 100 to 140 m of closely spaced, parallel reflections, labeled '?I in Figure 5.7, that are distinct from surrounding inclined reflections. Facies 2 and 3 are differentiated from facies 1 by a lack of inclined reflections. Radar facies 2 is characterized by laterally continuous, parallel wavy clinoforms that mirror surface scroll bars and facies 3 exhibits laterally continuous, stacked parallel reflections. Facies 3 is underlain by laterally continuous, wavy and subparallel clinoforms identified as radar facies 4. Facies 4 also exhibits two channel fills at 555 m and 670 m, the latter conforming to a prograded channel fill (see Figure 2.4).
Radar facies 5 is identified by the lack of radar returns below 8 - 10 m depth. Dashed lines at 300 m and 426 m identify the location of hyperbolic reflections, attributed to a stand of trees paralleling the radar profile.
Interpretation: The orientation of GPR survey line #3 with respect to the strike face of surface scroll bars varies between 0" and 22" along the profile (Figure 5.4). A significant increase in the dip angle of reflections in facies 1 from 3" to 18" at 230 m corresponds roughly with a change in scroll bar orientation on the floodplain surface (Figure 5.4), where profile orientation shifts from near 0" to 22" off the strike face of surface scroll bars. Thus, increased dip angle of inclined reflections can be attributed to this increased departure of the profile from the true strike direction of surface scroll bars. The set of reflections labeled '?' in facies 1 is interpreted as a layer of silt or clay at 5 m depth and is responsible for a set of multiple reflections below 5 m from 100 to 140 m. Radar facies 2 is characterized by surface ridge-and-swale (RAS) topography, vertically preserved at depth. Although lacking inclined stratification, facies 2 exhibits RAS units similar to those shown in Figure 2.1, although there are no inclined reflections below the RAS deposits in this case. Note that RAS structures appear elongate due to the orientation of the profile, nearly 65" from the true dip direction of surface scroll bars. Parallel reflections in radar facies 3 conform to a strike section along a surface scroll bar. Wavy clinoforms in facies 4 are interpreted as trough cross-stratified sands and gravels associated with channel bedform and lower point bar deposits, similar to those described in Section 2.2.1. Although channel fill patterns are identified at 550 m and 670 m in facies 4, they are only weakly defined and oriented normal to the strike direction of surface scroll topography, contrary to their expected orientation.
Radar Profile #4
Figure 5.8 shows the original and interpreted profiles of GPR survey line #4 (location in Figure 5.1). Macroscale patterns of radar reflections in the profile can be divided into two radar facies. Radar facies 1, uppermost in Figure 5.8, exhibits mainly continuous and semicontinuous subparallel reflections. A trough-shaped pattern of reflections from 66 to 124 m at 6 to 8 m depth marked 'CH' in the profile is identified as a channel fill. A distinctive, high-amplitude reflection from 66 to 402 m at 3.5 - 5 m depth is interpreted as the water table, shown by a dashed line labeled 'WT' in Figure 5.8. Locations marked 'Sf at 202 m, 228 m, and 326 m show the peaks of wavy clinoforms preserved at depth that are interpreted as preserved scroll ridge deposits. Rapid signal attenuation between 9 and 11 m depth defines the upper boundary of radar facies 2, a region devoid of reflections interpreted as signal attenuation through fine-grained silt or clay at 9 to 11 m depth. A hyperbolic reflection at 18 m in the profile is attributed to either a log or boulder at this location as no surface reflectors were near the area. Interpretation: The orientation of GPR survey line #4 with respect to the strike direction of surface scroll bars changes from 25" to 35" from north to south (Figure 5.4). Radar stratigraphy in Figure 5.8 exhibits inclined stratification dipping between 0" and 5",
expected along the dip direction of surface scroll bars. A set of more steeply inclined reflections, dipping up to 10' to the south, also appears from 60 to 200 m at 5 to 7 m
depth.. These reflections, and preserved RAS deposits at IS' locations in Figure 5.8, correlate well with Nanson and Croke's (1992) model of scrolled floodplain architecture (Figure 2.1), but inclined reflections are not as extensive as those shown by both Miall (1985, see also Figure 2.2) and Nanson and Croke (1992). Note that signal attenuation at the base of inclined reflections from 60 to 110 m is deeper than in adjacent areas. This is interpreted as channel scour where mixing of sand and gravel with underlying silt and clay has resulted in increased signal penetration. The location marked 'CH' from 70 to 100 m in Figure 5.8 is interpreted as an infilled swale as it lay between two scroll ridges on the floodplain surface. Identification of the water table reflector, 'WT' in Figure 5.8, is based on similar reflections verified as the water table in Section 5.1.
Radar Profile #5 Figure 5.9 shows the original and interpreted profiles of GPR survey line #5 (location in Figure 5.1). Macroscale patterns of radar reflections in the profile can be divided into six radar facies. Facies 1 is identified by laterally continuous, inclined reflections that dip toward the south between loand 18" and occupy much of the southern half of the profile
from 40 - 500 m. A high-amplitude reflection marked 'WT' in facies 1 and 5 is interpreted as the water table and indicated by a dashed line between 4 and 5 m depths in Figure 5.9. Radar facies 2 exhibits laterally continuous, parallel reflections interrupted at depth by
hyperbolic reflections from 600 to 820 m. An area marked 'CHI in facies 2 identifies a prograded channel fill sequence from 650 to 720 m. A second channel fill is also identified at 874 m in facies 3. Radar facies 3 is differentiated from facies 2 by a lack of parallel reflections and exhibits mainly semi-continuous, subparallel and wavy reflections. Radar facies 4 identifies a region virtually devoid of signal returns below 10 - 12 m depth. Radar facies 5 is characterized by semi-continuous, parallel and subparallel reflections. Facies 5 also exhibits a set of closely spaced, parallel reflections from 15 - 45
m at 5-9 m depth, marked '?I in the profile, similar to those observed in a corresponding location in survey line #3 (Figure 5.7). Radar facies 6 is identified by semi-continuous, wavy and trough-shaped reflections. The northern extent of facies 6 is interrupted by multiple hyperbolic reflections from 600 - 650 m. An absence of radar reflections from 683 to 695 m in Figure 5.9 was caused by a temporary system malfunction that was not observed during GPR profiling.
Interpretation: The orientation of GPR survey line #5 with respect to surface scroll topography is highly variable (Figure 5.4). The profile is oriented 30" to 35O from true strike direction of surface scroll ridges at 0 to 470 m and within 15" of true strike from
470 m to 894 m. The presence of inclined reflections from 0 to 496 m corresponds well with this shift in profile orientation, inclined reflections appear only where the profile comes within 60" of the true dip direction. Conversely, laterally continuous, parallel reflections in radar facies 2 correspond to profile orientation along the strike face of surface scroll bars, thus, as expected, no inclined stratification is observed.
Semicontinuous, wavy reflections in radar facies 3 are interpreted as trough cross- stratification. An erosional contact between facies 2 and 3 suggests that facies 3 might be part of a large channel fill although its orientation with respect to surface scroll bars (normal to strike direction) does not support this theory. Radar facies 4 is a result of underlying silt and clay in the region. A weak reflection between 14 and 18 m depth from
480 - 610 m in facies 4 is interpreted as a lithologic change although a lack of reflectors
in the area prevents identification of an erosional contact. Inclined reflections from 570 - 600 m in facies 4 show a possible prograded channel fill but signal attenuation limits the certainty of this interpretation. Radar facies 5 is oriented 55" to the true dip direction of surface scroll bars but exhibits no inclined stratification. Reasons for the lack of inclined stratification at this location are unclear, although the south end of this facies is located at the edge of a terrace that drops 2.2 m to the contemporary floodplain (Figures 4.12 and 3.4). Reflections in radar facies 6 are interpreted as trough cross-stratified sand and gravel and appear similar to lower point bar deposits described by Bridge et al. (in press, see Section 2.3.1). Hyperbolic reflections in the northern part of the profile correspond with a section of roadway followed by the survey line (Figure 4.12). Proximity to telephone poles, parked cars and fences parallel to the GPR survey line are considered most likely sources of hyperbolic reflections.
Radar Profile #6 Figure 5.10 shows the original and interpreted profiles of GPR survey line #6 (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into four radar facies. Facies 1 is distinguished by inclined reflections dipping between 3" and 12" to the south. A high-amplitude reflection marked 'WT' immediately below facies 1 is interpreted as the water table and indicated by a dashed line at 3 to 4 m depth in Figure 5.10. Radar facies 2 is also characterized by inclined reflections, dipping between lo and 15" toward the south. Facies 3 is identified by a pattern of laterally continuous, parallel and wavy reflections that mirror surface topography in the top 5 - 6 m of the floodplain and by semi-continuous subparallel and inclined reflections below 6 m.
