Geophysical, Geological and Hydrogeological Characterization of a Tributary Buried Bedrock Valley in Southern Ontario

Canadian Journal of Earth Sciences

Geophysical, geological and hydrogeological characterization of a tributary buried bedrock valley in southern Ontario

Journal: Canadian Journal of Earth Sciences

Manuscript ID cjes-2016-0120.R2

Manuscript Type: Article

Date Submitted by the Author: 05-Dec-2016

Complete List of Authors: Steelman, Colby; University of Guelph, School of Engineering Arnaud, Emmanuelle;Draft University of Guelph, Pehme, Peeter; University of Guelph Parker, Beth; University of Guelph, School of Engineering

buried bedrock valley, electrical resistivity tomography, karst, water Keyword: supply, Silurian dolostone

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TITLE : Geophysical, geological and hydrogeological characterization of a tributary buried bedrock valley in southern Ontario

AUTHORS :

Colby M. Steelman 1,2 , Emmanuelle Arnaud 1,3 , Peeter Pehme 1,2 and Beth L. Parker 1,2

AFFILIATIONS :

[1] G360 Centre for Applied Groundwater Research, University of Guelph, Guelph ON Canada [2] School of Engineering, University of Guelph, Guelph ON, Canada [3] School of Environmental Sciences, University of Guelph, Guelph ON, Canada *Correspondence to: C. M. Steelman ([email protected]) and B. L. Parker ([email protected])

Draft

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1 ABSTRACT

2 Buried bedrock valleys infilled with Quaternary-aged sediment have the potential to become productive

3 aquifers due to prevalent sand and gravel deposits often associated with these topographic lows. In

4 areas where groundwater is drawn from the underlying bedrock aquifer, buried bedrock channels may

5 significantly affect the spatial distribution of recharge and localized contaminant pathways. Therefore,

6 understanding the form, distribution and the nature of Quaternary infill sediments within these buried

7 bedrock river valleys, and their relationship to hydraulically-transmissive bedrock features is an important

8 aspect of groundwater resource management. Here, we evaluate the effectiveness of electrical resistivity

9 and seismic refraction collected over a partially urbanized 150 ha area with variable vegetation, roads,

10 and structures, to map the spatial distribution of sediments and delineation of a channel segment

11 associated with a regional bedrock valley. Electrical resistivity and seismic refraction was performed 12 along 13 (covering ~11.6 km) and 7 transects (coverDrafting ~0.9 km), respectively, to map and characterize 13 the bedrock surface morphology beneath a variable thickness of unconsolidated deposits. Three

14 continuously cored holes and downhole geophysical logs, supplemented with four nearby water well

15 records captured the in-channel as well as adjacent Quaternary stratigraphy (~15-40 m). Cores recorded

16 multiple glacial till deposits and ice marginal processes associated with ice advances and retreats.

17 Hydraulic transmissivity of the bedrock around the valley feature was evaluated using a FLUTe™ hydraulic

18 transmissivity profiling technique. This study demonstrates the potential of combining several surface

19 geophysical methods with sedimentological analysis of continuous cores and hydraulic data for

20 characterizing tributary bedrock channel morphology and Quaternary infill at a scale relevant to localized

21 studies of municipal production well recharge zones and contaminant transport and fate.

23 KEY WORDS : buried bedrock valley; electrical resistivity tomography; karst; water supply; Silurian dolostone

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24 1.0 INTRODUCTION

25 Buried bedrock valleys (BBVs) represent an important component of the hydrogeologic conceptual

26 model of southern Ontario and other previously glaciated terrains such as the Canadian Prairies, northern

27 United States, and Europe (e.g., Kehew and Boettger 1986; Sandersen and Jørgensen 2003; Russell et al.

28 2004; Cummings et al. 2012; Sharpe et al. 2013). In southern Ontario, these complex and variably

29 distributed features comprise an extensive network of bedrock valleys that formed through, or were

30 enhanced by, intense glacial and glaciofluvial erosion during the Pleistocene (Karrow 1979; Eyles et al.

31 1997; Kor and Cowell 1998; Gao 2011), and were subsequently infilled with Quaternary-age glacial and

32 interglacial sediment (Karrow 1968; Barnett 1992). Although province-wide mapping (Gao et al. 2006)

33 and regional mapping of some specific areas (e.g., Marich et al. 2011; Burt 2011; Bajc et al. 2012) have

34 been carried out, the location and geometry ofDraft these BBVs and their tributaries is not well constrained in

35 many parts of Ontario.

36 Source water protection has become an integral component of a community’s groundwater

37 management plan since the introduction of the Clean Water Act (2006), S.O. 2006, c.22. This has led to

38 significant investments in three-dimensional sediment and bedrock surface mapping across Ontario both

39 by the geological surveys (e.g., Sharpe and Russell, 2006; Bajc 2008; Davies et al. 2008; Bajc and Rainsford

40 2010; Gao et al. 2006; Bajc et al. 2012; Burt 2012; Bajc et al. 2014) and municipalities and conservation

41 authorities that were tasked with creating source water protection plans (e.g., Gartner Lee 2004; Golder

42 Associates 2006; AquaResource 2010). These activities have supported the conceptualization of aquifer

43 and aquitard units and the delineation of regional-scale groundwater flow systems within and around

44 BBVs.

45 In some cases, here in Ontario and elsewhere, thick successions of sand and gravel deposits have

46 infilled these valleys resulting in highly-productive aquifers (Kehew and Boettger 1986; Russell et al. 2004;

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47 Wiederhold 2009). Alternatively, buried valleys can be infilled or covered by low-permeable Quaternary-

48 age diamicton or a combination of sediment types with complex geometries and very different hydraulic

49 properties (Meyer and Eyles 2007; Jørgensen et al. 2003a; Cummings et al. 2012), further complicating

50 hydrogeologic conceptualizations (van der Kamp and Maathuis 2012, Cummings et al. 2012). Therefore,

51 these environments require more detailed characterization of both form and infill to ensure accurate

52 quantitative analysis of local groundwater flow system geometries.

53 The underlying sedimentary bedrock across southern Ontario contains ubiquitous interconnected

54 fractures with variable lengths and apertures (Johnson et al. 1992; Kennel 2008; Munn 2012). In addition

55 to these fractures, the Silurian dolostone aquifer – a prolific aquifer for private and municipal water

56 across southern Ontario – contains dissolution enhanced features (e.g., fractures or vugs) and conduits

57 that increase its groundwater resource potential,Draft but also its susceptibility to surface contamination (e.g.,

58 Perrin et al. 2011). Many communities across southern Ontario rely almost exclusively on these fractured

59 and dissolution enhanced bedrock aquifers for their drinking water supply. The complex interdependent

60 relationship between Quaternary deposits and these fractured sedimentary rocks has become an

61 important component in the development of effective groundwater management strategies for these

62 communities (e.g., Gartner Lee 2004; AquaResource 2010). Given the potential association between

63 BBVs and increased recharge and transmissivity of bedrock aquifers (Cole et al. 2009), successful

64 groundwater management strategies will depend on sufficiently high-resolution (spatial and temporal)

65 quantitative information, enabling accurate and well-constrained geologic and hydrogeologic

66 conceptualizations of buried valley features (e.g., Shaver and Pusc 1992; Smerdon et al. 2005).

67 Geophysical methods have the capacity to unravel complex geologic relationships and can play a

68 critical role in advancing our understanding of shallow groundwater flow systems in these settings (van

69 Dam 2012). Studies utilizing multiple geophysical methods combined with geologic and hydrogeologic

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70 data are particularly effective for three-dimensional mapping of hydrostratigraphic units (e.g., Gabriel et

71 al. 2003; Jørgensen et al. 2003a and 2003b; Steuer et al. 2009; Stumpf and Ismail 2013; Cassidy et al.

72 2014; Sapia et al. 2014). In Ontario, shallow seismic reflection, gravity and electromagnetic surveys have

73 been used at mostly regional scales to delineate the nature of the infilled sediment and bedrock valley

74 geometry (e.g., Greenhouse and Monier-Williams 1985; Greenhouse and Karrow 1994; Zweirs et al.

75 2008; Burt 2011; Bajc et al. 2012; Pugin et al. 2013). Elsewhere, airborne transient electromagnetic

76 (TEM) methods are mostly used (e.g., Sandersen and Jørgensen 2003; Jørgensen et al. 2003a and 2003b;

77 Sørensen and Auken 2004; Sapia et al. 2015).

78 This paper focuses on a local-scale high-resolution electrical resistivity study conducted within the

79 City of Guelph, Canada, with a primary goal of demonstrating the capability and limitations of the

80 technique for mapping a buried bedrock valleyDraft at a relatively local scale. Surface electrical resistivity and

81 seismic refraction results are combined with sedimentological analysis of cores, downhole geophysical

82 logs and borehole hydraulic information to develop a data-driven model of bedrock channel morphology

83 and Quaternary infill, and associated hydraulic characteristics at a scale relevant to localized studies of

84 municipal production-well recharge zones and flow and transport characteristics.

85 2.0 BACKGROUND

86 2.1 Regional geologic and hydrogeologic setting

87 The Guelph region is underlain by the Middle Silurian unsubdivided Amabel (i.e., locally identified as

88 the Irondequoit, Gasport, and lower Goat Island formations), Eramosa and Guelph Formation dolostone

89 (Brunton 2009, Armstrong and Carter 2010; Fig. 1). These massive and fossiliferous units represent an

90 extensive shallow water carbonate platform that formed over the Algonquin Arch, a tectonic high

91 situated between the subsiding Michigan and Appalachian basins (Brett et al. 1990; Johnson et al. 1992).

