Geological and geophysical studies of buried valleys and their fills near Elora and Rockwood, Ontario

Abstract

Buried bedrock valleys are of economic interest for groundwater and engineering studies, and of scientific interest for the sedimentary sequences they may contain. Two buried valleys, about 60 m deep, north and northeast of Guelph, Ontario, are tributary to the major Dundas Valley southwest of Guelph. Located initially from water well data, their locations and geometry were refined with surface geophysics, including very low frequency resistivity, terrain conductivity, resistivity, transient electromagnetic, gravity, and seismic methods. Useful data are obtained roughly in proportion to cost. Cored and rotary holes in the two valleys were followed by geophysical downhole logging with normal electric, spontaneous potential, single-point resistance, calliper, neutron -epithermal neutron, gamma-gamma, and natural gamma methods. Cores revealed multi-till sequences in the upper parts of the holes and, together with geophysical logs and their interpreted electrofacies, indicate the presence of underlying uniform, pollen-bearing (pine and spruce dominate) lacustrine fills. These fills and the buried valleys containing them are interpreted to be perhaps as old as the last interglacial. Buried valleys are fruitful targets for the discovery of older Quaternary sequences.Les vallCes encaissCes dans le roc et comblCes de dCp6ts meubles prtsentent un intCr6t Cconomique tant pour les eaux souterraines que pour les oeuvres de gCnie, et en outre un intCr6t scientifique pour les sCquences skdimentaires qu'elles peuvent renfermer. Deux valltes comblCes, d'une profondeur d'environ 60 m, localiskes au nord et nord-est de Guelph, Ontario, sont des tributaires de la vallCe principale de Dundas au sud-ouest de Guelph. Elles ont kt6 dCcouvertes g r k e aux donnkes recueillies lors des forages de puits pour l'eau, la localisation et la gkornktrie ont Ct C dkfinies plus finement par les travaux de gkophysique de surface, incluant les mtthodes de la rCsistivitC 2 1 trbs basse frkquence, de la conductivitC du terrain, de la rCsistivit6, d'tlectromagnCtisme transitoire, de la gravimktrie et de la sismicitk. Les donnkes utiles ont Ct C obtenues grossikrement en proportion des dCpenses consacrCes aux etudes. Les sondages rotatifs et carottCs dans les deux vallCes ont Ct C complCtCs par les diagraphies de parambtres gCophysiques mesurCs dans les trous de forage, incluant les mCthodes Clectrique normale, potentiel spontanC, rksistance de point unique, diamktrage, neutron -neutron Cpithermique, gamma-gamma et gamma naturel. L'Ctude des carottes rCv&le l'existence de skquences multiples de till dans les parties supCrieures des forages et, couplCe aux diagraphies gCophysiques et leurs klectrofac5s interprCtts, elle indique la prksence de dCp6ts lacustres de remplissage sous-jacents, uniformes et porteurs de pollens (surtout de pin et d'kpinette). Ces dCp6ts de remplissage et leu vallCes comblCes par eux sont interprCtCs comme datant possiblement du dernier interglaciaire. Les valltes combltes sont des cibles fructueuses pour la dtcouverte de vieill...

Evaluating the groundwater resource potential of the Dundas buried bedrock valley, southwestern Ontario: an integrated geological and hydrogeological case study

“…At least portions of the valley system are therefore inferred to be older than the last glaciation. Similar pre-Late Wisconsin sediment fills were observed in the Georgetown (Meyer and Eyles 2007), Rockwood (Greenhouse and Karrow 1994;Burt and Dodge 2016), and Hockley valleys (Bajc et al 2015) further to the north along the Niagara Escarpment. To the east, in the Niagara Peninsula, the St. David's buried gorge is infilled with sub-till, stratified deposits containing wood dated at 22.8-24.8 14 C ka BP (Hobson and Terasmae 1969;Barnett 1989;Bajc et al 2009).…”

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

“…3) indicated the presence of a local-scale BBV, which likely connected to the regional Guelph-Rockwood BBV ( Fig. 4; Karrow et al 1979;Greenhouse and Karrow 1994;Eyles et al 1997;Gao et al 2006). Based on these point data, the feature appears to be approximately 30 m deep, hundreds of metres wide, striking east-west with an approximate valley floor elevation of 290 m above sea level (masl).…”

“…Buried bedrock valleys and their Quaternary fills can be studied using a wide range of surface geophysical techniques (Greenhouse and Karrow 1994). However, the majority of studies to date have focused on the use of gravity (e.g., Greenhouse and Monier-Williams 1986;Gabriel et al 2003;Gabriel 2006;Møller et al 2007;Zweirs et al 2008;Bajc and Rainsford 2010;Burt and Rainsford 2010), which exploits changes in subsurface bulk density and ground and airborne transient electromagnetic methods (e.g., Jørgensen et al 2003aJørgensen et al , 2003bJørgensen et al , 2013Sørensen and Auken 2004;Steuer et al 2009;Høyer et al 2015;Oldenborger et al 2016).…”

“…In other words, the model that best matches the measured data is not necessarily an exact representation of the subsurface parameter distribution. Nevertheless, electrical imaging techniques using either induced or direct current sources have provided promising results for imaging valley morphology (e.g., Jørgensen et al 2003a;Ahmad et al 2009;Steuer et al 2009;Tye et al 2011), and in some cases, the infilled Quaternary sediment architecture (e.g., Baines et al 2002;Leopold et al 2013), particularly when combining multiple geophysical techniques with geological information (e.g., Greenhouse and Karrow 1994;Gabriel et al 2003;Jørgensen et al 2003b;Ahmad et al 2009). …”

“…Studies utilizing multiple geophysical methods combined with geologic and hydrogeologic data are particularly effective for 3D mapping of hydrostratigraphic units (e.g., Gabriel et al 2003;Jørgensen et al 2003aJørgensen et al , 2003bSteuer et al 2009; Stumpf and Ismail 2013;Cassidy et al 2014;Sapia et al 2014). In Ontario, shallow seismic reflection, gravity, and electromagnetic surveys have been used at mostly regional scales to delineate the nature of the infilled sediment and bedrock valley geometry (e.g., Greenhouse and Monier-Williams 1986;Greenhouse and Karrow 1994;Zweirs et al 2008;Burt 2011;Bajc et al 2012;Pugin et al 2013). Elsewhere, airborne transient electromagnetic methods are mostly used (e.g., Sandersen and Jørgensen 2003;Jørgensen et al 2003aJørgensen et al , 2003bSørensen and Auken 2004;Sapia et al 2015).…”