Note that a set of southward dipping reflections in facies 3 from 150 - 295 m at 7 - 12 m depth penetrates to the depth of a weak reflection at 13 m that remains visible from toward the south. This reflection is interpreted as a lithologic change as it remains visible in a region mostly devoid of radar returns. Significant variability exists in the depth to facies 4, which changes from 12 m at the north end of the profile to 9 m at the south end. Hyperbolic reflections, marked by dashed lines, from 580 to 680 m are attributed to buried point objects such as logs or boulders.
Interpretation: The orientation of GPR survey line #6 exhibits a fairly complex relationship with surface scroll topography (Figure 5.4). From 0 to 600 m the profile crosses the same curved scroll bars twice, both times at oblique angles and deviating from the strike direction of scroll bars by as much as 25". In contrast, the survey line from 600 to 864 m is only, at most, 10" from the strike direction of surface scroll topography.
The location of inclined stratification in the profile (facies 1 and 2 in Figure 5.10) shows little correspondence with these changes in profile orientation. Facies 1 was expected to show a southward increase in inclined stratification from 300 m to 0 m since the GPR survey line deviates increasingly from the true strike direction of surface scrolls through this section. However, inclined radar reflections show no change in frequency or thickness from 0 to 450 metres. Increased waviness of reflections at 5 - 10 m depth from 150 to 0 m, however, suggests well-preserved RAS topography. Inclined stratification is exhibited in facies 2, located parallel to a strike section of surface scroll bars. Thus, these IS deposits cannot be explained by lateral accretion of the floodplain. Episodic downstream accretions of the point bar in facies 2 may have resulted in these inclined reflections although no other strike section GPR profile exhibits such steep and continuous radar reflections in channel-parallel orientation.
Radar Profile #7 Figure 5.1 1 shows the original and interpreted profiles of GPR survey line #7 (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into five radar facies. Radar facies 1 and 2 are distinguished by southward dipping reflections within the top 4 m of the floodplain from 60 - 220 m and 255 - 365 m respectively. Radar facies 3 delineates a region of laterally continuous, parallel and subparallel reflections identified across the entire top 7 m of the profile. A divergent channel fill (see Figure 2.4) marked 'CH' in facies 3 is identified by a strong basal reflection from 760 - 850 m. Facies
4 identifies a zone with few radar returns below 7 - 8 m depth, although signal returns increase at depth from 220 m to 0 m. A reflection indicated by the dashed line in facies 4 is interpreted as a lithologic boundary at about 15 m depth. Facies 5 is differentiated from facies 4 by a lack signal attenuation and characterized by semicontinuous, wavy and subparallel reflections up to 15 m depth. Reflections marked by dashed lines at 4 10, 5 10, and 600 m in Figure 5.11 are attributed to surface reflectors, in this case fence posts 5 - 15 m from the GPR survey line.
Interpretation: GPR survey line 7 is oriented along the strike direction of surface scroll bars (Figure 5.4) except at its southern extreme (0 - 230 m) where it departs from true strike orientation by as much as 22". Parallel and subparallel reflections show a strike section of surface scroll bars from 350 - 850 m. Inclined reflections from 220 - 60 m (facies 1) conform to a departure of the profile from the strike direction of surface scroll bars in Figure 5.4. A lack of inclined beds from 60 to 0 m in facies 3 may be associated with channel adjustment to basal scour, exhibited in floodplain sediments from 96 - 0 m by a lack of signal attenuation in facies 5. Inclined reflections in facies 2 may also be a response to basal scour, resulting in steeper point bar deposits to develop as a response to increased depth of the thalweg. The channel fill, marked 'CH' in facies 3, is oriented along the dip face of surface scroll bars, thus its placement does not correlate with scroll bar orientation. Nonetheless, a strong basal reflection clearly shows what appears to be a channel form at this location. Radar Profile A Figure 5.12 shows the original and interpreted profiles of GPR survey line A (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into four radar facies. Facies 1 is characterized by laterally continuous, parallel clinoforms that mirror surface topography. A one metre rise in surface topography at 50 - 90 m marks a particularly large scroll bar at this location, and a change in the direction of floodplain accretion shown in Figure 5.4. Radar facies 2 is distinguished by inclined reflections dipping between 1" and 15" to the east. Facies 3 exhibits both hummocky and laterally continuous, parallel reflections. Two locations marked 'S' in facies 3 show preserved RAS deposits that are concordant with surface scroll bars. Radar facies 4 is identified by a region of few radar reflections and shows significant variation within the profile. At its western edge, this facies is 9 m from the surface while nearer the river channel, it lies 15 m from the surface. Facies 4 also shows noticeably less signal attenuation with depth from 170 - 310 m. Reflections between 50 and 100 m in facies 4 are interpreted as multiples and the hyperbolic reflection at 120 m is interpreted as a surface reflection from nearby trees.
Interpretation: Radar profile A is oriented between 35" and 60" from the strike direction of surface scroll bars (Figure 5.4). A bend in the profile at 446 m results in the large shift in orientation from 35" to 60". Thus, inclined stratification would be expected as the dominant feature beyond 446 m since this section is oriented only 30" from true dip of surface scroll bars. Examination of Figure 5.12, however, shows no pattern of inclined reflections in this section. Instead, facies 3 exhibits preserved RAS deposits that is concordant with surface topography. Inclined reflections are observed in facies 2 however, which is oriented 52" from true dip direction of surface scroll bars, although not in facies 1 which has nearly the same orientation. The deepening of floodplain sediments at the eastern edge of the profile is attributed to
channel scour. Somewhat weaker signal returns from 10 - 15 m depth likely reflect clay and silt mixing with coarser channel deposits. The cause of channel scour is attributed to deepening of the thalweg in adjustment to channel confinement immediately upstream (Figure 4.12).
Radar Profile B Figure 5.13 shows the original and interpreted profiles of GPR survey line B (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into seven radar facies. Facies 1 is distinguished by semi-continuous, subparallel and wavy reflections and is marked by a well-defined, eastward dipping reflector along its eastern boundary. Concordant reflections below the location marked IS' at 86 m in Figure 5.13 identify preserved scroll ridge deposits at depth. Facies 2 exhibits semi-continuous, subparallel and hummocky reflections. Facies 3 is identified as a divergent channel fill, extending from 210 - 340 m at 0 - 4 m depth. Facies 4 shows eastward dipping reflections
from 340 - 600 m at 0 - 4 m depth. A surface scroll ridge at 548 m in facies 4 also exhibits preserved reflections concordant with the modern scroll bar. Radar facies 5 is identified by a series of laterally continuous, parallel reflections that lie above a region of rapid signal attenuation. Radar facies 6 exhibits mainly hummocky reflections with some
eastward dipping reflections from 570 - 616 m at 5 - 12 m depth and facies 7 identifies a region with few radar reflections. A weak hyperbolic reflection indicated by dashed line at 88 m is attributed to a nearby tree. A similar reflection at 286 m is attributed to a buried object since no sources of surface scatter were found in the area. All locations marked 'S' indicate preserved scroll ridge deposits.
Interpretation: GPR profile B (Figure 5.13) is oriented between 30" and 70" from the
strike direction of surface scroll bars (Figure 5.4). From 183 m to 436 m, profile orientation is 30" to 40" from true strike and increases to near 70•‹ at either end of the profile. The eastern boundary of facies 1, identified as an eastward dipping clinoform below an unusually large scroll ridge, corresponds to a change in orientation of surface scroll bars and denotes a shift in the direction of floodplain accretion. Because facies 1 is oriented just 22" from the true dip direction of surface scroll bars, inclined stratification associated with lateral accretion was expected in this facies. However, facies 1 shows no large-scale inclined stratification and exhibits mainly large scale parallel wavy bedding interpreted as RAS deposits. Facies 2 exhibits similar structure, showing preserved scroll ridge deposits at locations marked 'S' in the profile.
Note that scroll bars are absent from 200 - 450 m in the profile, presumably due to an increased departure from dip direction (Figure 5.4). Reflections from 340 to 436 m in facies 4 exhibit an increase in the dip angle of inclined stratification from 3" to 18", this is in accordance with a bend in the profile at
440 m. Thus, the increase in dip angle from 440 - 616 m is attributed to a change in profile orientation, in this case more toward true dip direction. Basal reflections at the eastern edge of the profiles exhibit channel scouring similar to that observed in this area by profile A, and is also interpreted as adjustment to channel confinement immediately upstream.