92 Sedimentary facies across these dolostone units are considered varied and complex due to a diversity of

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93 paleoenvironments that ranged from high energy reefal to quiet water back-reef lagoons (Brunton 2009).

94 Wide spread till plains and a series of moraines have been deposited over the bedrock during the

95 Pleistocene through multiple ice advances and subsequent retreats (Barnett 1992). This glacial activity

96 resulted in highly variable drift thickness with bedrock valleys infilled with stacked successions of coarse

97 to fine-grained sediment of varying characteristics and geometries (e.g., Karrow 1973; Meyer and Eyles

98 2007; Burt 2011), preserving tills that represent several ice advances during the last Wisconinan (Fig. 2).

99 The bedrock topography across southern Ontario has been extensively sculpted by pre-glacial,

100 glacial and glaciofluvial erosion (Fig. 3) as evidenced by uneven and complex bedrock valleys and troughs

101 (Karrow 1973; Eyles et al. 1997; Gao 2011). In the Guelph region, the bedrock surface exhibits several

102 valleys trending in a southwest-northeast direction (Fig. 4) (Karrow et al. 1979; Cole et al. 2009) with drift

103 thicknesses ranging between 0 and 62 m. BasedDraft on regional mapping of the surficial geology, the study

104 site primarily consists of glaciofluvial outwash sand and gravel, poorly-sorted sandy silt to silty sand, and

105 drumlinized till plain deposits (Karrow 1968, 1987).

106 Using geological and hydraulic data sets from consultant reports completed for the City of Guelph,

107 Gautrey (2004) noted a spatial relationship between the location of high-capacity municipal production

108 wells and BBVs within the City of Guelph; based on these data and two complete bedrock cores, Cole et

109 al. (2009) hypothesized that bedrock near buried valleys in the Guelph region may possess greater

110 abundance of dissolution-enhanced fractures and conduits that would enhance hydraulic connectivity

111 with deeply buried aquifers and between fractured bedrock and Quaternary sediment. However, the

112 type and distribution of Quaternary sediment covering these BBVs as well as the valley’s geometry and

113 orientation with respect to groundwater flow direction will likely affect its significance to the overall flow

114 system. Perrin et al. (2011) examined the role of dissolution-enhanced flow features on the transport

115 and fate of contaminants within the same dolostone aquifer covered by up to 50 m of Quaternary

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116 deposits; their study found that deposition of Quaternary sediments altered the boundary conditions

117 governing the conduit flow system. A more recent study within the Guelph area showed the impact of

118 varying glacial sediment distribution on the migration and fate of agricultural contaminants to the

119 underlying fractured dolostone aquifer (Best et al. 2015). While the exact relationship between BBVs and

120 increased well productivity has yet to be fully understood, it stands to reason that the potential yield and

121 quality of water withdrawn from a bedrock aquifer near a Quaternary infilled channel will be influenced

122 by the composition and architecture of the unconsolidated sediments and the morphology of the bedrock

123 surface; hence, there is value to improving our understanding of these conditions.

124 2.2 Study site description

125 The study site is located within the City of Guelph and includes a large portion of the University of

126 Guelph’s Arboretum and an adjacent golf courseDraft to the north-northwest (Fig 2); various municipal roads

127 and private drives cross the site. The area largely consists of open grasslands, areas of dense brush

128 regrowth, manicured fairways, and is bounded by the Eramosa River to the northeast. Surface

129 topography is relatively flat toward the southeast and slowly decreases in elevation toward the Eramosa

130 River where it also begins to exhibit increasing variability.

131 Data from a number of existing water supply and irrigation wells are available in the study area

132 (Table 1). Well records indicated that the bedrock elevation is highly variable across the study site.

133 Available well records and regional bedrock elevation data compiled by the Ontario Geologic Survey (Fig.

134 3) indicated the presence of a local-scale BBV, which likely connected to the regional Guelph/Rockwood

135 buried bedrock valley (Fig. 4; Karrow 1979; Greenhouse and Karrow 1994; Eyles et al. 1997; Gao et al.

136 2006). Based on these point data, the feature appears to be approximately 30 m deep, 100’s of meters

137 wide, striking east-west with an approximate valley floor elevation of 290 masl.

138 2.3 Geophysical investigations of buried valleys

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139 BBVs and their Quaternary fills can be studied using a wide range of surface geophysical techniques

140 (Greenhouse and Karrow 1994). However, the majority of studies to date have focused on the use of

141 gravity (e.g., Greenhouse and Monier-Williams 1986; Gabriel et al. 2003; Gabriel 2006; Møller et al. 2007;

142 Zweirs et al. 2008; Burt and Rainsford 2010; Bajc and Rainsford 2010), which exploits changes in

143 subsurface bulk density and ground and airborne transient electromagnetic (TEM) methods (e.g.,

144 Jørgensen et al. 2003a, 2003b; Sørensen and Auken 2004; Steuer et al. 2009; Jørgensen et al. 2013; Høyer

145 et al. 2015; Oldenborger et al. 2016). While gravity methods can be highly effective in mapping changes

146 in bedrock elevation, they provide limited insight about Quaternary infill and architecture, and are

147 frequently accompanied by significant uncertainty in the true bedrock elevation in the absence of

148 borehole logs. Although TEM methods are relatively effective in the characterization of valley infills and 149 bedrock morphology, these methods have largeDraft sampling volumes making them more suited to deeper 150 and larger-scale investigations; these methods are also very susceptible to cultural electromagnetic

151 interference, which may limit applications in urbanized areas. Advancements in seismic reflection

152 technology (i.e., data processing equipment and software) have led to more frequent applications in

153 shallow (30-100 m) environments (Steeples 2000). Numerous studies demonstrate the capacity of

154 shallow seismic reflection to characterize the rock surface and Quaternary fill architecture (e.g., Boyce et

155 al. 1995; Büker et al. 1998; Gabriel et al. 2003; Jørgensen et al. 2003b; Ahmad et al. 2009; Ogunsuyi and

156 Schmitt 2010; Stumpf and Ismail 2013; Burt 2015). Although shallow seismic reflection has been

157 recognized as a powerful tool for investigating shallow bedrock and Quaternary deposit geometry (e.g.,

158 Hunter et al. 1984) to support or enhanced hydrogeologic characterization (e.g., Sharpe et al. 2003; Pugin

159 et al. 2009; Ogunsuyi and Schmitt 2010), data acquisition remains costly, can have poor resolution, and

160 in many cases data processing and interpretation can be challenging (Steeples et al. 1997; Steeples and

161 Miller 1998; Ismail et al. 2012) with varying quality in the results. These data are susceptible to cultural

162 noise making data acquisition in urban areas challenging.

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163 Surface electrical resistivity methods are an alternative to gravity, TEM and seismic reflection

164 surveys for imaging shallow (<100 m) geologic formations, especially in areas impacted by infrastructure,

165 cultural noise, and electromagnetic interference. Electrical methods also have considerably lower

166 operational costs, in some cases are easier to implement, and provide measurements that, in practice,

167 are more straightforward to interpret. Like all geophysical methods, it has its limitations; unlike seismic

168 reflection, electrical resistivity measurements do not provide a direct image of the subsurface, tend to

169 combine layers, and have relatively poor resolution of interfaces with increasing depth of investigation or

170 near the model boundaries.

171 Most modern apparent resistivity measurements are conducted with multicore cables and

172 interpretations that rely on a tomographic inversion process where measured data is reconstructed from

173 forward models of a specific physical property Draftdistribution (Snieder and Trampert 1999; Loke et al. 2013).

174 Although data inversion is a standard practice in the interpretation of most geophysical data, the

175 solutions of inverse problems in applied geophysics are not inherently unique. In other words, the model

176 that best matches the measured data is not necessarily an exact representation of the subsurface

177 parameter distribution. Nevertheless, electrical imaging techniques using either induced or direct current

178 (DC) sources have provided promising results for imaging valley morphology (e.g., Jørgensen et al. 2003a;

179 Steuer et al. 2009; Ahmad et al. 2009; Tye et al. 2011), and in some cases the infilled Quaternary

180 sediment architecture (e.g., Baines et al. 2002; Leopold et al. 2013); particularly when combining multiple

181 geophysical techniques with geological information (e.g., Greenhouse and Karrow 1994; Jørgensen et al.

182 2003b; Gabriel et al. 2003; Ahmad et al. 2009).

183 3.0 METHODOLOGY

184 3.1 Borehole lithology, geophysical logs and hydraulic profiling

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185 Continuous cores and downhole geophysics were collected at GDC-1, GDC-2B, GDC-6 (adjacent

186 bedrock cored hole to GDC-10A) and GDC-10A (Fig. 5), recording geologic characteristics of bedrock and

187 Quaternary sediments across the bedrock valley. Sedimentary characteristics of the unconsolidated

188 material overlying bedrock were logged in the field (cm scale) using standard sedimentological

189 techniques to record changes in texture, sorting, sedimentary structures, and clast characteristics with

190 depth based on PQ-mud-rotary (GDC-1, GDC-2B) and rotosonic (GDC-10A) continuous core records.

