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Buried Bedrock Valleys and Glacial and Subglacial Meltwater Erosion in Southern Ontario, Canada

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Buried Bedrock Valleys and Glacial and Subglacial Meltwater Erosion in Southern Ontario, Canada See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/232806604 Buried bedrock valleys and glacial and subglacial meltwater erosion in Southern Ontario, Canada Article in Canadian Journal of Earth Sciences · May 2011 DOI: 10.1139/E10-104 CITATIONS READS 3 100 1 author: Cunhai Gao Ontario Geological Survey 22 PUBLICATIONS 149 CITATIONS SEE PROFILE Available from: Cunhai Gao Retrieved on: 02 August 2016 801 Buried bedrock valleys and glacial and subglacial meltwater erosion in southern Ontario, Canada Cunhai Gao Abstract: Morphometric features from a recently compiled bedrock topography map by the Ontario Geological Survey sug- gest a glacial erosion origin for the buried large bedrock valleys and troughs in southern Ontario. The bedrock valleys at Milverton, Wingham and Mount Forest are tunnel valleys, resulting from subglacial meltwater erosion beneath the Huron ice lobe, probably during or shortly after the Late-Wisconsinan glacial maximum. Diagnostic features for this interpretation include abrupt valley beginning and termination, uneven longitudinal valley profiles and up-slope gradients. The Dundas bedrock valley is the western extension of the Lake Ontario Basin. No comparable bedrock valleys were found to connect it to the Milverton valley for a joint drainage system as previously suggested. The Laurentian bedrock trough is the southeast- ward extension of the Georgian Bay Basin, both developed along shale bedrock between the Precambrian shield highlands and the Niagara Escarpment, resulting from long-term mechanical weathering associated with Pleistocene glacial erosion. This bedrock low has a floor that exceeds 50 km in width and is 26 m and more below the current water level of Georgian Bay. It could drain Georgian Bay should the drift cover be removed. There is no evidence to suggest that a preglacial river channel, if it existed, is still preserved in the floor of the Laurentian trough as previously suggested. The framework for an intensely glacially sculpted bedrock surface differs from the previous view for simple modification of a preglacial landscape and is, therefore, important in regional subsurface geological mapping and groundwater studies. Résumé : Les caractéristiques morphométriques illustrées sur une carte topographique du socle rocheux faite par la Com- mission géologique de l’Ontario suggèrent que les grandes vallées et fosses enfouies du sud de l’Ontario aient une origine d’érosion glaciaire. Les vallées rocheuses à Milverton, Wingham et Mount Forest sont des vallées de tunnels et proviennent de l’érosion par l’eau de fonte sous les glaciers, sous le lobe glaciaire Huron, probablement durant ou peu de temps après le maximum glaciaire du Wisconsin tardif. Les caractéristiques de diagnostique pour cette interprétation comprennent des dé- buts et des fins de vallée abruptes, des profils longitudinaux et des gradients de pente amont irréguliers. La vallée rocheuse Dundas constitue l’extension vers l’ouest du bassin du lac Ontario. Aucune autre vallée rocheuse n’a été découverte la re- liant à la vallée Milverton pour constituer un système de drainage conjoint, tel que suggéré antérieurement. La fosse lauren- tienne dans le socle est le prolongement vers le sud-est du bassin de la baie Georgienne, les deux s’étant développés le long For personal use only. du socle de shale entre les hautes terres du bouclier précambrien et l’escarpement du Niagara par la météorisation méca- nique à long terme associée à l’érosion glaciaire au Pléistocène. Ce creux du socle rocheux a un plancher qui a une largeur de plus de 50 km et il est à 26 m ou plus sous le niveau d’eau actuel de la baie Georgienne. Si le couvert glacio-sédimen- taire devait être retiré, ce creux pourrait drainer la baie Georgienne. Il n’existe aucune preuve suggérant qu’un chenal de ri- vière préglaciaire, s’il avait existé, soit encore préservé sur le plancher de la fosse laurentienne, tel que déjà suggéré. Le cadre pour une surface de socle intensément sculpté par les glaciers diffère de l’ancienne vue d’une simple modification d’un paysage préglaciaire et il est donc important dans la cartographie géologique régionale subsurface et les études de l’eau souterraine. [Traduit par la Rédaction] Introduction the reason for the various interpretations proposed for their origins, including relict preglacial channels, glacial scours, Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by 142.141.31.173 on 05/05/11 Buried bedrock valleys have been reported in southern On- tectonically controlled Tertiary river valleys or subglacial tario since the late 19th century (Spencer 1881, 