Radar Profile C Figure 5.14 shows the original and interpreted profiles of GPR survey line C (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into five radar facies. Facies 1 exhibits mainly laterally continuous, parallel and wavy reflections interrupted by a prograded channel fill from 80 - 140 m at 0 - 3 m depth, labeled 'CHI in Figure 5.14. Facies 2 is characterized by semi-continuous and discontinuous, subparallel and hummocky reflections. A high-amplitude, laterally
continuous reflection at 4 - 5 m depth from 0 - 170 m is interpreted as the water table, 'WT' in Figure 5.14. Facies 3 is identified by a series of inclined reflections that dip to the east between 1" and 16" extending to a depth of up to 12 m, significantly deeper than surrounding reflections in the profile (Figure 5.14). Radar facies 4 manifests serni- continuous, hummocky and wavy clinoforms that exhibit no significant dip. Within this facies a strong, laterally continuous reflection from 500 - 570 m is also identified as the water table. Locations marked 'S' in the profile denote the locations of preserved scroll ridge deposits within the floodplain. Facies 5 is distinguished by rapid signal attenuation and an overall lack of radar reflections. Hyperbolic reflections at 225 m are interpreted as surface reflections from a stand of trees at this site. Hyperbolic reflections at 382, 585, and 612 m are attributed to buried objects since no surface objects were located here.
Interpretation: A bend at about 430 m in GPR survey line C and three abrupt changes in the orientation of surface scroll bars (Figure 5.4) result in significant variability in the orientation of the profile with respect to surface scroll bars. Radar facies 1 is oriented 55"
- 60" from the strike direction of surface scroll bars, thus the channel fill ('CH' in Figure
5.14) and wavy reflections in facies 1 are interpreted as a chute channel and preserved ridge-and-swale (RAS) deposits respectively. Locations marked IS' show the location of preserved scroll ridge deposits. A terrace at 220 m in Figure 5.14 suggests erosion of the northwestern section of the floodplain (see Figure 5.4). The cause of erosion is unclear although the channel fill at 106 m in Figure 5.14 seems a likely candidate. The three-dimensional extent of this channel fill is examined in section 5.5. Facies 2 shows signs of signal attenuation, interpreted as fine-grained sediments in this facies and confirmed by a core sample at this location (Section 5.1). Although the orientation of GPR survey line C from 220 to 440 m lies between 50" and 60" from true dip direction, inclined reflections in facies 3 still show RAS architecture, associated with lateral accretion of the floodplain. Wavy clinoforms, associated with preserved RAS deposits, are marked 'Sf in Figure 5.14 and appear as deep as 10 m below surface scroll bars. Facies 4 is oriented almost parallel to the dip direction of surface scrolls bars yet exhibits none of the large-scale inclined stratification expected of this orientation, instead weakly preserved RAS deposits are apparent.
Radar Profile D Figure 5.15 shows the original and interpreted profiles of GPR survey line D (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into four radar facies. Facies 1 is distinguished by discontinuous and oblique parallel reflections from 10 - 190 m at 0 - 4 m depth. A channel fill in this facies is identified from 50 - 110 m and shown by the 'CH' in Figure 5.15. Facies 2 exhibits laterally continuous, subparallel reflections from 0 - 250 m and parallel reflections from 250 - 495 m. A well-defined, laterally continuous reflection in facies 2 is interpreted as the water table, 'WT' in Figure 5.15. Facies 3 is characterized by laterally continuous, subparallel and wavy clinoforms that reflect surface scroll topography. Facies 4 denotes a region below rapid signal attenuation at 9 - 10 m depth. Note that an increase in reflections between 270 - 500 m in facies 4 is attributed to surface reflections from a Christmas tree farm crossed by the profile along this section. A hyperbolic reflection at 254 m is the result of a stand of trees at this location.
Interpretation: The orientation of GPR survey line D with respect to the strike direction of surface scroll topography varies throughout the profile. From 0 - 200 m the profile is oriented approximately 70 - 90" from the strike direction of surface scroll bars. Facies 1 and 2 in this section show laterally continuous, parallel and wavy bedding with no large- scale inclined stratification. Preserved scroll ridge deposits at 's' locations in the profile and a channel fill are also distinguished in facies 1. This channel fill appears to correspond roughly with the dimensions of a similar channel fill in profile C. Atop the terrace (250 - 490 m), radar facies 2 exhibits parallel bedding which corresponds to the stratification expected along a profile oriented 50" - 70" from the true dip direction of surface scroll bars (Figure 4.12). Facies 3 from 500 - 702 m roughly parallels the true dip direction of surface scroll topography, thus it represents a cross- section of the laterally accreting floodplain. Inclined and wavy reflections in facies 3 reflect the eastward, lateral accretion of the floodplain, similar to the inclined stratification and RAS architecture shown in Figure 2.1. Contrary to profiles A and B, facies 4 does not show a deepening of the channel thalweg at the eastern section of the profile, perhaps because profile C is located in an older section of the floodplain as compared to profiles A and B. Thus, the depth to facies 4 does not vary significantly from an estimated 8 - 11 throughout the entire profile.
Radar Profile E Figure 5.16 shows the original and interpreted profiles of GPR survey line E (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into four radar facies. Radar facies 1 is defined by semi-continuous, oblique reflections that dip both to the east and west from 20 - 220 m at 0 - 3 m depth. Radar stratigraphy from
50 - 110 m in facies 1 is identified as a prograded channel fill and a second set of reflections from 156 - 220 m exhibits similar properties. Radar facies 2 is characterized by continuous and semi-continuous subparallel reflections. A high-amplitude reflection between 0 and 334 m in facies 2 is identified as the water table, 'WT' in Figure 5.16. Facies 3 is characterized by semi-continuous and continuous wavy reflections that mirror surface scroll ridges, marked 'S' in Figure 5.16. Inclined dashed lines at 250 and 425 m are reflections caused by a row of trees bisecting the profile at 250 m and by a vertical steel pipe supporting part of a ginseng field next to the survey line at 450 m (see Figure
5.1). An increase in radar reflections from 260 - 480 m in facies 4 is attributed to a row of vertical steel pipes along the edge of the ginseng field paralleling the profile (see Figure 5.1).
Interpretation: The orientation of GPR survey line E with respect to the strike direction of surface scroll bars varies throughout the profile (Figure 5.4). the western portion of the profile, from 0 - 250 m, is oriented 50" - 60" from true strike direction thus, radar stratigraphy is expected to show inclined stratification in these deposits. Although radar facies 1 does show a pattern of eastward dipping reflections from 0 - 100 m, these reflections become parallel and then dip back toward the west from 170 - 240 m by as much as 15". This bowl-shaped pattern suggests a large-scale, divergent channel fill.
Similar smaller-scale patterns labeled 'CHI at 90 m and 180 m in facies 1 are also interpreted as channel fill deposits. The three-dimensional extent of these reflections is examined in Section 5.5. RAS architecture is not apparent in either facies 1 or 2 in this section of the floodplain, even though the profile is oriented within 30" of the dip direction of floodplain accretion.
It may be that patterns of apparent accretion topography in the air photo (Figure 5.4) are not scroll bars at all but are part of channel fills observed in GPR profiles. A lack of scroll topography along the surface of the profile in Figure 5.16 supports this notion. Moreover, erosion by chute channels in this part of the floodplain would explain the genetic origin of the terrace at 250 m in the profile. Note that depth to the interpreted water table reflector ('WT' in Figure 5.16) becomes increasingly shallow toward the west. This trend is consistent with information from a local resident on the property who obtains well water from this site within the top 2 - 3 m of the floodplain (Tessier, pers. cornrn.). Departure of profile E from the strike direction of surface scroll bars increases toward the east, thus the profile becomes increasingly oriented along the dip direction of surface scroll topography from 250 to 689 m. However, expected inclined stratification is not present in floodplain deposits. Instead, RAS deposits dominate, especially east of 480 m where the profile bends to become roughly parallel to the dip direction of surface scrolls (Figure 5.4). RAS deposits appear well preserved at depth in this section of the profile, although deeper reflections from 490 - 698 m in facies 4 that mirror RAS deposits are interpreted as multiples.