191 Quaternary cores were 7.6 cm (GDC-10A) to 12.7 cm (GDC-1 and GDC-2B) in diameter and ranged in

192 length from 10.4 m (GDC-10A) and 16.5 m (GDC-2B) to 43.4 m (GDC-1). Overall, core recovery was

193 excellent in both GDC-10A (97%) and GDC-1 (93%), and adequate in GDC-2B (80%). Disturbance in the

194 core is minimal and sedimentary bedding and structures well preserved once mud is scraped off the cores 195 recovered by PQ-mud rotary drilling, whereas Draftthe primary sedimentary structures tend to be 196 homogenized in the cores recovered by rotosonic drilling. Textural classification of diamict (a poorly

197 sorted deposit consisting of mud, sand and gravel that is commonly found in glaciated settings) is based

198 on Hambrey and Glasser’s (2003) classification where diamict is either clast poor or clast rich (1-5% or 5-

199 50% gravel content, respectively) and either sandy (> 66% sand), intermediate (33-66% sand) or muddy

200 (<33% sand).

201 Bedrock was continuously cored using a rotary rig and the HQ3 and PQ3 wireline coring technique,

202 producing a 6.1 cm (GDC-1, GDC-2B) to 8.3 cm (GDC-6) core, respectively. Cores ranged in length from

203 39.6 m (GDC-1) and 59.9 m (GDC-6) to 67 m (GDC-2B). Overall, core recovery was excellent in GDC-1

204 (96%), GDC-2B (99%) and GDC-6 (87%). Sedimentary characteristics of the bedrock were logged in a

205 similar fashion (10 cm scale) noting lithological properties such as rock type, colour, hardness,

206 crystallinity, fossil abundance, fossil type, dissolution/vug intensity, characteristics of fractures, and

207 presence of styolites (Fomenko 2015). Borehole geophysical logs included natural gamma, induction

208 conductivity (expressed here as the reciprocal apparent resistivity), and P-wave velocities within the

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209 bedrock. Steel casings were installed to bedrock with the exception of GDC-6, which was designed with a

210 PVC casing.

211 Open boreholes at GDC-1, GDC-2B and GDC-6 were hydraulically tested using a flexible downhole

212 liner method (Keller et al. 2013), which provides a measure of formation transmissivity (e.g., Quinn et al.

213 2015). With this method, flexible impermeable liners are everted into an open borehole while measuring

214 the rate of descent at a constant hydraulic head and associated forces (Keller et al. 2013). As the everting

215 liner passes and seals transmissive features or sections of the borehole, changes in the liner velocity

216 indicate the position of a permeable feature and provide estimates of total open borehole and depth-

217 discrete transmissivities using the Thiem equation for steady radial flow (Keller et al. 2013). 218 3.2 Electrical resistivity tomography Draft 219 The electrical resistivity tomography (ERT) method measures the apparent resistivity of the

220 subsurface by applying a DC electrical current between two electrodes inserted into the ground and

221 measuring the potential difference across two other electrodes at various spacings and separation

222 distances from the current source. These data were used to construct 2D apparent resistivity pseudo-

223 sections of the earth that were inverted to obtain a reasonable image of the subsurface materials (i.e.,

224 electrical property distribution). A more theoretical discussion of geophysical inversion and the surface

225 resistivity method can be found in Oldenburg and Li (2005) and Loke et al. (2013), respectively.

226 A fully-automated Syscal Jr. Switch resistivity system (Iris Instruments, Orleans, France) equipped

227 with 5 and 10 m electrode spacings connected to stainless steel electrodes inserted into the ground were

228 used to collect measurements. The system has the capacity to handle 48 electrodes for a total array

229 length of 235 m and 470 m for 5 and 10 m electrode spacing configurations, respectively. A maximum

230 investigation depth of 70 m was achieved using the 10 m electrode spacing with a Wenner-Schlumberger

231 electrode configuration; the Wenner-Schlumberger configuration was used for all resistivity surveys

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232 because of its optimal lateral and vertical sensitivity and higher signal-to-noise ratio relative to other

233 arrays (Dahlin and Zhou 2004). Full two-dimensional profiles were constructed by progressively rolling

234 along the profile in 115 m (for 5 m electrode spacing) and 110 m (for 10 m electrode spacing) increments

235 reaching maximum 2D survey lengths of 570 m to 1070 m across the site (Fig. 4). A total of 13 transects

236 were collected over the study area spanning ~11.6 km (Fig. 4).

237 Measured apparent resistivity data was manually filtered to remove bad data points and outliers

238 prior to being inverted using RES2DINV v.3.58 (Loke 2012). A robust inversion scheme was used with

239 moderate dampening parameters to reduce impacts of noise on the inverted model. Due to the distal

240 proximity of the boreholes with respect to the resistivity profile, the inversions were not constrained

241 using any depth-to-bedrock measurements from boreholes. Still, very reasonable agreement was

242 achieved between inverted models and nearbyDraft borehole information. Each resistivity data set was

243 inverted using the same set of parameters.

244 ERT arrays were collected along 2 orientations: lines 1-5 were collected in a northeastern direction

245 and lines 7-11 were collected in a northerly direction (Fig. 4). Given the minimum segment lengths (>570

246 m) needed to achieve the necessary depth of investigation and lateral coverage, as well as the added

247 complications of paved roadways with traffic, and restricted access to the adjacent golf course, the initial

248 resistivity measurements were collected in a southwest-to-northeast direction roughly parallel to College

249 Avenue. Based on these preliminary results, additional resistivity surveys were conducted with a north-

250 south orientation with a broader lateral coverage across the study area that included the adjacent golf

251 course.

252 3.3 Seismic refraction

253 A number of seismic refraction surveys were carried out in areas of low cultural noise to confirm the

254 apparent depth-to-bedrock interpreted from the resistivity models (Fig. 4). Refraction surveys were not

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255 conducted over the suspected valley given the large shot offset distances and strong energy source

256 needed to reach the bedrock surface. Seismic surveys were conducted using 24 and 48 channel Geode

257 seismographs (Geometrics, USA) connected to a laptop computer. A 10 kg sledge hammer and steel plate

258 were used to generate a P-wave source. A variable geophone array was used for all surveys, which

259 included 2 m geophone spacings on the ends and center of the array (48 channel arrays only) and wider 5

260 m spacings elsewhere. A variable spacing array was used to better detect the direct ground wave and the

261 cross-over position of the first refraction; standard forward and reverse shot procedures were used on-

262 end and off-end (Milsom 2003), as well as at various points along the array (e.g., middle of array and

263 opposite ends of 2 m spaced geophone segments). All data was recorded using a sample interval of

264 0.0625 milliseconds with a 0.25 second window; 100 Hz P-wave geophones were used for the surveys. A 265 total of 7 transects were collected over the studyDraft area with a total line length of ~0.9 km (Fig. 4).

266 Seismic data was processed using Reflex-Win v.7.0 (Sandmier 2012) seismic software, which

267 included basic signal processing: low-pass temporal filtering, automatic gain control, bandpass frequency

268 filter, and time cut. First-breaks corresponding to the arrival of the direct ground and refraction events

269 were manually identified at each geophone location. Refraction arrival times were analyzed using

270 Hagedoorn’s (1959) plus-minus “delay-time” method to provide a depth to bedrock estimate across the

271 survey area. These data were also interpreted using a more advanced refraction tomography technique

272 (i.e., simultaneous iterative reconstruction technique) available within Reflex-Win software. The seismic

273 tomographic method is similar to the resistivity tomography in that it optimizes a 2D velocity model to

274 the measured arrival times. As a result, this method can be more effective at incorporating spatial

275 heterogeneities and gradational boundaries relative to conventional interpretation methods (Sheehan et

276 al. 2005). Seismic inversion parameters were optimized for model stability and convergence.

277 4.0 RESULTS AND INTERPRETATION

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278 4.1. Geology, geophysical logging and hydraulic transmissivity profiles

279 Detailed Quaternary logging on either side of the bedrock valley (GDC-2B, GDC-10A), and from

280 within the valley (GDC-1), reveal relatively variable stratigraphy (Fig. 5). GDC-2B and GDC-1 are both

281 located in an area that is mapped as drumlinized till plain and are dominated by thick packages of

282 diamict, whereas GDC-10A is located in an area mapped as glaciofluvial outwash plain and is dominated

283 by variable stratified sediments. Bedrock was encountered at 318.4, 290.2 and 316.9 masl in GDC-2B,

284 GDC-1, GDC-6, respectively. Logging of the bedrock cores in GDC-1, GDC-2B and GDC-6 identified the

285 Eramosa (upper and lower), Goat Island, Gasport, Irondequoit, Rockway, Merritton and Cabot Head

286 formations. The Gasport formation is a regional semi-confined aquifer, which is locally overlain by the

287 regionally discontinuous Eramosa Formation, whereas the overlying Guelph Formation is an unconfined

288 bedrock aquifer, and the basal Cabot Head shaleDraft is a regional aquitard (Brunton 2009).

289 Quaternary sediments in GDC-2B (Fig. 5), on the SE edge of the valley, are dominated by multiple

290 layers of diamict (2 m bgs to 16.4 m bgs). Diamict beds (10’s of cm to ~3 m in thickness) are distinguished

291 by colour, degree of consolidation, matrix texture (sandy-intermediate and muddy), clast size, roundness

292 (subangular to subrounded), and amount (clast-rich to clast-poor) as well as minor sand interbeds. This

293 thick diamict package can be divided into three main units: a basal over-consolidated clast rich

294 intermediate diamict at the sediment bedrock interface, overlain by an over-consolidated clast poor

295 muddy diamict (~11.5-15 m bgs) with bullet shaped clasts, and a clast-rich intermediate diamict (~2-11 m

296 bgs). The diamict package is capped by a thin bed of gravel, a thick bed of gravelly (25%) very fine sand

297 and the current soil profile. The basal diamict and the overlying mud-rich diamict likely record deposition

298 in a subglacial environment based on their overconsolidated nature, presence of bullet shaped clasts and

299 stratigraphic position (Evans et al. 2006; Karrow 1968, 1987). In contrast, the layering, sand interbeds

300 and lack of consolidation in the coarser grained clast-rich upper diamict unit suggests that this unit

301 records deposition by melt out or mass flow (Boulton 1972; Evans et al. 2006). Based on its stratigraphic

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302 and topographic position relative to modern rivers, the uppermost pebbly sand and gravel likely record

303 glaciofluvial processes during deglaciation (Karrow 1968, 1987).