1890; Kar- meltwater valleys or channels (Spencer 1907; Straw 1968; row 1973; Flint and Lolcama 1986; Eyles et al. 1993, 1997). Karrow 1973; Brennand and Shaw 1994; Eyles et al. 1997; Early studies are based on hand-contoured maps, and details Kor and Cowell 1998). on the bedrock valleys are generally lacking as to the geome- Previous compilations for bedrock topography are lacking try, longitudinal profile, and their spatial relationships with quality controls on water-well records, the major source for other bedrock valleys in the vicinity. This is probably part of the depth-to-bedrock information. The current water-well da- Received 30 June 2010. Accepted 2 December 2010. Published tabase archived at the Ontario Ministry of Environment in at www.nrcresearchpress.com/cjes on 4 May 2011. Toronto has more than half a million records for southern Ontario. These records are notoriously inconsistent in quality, Paper handled by Associate Editor Timothy Fisher. containing georeferencing errors and incorrect geological de- C. Gao. Sedimentary Geoscience Section, Ontario Geological scriptions largely owing to the reporting procedure, inaccu- Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5, Canada. rate locations sketched and the lack of detailed material information because of the commonly used wash-bored drill- E-mail for correspondence: [email protected]. Can. J. Earth Sci. 48: 801–818 (2011) doi:10.1139/E10-104 Published by NRC Research Press 802 Can. J. Earth Sci., Vol. 48, 2011 ing method, and, lastly, the fact that most water-well drillers Methods are not trained professional geologists (Russell et al. 1998). Depth-to-bedrock information was extracted from water- The database has been systematically filtered for georeferenc- well, petroleum, and geotechnical drill records, as well as ing errors in recent subsurface mapping (Kenny et al. 1997; from published geological maps. Detailed descriptions of the Logan et al. 2005); however, the geological content of the re- methods have already been released, and the following is a cords has rarely been critiqued. summary of the quality control procedures adopted in this Recently, the Ontario Geological Survey developed proto- compilation. Readers can refer to Gao et al. (2006, 2007) for cols and a methodology to generate digital regional bedrock details. surface maps (Gao et al. 2006, 2007). Using this methodol- The water-well database that contains over half a million ogy, rigid quality control measures were employed to track records is the largest source for depth-to-bedrock information. and eliminate problematic data during the compilation. The The water-well records were systematically filtered to remove resultant map has enabled better delineation of the bedrock georeferencing errors, including unreliable locations, ground topography, in particular, the regional extent of significant surface elevations inconsistent with digital elevation model buried bedrock valleys or depressions. This paper introduces (DEM) values, and wells located within lake boundaries. Ap- briefly the methodology, describes in detail major bedrock plying these restrictions provided an initial database with valleys in southern Ontario, and discusses the possible causal more than 350 000 water-well records for southern Ontario. mechanisms. In the discussion that follows, some large bed- The drift–bedrock contact was then assigned through an auto- rock depressions with a size of 20 km and greater in width mation process (Gao et al. 2006, 2007). However, water-well are referred to as troughs. records containing ambiguous or questionable entries for bed- rock, such as basalt, conglomerate, greywacke, slate, sand- stone, and soapstone that do not occur or have limited Geological setting occurrence in southern Ontario, were inspected and the – Southern Ontario is underlain by a Precambrian basement drift bedrock contact was manually assigned. – containing Proterozoic gneissic rocks and an overlying Paleo- The borehole records with assigned drift bedrock contact zoic cover rock (Fig. 1A; Ontario Geological Survey 1991; were further filtered and those with inverted stratigraphy, e. Johnson et al. 1992). In the basement across southwestern g., a granite (Precambrian) overlying a limestone (Paleozoic) Ontario lie northeast-trending tectonic highs, referred to as or with duplicate locations, but having different depths to bedrock, were removed. Lastly, water-well records with ex- the Findlay and Algonquin Archs, which separate the Michi- cess depth to bedrock (>8 m) in the known thin-drift areas gan intracratonic basin to the northwest from the Appala- (<1 m) mapped by Ontario Geological Survey (2003) were chian foreland basin to the southeast (Fig. 1B). The inspected and many were removed because of the incorrectly Paleozoic bedrock thickens toward the basin centers, and, in assigned drift–bedrock
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