Radar Profile F Figure 5.17 shows the original and interpreted profiles of GPR survey line F (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into four radar facies. Facies 1 is characterized by inclined reflections that dip both east and west. Two prograded channel fills are identified in this facies, marked 'CHI in Figure
5.17. Radar facies 2 is located from 310 - 545 m at 0 - 4 m depth and distinguished by hummocky reflections inclined lo to 18' to the east. Radar facies 3 is characterized by laterally continuous, parallel and subparallel reflections from 0 to 550 m and wavy clinoforms from 550 to 730 m. Subparallel reflections from 150 - 260 m in facies 3 exhibit a distinct bowl-shaped symmetry at 10 - 11 m depth. A hyperbolic reflection at 30 m in facies 3 is denoted by a dashed line and attributed to multiple surface reflections from a shed, tree, and house near the GPR survey line. A high-amplitude reflection from
140 - 225 m is interpreted as the water table, 'WT' in Figure 5.17. From 550 m to 730 m, radar facies 3 exhibits laterally continuous, wavy reflections (marked IS' in Figure 5.17) that are interpreted as preserved scroll ridge deposits. Radar facies 4 is identified below a region of rapid signal attenuation that remains fairly constant at 8 - 10 m depth, except from 260 - 534 m where interference patterns appear at 8 - 20 m depth. This is attributed to evenly-spaced, steel poles on either side of the survey line that are part of ginseng fields in the area (see Figure 5.1). This region of interference patterns exactly matches the location of ginseng fields. Interpretation: The orientation of GPR survey line F varies with respect to surface scroll topography throughout the profile (Figure 5.4). Surface scroll bars are not apparent in
Figure 5.4 from 0 - 250 m although the direction of floodplain accretion from the air photo shows a 40" - 68" orientation of the profile with respect to the strike direction of floodplain accretion, increasing from west to east. Facies 1 and 3 from 0 - 250 m, however, do not show the expected inclined stratification or RAS architecture associated with the dip direction of floodplain accretion. Channel fill deposits in facies 1 similar to those observed in profiles C, D, and E, are the likely cause of a lack of scroll bars and preserved RAS deposits near the surface. It seems likely that interpreted patterns of floodplain accretion from the air photo are therefore a reflection of chute channel erosion on the floodplain surface west of the terrace at 270 m. Radar facies 2 is oriented between 35" and 50" from the strike direction of surface scroll bars. Inclined strata in this facies are interpreted as lateral accretion deposits associated with the dip direction of floodplain deposits. From 550 m to 730 m, radar facies 3 lies nearly parallel to the dip direction of surface scroll bars (Figure 5.4) and exhibits expected RAS architecture. The locations of preserved scroll ridges, marked IS' in Figure 5.17, also show vertical stacking of scroll bar deposits that are concordant with surface topography.
Radar Profile G Figure 5.18 shows the original and interpreted profiles of GPR survey line G (location in Figure 5.1). Macroscale patterns of radar reflections in this profile can be divided into four radar facies. Radar facies 1 distinguishes closely-spaced, laterally continuous, parallel reflections that mirror surface topography from 0 - 225 m at 0 - 5 m depth. Radar facies 2 is differentiated from facies 1 by less closely spaced (in the vertical) continuous and semi-continuous subparallel and wavy reflections. The location marked 'CH' in facies 2 is interpreted as a divergent channel fill. Locations marked IS' in facies 2 identify peaks of wavy clinoforms associated with scroll ridge deposits on the floodplain surface.
Strong, laterally continuous reflections from 370 - 450 m and 670 - 700 m, labeled 'WT' in Figure 5.18, are interpreted as the water table. Rapid signal loss below 6 m depth from
0 - 270 m characterizes facies 3. Facies 4 is differentiated from facies 3 by an increase in radar returns from 270 - 720 m at 6 - 14 m depth. Hyperbolic and inclined reflections at 200 m, 430 m, and 460 m are attributed to surface reflections along Dairy road, such as telephone poles and are noted in Figure 5.18 by dashed lines. The separation in the profile from 522 - 534 m is located along a steep, fenced in area that was not GPR surveyed.
Interpretation: GPR survey line G follows Dairy Road, the only section of roadway in the study area (see Figure 3.1). The first section of the profile, from 0 - 520 m is oriented 40" from the strike direction of surface scroll bars (see Figure 5.4).
Parallel reflections in radar facies 1 are uncharacteristic of the study area because of their close vertical stacking. They are interpreted as parallel tabular sands and may, in part, be attributed to the construction of Dairy road. Nonetheless, parallel reflections appear too deep ( 5 - 6 m) to suggest a strong association with road construction. East of 534 m, the profile changes orientation and parallels the dip direction of surface scroll bars. A change in the orientation of facies 2 from 534 - 724 m reveals RAS architecture along the dip direction of surface scroll bars, mirroring scroll ridges at locations marked 'St in the profile (Figure 5.18). An increase in radar reflections from facies 3 to facies 4 is interpreted as a decrease in basal silt and clay content at this location, although signal attenuation in the region remains significant compared to facies 2. Thus, facies 4 is not interpreted as channel scour but as a change in the character of underlying sediments. Note that a short section of the profile from 490-522 m below 5 m depth shows rapid signal attenuation. This is attributed to crossing the road surface (pavement) along this section of the profile. The divergent channel infill, 'CHI in facies 2, is oriented along the strike face of surface scroll bars, thus it is interpreted as a large chute channel striking across the floodplain.
Radar Profile H Figure 5.19 shows the original and interpreted profiles of GPR survey line H (location in Figure 5.1). The profile can be divided into two radar facies. The first facies exhibits a pattern of eastward dipping reflectors inclined between 1" and 22". The location marked
'CH' in facies 1 is interpreted as a channel fill. A strong, laterally continuous reflection from 10 - 110 m is interpreted as the water table, labeled 'WT' in Figure 5.19. Radar facies 2 identifies a region of signal attenuation above a basal reflection at 12 m from 50 - 156 m, interpreted as a lithologic change. Prominent hyperbolic reflections at 6 and 86 m are identified in Figure 5.19 by dashed lines and interpreted as buried point sources such as boulders or logs.
Interpretation: GPR survey line H is oriented along the dip direction of surface scroll bars (Figure 5.4) and exhibits the expected oblique clinoforms associated with a dip section of lateral accretion deposits. Facies 1 also exhibits RAS deposits, below a surface scroll ridge marked IS' in Figure 5.19. Radar facies 2 is differentiated from facies 1 by a decline in radar signal strength attributed to higher silt or clay content in this facies. An intermittent basal reflection at 12 m depth is interpreted as channel lag deposits as it corresponds to the depth of the modern thalweg in the adjacent river (see Section 3.1).
5.2.3 Interpretation of 200 MHz GPR Profiles
This section presents the results of 200 MHz GPR surveys of the study area. Their location in the survey grid (Figure 4.12) is shown in Figure 5.1. Note that GPR profiles in this section focus on greater detail of radar stratigraphy in true dip and strike sections along surface scroll bars from the most modern section of the floodplain (Figure 4.12). All GPR survey lines in this section are oriented either parallel or normal to surface scroll topography (Figure 5.4).
The 200 MHz GPR profiles are interpreted in terms of both macro- and meso-scale ( > 25 m horizontal distance) radar facies, distinguished by patterns of reflections whose appearance differs from that of adjacent units. Radar reflections which make up each radar facies are described using terminology from the Literature Review (Section 2.7). Radar reflection profiles are interpreted based on the expected internal architecture of a scrolled, meandering river floodplain (Section 2.1).
GPR Survey line dipl Figure 5.20 shows the original and interpreted profiles of GPR survey line dipl (location in Figure 5.1). Patterns of radar reflections in this profile can be divided into three radar facies. Facies 1 exhibits laterally continuous, subparallel and hummocky reflections from 0 m - 160 m that mirror surface topography. The locations of surface swales and scroll ridges (at 'St locations) are indicated in Figure 5.20. Facies 2 is identified by a region of multiple signal reflections from 0 - 80 m and by rapid signal attenuation from 80 - 192 m, increasing to 18 m depth at the south edge of the profile.
Evenly spaced weak reflections that mirror surface topography from 0 - 80 m at 7 - 10 m in facies 3 are identified as multiples caused by a lithologic change from sand and gravel to silt and clay at 7 m depth.
Interpretation: This profile is oriented parallel to the dip direction of surface scroll bars (Figure 5.4). Thus, radar stratigraphy in Figure 5.20 shows a cross-section of lateral accretion deposits in the scrolled floodplain. The majority of floodplain deposits show preserved RAS deposits concordant with surface topography, although the southern edge of facies 1 shows some inclined stratification without surface scroll bars. An increase in
the depth to facies 2 from 130 - 192 m is interpreted as channel scour in response to confinement at the concave bank bench (counterpoint bar) immediately upstream (see Figure 4.12). The pattern of inclined stratification in the southern section of facies 1 is interpreted as lateral accretion where a lack of scroll topography may, in part, be associated with the formation of an island in the river channel next to the profile (Figure
4.12).