304 The valley infill (GDC-1, Fig. 5) consists of just over 6 m of cobble gravel comprised of broken angular

305 black shale fragments at the base with trace sand, overlain by a thick package (~28 m) of very stiff clast

306 poor muddy diamict, with a silty clay and some medium-fine sand matrix. Crude layering within this thick

307 package of diamict is observed based on changes in colour (dark grey to olive grey to trace red), plasticity,

308 or size and composition of gravel sized clasts (ranging from granule to cobble and dolostone to shale

309 respectively). This thick package of diamict is in turn overlain by silty clay (2.3 m) and a clast-poor, stiff

310 diamict with a sandy clay matrix (2.2 m). The core is capped by a 4.2 m bed of gravel. This sequence is

311 interpreted to record accumulation of bedrock rubble over competent bedrock, deposition of till in a

312 subglacial environment, an interval of clay settlinDraftg in a ponded environment, another till deposited in a

313 subglacial environment and capped by gravel that most likely records glaciofluvial conditions in an ice

314 marginal environment.

315 GDC-10A, located on the NW edge of the valley (Fig. 5), is dominated by stratified sediments of

316 variable texture. A thin (60 cm) basal clast rich sandy diamict occurs at the bedrock sediment interface.

317 It is overlain by 3 m of crudely stratified pebbly medium sand with some cobbles; the stratification is

318 defined by changes in clast size, content and cohesiveness. The next 1.5 m includes beds of clast rich

319 intermediate diamict, pebble gravel and pebbly fine sand. The top 5 m is dominated by sandy silt and

320 clay with minor interbeds of pebbly sand and pebble gravel. Based on its stratigraphic position, the

321 glacial history of this area (Karrow 1968, 1987) and the variable types of sediments ranging from gravel to

322 clay, this succession is interpreted to record a dynamic environment with variable depositional and

323 erosional conditions ranging from higher energy glaciofluvial flows to quieter depositional conditions

324 where silts and clays were allowed to settle in a ponded setting. The depositional origin of the diamict is

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325 unclear; the lack of overconsolidation suggests these units record debris flows in an ice-proximal

326 environment.

327 In terms of regional till stratigraphy, the following tentative correlations are proposed. The basal

328 diamict in GDC-2B (~15-16.5 m bgs) is very distinct from the other tills in the area and is similar to that

329 observed by Burt (2011; borehole BH40-OF_2010) and interpreted to be a pre-Catfish Creek Till. The

330 overlying dense mud-rich grey to olive diamict (~11-15 m bgs) and the corresponding thick package of

331 dense grey diamict in GDC-1 (~9-37 m) is likely Catfish Creek Till (Late Wisconsinan, Nissouri Phase),

332 based on texture, colour and its overconsolidation. The depositional origin of the unconsolidated diamict

333 in the upper part of GDC-1 and GDC-2B is difficult to establish. Based on its stratigraphic position and

334 clay-rich nature, the uppermost diamict in GDC-1 is tentatively correlated with the regionally-mapped

335 Maryhill Till (Karrow 1987). Based on stratigraphiDraftc position, the absence of overconsolidation and the

336 coarse-grained nature of diamict in GDC-2B, it likely records sediment reworking and debris flows during

337 the retreat of Port Stanley ice (Late Wisconsinan; Port Bruce Phase; Karrow 1968, 1987).

338 The natural gamma logs exhibit a moderately variable response across the upper 10-15 m of

339 Quaternary sediment at GDC-1, GDC-2B and GDC-6 (27.4 m away from GDC-10A). The gamma response

340 at GDC-2B indicates sharply bounded Quaternary sediment subdivisions, including two clay-rich layers

341 between 0-2 m bgs (332.9-334.9 masl) and 11-14 m bgs (320.9-323.9 masl). In contrast, the response

342 within the Quaternary sediment portion of GDC-6 indicates gradational contacts with more subtle but

343 frequent subdivisions; however, the grout used behind the PCV casing at this location may be

344 contributing to the more erratic response across the unconsolidated sediments. The response observed

345 at GDC-1 within the upper 10 m shows sharper boundaries before transitioning into a relatively high and

346 uniform response between 15-32.5 m bgs (301.2-318.7 masl). The more variable gamma signal at the top

347 of the succession corresponds well with the gravel at surface and the clay rich interval at ~7 m, whereas

348 the more consistent signal between 15 and 32.5 m depth corresponds to the thick package of grey

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349 diamict that makes up the majority of the valley infill. A sharp change in gamma count was encountered

350 at a depth of 38 m (295.7 masl), which is consistent with the unit of broken black shale bedrock

351 fragments encountered below the diamict and above the more competent dolostone at the base of GDC-

352 1 (Fig. 5). Apparent resistivity and sonic logs were not collected across the Quaternary due to the

353 presence of a steel casing at GDC-1 and GDC-2B and a low water table at GDC-6 (sonic only).

354 The variable natural gamma responses observed within the bedrock between 288-307 masl in GDC-

355 2B, and between 292-309 masl in GDC-6 indicate increased stratigraphic subdivision; here, the high

356 gamma combined with low resistivity support the presence of a clay-rich unit consistent with the

357 regionally-discontinuous Eramosa Formation (Cole et al. 2009; Brunton 2009), which regionally separates

358 the underlying unsubdivided Amabel (Goat Island and Gasport formations as per Brunton 2009) from the

359 overlying Guelph Formation (absent in this area)Draft (Brunton 2009).

360 The sonic logs show variable changes in P-wave velocity (generally ranging between 2000-7000 m/s)

361 superimposed over more gradual trends. Here, the variable fluctuations reflect changes in mechanical

362 properties (e.g., fracture frequency), while the gradual trends demarcate the lithostratigraphic sequences

363 (e.g., Eramosa, Goat Island and Gasport formations). P-wave velocities of the shallow bedrock were

364 approximately 4500 m/s.

365 FLUTe TM hydraulic transmissivity profiling, as described by Keller et al. (2013), performed in GDC-2B,

366 GDC-1 and GDC-6, were used to compare hydraulic conditions at each of these locations with respect to

367 the BBV. These data illustrate a decrease in the hydraulic transmissivity of the borehole below the

368 bottom of the descending liner, which is expected as the liner seals flow features and reduces remaining

369 open hole transmissivity as it descends. The transmissivity T (and bulk hydraulic conductivity, Kb) of the

370 bedrock between the base of the valley (below the bottom of the casing at GDC-1 or 288 masl) and the

371 top of the Cabot Head shale was highest in GDC-2B, and lowest in GDC-1 (Fig. 5). The bedrock

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-3 2 372 transmissivity south of the valley at GDC-2B was relatively high at the base of the BBV ( T = 1×10 m /s; Kb

373 = 3 ×10 -5 m/s), with few changes in transmissivity, i.e., few inflections in the profile and nearly all the

374 reduction in T occurred at once near the top of the Cabot Head shale. The profile at GDC-1 within the

-5 2 -7 375 valley was comparatively low (T = 1×10 m /s; Kb = 3 ×10 m/s) and exhibited more noticeable declines in

376 transmissivity throughout the bottom of the Gasport compared to GDC-2B before T diminished to a non-

377 measurable value. Whereas the transmissivity at GDC-6 (north of the buried valley) exhibited an

-4 2 -5 378 intermediate value (T = 3×10 m /s; Kb = 1 ×10 m/s), the characteristics of the profile were distinct; for

379 instance a significant drop in T was observed in the middle of the Gasport at 273 masl, approximately 18

380 m above the bottom of the hole and well above the depth of the other two holes. This suggests that the

381 valley bottom did not receive much groundwater discharge from below but rather from the flanks of the 382 valley, with the bedrock on either side of the vallDraftey showing more transmissivity.

383 4.2 Surface electrical resistivity profiles

384 The Quaternary-bedrock contact was defined across the study area using inverted ERT models. This

385 approach is illustrated with the results from Line 10 (ERT-10A/B) (Fig. 6). The interpreted position of

386 bedrock surface (dotted white line) shown on the interpreted geo-electrical model was set according to

387 the largest upward electrical gradient in the inverted model section (i.e., approximately where the

388 resistivity exceeded 600 ohm ⋅m). Here, the upper zone is interpreted as less resistive Quaternary

389 deposits, whereas the lower zone represents the more resistive bedrock. It is important to note that

390 because the ERT model defines the spatial extent of ‘electrically similar’ material, it is possible for

391 unconsolidated materials to exhibit similar electrical properties to bedrock and vice versa (Reynolds,

392 2011). Further, the inversion process can result in resistivity artifacts, particularly around the sides and

393 bottom of the section that limits interpretability in those regions.

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394 Line 10 was collected using a 10 m electrode spacing and orientated such that it was orthogonal to

395 the suspected valley structure noted in lines 1-5 (Fig. 7a-e). These electrical data indicate a wide bedrock

396 depression or valley structure between 480 and 780 m, with a shallower valley-wall on the north-slope;

397 the interpreted bedrock surface is consistent with observations at GDC-1, GDC-2B and GDC-6. However,

398 it is apparent that the interpreted bedrock elevation of the valley floor is slightly elevated relative to GDC-

399 1. This is likely the result of a layer of bedrock rubble logged along the valley floor, which is electrically

400 more similar to the underlying competent bedrock than the overlying diamict . In addition, inverted

401 apparent resistivity values obtained between 0 to 250 m along line 10 within the interpreted bedrock

402 zone are lower than expected based on adjacent areas. These low values could be interpreted as a

403 decrease in bedrock elevation and/or another valley feature; however, available borehole data from Old 404 #8 (Table 1), approximately 138 m eastward intercepDraftts a shallow bedrock surface in that area.

405 A closer inspection of the measured apparent resistivity data reveals a resistivity decrease between

406 120 to 160 m, which extends down through the pseudo-section (upper image in Fig. 6). This response

407 coincides with the position of a perennial storm water pond (refer to Fig. 4), which collects runoff from a

408 parking lot and nearby roadway ditches during spring snowmelt. This pond might represent an area of

409 focused surface water infiltration with possibly higher total dissolved solids (e.g., road salt), which might

410 explain the relatively lower bedrock resistivity between 0-250 m (middle image in Fig. 6). Given the large

411 lateral offset of Old #8 east of the transect it is also possible that the resistivity response along the south

412 end of line 10 is indicative of a bedrock valley, corresponding to the southern edge of the valley identified

413 at the southern extent of lines 8 and 9; however, data coverage is limited in this area.

414 The bedrock surface can generally be identified across the study area using the 600 ohm ⋅m threshold in

415 the 10 m spaced resistivity profiles (Fig. 7). Occasionally, the bedrock surface was interpolated in areas of

416 low model sensitivity near the boundaries of the model (e.g., 7b, f, g, h) and/or where there was slight

417 disagreement between the delineation of the resistivity threshold and a nearby borehole (e.g., 7d, I, k, l).

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418 In those instances, available borehole information and careful interpretation of subtle electrical trends in

419 the apparent resistivity data sets were used to define the bedrock surface. Lower-resistivity trends,

420 marked by the green coloured zones in the inverted model, appear to delineate a valley floor consistent

421 with expected bedrock elevation; here, interpretation of the bedrock surface was more tentative given

422 the low electrical contrast likely attributed, in part, to a change in the Quaternary sediment (i.e.,

423 transition from predominantly conductive infill to relatively resistive infill or bedrock fragments).

424 The interpreted bedrock surface reveals numerous bedrock mound (bm) features (Fig. 7). These

425 smaller-scale (10’s of m) features suggest variable bedrock relief outside the valley. To better image these

426 features a series of higher-resolution resistivity surveys were conducted using 5 m electrode spacings

427 (Fig. 8). Resulting models also improved characterization of Quaternary deposits outside and within

428 portions of the valley structure, and led to a moreDraft accurate depiction of the bedrock mounds. Highly

429 variable Quaternary deposition is evident across these two profiles with sand and gravel-type units

430 (glaciofluvial) deposited within broader clay to sandy/silt clay-rich units (till, debris flow or

431 glaciolacustrine deposits) within the upper part of the succession. These higher resolution data also

432 define the geometry of the bedrock mound (bm) features (<50 m in diameter). Although reef mound

433 facies are present in the lower half of the Guelph Formation and the upper Gasport Formation (Brunton

434 2009), it is not known whether these apparent bedrock mounds represent exposed reef lithofacies

435 (residing above the Eramosa), or are simply elongated glacial erosional features. Nevertheless, these

436 mound features were identified throughout the broader study area outside the BBV (Fig. 7 and 8); thus,

437 these features are not considered to be an artifact of the inversion process.

438 4.3 Seismic refraction surveys

439 Bedrock depth estimates based on seismic refraction across the study area ranged from 9 to 20 m

440 bgs (Table 2), and were broadly consistent with ERT-based bedrock elevations. Results from two surveys

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441 carried out on the north-side of the study site are presented in Fig. 9 as a standard delay-time analysis

442 with off-end shots used to extrapolate the bedrock refraction to zero-offset (Redpath, 1973), and as an

443 inverse model (Sandmier 2012). Reported P-wave velocities in dolostones can range from 2600-6500 m/s

444 (Reynolds, 2011). Based on the range of velocities and smoothed nature of the tomographic models, and

445 measured sonic velocities within the Upper Eramosa Formation (~4500 m/s), a bedrock threshold velocity

446 of 3200 m/s was used to delineate the bedrock surface. The position of the interpreted bedrock surfaces

447 differ significantly between the two methods with the standard delay-time method consistently providing

448 ~5 m shallower depths to bedrock (Table 2) and the tomographic inversion providing a smoothed 2D

449 velocity model (Fig. 9). Although tomographic analyses will have a tendency to smooth sharp interfaces

450 (e.g., the contact between unconsolidated sediments and bedrock), the benefit of this method is its 451 ability to represent actual velocity gradients, whiDraftch, if significant, could impact the top of rock calculated 452 using the delay-time method. However, the absence of distinct boundaries in the 2D model means that a

453 more subjective analysis of the data is needed to mark the depth and thickness of geologic layers.

454 Refraction data along line 1 (SR-1A/B; Fig. 9a) and line 2 (SR-2A/B; Fig. 9b) (locations shown in Fig. 4)

455 was collected over an interpreted bedrock mound (bm) feature. The velocity models indicate a zone of

456 relatively lower velocity (3200-3800 m/s) extending downward within the bedrock between 308-316.5

457 masl (between 30-50 m along line 1 and 35-50 m along line 2). Its geometry is consistent with the

458 mounded feature identified in the resistivity data (Fig. 8a) over the same area. These lower velocities

459 suggest that the bedrock mounds are composed of less dense material compared to adjacent rock and

460 that they may be exposed reef mounds; however, further study is required to better constrain their

461 composition and origin. In summary, based on the corroborating evidence provided by the seismic

462 refraction survey and geological borehole data, the resistivity method has effectively identified the

463 bedrock surface across the study site, highlighting complex bedrock morphology and sediment

464 architecture.

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465 4.4 Interpolated buried bedrock valley model

466 We constructed a high-resolution, local-scale bedrock elevation model using ERT-derived bedrock

467 elevation data. 2D elevation profiles were interpolated using the kriging method with a coarse 125 m

468 grid-discretization; this grid size corresponds to approximately one-half the maximum ERT line

469 separation. Fig. 10a-c shows the topography from a regional 10 x 10 m digital elevation model (DEM),

470 spatially-interpolated bedrock elevations from the ERT data, and calculated overburden thicknesses (DEM

471 elevation minus ERT-bedrock elevation distribution); these data were used to construct a quasi-3D

472 conceptualization of the BBV (Fig. 10d). While the coarser grid discretization ensured a smooth

473 interpolation between adjacent transects reducing potential interpolation artifacts, the coarser

474 discretization ignored small-scale variations in bedrock elevation (e.g., bedrock mounds interpreted in

475 Figs. 5 and 6 are not captured in this model). Draft

476 These data indicate an east-west trending valley approximately 150 to >300 m wide and 27 to 51 m deep

477 (Fig. 11a). The valley geometry is narrow with an undulating channel base. The valley floor rises in

478 elevation from west to east, displaying an abrupt bend 150 m east of GDC-1 with a locally elevated

479 bedrock floor, before terminating east of the Taylor Center well. The valley incises the upper Goat Island

480 formation, exposing the full Eramosa Formation to the Quaternary infill (Fig. 11b). Based on the regional-

481 scale bedrock elevation data (Gao et al. 2006, Fig. 3 and 4), this bedrock valley is a tributary of the

482 regional Guelph valley mapped by Cole et al. (2009).

483 5.0 DISCUSSION

484 5.1 Buried bedrock valley: morphology and origin The development of BBVs can be affected by

485 the regional tectonic setting, structural elements and basin evolution, pre-glacial paleoenvironmental

486 conditions and the nature of glaciation in the area and associated changes in glacial hydraulics and

487 relative base level. In previously glaciated terrains, BBVs can be the result of a combination of pre-glacial,

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488 subglacial, and meltwater (proglacial or subglacial) erosional processes. In areas where BBVs are thought

489 to have been created in a subglacial environment (i.e., tunnel valleys), the relative importance of direct

490 glacial erosion and subglacial meltwater erosion as well as the steady-state versus catastrophic nature of

491 the meltwater erosion is often the subject of considerable debate (Jørgensen and Sandersen 2006;

492 Kehew et al. 2012).

493 BBVs in southern Ontario have been identified along broad valleys below escarpments, as

494 escarpment re-entrant and as individual or dendritic networks of channels developed across the

495 Paleozoic bedrock surface, with a combination of pre-glacial and glacial erosional processes and regional

496 tectonic stresses contributing to their development (Eyles et al. 1997; Russell et al. 2007; Gao 2011). As

497 elsewhere in Europe and North America, catastrophic meltwater erosion has been proposed for some

498 bedrock erosional forms associated with escarpmentsDraft northwest of the study area (Tinkler and Stenson

499 1992; Kor and Cowell 1998). Recent review papers have highlighted the controversial aspects of these

500 interpretations (Jørgensen and Sandersen 2006; Kehew et al. 2012), which are difficult to resolve for

501 BBVs in Ontario with current data sets. Additional studies of BBVs and their infill will help to more clearly

502 identify the relative importance of various erosional processes on the landscape.