GPR Survey line dip2 Figure 5.21 shows the original and interpreted profiles of GPR survey line dip2 (location in Figure 5.1). Patterns of radar reflections in this profile can be divided into two radar facies. Facies 1 exhibits laterally continuous wavy and inclined reflections that dip between 2 - 6". Wavy reflections in radar facies 1 are concordant with surface ridge- and-swale topography, indicated in Figure 5.21 where surface scroll ridges are marked 's'. Radar facies 2 is distinguished by a region of rapid signal attenuation at 7 - 8 m depth that declines to 18 m adjacent the modern channel at the south end of the profile. Note that the air and ground wave (top two reflections) show a departure from the radar profile from 38
- 100 m, caused by insufficient warm-up time of the GPR unit resulting in a departure of air and ground waves from the pre-set time domain. Their location is approximated with dark black lines in Figure 5.21.
Interpretation: Radar profile dip2 is oriented parallel to the dip direction of surface scroll bars (Figure 5.4) and illustrates a cross-section of lateral accretion deposits. Similar to profile dipl, profile dip2 shows an decline in the base of fluvial deposits near the river
channel, from 8 to 18 m depth, which is interpreted as a continuation of the channel scour observed in profile dipl. This profile does not show the expected macroscale inclined stratification of a laterally accreting floodplain (see Figures 2.1 and 2.2), although individual inclined beds dipping between 2 and 6" toward the river channel (Figure 5.21) do show signs of inclined stratification. Nonetheless, RAS architecture is the dominant feature in this section of floodplain deposits.
GPR Survey line dip3 Figure 5.22 shows the original and interpreted profiles of GPR survey line dip3 (location in Figure 5.1). This profile is divided into two sections by a steep terrace located at the division between profiles in Figure 5.22. Patterns of radar reflections are divided into three radar facies, identified in both sections of the profile. From 0 - 96 m, facies 1 exhibits laterally continuous, parallel and subparallel reflections that tend to mirror surface scroll bars, marked IS' in Figure 5.22. Facies 2 underlies facies 1 and is distinguished by subparallel, semi-continuous reflections. Facies 3 is characterized by thinly-spaced reflections from 5 - 8 m that mirror surface topography. These reflections are identified as multiples associated with an increase in silt and clay content at this depth. Below this the radar profile is virtually devoid of radar returns.
In the second section of the profile, from 110 - 213 m, radar facies 1 exhibits laterally continuous, parallel reflections that tend to follow surface scroll topography. Radar facies
2 is characterized by semi-continuous, subparallel and hummocky reflections. Facies 3 denotes a region below rapid signal attenuation at 8 m depth. A strong reflection at 15 m depth in facies 3 is interpreted as a lithologic change, based on signal strength in a region otherwise devoid of radar returns.
Interpretation: This profile follows the dip direction of surface scroll bars but does not extend to the edge of the modern river channel like profiles dipl and dip2 (Figure 5.4). Thus, it does not exhibit the channel scour indicated at the southern extremes of profiles dipl and dip2. The northern section of the profile (0 - 96 m) shows RAS deposits concordant with surface topography in facies 1. Facies 2 exhibits increased discontinuity of reflections and is interpreted as lower point bar deposits (Bridge et al., in press, see
Section 2.3.1). In contrast, the southern section of the profile (100 - 213 m) exhibits RAS deposits in addition to large-scale inclination of RAS deposits dipping toward the main
channel from 150 - 213 m. Radar facies 2, from 100 - 213 m, exhibits hummocky clinoforms interpreted as trough cross-stratification associated with downstream accretion of the point bar. Signal attenuation in facies 3 is interpreted as caused by silt and clay deposits underlying lower point-bar sand and gravel.
GPR Survey line dip4 Figure 5.23 shows the original and interpreted profiles of GPR survey line dip4 (location in Figure 5.1). Profile dip4 is oriented along the dip direction of surface scroll bars and is partly located on a terrace, the region north of 170 m in Figure 5.23. Patterns of radar reflections in this profile can be divided into three radar facies. Facies 1 exhibits laterally continuous, parallel reflectors that mirror surface scroll topography. Facies 2 is distinguished from facies 1 by a change in the character of radar returns where reflections in facies 2 are more closely spaced in the vertical direction, exhibiting a higher frequency (shorter wavelength) that suggests more closely-spaced or laminated strata. Radar facies 3 characterizes a region of reduced signal returns which attenuate and eventually disappear with depth.
Interpretation: For the most part, this profile shows RAS deposits associated with southward accretion of the floodplain. Surface scroll ridges and swales are indicated in the profile, the latter marked IS' in Figure 5.23. The division between facies 1 and 3 is interpreted as a lithologic change attributed to increased clay and silt sediments underlying the coarse floodplain alluvium. Although some inclined stratification is apparent in this profile, especially on the channelward side of surface scroll bars, macroscale stratification shows no clear pattern of inclined strata dipping toward the channel. Facies 2 is interpreted as clayey sand and gravel as the signal does not attenuate out as rapidly as in facies 3. Multiple reflections that persist at depth in facies 3 and mirror surface topography are interpreted as possible equipment noise because they exactly match the topographically corrected air and ground waves in Figure 5.23.
GPR survey line strike Figure 5.24 shows the original and interpreted profiles of GPR survey line strike (location in Figure 5.1). Patterns of radar reflections in this profile can be divided into three radar facies. Facies 1 exhibits laterally continuous, parallel reflections. Facies 2 is distinguished from facies 1 by less-continuous, subparallel reflections, some of which exhibit a southward dip of up to So. Facies 3 exhibits rapid signal attenuation below 10 m
and includes a series of equally-spaced reflections between 10 and 18 m depth that are interpreted as multiples caused by a sand and gravel contact with underlying silt and clay at 10 m depth.
Interpretation: GPR survey line strike is oriented along a surface scroll ridge (Figure 5.4), thus radar reflections in this profile reveal a strike section of scroll bar deposits. Radar facies 1 shows strike face deposits of RAS architecture observed in profiles dip2 and dip3. Facies 1 is interpreted as parallel tabular beds of stratified sand. Radar facies 2
exhibits inclined reflectors from 40 - 110 m at 6 - 8 m depth interpreted as downstream accretion the point bar. For the most part, however, macroscale radar stratigraphy is interpreted as cross-stratified sand and gravel that shows no overall dip. Based on signal attenuation, facies 3 is interpreted as a region of silt and clay below 10 m depth. 5.3 Three-dimensional (3-D) Distribution of Selected Radar Facies
This section examines the three-dimensional (3-D) extent of selected radar facies from 100 and 200 MHz GPR profiles in the previous two sections. Specifically, the 3-D distribution of ridge-and-swale (RAS), channel fill (CH), and inclined stratification (IS) radar stratigraphy as well as the basal contact between floodplain sediments and underlying Kamloops Lake Drift (as interpreted by rapid signal attenuation at depth) are examined. These radar facies are selected for 3-D interpretation for the following reasons: 1) the 3-D distribution of RAS and IS deposits within the study area is important for evaluating current models of scrolled, meandering river floodplains, where IS and RAS deposits predominate (Miall, 1985; Nanson and Croke, 1992); 2) an eroded area of the floodplain west of the terrace along GPR profile #3 (location in Figure 5.4) is potentially related to a 3-D channel fill deposit observed in the region; 3) rapid signal attenuation at depth, interpreted as the base of floodplain deposits, can be used to interpret regions of channel scour and fill in the floodplain. The remaining radar facies identified in Sections 5.3 and 5.4, most typically large-scale, horizontal radar stratigraphy (e.g. Figure 5.24) were not selected for 3-D examination since many of these are simply different orientations of RAS and IS deposits, shown to occupy much of the study area (Figure 5.25). RAS and IS deposits in the floodplain are not mutually exclusive, inclined stratification can exhibit preserved RAS structures (see Figures 2.1 and 2.3e). Thus, cross-sections showing the location of interpreted RAS, IS, and CH radar facies from 100 and 200 MHz GPR profiles, shown in Figures 5.26,5.27 and 5.28, may - and in this study almost always do
- exhibit both RAS and IS at the same location.