503 The BBV in this study is an example of an individual channel segment developed on the Paleozoic

504 bedrock surface that connects to a dendritic network of channels, namely the Guelph and Rockwood BBV

505 west of the Niagara Escarpment as indicated by the bedrock surface of Gao et al. (2006), and to the

506 Dundas valley further to the south (Eyles et al. 1997) (Fig. 3). The undulating morphology of the valley

507 floor, sudden change in valley axis direction and abrupt termination (Figs. 8 and 9) suggest Pleistocene

508 subglacial meltwater erosion was a significant process in the development of this BBV (Jørgensen and

509 Sandersen 2006; Gao 2011). It is difficult to assess the relative importance of other erosional processes

510 (proglacial meltwater, direct glacial and non-glacial) at this site with the current dataset, though all of

511 these processes are likely to have contributed at least in part considering the dendritic appearance of the

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512 channel network (Eyles et al. 1997), the multiple advances and retreats of ice in the area, the presence of

513 6 m of bedrock rubble at the base of the channel in GDC-1 (see next section) and the long period of

514 geological time represented by the regional unconformity at the sediment-bedrock interface.

515 5.2 Nature and origin of Quaternary valley infill

516 The valley in this study is primarily infilled with a thick (10’s of m) package of diamict, and

517 occasional laterally discontinuous coarse-grained lithologies at the top and bottom of the succession; this

518 is evident in the detailed log of GDC-1 (Fig. 5) and the borehole record of the Arboretum Centre well

519 (Table 1), and is further supported by the natural gamma log in GDC-1 (Fig. 5) and the nature of the

520 inverted modeled resistivity in the multiple ERT transects (Figs. 4, 5, 6). Most transects across the valley

521 are characterized by well-defined low resistivity zones (approximately <250 ohm m; Fig. 7), with several

522 transects showing laterally discontinuous unitsDraft of higher resistivity attributed to coarser grained materials

523 (>700-2000 ohm ⋅m; Fig. 7c, k, l) in the upper 10 m of the succession. Higher resolution ERT surveys (Fig.

524 8) captured more variability in the resistivity of the infill with slightly higher resistivity values of up to 500

525 ohm ⋅m; perhaps recording some of the textural variability within the thick package of diamict noted in

526 the detailed log of GDC-1, or the spatial variability in the infill sediment as noted in Fig. 7f, g, i.

527 Valley infills of BBVs tend to be variable (Kehew and Boettger 1986; Jørgensen and Sandersen

528 2006; Russell et al. 2007; Cummings et al. 2012; Szabo et al. 2013; Seyoum and Eckstein 2014). Russell et

529 al. (2004) classified valley infills as either: i) complex with variable lithologies and unconformities

530 recording a series of depositional and erosional events before and during several glaciations, or ii) simple

531 with one or two dominant lithologies (e.g., basal gravel capped with diamict or silt/clay), primarily

532 recording one event or a relatively short period of time. Some valley infills contain significant coarse-

533 grained colluvium along the sides of the valley, which interfinger with relatively finer-grained infill (Kehew

534 and Boettger 1986; Jørgensen and Sandersen 2006; Kehew et al. 2012; Lajeunesse 2014), suggesting mass

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535 movement was significant during a protracted period of valley infilling. In other instances, complex valley

536 infill of BBVs can include sediment hosted buried valleys (Jørgensen et al. 2003; Russell et al. 2004),

537 exhibiting lateral variability in sediments and architecture along the valley axis (Russell et al. 2004; Zweirs

538 et al. 2009; Burt 2011; Cummings et al. 2012). Recent studies in Ontario have shown that many BBVs

539 contain complex successions of sediments recording periods of erosion and associated unconformities,

540 multiple changes in depositional conditions over time, including changes from stadial to interstadial

541 conditions and in some cases multiple glacial periods (Meyer et al. 1997; Davies et al. 2008; Zweirs et al.

542 2009; Bajc et al. 2012). The nearby Rockwood BBV contains a record of two separate ice advances

543 (Catfish Creek and Canning Till) and a thick package of Lower Erie Phase predominantly sandy stratified

544 sediments (Burt 2011, 2012).

545 Although this is perhaps due to its sizeDraft and the limited high-resolution continuous core data

546 available at this time, the BBV infill in this study is comparatively simple (Fig. 11b) as the valley infill is

547 dominated by one thick till (28 m thick out of a total borehole depth of 43 m) attributed to Catfish Creek

548 ice (Nissouri phase). The 6 m of basal gravel that underlies this till consists of black shale fragments or

549 rubble, which most likely came from the Vinemount Member (lower Eramosa Formation) during

550 preglacial times following fluvial incision and/or during glacial times as a result of glacial erosion and

551 weathering of exposed bedrock, prior to its infill with Catfish Creek Till. Part of this rubble may be

552 colluvium from adjacent valley walls (Kehew and Boettger 1986) and/or part of it may result from in-situ

553 weathering of the bedrock valley floor as is evident in the modern Eramosa River. Additional subsurface

554 data within the valley to delineate the spatial distribution of this rubble is needed to confirm which of

555 these origins is most likely, as well as the timing of rubble deposition. At this time, the deposition of 6 m

556 of rubble and the erosional event that created the BBV is interpreted to predate the Catfish Creek ice

557 advance in the region prior to the Michigan Subepisode (Nissouri Phase), as is the case in the nearby

558 Rockwood BBV (Fig. 2; Burt 2011, 2012).

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559 Other minor lithologies overlying the thick Catfish Creek Till both within the valley and adjacent

560 to it, record ice marginal and glaciofluvial conditions. Stratigraphically, the clay and clay-rich diamict

561 between 4 and 8.5 m at the top of the succession in GDC-1 is tentatively correlated to the Maryhill Till of

562 Karrow (1987) based on its clay-rick texture and its stratigraphic position. Its depositional origin is

563 difficult to establish with this limited borehole data; the Maryhill Till has been interpreted as a till or a

564 product of sediment gravity flows in a glaciolacustrine environment (Karrow 1987). Although both the

565 clay and clay-rich diamict were both stiff, which could result from overconsolidation by overlying ice,

566 these sediments are laterally associated with sand and gravel interpreted as glaciofluvial deposit (Fig. 7.

567 and 6), such that accumulation in a subaqueous environment as a result of ponding in an ice marginal

568 environment is also plausible. Further studies that characterize the distribution of these fine grained 569 sediments and the interface between them andDraft the adjacent coarse-grained facies are needed to resolve 570 the depositional origin of the clay-rich interval in GDC-1.

571 The uppermost gravel unit in GDC-1 is associated laterally with the upper brown coloured diamict

572 in GDC-2B (Fig. 5, 2-8 m) and in the other water supply wells in the area (Table 1). The upper diamict in

573 GDC-2B has been interpreted as a debris flow. Laterally, the ERT transects (e.g., Fig. 7 and 8) provides

574 evidence that the upper 10 m of valley infill are locally comprised of discontinuous sand and gravel, which

575 likely record glaciofluvial conditions. This lateral facies association of debris flow and glaciofluvial

576 sediment in the valley infill, together with evidence of similar facies in laterally equivalent sediments

577 outside of the valley (Fig. 5, GDC-2B and GDC-10A) and ERT transects (line 3 in Fig. 7c and lines 12 and 13

578 in Fig. 8) are all consistent with an ice marginal setting associated with either Maryhill or Port Stanley ice

579 (Port Bruce Phase; Fig. 11b).

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580 5.3 Buried bedrock valleys and underlying aquifer conceptualization

581 A conceptual model for karst formation around BBVs in the Guelph area was proposed by Cole et

582 al. (2009). They observed a spatial relationship between productive municipal water supply wells

583 completed at ~60 m depth and proximity to the regional BBV. In their model, valley incision resulted in

584 steep hydraulic gradients, allowing (chemically) “aggressive” water to reach greater depths, enhancing

585 dissolution of vertical joints, fractures and conduits along flow paths, and ultimately discharging that

586 water at the base of the valley (Cole et al. 2009). Deposition of low-permeability glacial sediments during

587 subsequent ice advances reduced groundwater flow and discharge along the valleys, limiting the rate of

588 bedrock dissolution. They hypothesized that these dissolution features were responsible for the

589 increased production capacity of municipal supply wells near the BBV.

590 If we apply the Cole et al. (2009) modelDraft to this study, dissolution-enhanced features would have

591 developed as shallow and intermediate groundwater flowed through the Guelph and Eramosa

592 Formations towards the valley floor (Goat Island formation), prior to the valley being infilled with low-

593 permeable clay and clay rich diamict attributed to Catfish Creek ice (pre-Nissouri phase of the Michigan

594 Subepisode) (Fig. 11b). Whereas some of these pathways may be dissolution-enhanced features as

595 hypothesized by Cole et al (2009), most of the outcrop and cored hole data to date suggest well-

596 connected fracture networks are more significant (Perrin et al. 2011). Determining the relative

597 importance, and lateral and vertical connectivity of these pathways, is the subject of ongoing research as

598 it will likely impact flow paths and time of travel through the Guelph, Eramosa and Goat Island

599 formations, and consequently, the spatial distribution of recharge to the primary Gasport formation

600 aquifer.

601 In a separate study of GDC-6, dissolution-enhanced channelling features (vuggy conduits and

602 fractures) were observed in core and video logs primarily within two intervals (267-268 and 272-275

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603 masl, Fomenko 2015); these zones lie within the Gasport formation, roughly 17-25 m below the

604 interpreted bedrock valley floor. During the development of GDC-6 and adjacent boreholes, when

605 pressurized air was released from a compressor at the base of the Gasport formation (top of Cabot Head

606 shale), circulation was observed at the surface at other wells located 7.5-15 m away. This strong

607 transmissivity supports the existence of karst features and well-connected open fractures within the

608 Gasport formation that has been documented locally and regionally (e.g., Cole et al. 2009; Brunton 2009).

609 The average production zone interval across the City of Guelph based on data from 18 municipal supply

610 wells (as presented in Cole et al. 2009) was 268-278 masl; this broad regional zone encompasses the two

611 karst zones observed in GDC-6. The bulk of the dissolution that occurred within the Gasport may be the

612 result of relatively deeper groundwater flow discharging into the floor of the regional BBV system (Fig. 4), 613 if and where it intersects the Gasport formationDraft (i.e., conduit feature elevation) further down the BBV 614 thalweg. The presence of rubble at the base of the valley, if laterally extensive, would also be an

615 important transmissive feature in this three-dimensional flow system.

616 Further study of the distribution, morphology and nature of the Quaternary infill of these BBVs

617 relative to the underlying bedrock stratigraphy and hydrogeology is required to more fully understand

618 the relationship between BBVs, preferential pathways, recharge distribution and high capacity municipal

619 production wells. A better understanding of these aspects of the groundwater flow system will in turn

620 improve our characterization of groundwater residence time and capture zone geometries that are

621 central to groundwater resource management and source water protection.

622 6.0 CONCLUSIONS

623 This study demonstrates the synergistic integration of several surface geophysical methods

624 combined with sedimentological analysis of continuous cores and hydraulic data for characterization of

625 the impact of buried bedrock valleys (BBV) and their Quaternary infills on the present-day bedrock

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626 groundwater flow system. Similar to larger regional valleys, these local-scale features can be defined by

627 complex channel morphology and Quaternary infill, requiring a suite of tools at scales pertinent to

628 localized studies of municipal production well recharge zones and contaminant transport and fate.

629 The primary goal of this study was to demonstrate the capabilities and limitations of the electrical

630 resistivity tomography (ERT) technique for mapping a buried bedrock valley at a relatively local-scale.

631 Here, the method was used to delineate a bedrock surface beneath Quaternary deposits ranging from a

632 few meters to >50 m thick. ERT-derived bedrock depths were confirmed through refraction surveys and

633 available borehole logs. Although the ERT provided reliable estimates of bedrock depth, the existence of

634 a rubble layer along the valley floor may have led to an underestimation of the valley-floor extent. While

635 the ERT was generally effective at differentiating between the clay-rich diamict and underlying dolostone

636 at a local-scale, available borehole informationDraft did aid our interpretation of the bedrock interface in areas

637 near the edges and base of the models, or where Quaternary infill changed from clay-rich sediment to

638 coarser grained sand/gravel deposits near the bedrock surface.

639 The bedrock valley within the study area is thought to be a tributary to the Guelph and Rockwood

640 valleys west of the Niagara Escarpment and the Dundas valley to the south. The valley appears to

641 originate in the northeast corner of the study site, where it trends southwestward, widening and

642 deepening as it connects to the Eramosa River (current exposed section of the Guelph BBV). The width of

643 the valley ranged from 150 to >300 m in the southwest corner. The highly uneven and complex nature of

644 this valley morphology is consistent with subglacial meltwater erosion, though it is likely that other

645 erosional processes (proglacial meltwater, direct glacial erosion or pre-glacial fluvial incision) contributed

646 to its development.

647 A comprehensive analysis of unpublished geotechnical reports and hydraulic data sets within the

648 Guelph area by Gautrey (2004) revealed a statistical relationship between high capacity groundwater

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649 production wells and proximity to BBVs. This increase in production was attributed to the presence of

650 dissolution-enhanced fractures and karst conduit features that formed through progressive bedrock

651 incision prior to being covered by low-permeability Quaternary deposits (Cole et al. 2009). Our study

652 showed consistently higher bedrock transmissivity outside the BBV and relatively lower transmissivity

653 beneath the valley between the base of the valley floor and the top of the regional aquitard. These

654 differences in bulk hydraulic property may be explained by an increase in fracture connectivity and/or

655 karst conduit formation as deep groundwater discharged in the valley floor prior to being infilled with

656 Quaternary deposits.

657 The valley in this study is primarily infilled with a thick (10’s of m) package of diamict attributed

658 to Catfish Creek ice (Nissouri phase), and occasional laterally discontinuous coarse-grained lithologies in

659 the top and bottom of the succession. One otherDraft diamict unit within the succession is tentatively

660 correlated to the Maryhill Till (Port Bruce phase). Observed lateral facies association of debris flow and

661 glaciofluvial sediment in the uppermost valley infill, together with evidence of similar facies in laterally

662 equivalent sediments outside of the valley, and nearby ERT transects are all consistent with an ice

663 marginal setting associated with the ice retreat phase. The spatial distribution and geometry of BBVs and

664 their variable infill may play a role in the transmissivity characteristics of the incised bedrock formations.

665 The lateral and vertical connectivity of fractures and their 3D connectivity to karst conduit and

666 dissolution-enhanced features will likely impact the spatial distribution of recharge and groundwater flow

667 lines, ultimately affecting groundwater traveltimes and capture zone delineation.

668

669 ACKNOWLEDGEMENTS

670 This research was made possible through funding by the Ontario Research Fund (ORF-RE:

671 Sustainable Bedrock Water Supplies for Ontario Communities) grant to Drs. Beth Parker and Emmanuelle

672 Arnaud, as well as Southern Ontario Water Consortium (SOWC) and Canadian Foundation for Innovation

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673 (CFI) grants secured by Dr. Beth Parker. A NSERC PDF and NSERC Banting Fellowship supported Dr. Colby

674 Steelman. The authors would like to thank the many students and staff of G360 who helped with data

675 acquisition.

676

Draft

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931 emerging tools. Geomorphology, 137: 57–73. doi: 10.1016/j.geomorph.2010.09.005.

932 van der Kamp, G., and Maathuis, H. 2012. The unusual and large drawdown response of buried-valley

933 aquifers to pumping. Groundwater, 50 : 207–215.

934 Wiederhold, H. 2009. Buried valley aquifer systems. In Kirsch R, Groundwater Geophysics: A tool for

935 hydrogeology. Second Edition, Germany, Springer, pp. 398-413. 936 White, W.B. 1988. Geomorphology and hydrologyDraft of karst terrains. Oxford University Press, New York. 937 Zweirs, W.G., Rainsford, D.R.B., and Bajc, A.F. 2008. Investigation of the Dundas buried bedrock valley. In

938 Summary of Field work and other Activities, 2008. Ontario Geological Survey, Open File Report

939 6226, pp. 33–1 to 33–7.

940 Zweirs, W.G., Bajc, A.F., Priebe, E.H., Marich, A.S. 2009. Investigation of the Dundas buried bedrock

941 valley, Southern Ontario. In Summary of Field work and other Activities, 2009. Ontario

942 Geological Survey, Open File Report 6240, pp. 23–1 to 23–7.

943

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944 Tables

945

946

947 Table 1. Lithologic descriptions from on-site water supply wells.

Well ID Quaternary Quaternary Material † Bedrock Upper Bedrock Thickness Elevation Material † m m

Hilton 15.8 0-15.5 m : Clay with gravel 314.5 brown Center limestone Old #8 15.2 0-7.6 m : brown clay with stones 320.8 brown rock 7.6-15.2 m : brown clay with sand- gravel

Arboretum 39.3 0.3-8.5 m : brown clay with small 299.7 dark grey rock Center stones 8.5-29.0 m : gray clay with small stones 29.0-39.3 m: gray clay withDraft sand Taylor 15.2 0.3-9.1 m : brown clay with sand- 317.8 dark brown Center stones rock 9.1-15.2 m : gray clay with sand- stones

948 †According to drillers description of Quaternary and bedrock deposits.

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949

950

951

952 Table 2. Summary of seismic refraction results.

Tomographic Survey Geophones Spread Length Refractions Delay-Time (Plus-Minus) Inversion [m] Refractor Velocity Calculated Depth Interpreted Depth † [m/sec] [m bgs] [m bgs] 1 2 3 Bedrock 1 2 Bedrock Bedrock L-1 24 85 2 169 - 177 692 - 674 -- 5020 - 5320 1.0 -- 7 – 11 13 – 15 L-2 24 85 3 186 -186 454 - 809 1087 - 1410 4822 - 5091 0.9 4 9 – 15 12 – 15 L-3 48 190 3 99 - 152 491 - 671 720 - 987 5180 - 5278 0.7 3.1 8 – 11 16 – 20 L-4 48 158 2 329 - 381 Draft 869 - 1032 -- 4959 - 5203 3.3 -- 9 – 12 16 – 19 L-5 48 190 2 227 - 484 2409 - 2776 -- 5012 - 5176 2.2 -- 10 – 19 11 – 14 L-6 48 87.5 2 189 - 270 457 - 1244 -- 4680 - 4759 1.3 -- 4 – 7 11 – 12 L-7 48 87.5 2 219 - 300 1107 - 1582 -- 4336 - 4474 1.6 -- 4 – 8 9 – 11

953 †Based on a Quaternary-bedrock velocity transition of 3200 m/s.

954

955

956

957

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958 Figure Captions

959 Fig. 1. Lower and Middle Silurian traditional stratigraphic nomenclature for Guelph area based on Armstrong and 960 Carter (2010) and with proposed revisions by Brunton (2009). *Cabot Head Shale of the Cataract Group.

961 Fig. 2. Regional stratigraphy in the Guelph area based on mapping by Karrow (1987) and stratigraphy of the 962 nearby Rockwood buried bedrock valley based on borehole logging and regional mapping by Burt and Dodge 963 (2016); refer to Fig. 3 for location of Rockwood buried bedrock valley.