Examination of RAS, IS, and CH radar stratigraphy in 100 MHz profiles (Figures 5.26 and 5.27) shows that IS and RAS deposits are the dominant characteristics of floodplain deposits in the meander bend. As expected, RAS deposits appear less frequently with Figure 5.25 Locations of ridge and swale (RAS) and inclined stratification (IS) deposits in 100 MHz and 200 MHz radar profiles. Note that this does not necessarily imply RAS and IS within the entire vertical column.
lnter~retedRadar Facies Ridge and wale (RAS) Channel fill (CH)
Inclined stratification (IS) ...... ,.....,...... RAS and 1s
Figure 5.27 The location of interpreted ridge and swale (RAS), inclined stratification (IS) and channel fill (CH)radar stratigraphy in 100 MHz GPR profiles A - H (locations in Figure 5.1). GPR survey lines identified here are referred to individually in section 5.3. Vertical exaggeration is 2.54~.Block numbers 1 - 7 above each profile show intersection points with GPR profiles in Figure 5.1.
increasing deviation from true dip direction of surface scroll bars. In contrast, IS deposits persist even in true strike orientation, such as profile 6 in Figure 5.26 interpreted as the downstream accretion of a scroll bar. Another example of IS deposits resulting from
downstream accretion is shown in profile 2 from 200 - 480 m (Figure 5.26). In true dip direction, this area shows a series of channel fill deposits, observed in profiles F though B in Figure 5.27. Thus, IS deposits in this case are interpreted as downstream progradation of the channel fill, not the RAS and IS stratification interpreted independently from the original GPR profile (Figure 5.6). Note that profiles G through E in this region show no RAS or IS deposits in the direction of floodplain accretion, most likely due to only weakly observed scroll topography on the floodplain surface in this area, possibly not scroll bars at all but remnant banks of former chute channels (Brierley, 1991a). The depth of inclined stratification in profile 2 (Figure 5.26) matches the depth of channel deposits in profiles F through C (Figure 5.27). Thus, since each CH radar facies was interpreted independently of the other, their apparent congruence in three dimensions supports the interpretation of a channel fill in this region which, in all likelihood, is the cause of erosion in this section of the floodplain, resulting in a lower elevation of this section of the floodplain, bordered by a terrace to the east (see Figure 5.27). Apart from the 3-D channel fill identified above, no other interpreted CH deposit in GPR profiles shows continuity in three dimensions. Thus, it may be that these interpreted CH deposits do not exist in reality. Nonetheless, it is also possible that either a change in the orientation of a channel fill through three dimensions or variability in the lithology of channel boundaries makes it impossible to resolve these channel fills through their entire 3-D extent. Additional information is required to substantiate the presence of individual 2-D channel fill deposits that exhibit no 3-D consistency in Figures 5.26 and 5.27. Similar to 100 MHz profiles, Figure 5.28 shows continuous IS and RAS stratification in profiles oriented along the dip direction of surface scroll bars. Unlike the 100 MHz profiles, IS deposits in 200 MHz profiles are weakly developed and characterized by individual strata that dip channelward between 5 and 15" (Figure 5.29). An exception to this trend is observed in southern sections of profiles dip1 and dip2 where there is a deepening of floodplain deposits in accordance with a more well developed pattern of inclined stratification. Note that profile strike also exhibits some inclination of reflections in the downstream direction, up to an angle of 5" along the strike face of the surface scroll ridge, which is somewhat less than dipping reflections in true dip sections. IS in true strike orientation is interpreted as downstream accretion of the scroll bar, which on the surface dips to the south slightly and shows increasingly inclined beds with depth (Figure 5.29). For the most part, floodplain deposits vary in thickness from 8 to 12 m in the study area. The most evident change in the thickness of floodplain deposits is shown in profiles dip 1 and dip2 which exhibit a gradual deepening of floodplain deposits to the south, from 6 - 8 m to about 15 m in depth. This trend is also observed in the eastern sections of 100 MHz profiles A and B (Figure 5.27) which show a similar increase in depth. These regions show good continuity in three dimensions and are interpreted as the result of channel scour. The cause of this erosion is attributed to confinement of the bend immediately upstream which has created a deep scour hole (14 m) at this location (see Section 3.1). Note that the depth of the modern thalweg below bankfull level in the modern channel is between 9 and 14 m, conforming fairly well with the observed depth of floodplain deposits in these radar profiles.
CHAPTER SIX DISCUSSION
This chapter is divided into four sections. The first section identifies architectural elements in the floodplain and compares their radar stratigraphy with results from the literature. Section two addresses the ability of GPR to resolve inclined strata in floodplain deposits. Section three explores the three-dimensional (3-D) continuity of radar facies in the GPR survey grid and examines the use of macroscale inclined radar stratigraphy as an indicator of floodplain accretion direction. The final section discusses the limitations of GPR as they apply to this study.
6.1 Meandering River Floodplain Architecture and Comparisons with Existing Models
Based on Figures 5.25 through 5.28, two architectural elements can be identified in the study area: a single, extensive lateral accretion element (RAS and both IS and RAS in Figure 5.25) and a smaller channel fill element (successive CH radar facies in Figure 5.27). The 3-D extent of the CH element can be determined from successive 2-D CH radar facies in Figure 5.27, facies which appear as inclined beds in channel-parallel orientation (see Profile 2 in Figure 5.26). The LA element is larger and more complex, defined by multiple radar facies that form a single radar sequence including all RAS and both IS and RAS deposits from Figure 5.25. Macroscale radar stratigraphy in this element conforms well with existing models of scrolled, meandering river floodplains and with individual sections of lateral accretion deposits in the literature (see Figures 2.1, 2.2, and 2.3e). Representative samples of RAS and IS architecture in the lateral accretion element are shown in Figure 6.1 which identifies typical radar stratigraphy along dip and strike Figure 6.1 Typical radar stratigraphy of the scrolled floodplain in dip and strike sections. The location of each profile relative to surface scroll bars (dashed lines) is shown in the diagram above. B and D are dip direction GPR profiles and 7 is a strike direction profile. All three profiles show inclined stratification dipping in the direction of floodplain accretion and R and D also exhibit RAS stratification in dip direction. Vertical exaggeration is 2.54~. sections of the GPR survey grid. Radar stratigraphy in Figure 6.1 exhibits varying degrees of IS and RAS stratification in dip direction and some inclined stratification in strike orientation, dipping between 1 and 3" in the direction of floodplain accretion. Dip direction profiles, however, show greater dip angles, as high as 10" though typically less than 6". Inclined strata in strike section are interpreted as the downstream accretion of point bar deposits and similar to observations made by Sundborg (1956). Not all observed RAS stratification, however, is associated with lateral accretion of the river channel. Some RAS architecture, such as scroll ridge deposits at 72 and 123 m in GPR survey line C (see Figure 5.14), appear to be associated with the channel fill element. These scroll ridges exhibit characteristics similar to morphostratigraphic ridge elements, described by Brierley (1991a) as symmetrical mound-shaped features bordering preserved channel fills in the meandering Squamish River floodplain. Although most sections in the GPR survey grid exhibit inclined stratification similar to that expressed in models of laterally accreting floodplains described in the literature (see Figures 2.1 and 2.2), some sections of the survey grid do not. Figure 6.2 illustrates this point, showing only weakly developed inclined stratification in some dip direction profiles, occurring as individual bounding surfaces inclined from the top to the base of the floodplain in the direction of floodplain accretion (see Figure 5.29). These are most commonly observed among profiles that exhibit well-developed RAS deposits (i.e. eastern sections of profiles A through H and dip profiles in Figure 5.29). Indeed, Nanson (1980) observed this same form of stratification in an examination of floodplain deposits on the meandering Beatton River (see Figure 2.3e). Nonetheless, it does not appear in Nanson and Croke's (1992) generalized model of scrolled, meandering river floodplains which depicts inclined beds as a basal layer underlying RAS deposits (see Figure 2.1). Locations marked 'U' in Figure 6.2 indicate suspected unit bar deposits, identified because of their somewhat steeper channelward face and location at the base of preserved RAS deposits, similar to descriptions by Sundborg (1956) and Nanson (1980). The unit Figure 6.2 Examples of macroscale inclined reflections that occur along dip direction GPR profiles. The location of GPR survey lines dip1 and D is shown in Figure 5.4. Suspected unit bars are indicated at locations marked 'U' and lie immediately above the basal inclined reflector. bar in profile dip1 also exhibits somewhat coarser sediments than overlying deposits (see core C8 in Figure 5.2), corresponding with grain size descriptions of unit bars made by Sundborg (1956) and Nanson (1980). In addition, core C8 displays a coset of alternating coarse to fine-grained laminae from 1.4 to 1.6 m (see Figure 5.2) which is in agreement with Sundborg's (1956) description of preserved scroll ridge sediments. Genetically, inclined bounding surfaces in Figure 6.2 appear to separate successive floodplain forming events and are similar in structure to floodplain descriptions made by Puigdefabregas (1973) and Sundborg (1956), although there is no clear association between these events and the formation of individual scroll bars, some scroll bars exhibit these inclined strata and some do not. In summary, there exists significant variability in the macroscale radar stratigraphy of 2-D radar profiles in the GPR survey grid. As a result, interpreted floodplain architecture from one GPR survey line may represent only part of the range of macroscale radar stratigraphy found at the study site. Nevertheless, most GPR profiles in this study exhibit a common trend, that of IS or RAS stratification in dip (or near dip) direction profiles, allowing for an accurate assessment of channel planform from a single GPR profile.
Variability in the dip angle of lateral accretion deposits Based on the widtwdepth ratio of the main channel near the distal end of the meander bend (BC Environment, 1982), the average dip angle of lateral accretion deposits in the river floodplain is estimated as 4" to 5' (Leeder, 1973), a value which compares fairly well with observations from dip direction GPR profiles, although this is difficult to estimate since the dip angle of inclined reflections varies significantly about this range at any given location. Some areas of the floodplain, however, exhibit significantly steeper beds, such as in the top 4 m of floodplain deposits in GPR profiles 7 and B (Figure 6.3). This may be the result of a stepped channel profile, similar to Jackson's (1978) observation that smooth, sigmoidal ECS are rare in gravel-bed streams because their stepped cross-channel profiles from floodplain to thalweg make a smooth sigmoidal ECS improbable (Harms et al., 1963; McGowen and Garner, 1970). A stepped floodplain profile is clearly exhibited in this study by the terracing observed in floodplain topography (see Figure 3.5b). Steeply dipping IS deposits in Figure 6.3 are located on one of these steps, a location where radar profiles show signs of rapid channel scour3, likely a response to confinement and subsequent degradation of the river bed immediately upstream at the concave bank-bench (see Figure 3.1). If channel area remains roughly the same throughout this transition then the width-depth ratio must decrease, resulting in a greater dip angle of upper point bar sediments similar to that observed in Figure 6.3. An alternative hypothesis is that lateral accretion of chute and secondary channels in the river floodplain, such as those displayed in Figure 6.4, causes steeper inclined strata to be preserved in the upper 5 m of the floodplain. A representative sample combining all the features discussed in dip and strike GPR profiles in this section is shown in Figure 6.5. Though not necessarily occurring in this arrangement, this model serves to summarize large-scale radar stratigraphic trends and features interpreted in the floodplain along dip and strike direction profiles.
Geometry of Ridge-and-swale (RAS) stratification Sundborg (1956) and Gibling and Rust (1993) observed that scroll ridges are asymmetric and steeper on the streamward side. This study shows no clear trend. Some scroll bars are asymmetric, some not, and those that are asymmetric show steeper dips in variable directions, toward or away from the river channel. There is some evidence, however, of a trend in deeper preserved scroll ridge deposits which often do consistently dip more steeply toward the river channel (Figure 6.6). This corresponds with trends
radar profiles A and B (see Figure 5.27) show a decline in radar signal attenuation with depth along their eastern margins, interpreted as channel scour of underlying silt and clay sediments. Figure 6.4 Photographs of (a) a secondary channel next to the island in the meander bend (location in Figure 4.12) and (b) a chute channel, located immediately to the left of photo (a). Flow is out of the page. 1s J 4 - dip section -> , Macroscale inclined strata
Channel fill ...,.. :;>,:,,, c?.:...... :".:.>:.:.:: ;,.. Vertically preserved RAS ...AX+?..,...... 0.>'::>>".:::':...... stratification with well-developed IS .... Vertically preserved RAS stratification with weakly observed IS U Interpreted unit bar
,.>:.:.E,x.>,....:.:.:.: .... .,:.>= :::y: strike section I ...... 0...... Inclined stratification
Figure 6.5 Generalized schematic of radar stratigraphy and major features interpreted in dip and strike sections of GPR profiles. The direction of floodplain accretion in each section is shown by the arrow. dip 1
200 MHz VE=1.3x
100 MHz VE=2.54x
Figare 6.6 Examples of asymmetric scroll bar deposits that display steeper, channelward-dipping reflections at depth. Top and bottom examples are from 200 and 100 MHz GPR respectively. Each profile is labeled, according to the GPR survey line from which it derives. observed in interpreted unit bars in Figure 6.2, although not all reflections in Figure 6.6 occur at the base of floodplain deposits, the location where unit bars should appear in GPR profiles (Bridge et al., in press).
6.2 GPR Observation of Inclined Stratification
Although inclined stratification occurs in the majority of GPR profiles in this study, the predominance of RAS deposits and a lack of well-developed inclined stratification in some dip direction profiles remains a common observation. Since inclined stratification is possibly the most commonly described feature of laterally accreting floodplains in the literature, it might be suggested that the GPR simply does not accurately reveal inclined strata that are indeed present. Nonetheless, GPR data from other studies of meandering river floodplains show that inclined strata can easily be identified using GPR. For example, Gawthorpe et al. (1993) experienced no difficulty in observing well-developed IS in the Madison River floodplain (see Figure 2.6). Similar observations of inclined stratification in large-scale studies of deltas also demonstrate the ability of GPR to resolve
IS deposits (see Figure 4.3). Moreover, results from GPR calibration along gravel pit walls in this study show that GPR provides an accurate representation of subsurface stratal trends (see Figure 4.11). Thus, for the purpose of this discussion, we assume that radar stratigraphy indicating only weakly developed or absent inclined stratification is an accurate portrayal of underlying stratal trends.
6.3 The Identification of 3-D Radar Facies Boundaries
Although Jol (1993) defined radar facies as mappable 3-D sedimentary units, the 3-D extent of radar facies at intersection points in the GPR survey grid was often difficult to identify because of poor correlation between radar facies boundaries. Intersection points
122 were found to exhibit best correlation among these boundaries along the interpreted basal contact between floodplain alluvium and underlying silt and clay (see Section 3.0). Of the 48 intersection points in the GPR survey grid (Figure 5. I), 29 (or 60%) showed good 3-D correspondence (+I- 1 m) in the interpreted depth of this basal contact. Radar facies boundaries not associated with the basal contact, however, exhibited much poorer correspondence. Of 29 intersection points, only 5 (or 17%) showed good 3-D correspondence (+I- lm) between radar facies boundaries, the majority of which were associated with the channel fill element identified in the western part of the floodplain (see Figure 5.27). Good 3-D correlation of the interpreted depth of floodplain sediments in radar profiles was expected. The identification of basal silt and clay was not dependent on orientation of the GPR survey line but on the dielectric properties of silt and clay, which caused rapid signal attenuation at depth in all radar profiles. Radar facies boundaries delimiting different floodplain architectures, however, dependent on orientation of deposits because architectural elements vary in appearance depending on 2-D orientation, thus they may be more difficult to discern in some orientations than in others (Miall, 1985; Brierley, 1989, 1991a; Bridge, 1993). This remains a persistent problem not only in GPR interpretation but also in logging genuine floodplain sections (Wolman and Leopold, 1957; Brierley, 1991a). Another 3-D radar boundary identified throughout the GPR survey grid was the ground water table. It appeared as a prominent, high-amplitude reflection in some GPR profiles but not in others where it was indistinguishable from surrounding radar reflections. As a result, 3-D ground water table variability could not assessed due to a lack of prominent, high-amplitude reflections in GPR profiles. 6.3.1 The GPR Survey Grid as an Indicator of Floodplain Accretion Direction
Another useful application of the GPR survey grid is that the direction of floodplain accretion, known from the orientation of surface scroll bars (Hickin, 1974), can be compared with the resultant inclination of radar reflections at intersection points in the GPR survey grid (Figure 6.7). The usefulness of this form of analysis becomes clear in studies of ancient deposits where, if a GPR survey grid is used, the approximate accretion direction of a meander lobe can be established based on the resultant inclination (true dip) of radar reflections at intersection points in the survey grid. All directional arrows in Figure 6.7 were established trigonometrically, based on the resultant dip of radar reflections in the top 5 m of deposits at each intersection point. The dip angle of RAS deposits at an intersection point was always measured from the nearest reflection dipping toward the river channel, a direction ascertained by observing, at depth, dip direction of the steeper dip face of preserved scroll ridges (see Figure 6.6). Note that this observation illustrates a lack of correspondence between sedimentation direction on the lee-side of RAS deposits and the overall direction of floodplain accretion. Thus, inclined radar reflections do not necessarily provide data on the direction of floodplain sedimentation.
6.4 Limitations of GPR Methodology
There were three important methodological limitations of this study, 1) radar reflection profiling is only an indirect means of acquiring subsurface data, therefore the validity of results based on interpreted GPR profiles which differ from conventional models may be subject to question; 2) GPR survey design in this study focused only on large-scale radar stratigraphic trends in the floodplain, restricting the amount of Figure 6.7 Arrows indicate estimated floodplain accretion directions from IS deposits at intersection points in GPR profiles. Surface scroll topography is identified by dashed lines.