964 Fig. 3. Regional bedrock elevation showing major buried bedrock valley features across southern Ontario (Gao et 965 al. 2006). The study area within the City of Guelph examines a branch of the Guelph buried valley, which is an 966 extension of the larger Rockwood valley to the northeast toward Toronto. Coordinate system: NAD83 UTM zone 967 17N.

968 Fig. 4. Study area showing extent of geophysical transects, research boreholes and water supply wells owned 969 and operated by the University. Inset map shows areal extent of geophysical study area southeast of the 970 Eramosa River within the City of Guelph. Cole et al. (2009) identified multiple buried bedrock valleys within the 971 city limits, which they denoted as the Guelph buried bedrock valley; the primary Guelph valley connects to the 972 Rockwood buried valley to the northeast. Coordinate system: NAD83 UTM zone 17N.

973 Fig. 5. Stratigraphic Quaternary and bedrock logsDraft from three locations across the buried valley feature: GDC-2B 974 (south of valley); GDC-1 (within valley); GDC-6 and GDC-10A (north of valley). Downhole geophysical logs 975 include natural gamma, induced electrical resistivity and P-wave velocity; additional FLUTe™ transmissivity 976 profiles were performed within the open bedrock coreholes. The triangle symbol along the transmissivity 977 profiles indicates the transmissivity (T) of the bedrock between bottom of the valley (~288 masl) and the top of 978 the Cabot Head shale. The term *broken rock refers to either mechanically pulverized bedrock (GDC-10A) or 979 broken rock fragments (GDC-1). Note that the Amabel Formation is shown here with local informal subdivisions 980 of Irondequoit, Gasport and Goat Island formations as proposed by Brunton (2009).

981 Fig. 6. ERT results from line 10 using a 10 m electrode spacing show measured apparent resistivity pseudo- 982 section (top), inverted resistivity model (middle) and interpreted geo-electrical model (bottom). Long dotted line 983 on interpreted model (250-950 m position) depicts largest resistivity gradient (i.e., interpreted as Quaternary- 984 bedrock boundary), while the short dotted line (0-250 m position) represents an interpreted boundary based on 985 geo-electrical trends and available borehole logs. Given the lateral offset of Old #8 it is possible that the 986 resistivity response from 0-250 m identifies the edge of a valley feature that extends southwestward from lines 987 8 and 9. Core logs are superimposed to show measured unconsolidated sediment thickness (white) and depth of 988 bedrock (black); orthogonal offsets of core logs from resistivity transect are shown as “x” for into and “o” for out 989 the of 2D resistivity plane.

990 Fig. 7. Summary of 10 m ERT resistivity images with interpreted single layer over half-space models. Nearby logs 991 with measured depth to rock are shown with corresponding orthogonal offsets represented as “x” for into and 992 “o” for out of the two-dimensional plane. Results include (a-e) lines 1-5 orientated in a southwest-to-northeast 993 direction and (f-l) lines 7-11, 14 and 15 orientated in a south-to-north direction.

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994 Fig. 8. Higher-resolution ERT transects with 5 m electrode spacing corresponding to (a) line 13 and (b) line 12. 995 Resistivity models show spatially variable Quaternary deposits and an undulating bedrock surface with abrupt 996 ridges denoted as bedrock mounds (bm).

997 Fig. 9. Results of seismic refraction measurements collected along (a) SR-1 and (b) SR-2 with delay-time 998 interpretation (top) and tomographic inversion models (bottom). The + symbols on the tomographic inversion 999 corresponds to estimated bedrock depths using delay-time technique, while dotted line represents the 1000 interpreted bedrock interface based on a velocity threshold of 3200 m/s.

1001 Fig. 10. Conceptualization of buried bedrock valley based on (a) 10 x 10 m provincial digital elevation model 1002 (DEM) (Ontario Ministry of Natural Resources, 2006), (b) ERT-derived bedrock elevation model (black lines 1003 represent the location of the ERT transects) and (c) calculated Quaternary isopack map. (d) Vertically 1004 exaggerated bedrock valley shows a complex and hummocky channel bottom with abrupt changes in valley 1005 strike, which connects to the Guelph buried valley to the southwest and an abrupt termination towards the east. 1006 Coordinate system: NAD83 UTM zone 17N.

1007 Fig. 11. (a) Resistivity derived bedrock elevations showing extent of buried bedrock valley within the study area 1008 (coordinate system: NAD83 UTM zone 17N) with (b) cross-sectional view of the bedrock morphology and 1009 Quaternary infill along transect A-A’ based on resistivity and stratigraphic logs. Note that the Amabel Formation 1010 is shown here with local informal subdivisions Draftof Irondequoit, Gasport and Goat Island formations as proposed 1011 by Brunton (2009). The buried valley cuts into the upper Goat Island formation.

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Fig. 1. Lower and Middle Silurian traditional stratigraphic nomenclature for Guelph area based on Armstrong and Carter (2010) and with proposed revisions by Brunton (2009). *Cabot Head Shale of the Cataract Group.

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Fig. 2. Regional stratigraphy in the Guelph area based on mapping by Karrow (1987) and stratigraphy of the nearby Rockwood buried bedrock valley based on borehole logging and regional mapping by Burt and Dodge (2016); refer to Fig. 3 for location of Rockwood buried bedrock valley.

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Fig. 3. Regional bedrock elevation showing major buried bedrock valley features across southern Ontario (Gao et al. 2006). The study area within the City of Guelph examines a branch of the Guelph buried valley, which is an extension of the larger Rockwood valley to the northeast toward Toronto. Coordinate system: NAD83 UTM zone 17N.

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Fig. 4. Study area showing extent of geophysical transects, research boreholes and water supply wells owned and operated by the University. Inset map shows areal extent of geophysical study area southeast of the Eramosa River within the City of Guelph. Cole et al. (2009) identified multiple buried bedrock valleys within the city limits, which they denoted as the Guelph buried bedrock valley; the primary Guelph valley connects to the Rockwood buried valley to the northeast. Coordinate system: NAD83 UTM zone 17N.

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Fig. 5. Stratigraphic Quaternary and bedrock logs from three locations across the buried valley feature: GDC-2B (south of valley); GDC-1 (within valley); GDC-6 and GDC-10A (north of valley). Downhole geophysical logs include natural gamma, induced electrical resistivity and P-wave velocity; additional FLUTe™ transmissivity profiles were performed within the open bedrock coreholes. The triangle symbol along the transmissivity profiles indicates the transmi ssivity (T) of the bedrock between bottom of the valley (~288 masl) and the top of the Cabot Head shale. The term *broken rock refers to either mechanically pulverized bedrock (GDC-10A) or broken rock fragments (GDC-1). Note that the Amabel Formation is shown here with local informal subdivisions of Irondequoit, Gasport and Goat Island formations as proposed by Brunton (2009).

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Fig. 6. ERT results from line 10 using a 10 m electrode spacing show measured apparent resistivity pseudo- section (top), inverted resistivity model (middle) and interpreted geo-electrical model (bottom). Long dotted line on interpreted model (250-950 m position) depicts largest resistivity gradient (i.e., interpreted as Quaternary-bedrock boundary), while the short dotted line (0-250 m position) represents an interpreted boundary based on geo-electrical trends and available borehole logs. Given the lateral offset of Old #8 it is possible that the resistivity response from 0-250 m identifies the edge of a valley feature that extends southwestward from lines 8 and 9. Core logs are superimposed to show measured unconsolidated sediment thickness (white) and depth of bedrock (black); orthogonal offsets of core logs from resistivity transect are shown as “x” for into and “o” for out the of 2D resistivity plane.

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Fig. 7. Summary of 10 m ERT resistivity images with interpreted single layer over half-space models. Nearby logs with measured depth to rock are shown with corresponding orthogonal offsets represented as “x” for into and “o” for out of the two-dimensional plane. Results include (a-e) lines 1-5 orientated in a southwest- to-northeast direction and (f-l) lines 7-11, 14 and 15 orientated in a south-to-north direction.

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Fig. 8. Higher-resolution ERT transects with 5 m electrode spacing corresponding to (a) line 13 and (b) line 12. Resistivity models show spatially variable Quaternary deposits and an undulating bedrock surface with abrupt ridges denoted as bedrock mounds (bm).

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Fig. 9. Results of seismic refraction measurements collected along (a) SR-1 and (b) SR-2 with delay-time interpretation (top) and tomographic inversionDraft models (bottom). The + symbols on the tomographic inversion corresponds to estimated bedrock depths using delay-time technique, while dotted line represents the interpreted bedrock interface based on a velocity threshold of 3200 m/s.

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Fig. 10. Conceptualization of buried bedrock valley based on (a) 10 x 10 m provincial digital elevation model (DEM) (Ontario Ministry of Natural Resources, 2006), (b) ERT-derived bedrock elevation model (black lines represent the location of the ERT transects) and (c) calculated Quaternary isopack map. (d) Vertically exaggerated bedrock valley shows a complex and hummocky channel bottom with abrupt changes in valley strike, which connects to the Guelph buried valley to the southwest and an abrupt termination towards the east. Coordinate system: NAD83 UTM zone 17N.

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Fig. 11. (a) Resistivity derived bedrock elevations showing extent of buried bedrock valley within the study area (coordinate system: NAD83 UTM zone 17N) with (b) cross-sectional view of the bedrock morphology and Quaternary infill along transect A-A’ based on resistivity and stratigraphic logs. Note that the Amabel Formation is shown here with local informal subdivisions of Irondequoit, Gasport and Goat Island formations as proposed by Brunton (2009). The buried valley cuts into the upper Goat Island formation.

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