125 information gathered on smaller-scale structures; 3) a cartesian GPR survey grid design created complexity in the interpretation of GPR profiles, due to the highly variable orientation of GPR survey lines relative to surface scroll topography. Each of these limitations is discussed below. As a geophysical tool, GPR provides only indirect evidence of underlying stratal trends and is accompanied by two major limitations; i) the relationship between individual GPR reflections and subsurface strata remains to be understood, and ii) GPR is, for the most part, insensitive to variations in grain size other than those which cause a significant change in dielectric conductivity (Greenhouse et al., 1987). Nonetheless, an attempt was made in this study to minimize these limitations by focusing solely on large- scale patterns of radar stratigraphy, shown in Section 4.2.2 and in other studies to be the most reliable indicator of subsurface stratal trends in the absence of additional data (Smith and Jol, 1992b; Pratt and Miall, 1993; Huggenberger et al., 1994; Stephens, 1994). Although grain size variability was not an objective of this study, the ability of GPR to identify high concentrations of silt and clay was quite useful in determining the depth to Kamloops Lake Drift sediments in the study area. As a result of the macroscale approach of the GPR survey design (see Section 4.3.2), the resolution of GPR profiles at the study site was fairly coarse. The maximum vertical resolution of radar profiles was 0.4 m and 0.2 m for 100 and 200 MHz GPR profiles respectively (Jol, 1995). Horizontal resolution was somewhat less, 1.0 m and 0.5 m for 100 and 200 MHz profiles respectively. As a result, detailed paleocurrent and bedding thickness data could not be interpreted from GPR profiles. Furthermore, microscale (less than 10 m in the horizontal) internal architecture of trough cross-stratification, unit bars and other small-scale structures could not be resolved. The use of a cartesian grid pattern rather than a radial pattern for the GPR survey grid greatly increased the complexity of 2-D GPR profile interpretation, a result of highly variable orientation of GPR profiles relative to surface scroll topography (Figure 5.4). Because much of the GPR survey line descriptions were made based on their correspondence with dip or strike orientations, a radial grid design that followed surface scroll topography would have eliminated this complexity. Nonetheless, it was not possible to construct a radial grid in this study (discussed in Section 4.3.2), although it is recommended as a preferred method to curb the complexity of GPR profile interpretation in large-scale meander floodplains. The cartesian method, however, did reveal the appearance of radar stratigraphic sections in a variety of orientations, allowing clear observation of floodplain architecture in virtually any 2-D orientation. An additional limitation of this study lay in the difficulty of the vibracorer to penetrate through coarse-grained floodplain sediments, restricting information on grain size variability below 7 m depth, the limit of penetration by the vibracorer. As a result, only one sample of underlying Kamloops Lake Drift sediments was obtained for comparison with radar signal attenuation in GPR profiles (see Section 5.1). The vibracorer may have been able to core more deeply, however, if a single section of pipe were used rather than two pipes joined by a pipe connector, the method used in this study. The pipe connector was 1 cm wider than the core pipe and greatly impeded downward movement of the vibracorer. CHAPTER SEVEN CONCLUSIONS
7.1 Summary
The two primary goals of this study were: i) to assess the ability of GPR to identify stratal trends along visually documented exposures and ii) to interpret the meso- (> 25 m horizontal distance) and macro-scale (> 75 m horizontal distance) radar stratigraphy of a scrolled, meandering river floodplain. GPR calibration in a ground-truthing experiment against two gravel pit exposures showed that inferences of stratal trends in sand and gravel deposits made from GPR data are most accurate when based on patterns of radar reflections rather than single reflections. Individual reflections often did not conform well with corresponding stratal boundaries, and no consistently reliable correlation between radar reflections and individual stratal boundaries was observed. Varying GPR parameters along gravel pit exposures showed that high-resolution GPR profiling with less than 0.5 m station-spacing and step size provided little additional data on meso- and macro-scale stratigraphic trends. Therefore, a GPR survey design using 1.0 and 0.5 m antennae separation and step size, for 100 and 200 MHz antennae respectively, was selected for use at the meander floodplain. Radar stratigraphy of North Thompson River floodplain was similar in signal amplitude and reflection continuity to that observed in the GPR calibration study; radar reflections were generally either continuous or semi-continuous and typically exhibited inclined beds dipping less than 20".
Radar stratigraphy of the floodplain was divided into macro- and mesoscale radar facies, based solely on visually observed differences between patterns of radar reflections. Radar facies almost always showed poor 3-D continuity at intersection points between GPR profiles; however, the interpreted radar facies boundary between floodplain alluvium and underlying Kamloops Lake Drift sediments showed good 3-D consistency. Radar facies and facies sequences were used to identify the 3-D extent of a channel fill element and an extensive lateral accretion element in the floodplain, both of which corresponded with the expected architectural elements of a meandering river floodplain (Miall, 1985). The lateral accretion element exhibited both inclined stratification (IS) and ridge-and- swale (RAS) deposits in dip direction profiles and planar tabular and inclined stratification in strike direction profiles, in general agreement with existing models from the literature (Sundborg, 1956; Miall, 1985; Nanson and Croke, 1992). Two distinct forms of lateral accretion architecture in dip direction profiles were identified, i) well- developed RAS stratification separated by individual, channelward-dipping reflections and ii) well-developed IS, typically in locations where RAS was less well-developed. All channel-normal sections of the meander floodplain exhibited some form of macroscale IS or RAS architecture suggestive of a meandering river planform. The appearance of macroscale stratal trends was highly dependent on orientation of the GPR survey line with respect to surface scroll topography. In addition, GPR was able to identify, to a limited extent, the location of the water table and to a large extent the basal interface between floodplain alluvium and underlying silt and clay sediments of Kamloops Lake Drift.
7.2 Recommendations for Future Research
This study was restricted to the genetic interpretation of large-scale patterns of radar stratigraphy, preventing identification of microscale (10 m) structure of floodplain deposits (i.e. Bridge et al., in press), potentially useful for chronostratigraphic reconstruction of floodplain development, assessing paleocurrent directions, and measuring bedding thicknesses. Research involving multi-frequency antennae in the high- frequency range (500 to 1000 MHz) (Olhoeft et al., 1990) would allow not only for better stratigraphic interpretation of small-scale features, but also allow for the development of a series of related diagrams that show the ordering of bounding surfaces in radar profiles. Multi-frequency profiling also removes the trade-off encountered between resolution and depth penetration when using individual antennae frequencies. Perhaps the greatest challenge of GPR remains in identifying what actually causes radar reflections (Greenhouse et al., 1987). Although many GPR studies clearly reveal the correspondence between radar and real stratigraphy, the exact cause of radar reflections remains relatively undefined. It cannot be predicted, in an exposed section, what changes in grain size, porosity, or stratal variability will result in partial reflection of the radar signal (Greenhouse et al., 1987). The entire range of GPR applications in geomorphology can only be clarified through further case studies and refinement of the GPR technique. The results of this study show its applicability as a primary tool for the investigation of floodplain architecture in coarse- grained sediments, adding to the results of previous research and showing how GPR can contribute to the improvement of 3-D models in a variety of geomorphic environments. Allen, J.R.L. 1983. 'Studies in fluviatile sedimentation: bars, bar-complexes and sandstone sheets (low-sinuosity braided streams) in the Brownstones (L. Devonian), Welsh borders', Sedimentary Geology, 33,237-293. 1970. 'Studies in fluviatile sedimentation: A comparison of fining upwards cyclothems with special reference to coarse-member composition and interpretation', J. Sediment. Petrol., 40, 298-323. 1966. 'On bedforms and palaeocurrents', Sedimentology, 6, 153-190. 1965. 'A review of the origin and characteristics of recent alluvial sediments', Sedimentology, 5, 89-19 1. 1964. 'Studies in fluviatile sedimentation: six cyclothems from the Lower Old Red Sandstone', Anglo-Welsh Basin, Sedimentology, 3, 163-198. 1963. 'The classification of cross-stratified units, with notes on their origin', Sedimentology, 2,93-114.
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Young, R.A. and Sun, J. 1994. 'Recognition and removal of surface scattering in GPR data', Proc. Fifh Int'l. Con$ on Ground Penetrating Radar, vol. 2,735-746. APPENDIX 1 1:::;:::::::::: I::::::::: I::. ...:::
4 (SU ) aru 11 8 8 jsu )
LD : : sup 091 --A p : yadia ~_CCtCC.-.+-r..ct..*..~.*~.. Velocity calculation from common mid-point profiles
v = s/t v = s/t v = (14 m - 1 m)/(110 ns - 29 ns) v= (14 m -2m)/(117 ns-28 ns) v = 0.160 m/ns v = 0.1 35 mlns
Average velocity of all six CMP profiles: (0.175+0.163+0.152+0.173+ 0.1 6O+O. 1%)I6
= 0.160 mlns APPENDIX 2
Appendix 2 consists of 20 GPR survey lines located in the attached folder